2.2.0.M4

Copyright © 2012-2019

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1. Spring Boot Documentation

1.1. About the Documentation

The Spring Boot reference guide is available as

The latest copy is available at docs.spring.io/spring-boot/docs/current/reference.

Copies of this document may be made for your own use and for distribution to others, provided that you do not charge any fee for such copies and further provided that each copy contains this Copyright Notice, whether distributed in print or electronically.

1.2. Getting Help

If you have trouble with Spring Boot, we would like to help.

All of Spring Boot is open source, including the documentation. If you find problems with the docs or if you want to improve them, please get involved.

1.3. First Steps

If you are getting started with Spring Boot or 'Spring' in general, start with the following topics:

1.4. Working with Spring Boot

Ready to actually start using Spring Boot? We have you covered:

1.5. Learning about Spring Boot Features

Need more details about Spring Boot’s core features? The following content is for you:

1.6. Moving to Production

When you are ready to push your Spring Boot application to production, we have some tricks that you might like:

1.7. Advanced Topics

Finally, we have a few topics for more advanced users:

2. Getting Started

2.1. Introducing Spring Boot

Spring Boot makes it easy to create stand-alone, production-grade Spring-based Applications that you can run. We take an opinionated view of the Spring platform and third-party libraries, so that you can get started with minimum fuss. Most Spring Boot applications need very little Spring configuration.

You can use Spring Boot to create Java applications that can be started by using java -jar or more traditional war deployments. We also provide a command line tool that runs “spring scripts”.

Our primary goals are:

  • Provide a radically faster and widely accessible getting-started experience for all Spring development.

  • Be opinionated out of the box but get out of the way quickly as requirements start to diverge from the defaults.

  • Provide a range of non-functional features that are common to large classes of projects (such as embedded servers, security, metrics, health checks, and externalized configuration).

  • Absolutely no code generation and no requirement for XML configuration.

2.2. System Requirements

Spring Boot 2.2.0.M4 requires Java 8 and is compatible up to Java 11 (included). Spring Framework {spring-framework-version} or above is also required.

Explicit build support is provided for the following build tools:

Build Tool Version

Maven

3.3+

Gradle

5.x (4.10 is also supported but in a deprecated form)

2.2.1. Servlet Containers

Spring Boot supports the following embedded servlet containers:

Name Servlet Version

Tomcat 9.0

4.0

Jetty 9.4

3.1

Undertow 2.0

4.0

You can also deploy Spring Boot applications to any Servlet 3.1+ compatible container.

2.3. Installing Spring Boot

Spring Boot can be used with “classic” Java development tools or installed as a command line tool. Either way, you need Java SDK v1.8 or higher. Before you begin, you should check your current Java installation by using the following command:

$ java -version

If you are new to Java development or if you want to experiment with Spring Boot, you might want to try the Spring Boot CLI (Command Line Interface) first. Otherwise, read on for “classic” installation instructions.

2.3.1. Installation Instructions for the Java Developer

You can use Spring Boot in the same way as any standard Java library. To do so, include the appropriate spring-boot-*.jar files on your classpath. Spring Boot does not require any special tools integration, so you can use any IDE or text editor. Also, there is nothing special about a Spring Boot application, so you can run and debug a Spring Boot application as you would any other Java program.

Although you could copy Spring Boot jars, we generally recommend that you use a build tool that supports dependency management (such as Maven or Gradle).

Maven Installation

Spring Boot is compatible with Apache Maven 3.3 or above. If you do not already have Maven installed, you can follow the instructions at maven.apache.org.

On many operating systems, Maven can be installed with a package manager. If you use OSX Homebrew, try brew install maven. Ubuntu users can run sudo apt-get install maven. Windows users with Chocolatey can run choco install maven from an elevated (administrator) prompt.

Spring Boot dependencies use the org.springframework.boot groupId. Typically, your Maven POM file inherits from the spring-boot-starter-parent project and declares dependencies to one or more “Starters”. Spring Boot also provides an optional Maven plugin to create executable jars.

The following listing shows a typical pom.xml file:

<?xml version="1.0" encoding="UTF-8"?>
<project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 https://maven.apache.org/xsd/maven-4.0.0.xsd">
    <modelVersion>4.0.0</modelVersion>

    <groupId>com.example</groupId>
    <artifactId>myproject</artifactId>
    <version>0.0.1-SNAPSHOT</version>

    <!-- Inherit defaults from Spring Boot -->
    <parent>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-starter-parent</artifactId>
        <version>2.2.0.M4</version>
    </parent>

    <!-- Add typical dependencies for a web application -->
    <dependencies>
        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter-web</artifactId>
        </dependency>
    </dependencies>

    <!-- Package as an executable jar -->
    <build>
        <plugins>
            <plugin>
                <groupId>org.springframework.boot</groupId>
                <artifactId>spring-boot-maven-plugin</artifactId>
            </plugin>
        </plugins>
    </build>

    <!-- Add Spring repositories -->
    <!-- (you don't need this if you are using a .RELEASE version) -->
    <repositories>
        <repository>
            <id>spring-snapshots</id>
            <url>https://repo.spring.io/snapshot</url>
            <snapshots><enabled>true</enabled></snapshots>
        </repository>
        <repository>
            <id>spring-milestones</id>
            <url>https://repo.spring.io/milestone</url>
        </repository>
    </repositories>
    <pluginRepositories>
        <pluginRepository>
            <id>spring-snapshots</id>
            <url>https://repo.spring.io/snapshot</url>
        </pluginRepository>
        <pluginRepository>
            <id>spring-milestones</id>
            <url>https://repo.spring.io/milestone</url>
        </pluginRepository>
    </pluginRepositories>
</project>
The spring-boot-starter-parent is a great way to use Spring Boot, but it might not be suitable all of the time. Sometimes you may need to inherit from a different parent POM, or you might not like our default settings. In those cases, see Using Spring Boot without the Parent POM for an alternative solution that uses an import scope.
Gradle Installation

Spring Boot is compatible with 5.x. 4.10 is also supported but this support is deprecated and will be removed in a future release. If you do not already have Gradle installed, you can follow the instructions at gradle.org.

Spring Boot dependencies can be declared by using the org.springframework.boot group. Typically, your project declares dependencies to one or more “Starters”. Spring Boot provides a useful Gradle plugin that can be used to simplify dependency declarations and to create executable jars.

Gradle Wrapper

The Gradle Wrapper provides a nice way of “obtaining” Gradle when you need to build a project. It is a small script and library that you commit alongside your code to bootstrap the build process. See docs.gradle.org/4.2.1/userguide/gradle_wrapper.html for details.

More details on getting started with Spring Boot and Gradle can be found in the Getting Started section of the Gradle plugin’s reference guide.

2.3.2. Installing the Spring Boot CLI

The Spring Boot CLI (Command Line Interface) is a command line tool that you can use to quickly prototype with Spring. It lets you run Groovy scripts, which means that you have a familiar Java-like syntax without so much boilerplate code.

You do not need to use the CLI to work with Spring Boot, but it is definitely the quickest way to get a Spring application off the ground.

Manual Installation

You can download the Spring CLI distribution from the Spring software repository:

Cutting edge snapshot distributions are also available.

Once downloaded, follow the INSTALL.txt instructions from the unpacked archive. In summary, there is a spring script (spring.bat for Windows) in a bin/ directory in the .zip file. Alternatively, you can use java -jar with the .jar file (the script helps you to be sure that the classpath is set correctly).

Installation with SDKMAN!

SDKMAN! (The Software Development Kit Manager) can be used for managing multiple versions of various binary SDKs, including Groovy and the Spring Boot CLI. Get SDKMAN! from sdkman.io and install Spring Boot by using the following commands:

$ sdk install springboot
$ spring --version
Spring Boot v2.2.0.M4

If you develop features for the CLI and want easy access to the version you built, use the following commands:

$ sdk install springboot dev /path/to/spring-boot/spring-boot-cli/target/spring-boot-cli-2.2.0.M4-bin/spring-2.2.0.M4/
$ sdk default springboot dev
$ spring --version
Spring CLI v2.2.0.M4

The preceding instructions install a local instance of spring called the dev instance. It points at your target build location, so every time you rebuild Spring Boot, spring is up-to-date.

You can see it by running the following command:

$ sdk ls springboot

================================================================================
Available Springboot Versions
================================================================================
> + dev
* 2.2.0.M4

================================================================================
+ - local version
* - installed
> - currently in use
================================================================================
OSX Homebrew Installation

If you are on a Mac and use Homebrew, you can install the Spring Boot CLI by using the following commands:

$ brew tap pivotal/tap
$ brew install springboot

Homebrew installs spring to /usr/local/bin.

If you do not see the formula, your installation of brew might be out-of-date. In that case, run brew update and try again.
MacPorts Installation

If you are on a Mac and use MacPorts, you can install the Spring Boot CLI by using the following command:

$ sudo port install spring-boot-cli
Command-line Completion

The Spring Boot CLI includes scripts that provide command completion for the BASH and zsh shells. You can source the script (also named spring) in any shell or put it in your personal or system-wide bash completion initialization. On a Debian system, the system-wide scripts are in /shell-completion/bash and all scripts in that directory are executed when a new shell starts. For example, to run the script manually if you have installed by using SDKMAN!, use the following commands:

$ . ~/.sdkman/candidates/springboot/current/shell-completion/bash/spring
$ spring <HIT TAB HERE>
  grab  help  jar  run  test  version
If you install the Spring Boot CLI by using Homebrew or MacPorts, the command-line completion scripts are automatically registered with your shell.
Windows Scoop Installation

If you are on a Windows and use Scoop, you can install the Spring Boot CLI by using the following commands:

> scoop bucket add extras
> scoop install springboot

Scoop installs spring to ~/scoop/apps/springboot/current/bin.

If you do not see the app manifest, your installation of scoop might be out-of-date. In that case, run scoop update and try again.
Quick-start Spring CLI Example

You can use the following web application to test your installation. To start, create a file called app.groovy, as follows:

@RestController
class ThisWillActuallyRun {

    @RequestMapping("/")
    String home() {
        "Hello World!"
    }

}

Then run it from a shell, as follows:

$ spring run app.groovy
The first run of your application is slow, as dependencies are downloaded. Subsequent runs are much quicker.

Open localhost:8080 in your favorite web browser. You should see the following output:

Hello World!

2.3.3. Upgrading from an Earlier Version of Spring Boot

If you are upgrading from an earlier release of Spring Boot, check the “migration guide” on the project wiki that provides detailed upgrade instructions. Check also the “release notes” for a list of “new and noteworthy” features for each release.

When upgrading to a new feature release, some properties may have been renamed or removed. Spring Boot provides a way to analyze your application’s environment and print diagnostics at startup, but also temporarily migrate properties at runtime for you. To enable that feature, add the following dependency to your project:

<dependency>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-properties-migrator</artifactId>
    <scope>runtime</scope>
</dependency>
Properties that are added late to the environment, such as when using @PropertySource, will not be taken into account.
Once you’re done with the migration, please make sure to remove this module from your project’s dependencies.

To upgrade an existing CLI installation, use the appropriate package manager command (for example, brew upgrade) or, if you manually installed the CLI, follow the standard instructions, remembering to update your PATH environment variable to remove any older references.

2.4. Developing Your First Spring Boot Application

This section describes how to develop a simple “Hello World!” web application that highlights some of Spring Boot’s key features. We use Maven to build this project, since most IDEs support it.

The spring.io web site contains many “Getting Started” guides that use Spring Boot. If you need to solve a specific problem, check there first.

You can shortcut the steps below by going to start.spring.io and choosing the "Web" starter from the dependencies searcher. Doing so generates a new project structure so that you can start coding right away. Check the Spring Initializr documentation for more details.

Before we begin, open a terminal and run the following commands to ensure that you have valid versions of Java and Maven installed:

$ java -version
java version "1.8.0_102"
Java(TM) SE Runtime Environment (build 1.8.0_102-b14)
Java HotSpot(TM) 64-Bit Server VM (build 25.102-b14, mixed mode)
$ mvn -v
Apache Maven 3.5.4 (1edded0938998edf8bf061f1ceb3cfdeccf443fe; 2018-06-17T14:33:14-04:00)
Maven home: /usr/local/Cellar/maven/3.3.9/libexec
Java version: 1.8.0_102, vendor: Oracle Corporation
This sample needs to be created in its own folder. Subsequent instructions assume that you have created a suitable folder and that it is your current directory.

2.4.1. Creating the POM

We need to start by creating a Maven pom.xml file. The pom.xml is the recipe that is used to build your project. Open your favorite text editor and add the following:

<?xml version="1.0" encoding="UTF-8"?>
<project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 https://maven.apache.org/xsd/maven-4.0.0.xsd">
    <modelVersion>4.0.0</modelVersion>

    <groupId>com.example</groupId>
    <artifactId>myproject</artifactId>
    <version>0.0.1-SNAPSHOT</version>

    <parent>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-starter-parent</artifactId>
        <version>2.2.0.M4</version>
    </parent>

    <!-- Additional lines to be added here... -->

    <!-- (you don't need this if you are using a .RELEASE version) -->
    <repositories>
        <repository>
            <id>spring-snapshots</id>
            <url>https://repo.spring.io/snapshot</url>
            <snapshots><enabled>true</enabled></snapshots>
        </repository>
        <repository>
            <id>spring-milestones</id>
            <url>https://repo.spring.io/milestone</url>
        </repository>
    </repositories>
    <pluginRepositories>
        <pluginRepository>
            <id>spring-snapshots</id>
            <url>https://repo.spring.io/snapshot</url>
        </pluginRepository>
        <pluginRepository>
            <id>spring-milestones</id>
            <url>https://repo.spring.io/milestone</url>
        </pluginRepository>
    </pluginRepositories>
</project>

The preceding listing should give you a working build. You can test it by running mvn package (for now, you can ignore the “jar will be empty - no content was marked for inclusion!” warning).

At this point, you could import the project into an IDE (most modern Java IDEs include built-in support for Maven). For simplicity, we continue to use a plain text editor for this example.

2.4.2. Adding Classpath Dependencies

Spring Boot provides a number of “Starters” that let you add jars to your classpath. Our sample application has already used spring-boot-starter-parent in the parent section of the POM. The spring-boot-starter-parent is a special starter that provides useful Maven defaults. It also provides a dependency-management section so that you can omit version tags for “blessed” dependencies.

Other “Starters” provide dependencies that you are likely to need when developing a specific type of application. Since we are developing a web application, we add a spring-boot-starter-web dependency. Before that, we can look at what we currently have by running the following command:

$ mvn dependency:tree

[INFO] com.example:myproject:jar:0.0.1-SNAPSHOT

The mvn dependency:tree command prints a tree representation of your project dependencies. You can see that spring-boot-starter-parent provides no dependencies by itself. To add the necessary dependencies, edit your pom.xml and add the spring-boot-starter-web dependency immediately below the parent section:

<dependencies>
    <dependency>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-starter-web</artifactId>
    </dependency>
</dependencies>

If you run mvn dependency:tree again, you see that there are now a number of additional dependencies, including the Tomcat web server and Spring Boot itself.

2.4.3. Writing the Code

To finish our application, we need to create a single Java file. By default, Maven compiles sources from src/main/java, so you need to create that folder structure and then add a file named src/main/java/Example.java to contain the following code:

import org.springframework.boot.*;
import org.springframework.boot.autoconfigure.*;
import org.springframework.web.bind.annotation.*;

@RestController
@EnableAutoConfiguration
public class Example {

    @RequestMapping("/")
    String home() {
        return "Hello World!";
    }

    public static void main(String[] args) {
        SpringApplication.run(Example.class, args);
    }

}

Although there is not much code here, quite a lot is going on. We step through the important parts in the next few sections.

The @RestController and @RequestMapping Annotations

The first annotation on our Example class is @RestController. This is known as a stereotype annotation. It provides hints for people reading the code and for Spring that the class plays a specific role. In this case, our class is a web @Controller, so Spring considers it when handling incoming web requests.

The @RequestMapping annotation provides “routing” information. It tells Spring that any HTTP request with the / path should be mapped to the home method. The @RestController annotation tells Spring to render the resulting string directly back to the caller.

The @RestController and @RequestMapping annotations are Spring MVC annotations. (They are not specific to Spring Boot.) See the MVC section in the Spring Reference Documentation for more details.
The @EnableAutoConfiguration Annotation

The second class-level annotation is @EnableAutoConfiguration. This annotation tells Spring Boot to “guess” how you want to configure Spring, based on the jar dependencies that you have added. Since spring-boot-starter-web added Tomcat and Spring MVC, the auto-configuration assumes that you are developing a web application and sets up Spring accordingly.

Starters and Auto-configuration

Auto-configuration is designed to work well with “Starters”, but the two concepts are not directly tied. You are free to pick and choose jar dependencies outside of the starters. Spring Boot still does its best to auto-configure your application.

The “main” Method

The final part of our application is the main method. This is just a standard method that follows the Java convention for an application entry point. Our main method delegates to Spring Boot’s SpringApplication class by calling run. SpringApplication bootstraps our application, starting Spring, which, in turn, starts the auto-configured Tomcat web server. We need to pass Example.class as an argument to the run method to tell SpringApplication which is the primary Spring component. The args array is also passed through to expose any command-line arguments.

2.4.4. Running the Example

At this point, your application should work. Since you used the spring-boot-starter-parent POM, you have a useful run goal that you can use to start the application. Type mvn spring-boot:run from the root project directory to start the application. You should see output similar to the following:

$ mvn spring-boot:run

  .   ____          _            __ _ _
 /\\ / ___'_ __ _ _(_)_ __  __ _ \ \ \ \
( ( )\___ | '_ | '_| | '_ \/ _` | \ \ \ \
 \\/  ___)| |_)| | | | | || (_| |  ) ) ) )
  '  |____| .__|_| |_|_| |_\__, | / / / /
 =========|_|==============|___/=/_/_/_/
 :: Spring Boot ::  (v2.2.0.M4)
....... . . .
....... . . . (log output here)
....... . . .
........ Started Example in 2.222 seconds (JVM running for 6.514)

If you open a web browser to localhost:8080, you should see the following output:

Hello World!

To gracefully exit the application, press ctrl-c.

2.4.5. Creating an Executable Jar

We finish our example by creating a completely self-contained executable jar file that we could run in production. Executable jars (sometimes called “fat jars”) are archives containing your compiled classes along with all of the jar dependencies that your code needs to run.

Executable jars and Java

Java does not provide a standard way to load nested jar files (jar files that are themselves contained within a jar). This can be problematic if you are looking to distribute a self-contained application.

To solve this problem, many developers use “uber” jars. An uber jar packages all the classes from all the application’s dependencies into a single archive. The problem with this approach is that it becomes hard to see which libraries are in your application. It can also be problematic if the same filename is used (but with different content) in multiple jars.

Spring Boot takes a different approach and lets you actually nest jars directly.

To create an executable jar, we need to add the spring-boot-maven-plugin to our pom.xml. To do so, insert the following lines just below the dependencies section:

<build>
    <plugins>
        <plugin>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-maven-plugin</artifactId>
        </plugin>
    </plugins>
</build>
The spring-boot-starter-parent POM includes <executions> configuration to bind the repackage goal. If you do not use the parent POM, you need to declare this configuration yourself. See the plugin documentation for details.

Save your pom.xml and run mvn package from the command line, as follows:

$ mvn package

[INFO] Scanning for projects...
[INFO]
[INFO] ------------------------------------------------------------------------
[INFO] Building myproject 0.0.1-SNAPSHOT
[INFO] ------------------------------------------------------------------------
[INFO] .... ..
[INFO] --- maven-jar-plugin:2.4:jar (default-jar) @ myproject ---
[INFO] Building jar: /Users/developer/example/spring-boot-example/target/myproject-0.0.1-SNAPSHOT.jar
[INFO]
[INFO] --- spring-boot-maven-plugin:2.2.0.M4:repackage (default) @ myproject ---
[INFO] ------------------------------------------------------------------------
[INFO] BUILD SUCCESS
[INFO] ------------------------------------------------------------------------

If you look in the target directory, you should see myproject-0.0.1-SNAPSHOT.jar. The file should be around 10 MB in size. If you want to peek inside, you can use jar tvf, as follows:

$ jar tvf target/myproject-0.0.1-SNAPSHOT.jar

You should also see a much smaller file named myproject-0.0.1-SNAPSHOT.jar.original in the target directory. This is the original jar file that Maven created before it was repackaged by Spring Boot.

To run that application, use the java -jar command, as follows:

$ java -jar target/myproject-0.0.1-SNAPSHOT.jar

  .   ____          _            __ _ _
 /\\ / ___'_ __ _ _(_)_ __  __ _ \ \ \ \
( ( )\___ | '_ | '_| | '_ \/ _` | \ \ \ \
 \\/  ___)| |_)| | | | | || (_| |  ) ) ) )
  '  |____| .__|_| |_|_| |_\__, | / / / /
 =========|_|==============|___/=/_/_/_/
 :: Spring Boot ::  (v2.2.0.M4)
....... . . .
....... . . . (log output here)
....... . . .
........ Started Example in 2.536 seconds (JVM running for 2.864)

As before, to exit the application, press ctrl-c.

2.5. What to Read Next

Hopefully, this section provided some of the Spring Boot basics and got you on your way to writing your own applications. If you are a task-oriented type of developer, you might want to jump over to spring.io and check out some of the getting started guides that solve specific “How do I do that with Spring?” problems. We also have Spring Boot-specific “How-to” reference documentation.

The Spring Boot repository also has a bunch of samples you can run. The samples are independent of the rest of the code (that is, you do not need to build the rest to run or use the samples).

Otherwise, the next logical step is to read Using Spring Boot. If you are really impatient, you could also jump ahead and read about Spring Boot features.

3. Using Spring Boot

3.1. Build Systems

It is strongly recommended that you choose a build system that supports dependency management and that can consume artifacts published to the “Maven Central” repository. We would recommend that you choose Maven or Gradle. It is possible to get Spring Boot to work with other build systems (Ant, for example), but they are not particularly well supported.

3.1.1. Dependency Management

Each release of Spring Boot provides a curated list of dependencies that it supports. In practice, you do not need to provide a version for any of these dependencies in your build configuration, as Spring Boot manages that for you. When you upgrade Spring Boot itself, these dependencies are upgraded as well in a consistent way.

You can still specify a version and override Spring Boot’s recommendations if you need to do so.

The curated list contains all the spring modules that you can use with Spring Boot as well as a refined list of third party libraries. The list is available as a standard Bills of Materials (spring-boot-dependencies) that can be used with both Maven and Gradle.

Each release of Spring Boot is associated with a base version of the Spring Framework. We highly recommend that you not specify its version.

3.1.2. Maven

Maven users can inherit from the spring-boot-starter-parent project to obtain sensible defaults. The parent project provides the following features:

  • Java 1.8 as the default compiler level.

  • UTF-8 source encoding.

  • A Dependency Management section, inherited from the spring-boot-dependencies pom, that manages the versions of common dependencies. This dependency management lets you omit <version> tags for those dependencies when used in your own pom.

  • An execution of the repackage goal with a repackage execution id.

  • Sensible resource filtering.

  • Sensible plugin configuration (exec plugin, Git commit ID, and shade).

  • Sensible resource filtering for application.properties and application.yml including profile-specific files (for example, application-dev.properties and application-dev.yml)

Note that, since the application.properties and application.yml files accept Spring style placeholders (${…​}), the Maven filtering is changed to use @..@ placeholders. (You can override that by setting a Maven property called resource.delimiter.)

Inheriting the Starter Parent

To configure your project to inherit from the spring-boot-starter-parent, set the parent as follows:

<!-- Inherit defaults from Spring Boot -->
<parent>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-starter-parent</artifactId>
    <version>2.2.0.M4</version>
</parent>
You should need to specify only the Spring Boot version number on this dependency. If you import additional starters, you can safely omit the version number.

With that setup, you can also override individual dependencies by overriding a property in your own project. For instance, to upgrade to another Spring Data release train, you would add the following to your pom.xml:

<properties>
    <spring-data-releasetrain.version>Fowler-SR2</spring-data-releasetrain.version>
</properties>
Check the spring-boot-dependencies pom for a list of supported properties.
Using Spring Boot without the Parent POM

Not everyone likes inheriting from the spring-boot-starter-parent POM. You may have your own corporate standard parent that you need to use or you may prefer to explicitly declare all your Maven configuration.

If you do not want to use the spring-boot-starter-parent, you can still keep the benefit of the dependency management (but not the plugin management) by using a scope=import dependency, as follows:

<dependencyManagement>
    <dependencies>
        <dependency>
            <!-- Import dependency management from Spring Boot -->
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-dependencies</artifactId>
            <version>2.2.0.M4</version>
            <type>pom</type>
            <scope>import</scope>
        </dependency>
    </dependencies>
</dependencyManagement>

The preceding sample setup does not let you override individual dependencies by using a property, as explained above. To achieve the same result, you need to add an entry in the dependencyManagement of your project before the spring-boot-dependencies entry. For instance, to upgrade to another Spring Data release train, you could add the following element to your pom.xml:

<dependencyManagement>
    <dependencies>
        <!-- Override Spring Data release train provided by Spring Boot -->
        <dependency>
            <groupId>org.springframework.data</groupId>
            <artifactId>spring-data-releasetrain</artifactId>
            <version>Fowler-SR2</version>
            <type>pom</type>
            <scope>import</scope>
        </dependency>
        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-dependencies</artifactId>
            <version>2.2.0.M4</version>
            <type>pom</type>
            <scope>import</scope>
        </dependency>
    </dependencies>
</dependencyManagement>
In the preceding example, we specify a BOM, but any dependency type can be overridden in the same way.
Using the Spring Boot Maven Plugin

Spring Boot includes a Maven plugin that can package the project as an executable jar. Add the plugin to your <plugins> section if you want to use it, as shown in the following example:

<build>
    <plugins>
        <plugin>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-maven-plugin</artifactId>
        </plugin>
    </plugins>
</build>
If you use the Spring Boot starter parent pom, you need to add only the plugin. There is no need to configure it unless you want to change the settings defined in the parent.

3.1.3. Gradle

To learn about using Spring Boot with Gradle, please refer to the documentation for Spring Boot’s Gradle plugin:

3.1.4. Ant

It is possible to build a Spring Boot project using Apache Ant+Ivy. The spring-boot-antlib “AntLib” module is also available to help Ant create executable jars.

To declare dependencies, a typical ivy.xml file looks something like the following example:

<ivy-module version="2.0">
    <info organisation="org.springframework.boot" module="spring-boot-sample-ant" />
    <configurations>
        <conf name="compile" description="everything needed to compile this module" />
        <conf name="runtime" extends="compile" description="everything needed to run this module" />
    </configurations>
    <dependencies>
        <dependency org="org.springframework.boot" name="spring-boot-starter"
            rev="${spring-boot.version}" conf="compile" />
    </dependencies>
</ivy-module>

A typical build.xml looks like the following example:

<project
    xmlns:ivy="antlib:org.apache.ivy.ant"
    xmlns:spring-boot="antlib:org.springframework.boot.ant"
    name="myapp" default="build">

    <property name="spring-boot.version" value="2.2.0.M4" />

    <target name="resolve" description="--> retrieve dependencies with ivy">
        <ivy:retrieve pattern="lib/[conf]/[artifact]-[type]-[revision].[ext]" />
    </target>

    <target name="classpaths" depends="resolve">
        <path id="compile.classpath">
            <fileset dir="lib/compile" includes="*.jar" />
        </path>
    </target>

    <target name="init" depends="classpaths">
        <mkdir dir="build/classes" />
    </target>

    <target name="compile" depends="init" description="compile">
        <javac srcdir="src/main/java" destdir="build/classes" classpathref="compile.classpath" />
    </target>

    <target name="build" depends="compile">
        <spring-boot:exejar destfile="build/myapp.jar" classes="build/classes">
            <spring-boot:lib>
                <fileset dir="lib/runtime" />
            </spring-boot:lib>
        </spring-boot:exejar>
    </target>
</project>
If you do not want to use the spring-boot-antlib module, see the Build an Executable Archive from Ant without Using spring-boot-antlib “How-to” .

3.1.5. Starters

Starters are a set of convenient dependency descriptors that you can include in your application. You get a one-stop shop for all the Spring and related technologies that you need without having to hunt through sample code and copy-paste loads of dependency descriptors. For example, if you want to get started using Spring and JPA for database access, include the spring-boot-starter-data-jpa dependency in your project.

The starters contain a lot of the dependencies that you need to get a project up and running quickly and with a consistent, supported set of managed transitive dependencies.

What’s in a name

All official starters follow a similar naming pattern; spring-boot-starter-*, where * is a particular type of application. This naming structure is intended to help when you need to find a starter. The Maven integration in many IDEs lets you search dependencies by name. For example, with the appropriate Eclipse or STS plugin installed, you can press ctrl-space in the POM editor and type “spring-boot-starter” for a complete list.

As explained in the “Creating Your Own Starter” section, third party starters should not start with spring-boot, as it is reserved for official Spring Boot artifacts. Rather, a third-party starter typically starts with the name of the project. For example, a third-party starter project called thirdpartyproject would typically be named thirdpartyproject-spring-boot-starter.

The following application starters are provided by Spring Boot under the org.springframework.boot group:

Table 1. Spring Boot application starters
Name Description Pom

spring-boot-starter

Core starter, including auto-configuration support, logging and YAML

Pom

spring-boot-starter-activemq

Starter for JMS messaging using Apache ActiveMQ

Pom

spring-boot-starter-amqp

Starter for using Spring AMQP and Rabbit MQ

Pom

spring-boot-starter-aop

Starter for aspect-oriented programming with Spring AOP and AspectJ

Pom

spring-boot-starter-artemis

Starter for JMS messaging using Apache Artemis

Pom

spring-boot-starter-batch

Starter for using Spring Batch

Pom

spring-boot-starter-cache

Starter for using Spring Framework’s caching support

Pom

spring-boot-starter-cloud-connectors

Starter for using Spring Cloud Connectors which simplifies connecting to services in cloud platforms like Cloud Foundry and Heroku

Pom

spring-boot-starter-data-cassandra

Starter for using Cassandra distributed database and Spring Data Cassandra

Pom

spring-boot-starter-data-cassandra-reactive

Starter for using Cassandra distributed database and Spring Data Cassandra Reactive

Pom

spring-boot-starter-data-couchbase

Starter for using Couchbase document-oriented database and Spring Data Couchbase

Pom

spring-boot-starter-data-couchbase-reactive

Starter for using Couchbase document-oriented database and Spring Data Couchbase Reactive

Pom

spring-boot-starter-data-elasticsearch

Starter for using Elasticsearch search and analytics engine and Spring Data Elasticsearch

Pom

spring-boot-starter-data-jdbc

Starter for using Spring Data JDBC

Pom

spring-boot-starter-data-jpa

Starter for using Spring Data JPA with Hibernate

Pom

spring-boot-starter-data-ldap

Starter for using Spring Data LDAP

Pom

spring-boot-starter-data-mongodb

Starter for using MongoDB document-oriented database and Spring Data MongoDB

Pom

spring-boot-starter-data-mongodb-reactive

Starter for using MongoDB document-oriented database and Spring Data MongoDB Reactive

Pom

spring-boot-starter-data-neo4j

Starter for using Neo4j graph database and Spring Data Neo4j

Pom

spring-boot-starter-data-redis

Starter for using Redis key-value data store with Spring Data Redis and the Lettuce client

Pom

spring-boot-starter-data-redis-reactive

Starter for using Redis key-value data store with Spring Data Redis reactive and the Lettuce client

Pom

spring-boot-starter-data-rest

Starter for exposing Spring Data repositories over REST using Spring Data REST

Pom

spring-boot-starter-data-solr

Starter for using the Apache Solr search platform with Spring Data Solr

Pom

spring-boot-starter-freemarker

Starter for building MVC web applications using FreeMarker views

Pom

spring-boot-starter-groovy-templates

Starter for building MVC web applications using Groovy Templates views

Pom

spring-boot-starter-hateoas

Starter for building hypermedia-based RESTful web application with Spring MVC and Spring HATEOAS

Pom

spring-boot-starter-integration

Starter for using Spring Integration

Pom

spring-boot-starter-jdbc

Starter for using JDBC with the HikariCP connection pool

Pom

spring-boot-starter-jersey

Starter for building RESTful web applications using JAX-RS and Jersey. An alternative to spring-boot-starter-web

Pom

spring-boot-starter-jooq

Starter for using jOOQ to access SQL databases. An alternative to spring-boot-starter-data-jpa or spring-boot-starter-jdbc

Pom

spring-boot-starter-json

Starter for reading and writing json

Pom

spring-boot-starter-jta-atomikos

Starter for JTA transactions using Atomikos

Pom

spring-boot-starter-jta-bitronix

Starter for JTA transactions using Bitronix

Pom

spring-boot-starter-mail

Starter for using Java Mail and Spring Framework’s email sending support

Pom

spring-boot-starter-mustache

Starter for building web applications using Mustache views

Pom

spring-boot-starter-oauth2-client

Starter for using Spring Security’s OAuth2/OpenID Connect client features

Pom

spring-boot-starter-oauth2-resource-server

Starter for using Spring Security’s OAuth2 resource server features

Pom

spring-boot-starter-quartz

Starter for using the Quartz scheduler

Pom

spring-boot-starter-rsocket

Starter for building RSocket clients and servers.

Pom

spring-boot-starter-security

Starter for using Spring Security

Pom

spring-boot-starter-test

Starter for testing Spring Boot applications with libraries including JUnit, Hamcrest and Mockito

Pom

spring-boot-starter-thymeleaf

Starter for building MVC web applications using Thymeleaf views

Pom

spring-boot-starter-validation

Starter for using Java Bean Validation with Hibernate Validator

Pom

spring-boot-starter-web

Starter for building web, including RESTful, applications using Spring MVC. Uses Tomcat as the default embedded container

Pom

spring-boot-starter-web-services

Starter for using Spring Web Services

Pom

spring-boot-starter-webflux

Starter for building WebFlux applications using Spring Framework’s Reactive Web support

Pom

spring-boot-starter-websocket

Starter for building WebSocket applications using Spring Framework’s WebSocket support

Pom

In addition to the application starters, the following starters can be used to add production ready features:

Table 2. Spring Boot production starters
Name Description Pom

spring-boot-starter-actuator

Starter for using Spring Boot’s Actuator which provides production ready features to help you monitor and manage your application

Pom

Finally, Spring Boot also includes the following starters that can be used if you want to exclude or swap specific technical facets:

Table 3. Spring Boot technical starters
Name Description Pom

spring-boot-starter-jetty

Starter for using Jetty as the embedded servlet container. An alternative to spring-boot-starter-tomcat

Pom

spring-boot-starter-log4j2

Starter for using Log4j2 for logging. An alternative to spring-boot-starter-logging

Pom

spring-boot-starter-logging

Starter for logging using Logback. Default logging starter

Pom

spring-boot-starter-reactor-netty

Starter for using Reactor Netty as the embedded reactive HTTP server.

Pom

spring-boot-starter-tomcat

Starter for using Tomcat as the embedded servlet container. Default servlet container starter used by spring-boot-starter-web

Pom

spring-boot-starter-undertow

Starter for using Undertow as the embedded servlet container. An alternative to spring-boot-starter-tomcat

Pom

For a list of additional community contributed starters, see the README file in the spring-boot-starters module on GitHub.

3.2. Structuring Your Code

Spring Boot does not require any specific code layout to work. However, there are some best practices that help.

3.2.1. Using the “default” Package

When a class does not include a package declaration, it is considered to be in the “default package”. The use of the “default package” is generally discouraged and should be avoided. It can cause particular problems for Spring Boot applications that use the @ComponentScan, @ConfigurationPropertiesScan, @EntityScan, or @SpringBootApplication annotations, since every class from every jar is read.

We recommend that you follow Java’s recommended package naming conventions and use a reversed domain name (for example, com.example.project).

3.2.2. Locating the Main Application Class

We generally recommend that you locate your main application class in a root package above other classes. The @SpringBootApplication annotation is often placed on your main class, and it implicitly defines a base “search package” for certain items. For example, if you are writing a JPA application, the package of the @SpringBootApplication annotated class is used to search for @Entity items. Using a root package also allows component scan to apply only on your project.

If you don’t want to use @SpringBootApplication, the @EnableAutoConfiguration @ComponentScan, and @ConfigurationPropertiesScan annotations that it imports defines that behaviour so you can also use those instead.

The following listing shows a typical layout:

com
 +- example
     +- myapplication
         +- Application.java
         |
         +- customer
         |   +- Customer.java
         |   +- CustomerController.java
         |   +- CustomerService.java
         |   +- CustomerRepository.java
         |
         +- order
             +- Order.java
             +- OrderController.java
             +- OrderService.java
             +- OrderRepository.java

The Application.java file would declare the main method, along with the basic @SpringBootApplication, as follows:

package com.example.myapplication;

import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;

@SpringBootApplication
public class Application {

    public static void main(String[] args) {
        SpringApplication.run(Application.class, args);
    }

}

3.3. Configuration Classes

Spring Boot favors Java-based configuration. Although it is possible to use SpringApplication with XML sources, we generally recommend that your primary source be a single @Configuration class. Usually the class that defines the main method is a good candidate as the primary @Configuration.

Many Spring configuration examples have been published on the Internet that use XML configuration. If possible, always try to use the equivalent Java-based configuration. Searching for Enable* annotations can be a good starting point.

3.3.1. Importing Additional Configuration Classes

You need not put all your @Configuration into a single class. The @Import annotation can be used to import additional configuration classes. Alternatively, you can use @ComponentScan to automatically pick up all Spring components, including @Configuration classes.

3.3.2. Importing XML Configuration

If you absolutely must use XML based configuration, we recommend that you still start with a @Configuration class. You can then use an @ImportResource annotation to load XML configuration files.

3.4. Auto-configuration

Spring Boot auto-configuration attempts to automatically configure your Spring application based on the jar dependencies that you have added. For example, if HSQLDB is on your classpath, and you have not manually configured any database connection beans, then Spring Boot auto-configures an in-memory database.

You need to opt-in to auto-configuration by adding the @EnableAutoConfiguration or @SpringBootApplication annotations to one of your @Configuration classes.

You should only ever add one @SpringBootApplication or @EnableAutoConfiguration annotation. We generally recommend that you add one or the other to your primary @Configuration class only.

3.4.1. Gradually Replacing Auto-configuration

Auto-configuration is non-invasive. At any point, you can start to define your own configuration to replace specific parts of the auto-configuration. For example, if you add your own DataSource bean, the default embedded database support backs away.

If you need to find out what auto-configuration is currently being applied, and why, start your application with the --debug switch. Doing so enables debug logs for a selection of core loggers and logs a conditions report to the console.

3.4.2. Disabling Specific Auto-configuration Classes

If you find that specific auto-configuration classes that you do not want are being applied, you can use the exclude attribute of @EnableAutoConfiguration to disable them, as shown in the following example:

import org.springframework.boot.autoconfigure.*;
import org.springframework.boot.autoconfigure.jdbc.*;
import org.springframework.context.annotation.*;

@Configuration(proxyBeanMethods = false)
@EnableAutoConfiguration(exclude={DataSourceAutoConfiguration.class})
public class MyConfiguration {
}

If the class is not on the classpath, you can use the excludeName attribute of the annotation and specify the fully qualified name instead. Finally, you can also control the list of auto-configuration classes to exclude by using the spring.autoconfigure.exclude property.

You can define exclusions both at the annotation level and by using the property.

3.5. Spring Beans and Dependency Injection

You are free to use any of the standard Spring Framework techniques to define your beans and their injected dependencies. For simplicity, we often find that using @ComponentScan (to find your beans) and using @Autowired (to do constructor injection) works well.

If you structure your code as suggested above (locating your application class in a root package), you can add @ComponentScan without any arguments. All of your application components (@Component, @Service, @Repository, @Controller etc.) are automatically registered as Spring Beans.

The following example shows a @Service Bean that uses constructor injection to obtain a required RiskAssessor bean:

package com.example.service;

import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.stereotype.Service;

@Service
public class DatabaseAccountService implements AccountService {

    private final RiskAssessor riskAssessor;

    @Autowired
    public DatabaseAccountService(RiskAssessor riskAssessor) {
        this.riskAssessor = riskAssessor;
    }

    // ...

}

If a bean has one constructor, you can omit the @Autowired, as shown in the following example:

@Service
public class DatabaseAccountService implements AccountService {

    private final RiskAssessor riskAssessor;

    public DatabaseAccountService(RiskAssessor riskAssessor) {
        this.riskAssessor = riskAssessor;
    }

    // ...

}
Notice how using constructor injection lets the riskAssessor field be marked as final, indicating that it cannot be subsequently changed.

3.6. Using the @SpringBootApplication Annotation

Many Spring Boot developers like their apps to use auto-configuration, component scan and be able to define extra configuration on their "application class". A single @SpringBootApplication annotation can be used to enable those three features, that is:

  • @EnableAutoConfiguration: enable Spring Boot’s auto-configuration mechanism

  • @ComponentScan: enable @Component scan on the package where the application is located (see the best practices)

  • @ConfigurationPropertiesScan: enable @ConfigurationProperties scan on the package where the application is located (see the best practices)

  • @Configuration: allow to register extra beans in the context or import additional configuration classes

package com.example.myapplication;

import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;

@SpringBootApplication // same as @Configuration @EnableAutoConfiguration @ComponentScan @ConfigurationPropertiesScan
public class Application {

    public static void main(String[] args) {
        SpringApplication.run(Application.class, args);
    }

}
@SpringBootApplication also provides aliases to customize the attributes of @EnableAutoConfiguration and @ComponentScan.

None of these features are mandatory and you may choose to replace this single annotation by any of the features that it enables. For instance, you may not want to use component scan or configuration properties scan in your application:

package com.example.myapplication;

import org.springframework.boot.SpringApplication;
import org.springframework.context.annotation.ComponentScan
import org.springframework.context.annotation.Configuration;
import org.springframework.context.annotation.Import;

@Configuration(proxyBeanMethods = false)
@EnableAutoConfiguration
@Import({ MyConfig.class, MyAnotherConfig.class })
public class Application {

    public static void main(String[] args) {
            SpringApplication.run(Application.class, args);
    }

}

In this example, Application is just like any other Spring Boot application except that @Component-annotated classes and @ConfigurationProperties-annotated classes are not detected automatically and the user-defined beans are imported explicitly (see @Import).

3.7. Running Your Application

One of the biggest advantages of packaging your application as a jar and using an embedded HTTP server is that you can run your application as you would any other. Debugging Spring Boot applications is also easy. You do not need any special IDE plugins or extensions.

This section only covers jar based packaging. If you choose to package your application as a war file, you should refer to your server and IDE documentation.

3.7.1. Running from an IDE

You can run a Spring Boot application from your IDE as a simple Java application. However, you first need to import your project. Import steps vary depending on your IDE and build system. Most IDEs can import Maven projects directly. For example, Eclipse users can select Import…​Existing Maven Projects from the File menu.

If you cannot directly import your project into your IDE, you may be able to generate IDE metadata by using a build plugin. Maven includes plugins for Eclipse and IDEA. Gradle offers plugins for various IDEs.

If you accidentally run a web application twice, you see a “Port already in use” error. STS users can use the Relaunch button rather than the Run button to ensure that any existing instance is closed.

3.7.2. Running as a Packaged Application

If you use the Spring Boot Maven or Gradle plugins to create an executable jar, you can run your application using java -jar, as shown in the following example:

$ java -jar target/myapplication-0.0.1-SNAPSHOT.jar

It is also possible to run a packaged application with remote debugging support enabled. Doing so lets you attach a debugger to your packaged application, as shown in the following example:

$ java -Xdebug -Xrunjdwp:server=y,transport=dt_socket,address=8000,suspend=n \
       -jar target/myapplication-0.0.1-SNAPSHOT.jar

3.7.3. Using the Maven Plugin

The Spring Boot Maven plugin includes a run goal that can be used to quickly compile and run your application. Applications run in an exploded form, as they do in your IDE. The following example shows a typical Maven command to run a Spring Boot application:

$ mvn spring-boot:run

You might also want to use the MAVEN_OPTS operating system environment variable, as shown in the following example:

$ export MAVEN_OPTS=-Xmx1024m

3.7.4. Using the Gradle Plugin

The Spring Boot Gradle plugin also includes a bootRun task that can be used to run your application in an exploded form. The bootRun task is added whenever you apply the org.springframework.boot and java plugins and is shown in the following example:

$ gradle bootRun

You might also want to use the JAVA_OPTS operating system environment variable, as shown in the following example:

$ export JAVA_OPTS=-Xmx1024m

3.7.5. Hot Swapping

Since Spring Boot applications are just plain Java applications, JVM hot-swapping should work out of the box. JVM hot swapping is somewhat limited with the bytecode that it can replace. For a more complete solution, JRebel can be used.

The spring-boot-devtools module also includes support for quick application restarts. See the Developer Tools section later in this chapter and the Hot swapping “How-to” for details.

3.8. Developer Tools

Spring Boot includes an additional set of tools that can make the application development experience a little more pleasant. The spring-boot-devtools module can be included in any project to provide additional development-time features. To include devtools support, add the module dependency to your build, as shown in the following listings for Maven and Gradle:

Maven
<dependencies>
    <dependency>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-devtools</artifactId>
        <optional>true</optional>
    </dependency>
</dependencies>
Gradle
configurations {
    developmentOnly
    runtimeClasspath {
        extendsFrom developmentOnly
    }
}
dependencies {
    developmentOnly("org.springframework.boot:spring-boot-devtools")
}
Developer tools are automatically disabled when running a fully packaged application. If your application is launched from java -jar or if it is started from a special classloader, then it is considered a “production application”. If that does not apply to you (i.e. if you run your application from a container), consider excluding devtools or set the -Dspring.devtools.restart.enabled=false system property.
Flagging the dependency as optional in Maven or using a custom developmentOnly configuration in Gradle (as shown above) is a best practice that prevents devtools from being transitively applied to other modules that use your project.
Repackaged archives do not contain devtools by default. If you want to use a certain remote devtools feature, you need to disable the excludeDevtools build property to include it. The property is supported with both the Maven and Gradle plugins.

3.8.1. Property Defaults

Several of the libraries supported by Spring Boot use caches to improve performance. For example, template engines cache compiled templates to avoid repeatedly parsing template files. Also, Spring MVC can add HTTP caching headers to responses when serving static resources.

While caching is very beneficial in production, it can be counter-productive during development, preventing you from seeing the changes you just made in your application. For this reason, spring-boot-devtools disables the caching options by default.

Cache options are usually configured by settings in your application.properties file. For example, Thymeleaf offers the spring.thymeleaf.cache property. Rather than needing to set these properties manually, the spring-boot-devtools module automatically applies sensible development-time configuration.

Because you need more information about web requests while developing Spring MVC and Spring WebFlux applications, developer tools will enable DEBUG logging for the web logging group. This will give you information about the incoming request, which handler is processing it, the response outcome, etc. If you wish to log all request details (including potentially sensitive information), you can turn on the spring.http.log-request-details configuration property.

If you don’t want property defaults to be applied you can set spring.devtools.add-properties to false in your application.properties.
For a complete list of the properties that are applied by the devtools, see DevToolsPropertyDefaultsPostProcessor.

3.8.2. Automatic Restart

Applications that use spring-boot-devtools automatically restart whenever files on the classpath change. This can be a useful feature when working in an IDE, as it gives a very fast feedback loop for code changes. By default, any entry on the classpath that points to a folder is monitored for changes. Note that certain resources, such as static assets and view templates, do not need to restart the application.

Triggering a restart

As DevTools monitors classpath resources, the only way to trigger a restart is to update the classpath. The way in which you cause the classpath to be updated depends on the IDE that you are using. In Eclipse, saving a modified file causes the classpath to be updated and triggers a restart. In IntelliJ IDEA, building the project (Build +→+ Build Project) has the same effect.

As long as forking is enabled, you can also start your application by using the supported build plugins (Maven and Gradle), since DevTools needs an isolated application classloader to operate properly. By default, the Gradle and Maven plugins fork the application process.

Automatic restart works very well when used with LiveReload. See the LiveReload section for details. If you use JRebel, automatic restarts are disabled in favor of dynamic class reloading. Other devtools features (such as LiveReload and property overrides) can still be used.
DevTools relies on the application context’s shutdown hook to close it during a restart. It does not work correctly if you have disabled the shutdown hook (SpringApplication.setRegisterShutdownHook(false)).
When deciding if an entry on the classpath should trigger a restart when it changes, DevTools automatically ignores projects named spring-boot, spring-boot-devtools, spring-boot-autoconfigure, spring-boot-actuator, and spring-boot-starter.
DevTools needs to customize the ResourceLoader used by the ApplicationContext. If your application provides one already, it is going to be wrapped. Direct override of the getResource method on the ApplicationContext is not supported.
Restart vs Reload

The restart technology provided by Spring Boot works by using two classloaders. Classes that do not change (for example, those from third-party jars) are loaded into a base classloader. Classes that you are actively developing are loaded into a restart classloader. When the application is restarted, the restart classloader is thrown away and a new one is created. This approach means that application restarts are typically much faster than “cold starts”, since the base classloader is already available and populated.

If you find that restarts are not quick enough for your applications or you encounter classloading issues, you could consider reloading technologies such as JRebel from ZeroTurnaround. These work by rewriting classes as they are loaded to make them more amenable to reloading.

Logging changes in condition evaluation

By default, each time your application restarts, a report showing the condition evaluation delta is logged. The report shows the changes to your application’s auto-configuration as you make changes such as adding or removing beans and setting configuration properties.

To disable the logging of the report, set the following property:

spring.devtools.restart.log-condition-evaluation-delta=false
Excluding Resources

Certain resources do not necessarily need to trigger a restart when they are changed. For example, Thymeleaf templates can be edited in-place. By default, changing resources in /META-INF/maven, /META-INF/resources, /resources, /static, /public, or /templates does not trigger a restart but does trigger a live reload. If you want to customize these exclusions, you can use the spring.devtools.restart.exclude property. For example, to exclude only /static and /public you would set the following property:

spring.devtools.restart.exclude=static/**,public/**
If you want to keep those defaults and add additional exclusions, use the spring.devtools.restart.additional-exclude property instead.
Watching Additional Paths

You may want your application to be restarted or reloaded when you make changes to files that are not on the classpath. To do so, use the spring.devtools.restart.additional-paths property to configure additional paths to watch for changes. You can use the spring.devtools.restart.exclude property described earlier to control whether changes beneath the additional paths trigger a full restart or a live reload.

Disabling Restart

If you do not want to use the restart feature, you can disable it by using the spring.devtools.restart.enabled property. In most cases, you can set this property in your application.properties (doing so still initializes the restart classloader, but it does not watch for file changes).

If you need to completely disable restart support (for example, because it does not work with a specific library), you need to set the spring.devtools.restart.enabled System property to false before calling SpringApplication.run(…​), as shown in the following example:

public static void main(String[] args) {
    System.setProperty("spring.devtools.restart.enabled", "false");
    SpringApplication.run(MyApp.class, args);
}
Using a Trigger File

If you work with an IDE that continuously compiles changed files, you might prefer to trigger restarts only at specific times. To do so, you can use a “trigger file”, which is a special file that must be modified when you want to actually trigger a restart check. Changing the file only triggers the check and the restart only occurs if Devtools has detected it has to do something. The trigger file can be updated manually or with an IDE plugin.

To use a trigger file, set the spring.devtools.restart.trigger-file property to the path of your trigger file.

You might want to set spring.devtools.restart.trigger-file as a global setting, so that all your projects behave in the same way.
Customizing the Restart Classloader

As described earlier in the Restart vs Reload section, restart functionality is implemented by using two classloaders. For most applications, this approach works well. However, it can sometimes cause classloading issues.

By default, any open project in your IDE is loaded with the “restart” classloader, and any regular .jar file is loaded with the “base” classloader. If you work on a multi-module project, and not every module is imported into your IDE, you may need to customize things. To do so, you can create a META-INF/spring-devtools.properties file.

The spring-devtools.properties file can contain properties prefixed with restart.exclude and restart.include. The include elements are items that should be pulled up into the “restart” classloader, and the exclude elements are items that should be pushed down into the “base” classloader. The value of the property is a regex pattern that is applied to the classpath, as shown in the following example:

restart.exclude.companycommonlibs=/mycorp-common-[\\w\\d-\.]+\.jar
restart.include.projectcommon=/mycorp-myproj-[\\w\\d-\.]+\.jar
All property keys must be unique. As long as a property starts with restart.include. or restart.exclude. it is considered.
All META-INF/spring-devtools.properties from the classpath are loaded. You can package files inside your project, or in the libraries that the project consumes.
Known Limitations

Restart functionality does not work well with objects that are deserialized by using a standard ObjectInputStream. If you need to deserialize data, you may need to use Spring’s ConfigurableObjectInputStream in combination with Thread.currentThread().getContextClassLoader().

Unfortunately, several third-party libraries deserialize without considering the context classloader. If you find such a problem, you need to request a fix with the original authors.

3.8.3. LiveReload

The spring-boot-devtools module includes an embedded LiveReload server that can be used to trigger a browser refresh when a resource is changed. LiveReload browser extensions are freely available for Chrome, Firefox and Safari from livereload.com.

If you do not want to start the LiveReload server when your application runs, you can set the spring.devtools.livereload.enabled property to false.

You can only run one LiveReload server at a time. Before starting your application, ensure that no other LiveReload servers are running. If you start multiple applications from your IDE, only the first has LiveReload support.

3.8.4. Global Settings

You can configure global devtools settings by adding a file named .spring-boot-devtools.properties to your $HOME folder (note that the filename starts with “.”). Any properties added to this file apply to all Spring Boot applications on your machine that use devtools. For example, to configure restart to always use a trigger file, you would add the following property:

~/.spring-boot-devtools.properties
spring.devtools.reload.trigger-file=.reloadtrigger
Profiles activated in .spring-boot-devtools.properties will not affect the loading of profile-specific configuration files.

3.8.5. Remote Applications

The Spring Boot developer tools are not limited to local development. You can also use several features when running applications remotely. Remote support is opt-in. To enable it, you need to make sure that devtools is included in the repackaged archive, as shown in the following listing:

<build>
    <plugins>
        <plugin>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-maven-plugin</artifactId>
            <configuration>
                <excludeDevtools>false</excludeDevtools>
            </configuration>
        </plugin>
    </plugins>
</build>

Then you need to set a spring.devtools.remote.secret property, as shown in the following example:

spring.devtools.remote.secret=mysecret
Enabling spring-boot-devtools on a remote application is a security risk. You should never enable support on a production deployment.

Remote devtools support is provided in two parts: a server-side endpoint that accepts connections and a client application that you run in your IDE. The server component is automatically enabled when the spring.devtools.remote.secret property is set. The client component must be launched manually.

Running the Remote Client Application

The remote client application is designed to be run from within your IDE. You need to run org.springframework.boot.devtools.RemoteSpringApplication with the same classpath as the remote project that you connect to. The application’s single required argument is the remote URL to which it connects.

For example, if you are using Eclipse or STS and you have a project named my-app that you have deployed to Cloud Foundry, you would do the following:

  • Select Run Configurations…​ from the Run menu.

  • Create a new Java Application “launch configuration”.

  • Browse for the my-app project.

  • Use org.springframework.boot.devtools.RemoteSpringApplication as the main class.

  • Add https://myapp.cfapps.io to the Program arguments (or whatever your remote URL is).

A running remote client might resemble the following listing:

  .   ____          _                                              __ _ _
 /\\ / ___'_ __ _ _(_)_ __  __ _          ___               _      \ \ \ \
( ( )\___ | '_ | '_| | '_ \/ _` |        | _ \___ _ __  ___| |_ ___ \ \ \ \
 \\/  ___)| |_)| | | | | || (_| []::::::[]   / -_) '  \/ _ \  _/ -_) ) ) ) )
  '  |____| .__|_| |_|_| |_\__, |        |_|_\___|_|_|_\___/\__\___|/ / / /
 =========|_|==============|___/===================================/_/_/_/
 :: Spring Boot Remote :: 2.2.0.M4

2015-06-10 18:25:06.632  INFO 14938 --- [           main] o.s.b.devtools.RemoteSpringApplication   : Starting RemoteSpringApplication on pwmbp with PID 14938 (/Users/pwebb/projects/spring-boot/code/spring-boot-devtools/target/classes started by pwebb in /Users/pwebb/projects/spring-boot/code/spring-boot-samples/spring-boot-sample-devtools)
2015-06-10 18:25:06.671  INFO 14938 --- [           main] s.c.a.AnnotationConfigApplicationContext : Refreshing org.springframework.context.annotation.AnnotationConfigApplicationContext@2a17b7b6: startup date [Wed Jun 10 18:25:06 PDT 2015]; root of context hierarchy
2015-06-10 18:25:07.043  WARN 14938 --- [           main] o.s.b.d.r.c.RemoteClientConfiguration    : The connection to http://localhost:8080 is insecure. You should use a URL starting with 'https://'.
2015-06-10 18:25:07.074  INFO 14938 --- [           main] o.s.b.d.a.OptionalLiveReloadServer       : LiveReload server is running on port 35729
2015-06-10 18:25:07.130  INFO 14938 --- [           main] o.s.b.devtools.RemoteSpringApplication   : Started RemoteSpringApplication in 0.74 seconds (JVM running for 1.105)
Because the remote client is using the same classpath as the real application it can directly read application properties. This is how the spring.devtools.remote.secret property is read and passed to the server for authentication.
It is always advisable to use https:// as the connection protocol, so that traffic is encrypted and passwords cannot be intercepted.
If you need to use a proxy to access the remote application, configure the spring.devtools.remote.proxy.host and spring.devtools.remote.proxy.port properties.
Remote Update

The remote client monitors your application classpath for changes in the same way as the local restart. Any updated resource is pushed to the remote application and (if required) triggers a restart. This can be helpful if you iterate on a feature that uses a cloud service that you do not have locally. Generally, remote updates and restarts are much quicker than a full rebuild and deploy cycle.

Files are only monitored when the remote client is running. If you change a file before starting the remote client, it is not pushed to the remote server.

3.9. Packaging Your Application for Production

Executable jars can be used for production deployment. As they are self-contained, they are also ideally suited for cloud-based deployment.

For additional “production ready” features, such as health, auditing, and metric REST or JMX end-points, consider adding spring-boot-actuator. See Spring Boot Actuator: Production-ready Features for details.

3.10. What to Read Next

You should now understand how you can use Spring Boot and some best practices that you should follow. You can now go on to learn about specific Spring Boot features in depth, or you could skip ahead and read about the “production ready” aspects of Spring Boot.

4. Spring Boot Features

4.1. SpringApplication

The SpringApplication class provides a convenient way to bootstrap a Spring application that is started from a main() method. In many situations, you can delegate to the static SpringApplication.run method, as shown in the following example:

public static void main(String[] args) {
    SpringApplication.run(MySpringConfiguration.class, args);
}

When your application starts, you should see something similar to the following output:

  .   ____          _            __ _ _
 /\\ / ___'_ __ _ _(_)_ __  __ _ \ \ \ \
( ( )\___ | '_ | '_| | '_ \/ _` | \ \ \ \
 \\/  ___)| |_)| | | | | || (_| |  ) ) ) )
  '  |____| .__|_| |_|_| |_\__, | / / / /
 =========|_|==============|___/=/_/_/_/
 :: Spring Boot ::   v2.2.0.M4

2013-07-31 00:08:16.117  INFO 56603 --- [           main] o.s.b.s.app.SampleApplication            : Starting SampleApplication v0.1.0 on mycomputer with PID 56603 (/apps/myapp.jar started by pwebb)
2013-07-31 00:08:16.166  INFO 56603 --- [           main] ationConfigServletWebServerApplicationContext : Refreshing org.springframework.boot.web.servlet.context.AnnotationConfigServletWebServerApplicationContext@6e5a8246: startup date [Wed Jul 31 00:08:16 PDT 2013]; root of context hierarchy
2014-03-04 13:09:54.912  INFO 41370 --- [           main] .t.TomcatServletWebServerFactory : Server initialized with port: 8080
2014-03-04 13:09:56.501  INFO 41370 --- [           main] o.s.b.s.app.SampleApplication            : Started SampleApplication in 2.992 seconds (JVM running for 3.658)

By default, INFO logging messages are shown, including some relevant startup details, such as the user that launched the application. If you need a log level other than INFO, you can set it, as described in Log Levels,

4.1.1. Startup Failure

If your application fails to start, registered FailureAnalyzers get a chance to provide a dedicated error message and a concrete action to fix the problem. For instance, if you start a web application on port 8080 and that port is already in use, you should see something similar to the following message:

***************************
APPLICATION FAILED TO START
***************************

Description:

Embedded servlet container failed to start. Port 8080 was already in use.

Action:

Identify and stop the process that's listening on port 8080 or configure this application to listen on another port.
Spring Boot provides numerous FailureAnalyzer implementations, and you can add your own.

If no failure analyzers are able to handle the exception, you can still display the full conditions report to better understand what went wrong. To do so, you need to enable the debug property or enable DEBUG logging for org.springframework.boot.autoconfigure.logging.ConditionEvaluationReportLoggingListener.

For instance, if you are running your application by using java -jar, you can enable the debug property as follows:

$ java -jar myproject-0.0.1-SNAPSHOT.jar --debug

4.1.2. Lazy Initialization

SpringApplication allows an application to be initialized lazily. When lazy initialization is enabled, beans are created as they are needed rather than during application startup. As a result, enabling lazy initialization can reduce the time that it takes your application to start. In a web application, enabling lazy initialization will result in many web-related beans not being initialized until an HTTP request is received.

A downside of lazy initialization is that it can delay the discovery of a problem with the application. If a misconfigured bean is initialized lazily, a failure will no longer occur during startup and the problem will only become apparent when the bean is initialized. Care must also be taken to ensure that the JVM has sufficient memory to accommodate all of the application’s beans and not just those that are initialized during startup. For these reasons, lazy initialization is not enabled by default and it is recommended that fine-tuning of the JVM’s heap size is done before enabling lazy initialization.

Lazy initialization can be enabled programatically using the lazyInitialization method on SpringApplicationBuilder or the setLazyInitialization method on SpringApplication. Alternatively, it can be enabled using the spring.main.lazy-initialization property as shown in the following example:

spring.main.lazy-initialization=true
If you want to disable lazy initialization for certain beans while using lazy initialization for the rest of the application, you can explicitly set their lazy attribute to false using the @Lazy(false) annotation.

4.1.3. Customizing the Banner

The banner that is printed on start up can be changed by adding a banner.txt file to your classpath or by setting the spring.banner.location property to the location of such a file. If the file has an encoding other than UTF-8, you can set spring.banner.charset. In addition to a text file, you can also add a banner.gif, banner.jpg, or banner.png image file to your classpath or set the spring.banner.image.location property. Images are converted into an ASCII art representation and printed above any text banner.

Inside your banner.txt file, you can use any of the following placeholders:

Table 4. Banner variables
Variable Description

${application.version}

The version number of your application, as declared in MANIFEST.MF. For example, Implementation-Version: 1.0 is printed as 1.0.

${application.formatted-version}

The version number of your application, as declared in MANIFEST.MF and formatted for display (surrounded with brackets and prefixed with v). For example (v1.0).

${spring-boot.version}

The Spring Boot version that you are using. For example 2.2.0.M4.

${spring-boot.formatted-version}

The Spring Boot version that you are using, formatted for display (surrounded with brackets and prefixed with v). For example (v2.2.0.M4).

${Ansi.NAME} (or ${AnsiColor.NAME}, ${AnsiBackground.NAME}, ${AnsiStyle.NAME})

Where NAME is the name of an ANSI escape code. See AnsiPropertySource for details.

${application.title}

The title of your application, as declared in MANIFEST.MF. For example Implementation-Title: MyApp is printed as MyApp.

The SpringApplication.setBanner(…​) method can be used if you want to generate a banner programmatically. Use the org.springframework.boot.Banner interface and implement your own printBanner() method.

You can also use the spring.main.banner-mode property to determine if the banner has to be printed on System.out (console), sent to the configured logger (log), or not produced at all (off).

The printed banner is registered as a singleton bean under the following name: springBootBanner.

YAML maps off to false, so be sure to add quotes if you want to disable the banner in your application, as shown in the following example:

spring:
    main:
        banner-mode: "off"

4.1.4. Customizing SpringApplication

If the SpringApplication defaults are not to your taste, you can instead create a local instance and customize it. For example, to turn off the banner, you could write:

public static void main(String[] args) {
    SpringApplication app = new SpringApplication(MySpringConfiguration.class);
    app.setBannerMode(Banner.Mode.OFF);
    app.run(args);
}
The constructor arguments passed to SpringApplication are configuration sources for Spring beans. In most cases, these are references to @Configuration classes, but they could also be references to XML configuration or to packages that should be scanned.

It is also possible to configure the SpringApplication by using an application.properties file. See Externalized Configuration for details.

For a complete list of the configuration options, see the SpringApplication Javadoc.

4.1.5. Fluent Builder API

If you need to build an ApplicationContext hierarchy (multiple contexts with a parent/child relationship) or if you prefer using a “fluent” builder API, you can use the SpringApplicationBuilder.

The SpringApplicationBuilder lets you chain together multiple method calls and includes parent and child methods that let you create a hierarchy, as shown in the following example:

new SpringApplicationBuilder()
        .sources(Parent.class)
        .child(Application.class)
        .bannerMode(Banner.Mode.OFF)
        .run(args);
There are some restrictions when creating an ApplicationContext hierarchy. For example, Web components must be contained within the child context, and the same Environment is used for both parent and child contexts. See the SpringApplicationBuilder Javadoc for full details.

4.1.6. Application Events and Listeners

In addition to the usual Spring Framework events, such as ContextRefreshedEvent, a SpringApplication sends some additional application events.

Some events are actually triggered before the ApplicationContext is created, so you cannot register a listener on those as a @Bean. You can register them with the SpringApplication.addListeners(…​) method or the SpringApplicationBuilder.listeners(…​) method.

If you want those listeners to be registered automatically, regardless of the way the application is created, you can add a META-INF/spring.factories file to your project and reference your listener(s) by using the org.springframework.context.ApplicationListener key, as shown in the following example:

org.springframework.context.ApplicationListener=com.example.project.MyListener

Application events are sent in the following order, as your application runs:

  1. An ApplicationStartingEvent is sent at the start of a run but before any processing, except for the registration of listeners and initializers.

  2. An ApplicationEnvironmentPreparedEvent is sent when the Environment to be used in the context is known but before the context is created.

  3. An ApplicationContextInitializedEvent is sent when the ApplicationContext is prepared and ApplicationContextInitializers have been called but before any bean definitions are loaded.

  4. An ApplicationPreparedEvent is sent just before the refresh is started but after bean definitions have been loaded.

  5. An ApplicationStartedEvent is sent after the context has been refreshed but before any application and command-line runners have been called.

  6. An ApplicationReadyEvent is sent after any application and command-line runners have been called. It indicates that the application is ready to service requests.

  7. An ApplicationFailedEvent is sent if there is an exception on startup.

The above list only includes SpringApplicationEvent`s that are tied to a `SpringApplication. In addition to these, the following events are also published after ApplicationPreparedEvent and before ApplicationStartedEvent:

  1. A ContextRefreshedEvent is sent when an ApplicationContext is refreshed.

  2. A WebServerInitializedEvent is sent after the WebServer is ready. ServletWebServerInitializedEvent and ReactiveWebServerInitializedEvent are the servlet and reactive variants respectively.

You often need not use application events, but it can be handy to know that they exist. Internally, Spring Boot uses events to handle a variety of tasks.

Application events are sent by using Spring Framework’s event publishing mechanism. Part of this mechanism ensures that an event published to the listeners in a child context is also published to the listeners in any ancestor contexts. As a result of this, if your application uses a hierarchy of SpringApplication instances, a listener may receive multiple instances of the same type of application event.

To allow your listener to distinguish between an event for its context and an event for a descendant context, it should request that its application context is injected and then compare the injected context with the context of the event. The context can be injected by implementing ApplicationContextAware or, if the listener is a bean, by using @Autowired.

4.1.7. Web Environment

A SpringApplication attempts to create the right type of ApplicationContext on your behalf. The algorithm used to determine a WebApplicationType is fairly simple:

  • If Spring MVC is present, an AnnotationConfigServletWebServerApplicationContext is used

  • If Spring MVC is not present and Spring WebFlux is present, an AnnotationConfigReactiveWebServerApplicationContext is used

  • Otherwise, AnnotationConfigApplicationContext is used

This means that if you are using Spring MVC and the new WebClient from Spring WebFlux in the same application, Spring MVC will be used by default. You can override that easily by calling setWebApplicationType(WebApplicationType).

It is also possible to take complete control of the ApplicationContext type that is used by calling setApplicationContextClass(…​).

It is often desirable to call setWebApplicationType(WebApplicationType.NONE) when using SpringApplication within a JUnit test.

4.1.8. Accessing Application Arguments

If you need to access the application arguments that were passed to SpringApplication.run(…​), you can inject a org.springframework.boot.ApplicationArguments bean. The ApplicationArguments interface provides access to both the raw String[] arguments as well as parsed option and non-option arguments, as shown in the following example:

import org.springframework.boot.*;
import org.springframework.beans.factory.annotation.*;
import org.springframework.stereotype.*;

@Component
public class MyBean {

    @Autowired
    public MyBean(ApplicationArguments args) {
        boolean debug = args.containsOption("debug");
        List<String> files = args.getNonOptionArgs();
        // if run with "--debug logfile.txt" debug=true, files=["logfile.txt"]
    }

}
Spring Boot also registers a CommandLinePropertySource with the Spring Environment. This lets you also inject single application arguments by using the @Value annotation.

4.1.9. Using the ApplicationRunner or CommandLineRunner

If you need to run some specific code once the SpringApplication has started, you can implement the ApplicationRunner or CommandLineRunner interfaces. Both interfaces work in the same way and offer a single run method, which is called just before SpringApplication.run(…​) completes.

The CommandLineRunner interfaces provides access to application arguments as a simple string array, whereas the ApplicationRunner uses the ApplicationArguments interface discussed earlier. The following example shows a CommandLineRunner with a run method:

import org.springframework.boot.*;
import org.springframework.stereotype.*;

@Component
public class MyBean implements CommandLineRunner {

    public void run(String... args) {
        // Do something...
    }

}

If several CommandLineRunner or ApplicationRunner beans are defined that must be called in a specific order, you can additionally implement the org.springframework.core.Ordered interface or use the org.springframework.core.annotation.Order annotation.

4.1.10. Application Exit

Each SpringApplication registers a shutdown hook with the JVM to ensure that the ApplicationContext closes gracefully on exit. All the standard Spring lifecycle callbacks (such as the DisposableBean interface or the @PreDestroy annotation) can be used.

In addition, beans may implement the org.springframework.boot.ExitCodeGenerator interface if they wish to return a specific exit code when SpringApplication.exit() is called. This exit code can then be passed to System.exit() to return it as a status code, as shown in the following example:

@SpringBootApplication
public class ExitCodeApplication {

    @Bean
    public ExitCodeGenerator exitCodeGenerator() {
        return () -> 42;
    }

    public static void main(String[] args) {
        System.exit(SpringApplication.exit(SpringApplication.run(ExitCodeApplication.class, args)));
    }

}

Also, the ExitCodeGenerator interface may be implemented by exceptions. When such an exception is encountered, Spring Boot returns the exit code provided by the implemented getExitCode() method.

4.1.11. Admin Features

It is possible to enable admin-related features for the application by specifying the spring.application.admin.enabled property. This exposes the SpringApplicationAdminMXBean on the platform MBeanServer. You could use this feature to administer your Spring Boot application remotely. This feature could also be useful for any service wrapper implementation.

If you want to know on which HTTP port the application is running, get the property with a key of local.server.port.
Take care when enabling this feature, as the MBean exposes a method to shutdown the application.

4.2. Externalized Configuration

Spring Boot lets you externalize your configuration so that you can work with the same application code in different environments. You can use properties files, YAML files, environment variables, and command-line arguments to externalize configuration. Property values can be injected directly into your beans by using the @Value annotation, accessed through Spring’s Environment abstraction, or be bound to structured objects through @ConfigurationProperties.

Spring Boot uses a very particular PropertySource order that is designed to allow sensible overriding of values. Properties are considered in the following order:

  1. Devtools global settings properties on your home directory (~/.spring-boot-devtools.properties when devtools is active).

  2. @TestPropertySource annotations on your tests.

  3. properties attribute on your tests. Available on @SpringBootTest and the test annotations for testing a particular slice of your application.

  4. Command line arguments.

  5. Properties from SPRING_APPLICATION_JSON (inline JSON embedded in an environment variable or system property).

  6. ServletConfig init parameters.

  7. ServletContext init parameters.

  8. JNDI attributes from java:comp/env.

  9. Java System properties (System.getProperties()).

  10. OS environment variables.

  11. A RandomValuePropertySource that has properties only in random.*.

  12. Profile-specific application properties outside of your packaged jar (application-{profile}.properties and YAML variants).

  13. Profile-specific application properties packaged inside your jar (application-{profile}.properties and YAML variants).

  14. Application properties outside of your packaged jar (application.properties and YAML variants).

  15. Application properties packaged inside your jar (application.properties and YAML variants).

  16. @PropertySource annotations on your @Configuration classes.

  17. Default properties (specified by setting SpringApplication.setDefaultProperties).

To provide a concrete example, suppose you develop a @Component that uses a name property, as shown in the following example:

import org.springframework.stereotype.*;
import org.springframework.beans.factory.annotation.*;

@Component
public class MyBean {

    @Value("${name}")
    private String name;

    // ...

}

On your application classpath (for example, inside your jar) you can have an application.properties file that provides a sensible default property value for name. When running in a new environment, an application.properties file can be provided outside of your jar that overrides the name. For one-off testing, you can launch with a specific command line switch (for example, java -jar app.jar --name="Spring").

The SPRING_APPLICATION_JSON properties can be supplied on the command line with an environment variable. For example, you could use the following line in a UN*X shell:

$ SPRING_APPLICATION_JSON='{"acme":{"name":"test"}}' java -jar myapp.jar

In the preceding example, you end up with acme.name=test in the Spring Environment. You can also supply the JSON as spring.application.json in a System property, as shown in the following example:

$ java -Dspring.application.json='{"name":"test"}' -jar myapp.jar

You can also supply the JSON by using a command line argument, as shown in the following example:

$ java -jar myapp.jar --spring.application.json='{"name":"test"}'

You can also supply the JSON as a JNDI variable, as follows: java:comp/env/spring.application.json.

4.2.1. Configuring Random Values

The RandomValuePropertySource is useful for injecting random values (for example, into secrets or test cases). It can produce integers, longs, uuids, or strings, as shown in the following example:

my.secret=${random.value}
my.number=${random.int}
my.bignumber=${random.long}
my.uuid=${random.uuid}
my.number.less.than.ten=${random.int(10)}
my.number.in.range=${random.int[1024,65536]}

The random.int* syntax is OPEN value (,max) CLOSE where the OPEN,CLOSE are any character and value,max are integers. If max is provided, then value is the minimum value and max is the maximum value (exclusive).

4.2.2. Accessing Command Line Properties

By default, SpringApplication converts any command line option arguments (that is, arguments starting with --, such as --server.port=9000) to a property and adds them to the Spring Environment. As mentioned previously, command line properties always take precedence over other property sources.

If you do not want command line properties to be added to the Environment, you can disable them by using SpringApplication.setAddCommandLineProperties(false).

4.2.3. Application Property Files

SpringApplication loads properties from application.properties files in the following locations and adds them to the Spring Environment:

  1. A /config subdirectory of the current directory

  2. The current directory

  3. A classpath /config package

  4. The classpath root

The list is ordered by precedence (properties defined in locations higher in the list override those defined in lower locations).

You can also use YAML ('.yml') files as an alternative to '.properties'.

If you do not like application.properties as the configuration file name, you can switch to another file name by specifying a spring.config.name environment property. You can also refer to an explicit location by using the spring.config.location environment property (which is a comma-separated list of directory locations or file paths). The following example shows how to specify a different file name:

$ java -jar myproject.jar --spring.config.name=myproject

The following example shows how to specify two locations:

$ java -jar myproject.jar --spring.config.location=classpath:/default.properties,classpath:/override.properties
spring.config.name and spring.config.location are used very early to determine which files have to be loaded, so they must be defined as an environment property (typically an OS environment variable, a system property, or a command-line argument).

If spring.config.location contains directories (as opposed to files), they should end in / (and, at runtime, be appended with the names generated from spring.config.name before being loaded, including profile-specific file names). Files specified in spring.config.location are used as-is, with no support for profile-specific variants, and are overridden by any profile-specific properties.

Config locations are searched in reverse order. By default, the configured locations are classpath:/,classpath:/config/,file:./,file:./config/. The resulting search order is the following:

  1. file:./config/

  2. file:./

  3. classpath:/config/

  4. classpath:/

When custom config locations are configured by using spring.config.location, they replace the default locations. For example, if spring.config.location is configured with the value classpath:/custom-config/,file:./custom-config/, the search order becomes the following:

  1. file:./custom-config/

  2. classpath:custom-config/

Alternatively, when custom config locations are configured by using spring.config.additional-location, they are used in addition to the default locations. Additional locations are searched before the default locations. For example, if additional locations of classpath:/custom-config/,file:./custom-config/ are configured, the search order becomes the following:

  1. file:./custom-config/

  2. classpath:custom-config/

  3. file:./config/

  4. file:./

  5. classpath:/config/

  6. classpath:/

This search ordering lets you specify default values in one configuration file and then selectively override those values in another. You can provide default values for your application in application.properties (or whatever other basename you choose with spring.config.name) in one of the default locations. These default values can then be overridden at runtime with a different file located in one of the custom locations.

If you use environment variables rather than system properties, most operating systems disallow period-separated key names, but you can use underscores instead (for example, SPRING_CONFIG_NAME instead of spring.config.name).
If your application runs in a container, then JNDI properties (in java:comp/env) or servlet context initialization parameters can be used instead of, or as well as, environment variables or system properties.

4.2.4. Profile-specific Properties

In addition to application.properties files, profile-specific properties can also be defined by using the following naming convention: application-{profile}.properties. The Environment has a set of default profiles (by default, [default]) that are used if no active profiles are set. In other words, if no profiles are explicitly activated, then properties from application-default.properties are loaded.

Profile-specific properties are loaded from the same locations as standard application.properties, with profile-specific files always overriding the non-specific ones, whether or not the profile-specific files are inside or outside your packaged jar.

If several profiles are specified, a last-wins strategy applies. For example, profiles specified by the spring.profiles.active property are added after those configured through the SpringApplication API and therefore take precedence.

If you have specified any files in spring.config.location, profile-specific variants of those files are not considered. Use directories in spring.config.location if you want to also use profile-specific properties.

4.2.5. Placeholders in Properties

The values in application.properties are filtered through the existing Environment when they are used, so you can refer back to previously defined values (for example, from System properties).

app.name=MyApp
app.description=${app.name} is a Spring Boot application
You can also use this technique to create “short” variants of existing Spring Boot properties. See the Use ‘Short’ Command Line Arguments how-to for details.

4.2.6. Encrypting Properties

Spring Boot does not provide any built in support for encrypting property values, however, it does provide the hook points necessary to modify values contained in the Spring Environment. The EnvironmentPostProcessor interface allows you to manipulate the Environment before the application starts. See Customize the Environment or ApplicationContext Before It Starts for details.

If you’re looking for a secure way to store credentials and passwords, the Spring Cloud Vault project provides support for storing externalized configuration in HashiCorp Vault.

4.2.7. Using YAML Instead of Properties

YAML is a superset of JSON and, as such, is a convenient format for specifying hierarchical configuration data. The SpringApplication class automatically supports YAML as an alternative to properties whenever you have the SnakeYAML library on your classpath.

If you use “Starters”, SnakeYAML is automatically provided by spring-boot-starter.
Loading YAML

Spring Framework provides two convenient classes that can be used to load YAML documents. The YamlPropertiesFactoryBean loads YAML as Properties and the YamlMapFactoryBean loads YAML as a Map.

For example, consider the following YAML document:

environments:
    dev:
        url: https://dev.example.com
        name: Developer Setup
    prod:
        url: https://another.example.com
        name: My Cool App

The preceding example would be transformed into the following properties:

environments.dev.url=https://dev.example.com
environments.dev.name=Developer Setup
environments.prod.url=https://another.example.com
environments.prod.name=My Cool App

YAML lists are represented as property keys with [index] dereferencers. For example, consider the following YAML:

my:
   servers:
       - dev.example.com
       - another.example.com

The preceding example would be transformed into these properties:

my.servers[0]=dev.example.com
my.servers[1]=another.example.com

To bind to properties like that by using Spring Boot’s Binder utilities (which is what @ConfigurationProperties does), you need to have a property in the target bean of type java.util.List (or Set) and you either need to provide a setter or initialize it with a mutable value. For example, the following example binds to the properties shown previously:

@ConfigurationProperties(prefix="my")
public class Config {

    private List<String> servers = new ArrayList<String>();

    public List<String> getServers() {
        return this.servers;
    }
}
Exposing YAML as Properties in the Spring Environment

The YamlPropertySourceLoader class can be used to expose YAML as a PropertySource in the Spring Environment. Doing so lets you use the @Value annotation with placeholders syntax to access YAML properties.

Multi-profile YAML Documents

You can specify multiple profile-specific YAML documents in a single file by using a spring.profiles key to indicate when the document applies, as shown in the following example:

server:
    address: 192.168.1.100
---
spring:
    profiles: development
server:
    address: 127.0.0.1
---
spring:
    profiles: production & eu-central
server:
    address: 192.168.1.120

In the preceding example, if the development profile is active, the server.address property is 127.0.0.1. Similarly, if the production and eu-central profiles are active, the server.address property is 192.168.1.120. If the development, production and eu-central profiles are not enabled, then the value for the property is 192.168.1.100.

spring.profiles can therefore contain a simple profile name (for example production) or a profile expression. A profile expression allows for more complicated profile logic to be expressed, for example production & (eu-central | eu-west). Check the reference guide for more details.

If none are explicitly active when the application context starts, the default profiles are activated. So, in the following YAML, we set a value for spring.security.user.password that is available only in the "default" profile:

server:
  port: 8000
---
spring:
  profiles: default
  security:
    user:
      password: weak

Whereas, in the following example, the password is always set because it is not attached to any profile, and it would have to be explicitly reset in all other profiles as necessary:

server:
  port: 8000
spring:
  security:
    user:
      password: weak

Spring profiles designated by using the spring.profiles element may optionally be negated by using the ! character. If both negated and non-negated profiles are specified for a single document, at least one non-negated profile must match, and no negated profiles may match.

YAML Shortcomings

YAML files cannot be loaded by using the @PropertySource annotation. So, in the case that you need to load values that way, you need to use a properties file.

Using the multi YAML document syntax in profile-specific YAML files can lead to unexpected behavior. For example, consider the following config in a file called application-dev.yml, with the dev profile being active:

server:
  port: 8000
---
spring:
  profiles: !test
  security:
    user:
      password: weak

In the example above, profile negation and profile expressions will not behave as expected. We recommend that you don’t combine profile-specific YAML files and multiple YAML documents and stick to using only one of them.

4.2.8. Type-safe Configuration Properties

Using the @Value("${property}") annotation to inject configuration properties can sometimes be cumbersome, especially if you are working with multiple properties or your data is hierarchical in nature. Spring Boot provides an alternative method of working with properties that lets strongly typed beans govern and validate the configuration of your application.

JavaBean properties binding

It is possible to bind a bean declaring standard JavaBean properties as shown in the following example:

package com.example;

import java.net.InetAddress;
import java.util.ArrayList;
import java.util.Collections;
import java.util.List;

import org.springframework.boot.context.properties.ConfigurationProperties;

@ConfigurationProperties("acme")
public class AcmeProperties {

    private boolean enabled;

    private InetAddress remoteAddress;

    private final Security security = new Security();

    public boolean isEnabled() { ... }

    public void setEnabled(boolean enabled) { ... }

    public InetAddress getRemoteAddress() { ... }

    public void setRemoteAddress(InetAddress remoteAddress) { ... }

    public Security getSecurity() { ... }

    public static class Security {

        private String username;

        private String password;

        private List<String> roles = new ArrayList<>(Collections.singleton("USER"));

        public String getUsername() { ... }

        public void setUsername(String username) { ... }

        public String getPassword() { ... }

        public void setPassword(String password) { ... }

        public List<String> getRoles() { ... }

        public void setRoles(List<String> roles) { ... }

    }
}

The preceding POJO defines the following properties:

  • acme.enabled, with a value of false by default.

  • acme.remote-address, with a type that can be coerced from String.

  • acme.security.username, with a nested "security" object whose name is determined by the name of the property. In particular, the return type is not used at all there and could have been SecurityProperties.

  • acme.security.password.

  • acme.security.roles, with a collection of String that defaults to USER.

Such arrangement relies on a default empty constructor and getters and setters are usually mandatory, since binding is through standard Java Beans property descriptors, just like in Spring MVC. A setter may be omitted in the following cases:

  • Maps, as long as they are initialized, need a getter but not necessarily a setter, since they can be mutated by the binder.

  • Collections and arrays can be accessed either through an index (typically with YAML) or by using a single comma-separated value (properties). In the latter case, a setter is mandatory. We recommend to always add a setter for such types. If you initialize a collection, make sure it is not immutable (as in the preceding example).

  • If nested POJO properties are initialized (like the Security field in the preceding example), a setter is not required. If you want the binder to create the instance on the fly by using its default constructor, you need a setter.

Some people use Project Lombok to add getters and setters automatically. Make sure that Lombok does not generate any particular constructor for such a type, as it is used automatically by the container to instantiate the object.

Finally, only standard Java Bean properties are considered and binding on static properties is not supported.

Constructor binding

The example in the previous section can be rewritten in an immutable fashion as shown in the following example:

package com.example;

import java.net.InetAddress;
import java.util.List;

import org.springframework.boot.context.properties.ConfigurationProperties;
import org.springframework.boot.context.properties.DefaultValue;

@ConfigurationProperties("acme")
public class AcmeProperties {

    private final boolean enabled;

    private final InetAddress remoteAddress;

    private final Security security;

    public AcmeProperties(boolean enabled, InetAddress remoteAddress, Security security) {
        this.enabled = enabled;
        this.remoteAddress = remoteAddress;
        this.security = security;
    }

    public boolean isEnabled() { ... }

    public InetAddress getRemoteAddress() { ... }

    public Security getSecurity() { ... }

    public static class Security {

        private final String username;

        private final String password;

        private final List<String> roles;

        public Security(String username, String password,
                @DefaultValue("USER") List<String> roles) {
            this.username = username;
            this.password = password;
            this.roles = roles;
        }

        public String getUsername() { ... }

        public String getPassword() { ... }

        public List<String> getRoles() { ... }

    }

}

In this setup one, and only one constructor must be defined with the list of properties that you wish to bind and not other properties than the ones in the constructor are bound.

Default values can be specified using @DefaultValue and the same conversion service will be applied to coerce the String value to the target type of a missing property.

Enabling @ConfigurationProperties-annotated types

Spring Boot provides an infrastructure to bind such types and register them as beans automatically. If your application uses @SpringBootApplication, classes annotated with @ConfigurationProperties will automatically be scanned and registered as beans. By default, scanning will occur from the package of the class that declares this annotation. If you want to define specific packages to scan, you can do so using an explicit @ConfigurationPropertiesScan directive on your @SpringBootApplication-annotated class as shown in the following example:

@SpringBootApplication
@ConfigurationPropertiesScan({ "com.example.app", "org.acme.another" })
public class MyApplication {
}

Sometimes, classes annotated with @ConfigurationProperties might not be suitable for scanning, for example, if you’re developing your own auto-configuration. In these cases, you can specify the list of types to process on any @Configuration class as shown in the following example:

@Configuration(proxyBeanMethods = false)
@EnableConfigurationProperties(AcmeProperties.class)
public class MyConfiguration {
}

When the @ConfigurationProperties bean is registered using scanning or via @EnableConfigurationProperties, the bean has a conventional name: <prefix>-<fqn>, where <prefix> is the environment key prefix specified in the @ConfigurationProperties annotation and <fqn> is the fully qualified name of the bean. If the annotation does not provide any prefix, only the fully qualified name of the bean is used.

The bean name in the example above is acme-com.example.AcmeProperties.

We recommend that @ConfigurationProperties only deal with the environment and, in particular, does not inject other beans from the context. In particular, it is not possible to inject other beans using the constructor as this would trigger the constructor binder that only deals with the environment.

For corner cases, setter injection can be used or any of the *Aware interfaces provided by the framework (such as EnvironmentAware if you need access to the Environment).

Annotating a @ConfigurationProperties type with @Component will result in two beans of the same type if the type is also scanned as part of classpath scanning. If you want to register the bean yourself using @Component, consider disabling scanning of @ConfigurationProperties.
Using @ConfigurationProperties-annotated types

This style of configuration works particularly well with the SpringApplication external YAML configuration, as shown in the following example:

# application.yml

acme:
    remote-address: 192.168.1.1
    security:
        username: admin
        roles:
          - USER
          - ADMIN

# additional configuration as required

To work with @ConfigurationProperties beans, you can inject them in the same way as any other bean, as shown in the following example:

@Service
public class MyService {

    private final AcmeProperties properties;

    @Autowired
    public MyService(AcmeProperties properties) {
        this.properties = properties;
    }

    //...

    @PostConstruct
    public void openConnection() {
        Server server = new Server(this.properties.getRemoteAddress());
        // ...
    }

}
Using @ConfigurationProperties also lets you generate metadata files that can be used by IDEs to offer auto-completion for your own keys. See the appendix for details.
Third-party Configuration

As well as using @ConfigurationProperties to annotate a class, you can also use it on public @Bean methods. Doing so can be particularly useful when you want to bind properties to third-party components that are outside of your control.

To configure a bean from the Environment properties, add @ConfigurationProperties to its bean registration, as shown in the following example:

@ConfigurationProperties(prefix = "another")
@Bean
public AnotherComponent anotherComponent() {
    ...
}

Any JavaBean property defined with the another prefix is mapped onto that AnotherComponent bean in manner similar to the preceding AcmeProperties example.

Relaxed Binding

Spring Boot uses some relaxed rules for binding Environment properties to @ConfigurationProperties beans, so there does not need to be an exact match between the Environment property name and the bean property name. Common examples where this is useful include dash-separated environment properties (for example, context-path binds to contextPath), and capitalized environment properties (for example, PORT binds to port).

For example, consider the following @ConfigurationProperties class:

@ConfigurationProperties(prefix="acme.my-project.person")
public class OwnerProperties {

    private String firstName;

    public String getFirstName() {
        return this.firstName;
    }

    public void setFirstName(String firstName) {
        this.firstName = firstName;
    }

}

In the preceding example, the following properties names can all be used:

Table 5. relaxed binding
Property Note

acme.my-project.person.first-name

Kebab case, which is recommended for use in .properties and .yml files.

acme.myProject.person.firstName

Standard camel case syntax.

acme.my_project.person.first_name

Underscore notation, which is an alternative format for use in .properties and .yml files.

ACME_MYPROJECT_PERSON_FIRSTNAME

Upper case format, which is recommended when using system environment variables.

The prefix value for the annotation must be in kebab case (lowercase and separated by -, such as acme.my-project.person).
Table 6. relaxed binding rules per property source
Property Source Simple List

Properties Files

Camel case, kebab case, or underscore notation

Standard list syntax using [ ] or comma-separated values

YAML Files

Camel case, kebab case, or underscore notation

Standard YAML list syntax or comma-separated values

Environment Variables

Upper case format with underscore as the delimiter. _ should not be used within a property name

Numeric values surrounded by underscores, such as MY_ACME_1_OTHER = my.acme[1].other

System properties

Camel case, kebab case, or underscore notation

Standard list syntax using [ ] or comma-separated values

We recommend that, when possible, properties are stored in lower-case kebab format, such as my.property-name=acme.

When binding to Map properties, if the key contains anything other than lowercase alpha-numeric characters or -, you need to use the bracket notation so that the original value is preserved. If the key is not surrounded by [], any characters that are not alpha-numeric or - are removed. For example, consider binding the following properties to a Map:

acme:
  map:
    "[/key1]": value1
    "[/key2]": value2
    /key3: value3

The properties above will bind to a Map with /key1, /key2 and key3 as the keys in the map.

Merging Complex Types

When lists are configured in more than one place, overriding works by replacing the entire list.

For example, assume a MyPojo object with name and description attributes that are null by default. The following example exposes a list of MyPojo objects from AcmeProperties:

@ConfigurationProperties("acme")
public class AcmeProperties {

    private final List<MyPojo> list = new ArrayList<>();

    public List<MyPojo> getList() {
        return this.list;
    }

}

Consider the following configuration:

acme:
  list:
    - name: my name
      description: my description
---
spring:
  profiles: dev
acme:
  list:
    - name: my another name

If the dev profile is not active, AcmeProperties.list contains one MyPojo entry, as previously defined. If the dev profile is enabled, however, the list still contains only one entry (with a name of my another name and a description of null). This configuration does not add a second MyPojo instance to the list, and it does not merge the items.

When a List is specified in multiple profiles, the one with the highest priority (and only that one) is used. Consider the following example:

acme:
  list:
    - name: my name
      description: my description
    - name: another name
      description: another description
---
spring:
  profiles: dev
acme:
  list:
    - name: my another name

In the preceding example, if the dev profile is active, AcmeProperties.list contains one MyPojo entry (with a name of my another name and a description of null). For YAML, both comma-separated lists and YAML lists can be used for completely overriding the contents of the list.

For Map properties, you can bind with property values drawn from multiple sources. However, for the same property in multiple sources, the one with the highest priority is used. The following example exposes a Map<String, MyPojo> from AcmeProperties:

@ConfigurationProperties("acme")
public class AcmeProperties {

    private final Map<String, MyPojo> map = new HashMap<>();

    public Map<String, MyPojo> getMap() {
        return this.map;
    }

}

Consider the following configuration:

acme:
  map:
    key1:
      name: my name 1
      description: my description 1
---
spring:
  profiles: dev
acme:
  map:
    key1:
      name: dev name 1
    key2:
      name: dev name 2
      description: dev description 2

If the dev profile is not active, AcmeProperties.map contains one entry with key key1 (with a name of my name 1 and a description of my description 1). If the dev profile is enabled, however, map contains two entries with keys key1 (with a name of dev name 1 and a description of my description 1) and key2 (with a name of dev name 2 and a description of dev description 2).

The preceding merging rules apply to properties from all property sources and not just YAML files.
Properties Conversion

Spring Boot attempts to coerce the external application properties to the right type when it binds to the @ConfigurationProperties beans. If you need custom type conversion, you can provide a ConversionService bean (with a bean named conversionService) or custom property editors (through a CustomEditorConfigurer bean) or custom Converters (with bean definitions annotated as @ConfigurationPropertiesBinding).

As this bean is requested very early during the application lifecycle, make sure to limit the dependencies that your ConversionService is using. Typically, any dependency that you require may not be fully initialized at creation time. You may want to rename your custom ConversionService if it is not required for configuration keys coercion and only rely on custom converters qualified with @ConfigurationPropertiesBinding.
Converting durations

Spring Boot has dedicated support for expressing durations. If you expose a java.time.Duration property, the following formats in application properties are available:

  • A regular long representation (using milliseconds as the default unit unless a @DurationUnit has been specified)

  • The standard ISO-8601 format used by java.time.Duration

  • A more readable format where the value and the unit are coupled (e.g. 10s means 10 seconds)

Consider the following example:

@ConfigurationProperties("app.system")
public class AppSystemProperties {

    @DurationUnit(ChronoUnit.SECONDS)
    private Duration sessionTimeout = Duration.ofSeconds(30);

    private Duration readTimeout = Duration.ofMillis(1000);

    public Duration getSessionTimeout() {
        return this.sessionTimeout;
    }

    public void setSessionTimeout(Duration sessionTimeout) {
        this.sessionTimeout = sessionTimeout;
    }

    public Duration getReadTimeout() {
        return this.readTimeout;
    }

    public void setReadTimeout(Duration readTimeout) {
        this.readTimeout = readTimeout;
    }

}

To specify a session timeout of 30 seconds, 30, PT30S and 30s are all equivalent. A read timeout of 500ms can be specified in any of the following form: 500, PT0.5S and 500ms.

You can also use any of the supported units. These are:

  • ns for nanoseconds

  • us for microseconds

  • ms for milliseconds

  • s for seconds

  • m for minutes

  • h for hours

  • d for days

The default unit is milliseconds and can be overridden using @DurationUnit as illustrated in the sample above.

If you are upgrading from a previous version that is simply using Long to express the duration, make sure to define the unit (using @DurationUnit) if it isn’t milliseconds alongside the switch to Duration. Doing so gives a transparent upgrade path while supporting a much richer format.
Converting Data Sizes

Spring Framework has a DataSize value type that allows to express size in bytes. If you expose a DataSize property, the following formats in application properties are available:

  • A regular long representation (using bytes as the default unit unless a @DataSizeUnit has been specified)

  • A more readable format where the value and the unit are coupled (e.g. 10MB means 10 megabytes)

Consider the following example:

@ConfigurationProperties("app.io")
public class AppIoProperties {

    @DataSizeUnit(DataUnit.MEGABYTES)
    private DataSize bufferSize = DataSize.ofMegabytes(2);

    private DataSize sizeThreshold = DataSize.ofBytes(512);

    public DataSize getBufferSize() {
        return this.bufferSize;
    }

    public void setBufferSize(DataSize bufferSize) {
        this.bufferSize = bufferSize;
    }

    public DataSize getSizeThreshold() {
        return this.sizeThreshold;
    }

    public void setSizeThreshold(DataSize sizeThreshold) {
        this.sizeThreshold = sizeThreshold;
    }

}

To specify a buffer size of 10 megabytes, 10 and 10MB are equivalent. A size threshold of 256 bytes can be specified as 256 or 256B.

You can also use any of the supported units. These are:

  • B for bytes

  • KB for kilobytes

  • MB for megabytes

  • GB for gigabytes

  • TB for terabytes

The default unit is bytes and can be overridden using @DataSizeUnit as illustrated in the sample above.

If you are upgrading from a previous version that is simply using Long to express the size, make sure to define the unit (using @DataSizeUnit) if it isn’t bytes alongside the switch to DataSize. Doing so gives a transparent upgrade path while supporting a much richer format.
@ConfigurationProperties Validation

Spring Boot attempts to validate @ConfigurationProperties classes whenever they are annotated with Spring’s @Validated annotation. You can use JSR-303 javax.validation constraint annotations directly on your configuration class. To do so, ensure that a compliant JSR-303 implementation is on your classpath and then add constraint annotations to your fields, as shown in the following example:

@ConfigurationProperties(prefix="acme")
@Validated
public class AcmeProperties {

    @NotNull
    private InetAddress remoteAddress;

    // ... getters and setters

}
You can also trigger validation by annotating the @Bean method that creates the configuration properties with @Validated.

Although nested properties will also be validated when bound, it’s good practice to also annotate the associated field as @Valid. This ensure that validation is triggered even if no nested properties are found. The following example builds on the preceding AcmeProperties example:

@ConfigurationProperties(prefix="acme")
@Validated
public class AcmeProperties {

    @NotNull
    private InetAddress remoteAddress;

    @Valid
    private final Security security = new Security();

    // ... getters and setters

    public static class Security {

        @NotEmpty
        public String username;

        // ... getters and setters

    }

}

You can also add a custom Spring Validator by creating a bean definition called configurationPropertiesValidator. The @Bean method should be declared static. The configuration properties validator is created very early in the application’s lifecycle, and declaring the @Bean method as static lets the bean be created without having to instantiate the @Configuration class. Doing so avoids any problems that may be caused by early instantiation. There is a property validation sample that shows how to set things up.

The spring-boot-actuator module includes an endpoint that exposes all @ConfigurationProperties beans. Point your web browser to /actuator/configprops or use the equivalent JMX endpoint. See the "Production ready features" section for details.
@ConfigurationProperties vs. @Value

The @Value annotation is a core container feature, and it does not provide the same features as type-safe configuration properties. The following table summarizes the features that are supported by @ConfigurationProperties and @Value:

Feature @ConfigurationProperties @Value

Relaxed binding

Yes

No

Meta-data support

Yes

No

SpEL evaluation

No

Yes

If you define a set of configuration keys for your own components, we recommend you group them in a POJO annotated with @ConfigurationProperties. You should also be aware that, since @Value does not support relaxed binding, it is not a good candidate if you need to provide the value by using environment variables.

Finally, while you can write a SpEL expression in @Value, such expressions are not processed from application property files.

4.3. Profiles

Spring Profiles provide a way to segregate parts of your application configuration and make it be available only in certain environments. Any @Component, @Configuration or @ConfigurationProperties can be marked with @Profile to limit when it is loaded, as shown in the following example:

@Configuration(proxyBeanMethods = false)
@Profile("production")
public class ProductionConfiguration {

    // ...

}
If @ConfigurationProperties beans are registered via @EnableConfigurationProperties instead of automatic scanning, the @Profile annotation needs to be specified on the @Configuration class that has the @EnableConfigurationProperties annotation. In the case where @ConfigurationProperties are scanned, @Profile can be specified on the @ConfigurationProperties class itself.

You can use a spring.profiles.active Environment property to specify which profiles are active. You can specify the property in any of the ways described earlier in this chapter. For example, you could include it in your application.properties, as shown in the following example:

spring.profiles.active=dev,hsqldb

You could also specify it on the command line by using the following switch: --spring.profiles.active=dev,hsqldb.

4.3.1. Adding Active Profiles

The spring.profiles.active property follows the same ordering rules as other properties: The highest PropertySource wins. This means that you can specify active profiles in application.properties and then replace them by using the command line switch.

Sometimes, it is useful to have profile-specific properties that add to the active profiles rather than replace them. The spring.profiles.include property can be used to unconditionally add active profiles. The SpringApplication entry point also has a Java API for setting additional profiles (that is, on top of those activated by the spring.profiles.active property). See the setAdditionalProfiles() method in SpringApplication.

For example, when an application with the following properties is run by using the switch, --spring.profiles.active=prod, the proddb and prodmq profiles are also activated:

---
my.property: fromyamlfile
---
spring.profiles: prod
spring.profiles.include:
  - proddb
  - prodmq
Remember that the spring.profiles property can be defined in a YAML document to determine when this particular document is included in the configuration. See Change Configuration Depending on the Environment for more details.

4.3.2. Programmatically Setting Profiles

You can programmatically set active profiles by calling SpringApplication.setAdditionalProfiles(…​) before your application runs. It is also possible to activate profiles by using Spring’s ConfigurableEnvironment interface.

4.3.3. Profile-specific Configuration Files

Profile-specific variants of both application.properties (or application.yml) and files referenced through @ConfigurationProperties are considered as files and loaded. See "Profile-specific Properties" for details.

4.4. Logging

Spring Boot uses Commons Logging for all internal logging but leaves the underlying log implementation open. Default configurations are provided for Java Util Logging, Log4J2, and Logback. In each case, loggers are pre-configured to use console output with optional file output also available.

By default, if you use the “Starters”, Logback is used for logging. Appropriate Logback routing is also included to ensure that dependent libraries that use Java Util Logging, Commons Logging, Log4J, or SLF4J all work correctly.

There are a lot of logging frameworks available for Java. Do not worry if the above list seems confusing. Generally, you do not need to change your logging dependencies and the Spring Boot defaults work just fine.

4.4.1. Log Format

The default log output from Spring Boot resembles the following example:

2014-03-05 10:57:51.112  INFO 45469 --- [           main] org.apache.catalina.core.StandardEngine  : Starting Servlet Engine: Apache Tomcat/7.0.52
2014-03-05 10:57:51.253  INFO 45469 --- [ost-startStop-1] o.a.c.c.C.[Tomcat].[localhost].[/]       : Initializing Spring embedded WebApplicationContext
2014-03-05 10:57:51.253  INFO 45469 --- [ost-startStop-1] o.s.web.context.ContextLoader            : Root WebApplicationContext: initialization completed in 1358 ms
2014-03-05 10:57:51.698  INFO 45469 --- [ost-startStop-1] o.s.b.c.e.ServletRegistrationBean        : Mapping servlet: 'dispatcherServlet' to [/]
2014-03-05 10:57:51.702  INFO 45469 --- [ost-startStop-1] o.s.b.c.embedded.FilterRegistrationBean  : Mapping filter: 'hiddenHttpMethodFilter' to: [/*]

The following items are output:

  • Date and Time: Millisecond precision and easily sortable.

  • Log Level: ERROR, WARN, INFO, DEBUG, or TRACE.

  • Process ID.

  • A --- separator to distinguish the start of actual log messages.

  • Thread name: Enclosed in square brackets (may be truncated for console output).

  • Logger name: This is usually the source class name (often abbreviated).

  • The log message.

Logback does not have a FATAL level. It is mapped to ERROR.

4.4.2. Console Output

The default log configuration echoes messages to the console as they are written. By default, ERROR-level, WARN-level, and INFO-level messages are logged. You can also enable a “debug” mode by starting your application with a --debug flag.

$ java -jar myapp.jar --debug
You can also specify debug=true in your application.properties.

When the debug mode is enabled, a selection of core loggers (embedded container, Hibernate, and Spring Boot) are configured to output more information. Enabling the debug mode does not configure your application to log all messages with DEBUG level.

Alternatively, you can enable a “trace” mode by starting your application with a --trace flag (or trace=true in your application.properties). Doing so enables trace logging for a selection of core loggers (embedded container, Hibernate schema generation, and the whole Spring portfolio).

Color-coded Output

If your terminal supports ANSI, color output is used to aid readability. You can set spring.output.ansi.enabled to a supported value to override the auto detection.

Color coding is configured by using the %clr conversion word. In its simplest form, the converter colors the output according to the log level, as shown in the following example:

%clr(%5p)

The following table describes the mapping of log levels to colors:

Level Color

FATAL

Red

ERROR

Red

WARN

Yellow

INFO

Green

DEBUG

Green

TRACE

Green

Alternatively, you can specify the color or style that should be used by providing it as an option to the conversion. For example, to make the text yellow, use the following setting:

%clr(%d{yyyy-MM-dd HH:mm:ss.SSS}){yellow}

The following colors and styles are supported:

  • blue

  • cyan

  • faint

  • green

  • magenta

  • red

  • yellow

4.4.3. File Output

By default, Spring Boot logs only to the console and does not write log files. If you want to write log files in addition to the console output, you need to set a logging.file.name or logging.file.path property (for example, in your application.properties).

The following table shows how the logging.* properties can be used together:

Table 7. Logging properties
logging.file.name logging.file.path Example Description

(none)

(none)

Console only logging.

Specific file

(none)

my.log

Writes to the specified log file. Names can be an exact location or relative to the current directory.

(none)

Specific directory

/var/log

Writes spring.log to the specified directory. Names can be an exact location or relative to the current directory.

Log files rotate when they reach 10 MB and, as with console output, ERROR-level, WARN-level, and INFO-level messages are logged by default. Size limits can be changed using the logging.file.max-size property. Previously rotated files are archived indefinitely unless the logging.file.max-history property has been set. The total size of log archives can be capped using logging.file.total-size-cap. When the total size of log archives exceeds that threshold, backups will be deleted. To force log archive cleanup on application startup, use the logging.file.clean-history-on-start property.

The logging system is initialized early in the application lifecycle. Consequently, logging properties are not found in property files loaded through @PropertySource annotations.
Logging properties are independent of the actual logging infrastructure. As a result, specific configuration keys (such as logback.configurationFile for Logback) are not managed by spring Boot.

4.4.4. Log Levels

All the supported logging systems can have the logger levels set in the Spring Environment (for example, in application.properties) by using logging.level.<logger-name>=<level> where level is one of TRACE, DEBUG, INFO, WARN, ERROR, FATAL, or OFF. The root logger can be configured by using logging.level.root.

The following example shows potential logging settings in application.properties:

logging.level.root=WARN
logging.level.org.springframework.web=DEBUG
logging.level.org.hibernate=ERROR

4.4.5. Log Groups

It’s often useful to be able to group related loggers together so that they can all be configured at the same time. For example, you might commonly change the logging levels for all Tomcat related loggers, but you can’t easily remember top level packages.

To help with this, Spring Boot allows you to define logging groups in your Spring Environment. For example, here’s how you could define a “tomcat” group by adding it to your application.properties:

logging.group.tomcat=org.apache.catalina, org.apache.coyote, org.apache.tomcat

Once defined, you can change the level for all the loggers in the group with a single line:

logging.level.tomcat=TRACE

Spring Boot includes the following pre-defined logging groups that can be used out-of-the-box:

Name Loggers

web

org.springframework.core.codec, org.springframework.http, org.springframework.web

sql

org.springframework.jdbc.core, org.hibernate.SQL, org.jooq.tools.LoggerListener

4.4.6. Custom Log Configuration

The various logging systems can be activated by including the appropriate libraries on the classpath and can be further customized by providing a suitable configuration file in the root of the classpath or in a location specified by the following Spring Environment property: logging.config.

You can force Spring Boot to use a particular logging system by using the org.springframework.boot.logging.LoggingSystem system property. The value should be the fully qualified class name of a LoggingSystem implementation. You can also disable Spring Boot’s logging configuration entirely by using a value of none.

Since logging is initialized before the ApplicationContext is created, it is not possible to control logging from @PropertySources in Spring @Configuration files. The only way to change the logging system or disable it entirely is via System properties.

Depending on your logging system, the following files are loaded:

Logging System Customization

Logback

logback-spring.xml, logback-spring.groovy, logback.xml, or logback.groovy

Log4j2

log4j2-spring.xml or log4j2.xml

JDK (Java Util Logging)

logging.properties

When possible, we recommend that you use the -spring variants for your logging configuration (for example, logback-spring.xml rather than logback.xml). If you use standard configuration locations, Spring cannot completely control log initialization.
There are known classloading issues with Java Util Logging that cause problems when running from an 'executable jar'. We recommend that you avoid it when running from an 'executable jar' if at all possible.

To help with the customization, some other properties are transferred from the Spring Environment to System properties, as described in the following table:

Spring Environment System Property Comments

logging.exception-conversion-word

LOG_EXCEPTION_CONVERSION_WORD

The conversion word used when logging exceptions.

logging.file.clean-history-on-start

LOG_FILE_CLEAN_HISTORY_ON_START

Whether to clean the archive log files on startup (if LOG_FILE enabled). (Only supported with the default Logback setup.)

logging.file.name

LOG_FILE

If defined, it is used in the default log configuration.

logging.file.max-size

LOG_FILE_MAX_SIZE

Maximum log file size (if LOG_FILE enabled). (Only supported with the default Logback setup.)

logging.file.max-history

LOG_FILE_MAX_HISTORY

Maximum number of archive log files to keep (if LOG_FILE enabled). (Only supported with the default Logback setup.)

logging.file.path

LOG_PATH

If defined, it is used in the default log configuration.

logging.file.total-size-cap

LOG_FILE_TOTAL_SIZE_CAP

Total size of log backups to be kept (if LOG_FILE enabled). (Only supported with the default Logback setup.)

logging.pattern.console

CONSOLE_LOG_PATTERN

The log pattern to use on the console (stdout). (Only supported with the default Logback setup.)

logging.pattern.dateformat

LOG_DATEFORMAT_PATTERN

Appender pattern for log date format. (Only supported with the default Logback setup.)

logging.pattern.file

FILE_LOG_PATTERN

The log pattern to use in a file (if LOG_FILE is enabled). (Only supported with the default Logback setup.)

logging.pattern.level

LOG_LEVEL_PATTERN

The format to use when rendering the log level (default %5p). (Only supported with the default Logback setup.)

PID

PID

The current process ID (discovered if possible and when not already defined as an OS environment variable).

All the supported logging systems can consult System properties when parsing their configuration files. See the default configurations in spring-boot.jar for examples:

If you want to use a placeholder in a logging property, you should use Spring Boot’s syntax and not the syntax of the underlying framework. Notably, if you use Logback, you should use : as the delimiter between a property name and its default value and not use :-.

You can add MDC and other ad-hoc content to log lines by overriding only the LOG_LEVEL_PATTERN (or logging.pattern.level with Logback). For example, if you use logging.pattern.level=user:%X{user} %5p, then the default log format contains an MDC entry for "user", if it exists, as shown in the following example.

2015-09-30 12:30:04.031 user:someone INFO 22174 --- [  nio-8080-exec-0] demo.Controller
Handling authenticated request

4.4.7. Logback Extensions

Spring Boot includes a number of extensions to Logback that can help with advanced configuration. You can use these extensions in your logback-spring.xml configuration file.

Because the standard logback.xml configuration file is loaded too early, you cannot use extensions in it. You need to either use logback-spring.xml or define a logging.config property.
The extensions cannot be used with Logback’s configuration scanning. If you attempt to do so, making changes to the configuration file results in an error similar to one of the following being logged:
ERROR in ch.qos.logback.core.joran.spi.Interpreter@4:71 - no applicable action for [springProperty], current ElementPath is [[configuration][springProperty]]
ERROR in ch.qos.logback.core.joran.spi.Interpreter@4:71 - no applicable action for [springProfile], current ElementPath is [[configuration][springProfile]]
Profile-specific Configuration

The <springProfile> tag lets you optionally include or exclude sections of configuration based on the active Spring profiles. Profile sections are supported anywhere within the <configuration> element. Use the name attribute to specify which profile accepts the configuration. The <springProfile> tag can contain a simple profile name (for example staging) or a profile expression. A profile expression allows for more complicated profile logic to be expressed, for example production & (eu-central | eu-west). Check the reference guide for more details. The following listing shows three sample profiles:

<springProfile name="staging">
    <!-- configuration to be enabled when the "staging" profile is active -->
</springProfile>

<springProfile name="dev | staging">
    <!-- configuration to be enabled when the "dev" or "staging" profiles are active -->
</springProfile>

<springProfile name="!production">
    <!-- configuration to be enabled when the "production" profile is not active -->
</springProfile>
Environment Properties

The <springProperty> tag lets you expose properties from the Spring Environment for use within Logback. Doing so can be useful if you want to access values from your application.properties file in your Logback configuration. The tag works in a similar way to Logback’s standard <property> tag. However, rather than specifying a direct value, you specify the source of the property (from the Environment). If you need to store the property somewhere other than in local scope, you can use the scope attribute. If you need a fallback value (in case the property is not set in the Environment), you can use the defaultValue attribute. The following example shows how to expose properties for use within Logback:

<springProperty scope="context" name="fluentHost" source="myapp.fluentd.host"
        defaultValue="localhost"/>
<appender name="FLUENT" class="ch.qos.logback.more.appenders.DataFluentAppender">
    <remoteHost>${fluentHost}</remoteHost>
    ...
</appender>
The source must be specified in kebab case (such as my.property-name). However, properties can be added to the Environment by using the relaxed rules.

4.5. Internationalization

Spring Boot supports localized messages so that your application can cater to users of different language preferences. By default, Spring Boot looks for the presence of a messages resource bundle at the root of the classpath.

The auto-configuration applies when the default properties file for the configured resource bundle is available (i.e. messages.properties by default). If your resource bundle contains only language-specific properties files, you are required to add the default.

The basename of the resource bundle as well as several other attributes can be configured using the spring.messages namespace, as shown in the following example:

spring.messages.basename=messages,config.i18n.messages
spring.messages.fallback-to-system-locale=false
spring.messages.basename supports comma-separated list of locations, either a package qualifier or a resource resolved from the classpath root.

See MessageSourceProperties for more supported options.

4.6. JSON

Spring Boot provides integration with three JSON mapping libraries:

  • Gson

  • Jackson

  • JSON-B

Jackson is the preferred and default library.

4.6.1. Jackson

Auto-configuration for Jackson is provided and Jackson is part of spring-boot-starter-json. When Jackson is on the classpath an ObjectMapper bean is automatically configured. Several configuration properties are provided for customizing the configuration of the ObjectMapper.

4.6.2. Gson

Auto-configuration for Gson is provided. When Gson is on the classpath a Gson bean is automatically configured. Several spring.gson.* configuration properties are provided for customizing the configuration. To take more control, one or more GsonBuilderCustomizer beans can be used.

4.6.3. JSON-B

Auto-configuration for JSON-B is provided. When the JSON-B API and an implementation are on the classpath a Jsonb bean will be automatically configured. The preferred JSON-B implementation is Apache Johnzon for which dependency management is provided.

4.7. Developing Web Applications

Spring Boot is well suited for web application development. You can create a self-contained HTTP server by using embedded Tomcat, Jetty, Undertow, or Netty. Most web applications use the spring-boot-starter-web module to get up and running quickly. You can also choose to build reactive web applications by using the spring-boot-starter-webflux module.

If you have not yet developed a Spring Boot web application, you can follow the "Hello World!" example in the Getting started section.

4.7.1. The “Spring Web MVC Framework”

The Spring Web MVC framework (often referred to as simply “Spring MVC”) is a rich “model view controller” web framework. Spring MVC lets you create special @Controller or @RestController beans to handle incoming HTTP requests. Methods in your controller are mapped to HTTP by using @RequestMapping annotations.

The following code shows a typical @RestController that serves JSON data:

@RestController
@RequestMapping(value="/users")
public class MyRestController {

    @RequestMapping(value="/{user}", method=RequestMethod.GET)
    public User getUser(@PathVariable Long user) {
        // ...
    }

    @RequestMapping(value="/{user}/customers", method=RequestMethod.GET)
    List<Customer> getUserCustomers(@PathVariable Long user) {
        // ...
    }

    @RequestMapping(value="/{user}", method=RequestMethod.DELETE)
    public User deleteUser(@PathVariable Long user) {
        // ...
    }

}

Spring MVC is part of the core Spring Framework, and detailed information is available in the reference documentation. There are also several guides that cover Spring MVC available at spring.io/guides.

Spring MVC Auto-configuration

Spring Boot provides auto-configuration for Spring MVC that works well with most applications.

The auto-configuration adds the following features on top of Spring’s defaults:

  • Inclusion of ContentNegotiatingViewResolver and BeanNameViewResolver beans.

  • Support for serving static resources, including support for WebJars (covered later in this document)).

  • Automatic registration of Converter, GenericConverter, and Formatter beans.

  • Support for HttpMessageConverters (covered later in this document).

  • Automatic registration of MessageCodesResolver (covered later in this document).

  • Static index.html support.

  • Custom Favicon support (covered later in this document).

  • Automatic use of a ConfigurableWebBindingInitializer bean (covered later in this document).

If you want to keep Spring Boot MVC features and you want to add additional MVC configuration (interceptors, formatters, view controllers, and other features), you can add your own @Configuration class of type WebMvcConfigurer but without @EnableWebMvc. If you wish to provide custom instances of RequestMappingHandlerMapping, RequestMappingHandlerAdapter, or ExceptionHandlerExceptionResolver, you can declare a WebMvcRegistrationsAdapter instance to provide such components.

If you want to take complete control of Spring MVC, you can add your own @Configuration annotated with @EnableWebMvc.

HttpMessageConverters

Spring MVC uses the HttpMessageConverter interface to convert HTTP requests and responses. Sensible defaults are included out of the box. For example, objects can be automatically converted to JSON (by using the Jackson library) or XML (by using the Jackson XML extension, if available, or by using JAXB if the Jackson XML extension is not available). By default, strings are encoded in UTF-8.

If you need to add or customize converters, you can use Spring Boot’s HttpMessageConverters class, as shown in the following listing:

import org.springframework.boot.autoconfigure.http.HttpMessageConverters;
import org.springframework.context.annotation.*;
import org.springframework.http.converter.*;

@Configuration(proxyBeanMethods = false)
public class MyConfiguration {

    @Bean
    public HttpMessageConverters customConverters() {
        HttpMessageConverter<?> additional = ...
        HttpMessageConverter<?> another = ...
        return new HttpMessageConverters(additional, another);
    }

}

Any HttpMessageConverter bean that is present in the context is added to the list of converters. You can also override default converters in the same way.

Custom JSON Serializers and Deserializers

If you use Jackson to serialize and deserialize JSON data, you might want to write your own JsonSerializer and JsonDeserializer classes. Custom serializers are usually registered with Jackson through a module, but Spring Boot provides an alternative @JsonComponent annotation that makes it easier to directly register Spring Beans.

You can use the @JsonComponent annotation directly on JsonSerializer, JsonDeserializer or KeyDeserializer implementations. You can also use it on classes that contain serializers/deserializers as inner classes, as shown in the following example:

import java.io.*;
import com.fasterxml.jackson.core.*;
import com.fasterxml.jackson.databind.*;
import org.springframework.boot.jackson.*;

@JsonComponent
public class Example {

    public static class Serializer extends JsonSerializer<SomeObject> {
        // ...
    }

    public static class Deserializer extends JsonDeserializer<SomeObject> {
        // ...
    }

}

All @JsonComponent beans in the ApplicationContext are automatically registered with Jackson. Because @JsonComponent is meta-annotated with @Component, the usual component-scanning rules apply.

Spring Boot also provides JsonObjectSerializer and JsonObjectDeserializer base classes that provide useful alternatives to the standard Jackson versions when serializing objects. See JsonObjectSerializer and JsonObjectDeserializer in the Javadoc for details.

MessageCodesResolver

Spring MVC has a strategy for generating error codes for rendering error messages from binding errors: MessageCodesResolver. If you set the spring.mvc.message-codes-resolver.format property PREFIX_ERROR_CODE or POSTFIX_ERROR_CODE, Spring Boot creates one for you (see the enumeration in DefaultMessageCodesResolver.Format).

Static Content

By default, Spring Boot serves static content from a directory called /static (or /public or /resources or /META-INF/resources) in the classpath or from the root of the ServletContext. It uses the ResourceHttpRequestHandler from Spring MVC so that you can modify that behavior by adding your own WebMvcConfigurer and overriding the addResourceHandlers method.

In a stand-alone web application, the default servlet from the container is also enabled and acts as a fallback, serving content from the root of the ServletContext if Spring decides not to handle it. Most of the time, this does not happen (unless you modify the default MVC configuration), because Spring can always handle requests through the DispatcherServlet.

By default, resources are mapped on /**, but you can tune that with the spring.mvc.static-path-pattern property. For instance, relocating all resources to /resources/** can be achieved as follows:

spring.mvc.static-path-pattern=/resources/**

You can also customize the static resource locations by using the spring.resources.static-locations property (replacing the default values with a list of directory locations). The root Servlet context path, "/", is automatically added as a location as well.

In addition to the “standard” static resource locations mentioned earlier, a special case is made for Webjars content. Any resources with a path in /webjars/** are served from jar files if they are packaged in the Webjars format.

Do not use the src/main/webapp directory if your application is packaged as a jar. Although this directory is a common standard, it works only with war packaging, and it is silently ignored by most build tools if you generate a jar.

Spring Boot also supports the advanced resource handling features provided by Spring MVC, allowing use cases such as cache-busting static resources or using version agnostic URLs for Webjars.

To use version agnostic URLs for Webjars, add the webjars-locator-core dependency. Then declare your Webjar. Using jQuery as an example, adding "/webjars/jquery/jquery.min.js" results in "/webjars/jquery/x.y.z/jquery.min.js". where x.y.z is the Webjar version.

If you use JBoss, you need to declare the webjars-locator-jboss-vfs dependency instead of the webjars-locator-core. Otherwise, all Webjars resolve as a 404.

To use cache busting, the following configuration configures a cache busting solution for all static resources, effectively adding a content hash, such as <link href="/css/spring-2a2d595e6ed9a0b24f027f2b63b134d6.css"/>, in URLs:

spring.resources.chain.strategy.content.enabled=true
spring.resources.chain.strategy.content.paths=/**
Links to resources are rewritten in templates at runtime, thanks to a ResourceUrlEncodingFilter that is auto-configured for Thymeleaf and FreeMarker. You should manually declare this filter when using JSPs. Other template engines are currently not automatically supported but can be with custom template macros/helpers and the use of the ResourceUrlProvider.

When loading resources dynamically with, for example, a JavaScript module loader, renaming files is not an option. That is why other strategies are also supported and can be combined. A "fixed" strategy adds a static version string in the URL without changing the file name, as shown in the following example:

spring.resources.chain.strategy.content.enabled=true
spring.resources.chain.strategy.content.paths=/**
spring.resources.chain.strategy.fixed.enabled=true
spring.resources.chain.strategy.fixed.paths=/js/lib/
spring.resources.chain.strategy.fixed.version=v12

With this configuration, JavaScript modules located under "/js/lib/" use a fixed versioning strategy ("/v12/js/lib/mymodule.js"), while other resources still use the content one (<link href="/css/spring-2a2d595e6ed9a0b24f027f2b63b134d6.css"/>).

See ResourceProperties for more supported options.

This feature has been thoroughly described in a dedicated blog post and in Spring Framework’s reference documentation.

Welcome Page

Spring Boot supports both static and templated welcome pages. It first looks for an index.html file in the configured static content locations. If one is not found, it then looks for an index template. If either is found, it is automatically used as the welcome page of the application.

Custom Favicon

Spring Boot looks for a favicon.ico in the configured static content locations and the root of the classpath (in that order). If such a file is present, it is automatically used as the favicon of the application.

Path Matching and Content Negotiation

Spring MVC can map incoming HTTP requests to handlers by looking at the request path and matching it to the mappings defined in your application (for example, @GetMapping annotations on Controller methods).

Spring Boot chooses to disable suffix pattern matching by default, which means that requests like "GET /projects/spring-boot.json" won’t be matched to @GetMapping("/projects/spring-boot") mappings. This is considered as a best practice for Spring MVC applications. This feature was mainly useful in the past for HTTP clients which did not send proper "Accept" request headers; we needed to make sure to send the correct Content Type to the client. Nowadays, Content Negotiation is much more reliable.

There are other ways to deal with HTTP clients that don’t consistently send proper "Accept" request headers. Instead of using suffix matching, we can use a query parameter to ensure that requests like "GET /projects/spring-boot?format=json" will be mapped to @GetMapping("/projects/spring-boot"):

spring.mvc.contentnegotiation.favor-parameter=true

# We can change the parameter name, which is "format" by default:
# spring.mvc.contentnegotiation.parameter-name=myparam

# We can also register additional file extensions/media types with:
spring.mvc.contentnegotiation.media-types.markdown=text/markdown

If you understand the caveats and would still like your application to use suffix pattern matching, the following configuration is required:

spring.mvc.contentnegotiation.favor-path-extension=true
spring.mvc.pathmatch.use-suffix-pattern=true

Alternatively, rather than open all suffix patterns, it’s more secure to just support registered suffix patterns:

spring.mvc.contentnegotiation.favor-path-extension=true
spring.mvc.pathmatch.use-registered-suffix-pattern=true

# You can also register additional file extensions/media types with:
# spring.mvc.contentnegotiation.media-types.adoc=text/asciidoc
ConfigurableWebBindingInitializer

Spring MVC uses a WebBindingInitializer to initialize a WebDataBinder for a particular request. If you create your own ConfigurableWebBindingInitializer @Bean, Spring Boot automatically configures Spring MVC to use it.

Template Engines

As well as REST web services, you can also use Spring MVC to serve dynamic HTML content. Spring MVC supports a variety of templating technologies, including Thymeleaf, FreeMarker, and JSPs. Also, many other templating engines include their own Spring MVC integrations.

Spring Boot includes auto-configuration support for the following templating engines:

If possible, JSPs should be avoided. There are several known limitations when using them with embedded servlet containers.

When you use one of these templating engines with the default configuration, your templates are picked up automatically from src/main/resources/templates.

Depending on how you run your application, IntelliJ IDEA orders the classpath differently. Running your application in the IDE from its main method results in a different ordering than when you run your application by using Maven or Gradle or from its packaged jar. This can cause Spring Boot to fail to find the templates on the classpath. If you have this problem, you can reorder the classpath in the IDE to place the module’s classes and resources first. Alternatively, you can configure the template prefix to search every templates directory on the classpath, as follows: classpath*:/templates/.
Error Handling

By default, Spring Boot provides an /error mapping that handles all errors in a sensible way, and it is registered as a “global” error page in the servlet container. For machine clients, it produces a JSON response with details of the error, the HTTP status, and the exception message. For browser clients, there is a “whitelabel” error view that renders the same data in HTML format (to customize it, add a View that resolves to error). To replace the default behavior completely, you can implement ErrorController and register a bean definition of that type or add a bean of type ErrorAttributes to use the existing mechanism but replace the contents.

The BasicErrorController can be used as a base class for a custom ErrorController. This is particularly useful if you want to add a handler for a new content type (the default is to handle text/html specifically and provide a fallback for everything else). To do so, extend BasicErrorController, add a public method with a @RequestMapping that has a produces attribute, and create a bean of your new type.

You can also define a class annotated with @ControllerAdvice to customize the JSON document to return for a particular controller and/or exception type, as shown in the following example:

@ControllerAdvice(basePackageClasses = AcmeController.class)
public class AcmeControllerAdvice extends ResponseEntityExceptionHandler {

    @ExceptionHandler(YourException.class)
    @ResponseBody
    ResponseEntity<?> handleControllerException(HttpServletRequest request, Throwable ex) {
        HttpStatus status = getStatus(request);
        return new ResponseEntity<>(new CustomErrorType(status.value(), ex.getMessage()), status);
    }

    private HttpStatus getStatus(HttpServletRequest request) {
        Integer statusCode = (Integer) request.getAttribute("javax.servlet.error.status_code");
        if (statusCode == null) {
            return HttpStatus.INTERNAL_SERVER_ERROR;
        }
        return HttpStatus.valueOf(statusCode);
    }

}

In the preceding example, if YourException is thrown by a controller defined in the same package as AcmeController, a JSON representation of the CustomErrorType POJO is used instead of the ErrorAttributes representation.

Custom Error Pages

If you want to display a custom HTML error page for a given status code, you can add a file to an /error folder. Error pages can either be static HTML (that is, added under any of the static resource folders) or be built by using templates. The name of the file should be the exact status code or a series mask.

For example, to map 404 to a static HTML file, your folder structure would be as follows:

src/
 +- main/
     +- java/
     |   + <source code>
     +- resources/
         +- public/
             +- error/
             |   +- 404.html
             +- <other public assets>

To map all 5xx errors by using a FreeMarker template, your folder structure would be as follows:

src/
 +- main/
     +- java/
     |   + <source code>
     +- resources/
         +- templates/
             +- error/
             |   +- 5xx.ftlh
             +- <other templates>

For more complex mappings, you can also add beans that implement the ErrorViewResolver interface, as shown in the following example:

public class MyErrorViewResolver implements ErrorViewResolver {

    @Override
    public ModelAndView resolveErrorView(HttpServletRequest request,
            HttpStatus status, Map<String, Object> model) {
        // Use the request or status to optionally return a ModelAndView
        return ...
    }

}

You can also use regular Spring MVC features such as @ExceptionHandler methods and @ControllerAdvice. The ErrorController then picks up any unhandled exceptions.

Mapping Error Pages outside of Spring MVC

For applications that do not use Spring MVC, you can use the ErrorPageRegistrar interface to directly register ErrorPages. This abstraction works directly with the underlying embedded servlet container and works even if you do not have a Spring MVC DispatcherServlet.

@Bean
public ErrorPageRegistrar errorPageRegistrar(){
    return new MyErrorPageRegistrar();
}

// ...

private static class MyErrorPageRegistrar implements ErrorPageRegistrar {

    @Override
    public void registerErrorPages(ErrorPageRegistry registry) {
        registry.addErrorPages(new ErrorPage(HttpStatus.BAD_REQUEST, "/400"));
    }

}
If you register an ErrorPage with a path that ends up being handled by a Filter (as is common with some non-Spring web frameworks, like Jersey and Wicket), then the Filter has to be explicitly registered as an ERROR dispatcher, as shown in the following example:
@Bean
public FilterRegistrationBean myFilter() {
    FilterRegistrationBean registration = new FilterRegistrationBean();
    registration.setFilter(new MyFilter());
    ...
    registration.setDispatcherTypes(EnumSet.allOf(DispatcherType.class));
    return registration;
}

Note that the default FilterRegistrationBean does not include the ERROR dispatcher type.

CAUTION:When deployed to a servlet container, Spring Boot uses its error page filter to forward a request with an error status to the appropriate error page. The request can only be forwarded to the correct error page if the response has not already been committed. By default, WebSphere Application Server 8.0 and later commits the response upon successful completion of a servlet’s service method. You should disable this behavior by setting com.ibm.ws.webcontainer.invokeFlushAfterService to false.

Spring HATEOAS

If you develop a RESTful API that makes use of hypermedia, Spring Boot provides auto-configuration for Spring HATEOAS that works well with most applications. The auto-configuration replaces the need to use @EnableHypermediaSupport and registers a number of beans to ease building hypermedia-based applications, including a LinkDiscoverers (for client side support) and an ObjectMapper configured to correctly marshal responses into the desired representation. The ObjectMapper is customized by setting the various spring.jackson.* properties or, if one exists, by a Jackson2ObjectMapperBuilder bean.

You can take control of Spring HATEOAS’s configuration by using @EnableHypermediaSupport. Note that doing so disables the ObjectMapper customization described earlier.

CORS Support

Cross-origin resource sharing (CORS) is a W3C specification implemented by most browsers that lets you specify in a flexible way what kind of cross-domain requests are authorized, instead of using some less secure and less powerful approaches such as IFRAME or JSONP.

As of version 4.2, Spring MVC supports CORS. Using controller method CORS configuration with @CrossOrigin annotations in your Spring Boot application does not require any specific configuration. Global CORS configuration can be defined by registering a WebMvcConfigurer bean with a customized addCorsMappings(CorsRegistry) method, as shown in the following example:

@Configuration(proxyBeanMethods = false)
public class MyConfiguration {

    @Bean
    public WebMvcConfigurer corsConfigurer() {
        return new WebMvcConfigurer() {
            @Override
            public void addCorsMappings(CorsRegistry registry) {
                registry.addMapping("/api/**");
            }
        };
    }
}

4.7.2. The “Spring WebFlux Framework”

Spring WebFlux is the new reactive web framework introduced in Spring Framework 5.0. Unlike Spring MVC, it does not require the Servlet API, is fully asynchronous and non-blocking, and implements the Reactive Streams specification through the Reactor project.

Spring WebFlux comes in two flavors: functional and annotation-based. The annotation-based one is quite close to the Spring MVC model, as shown in the following example:

@RestController
@RequestMapping("/users")
public class MyRestController {

    @GetMapping("/{user}")
    public Mono<User> getUser(@PathVariable Long user) {
        // ...
    }

    @GetMapping("/{user}/customers")
    public Flux<Customer> getUserCustomers(@PathVariable Long user) {
        // ...
    }

    @DeleteMapping("/{user}")
    public Mono<User> deleteUser(@PathVariable Long user) {
        // ...
    }

}

“WebFlux.fn”, the functional variant, separates the routing configuration from the actual handling of the requests, as shown in the following example:

@Configuration(proxyBeanMethods = false)
public class RoutingConfiguration {

    @Bean
    public RouterFunction<ServerResponse> monoRouterFunction(UserHandler userHandler) {
        return route(GET("/{user}").and(accept(APPLICATION_JSON)), userHandler::getUser)
                .andRoute(GET("/{user}/customers").and(accept(APPLICATION_JSON)), userHandler::getUserCustomers)
                .andRoute(DELETE("/{user}").and(accept(APPLICATION_JSON)), userHandler::deleteUser);
    }

}

@Component
public class UserHandler {

    public Mono<ServerResponse> getUser(ServerRequest request) {
        // ...
    }

    public Mono<ServerResponse> getUserCustomers(ServerRequest request) {
        // ...
    }

    public Mono<ServerResponse> deleteUser(ServerRequest request) {
        // ...
    }
}

WebFlux is part of the Spring Framework and detailed information is available in its reference documentation.

You can define as many RouterFunction beans as you like to modularize the definition of the router. Beans can be ordered if you need to apply a precedence.

To get started, add the spring-boot-starter-webflux module to your application.

Adding both spring-boot-starter-web and spring-boot-starter-webflux modules in your application results in Spring Boot auto-configuring Spring MVC, not WebFlux. This behavior has been chosen because many Spring developers add spring-boot-starter-webflux to their Spring MVC application to use the reactive WebClient. You can still enforce your choice by setting the chosen application type to SpringApplication.setWebApplicationType(WebApplicationType.REACTIVE).
Spring WebFlux Auto-configuration

Spring Boot provides auto-configuration for Spring WebFlux that works well with most applications.

The auto-configuration adds the following features on top of Spring’s defaults:

If you want to keep Spring Boot WebFlux features and you want to add additional WebFlux configuration, you can add your own @Configuration class of type WebFluxConfigurer but without @EnableWebFlux.

If you want to take complete control of Spring WebFlux, you can add your own @Configuration annotated with @EnableWebFlux.

HTTP Codecs with HttpMessageReaders and HttpMessageWriters

Spring WebFlux uses the HttpMessageReader and HttpMessageWriter interfaces to convert HTTP requests and responses. They are configured with CodecConfigurer to have sensible defaults by looking at the libraries available in your classpath.

Spring Boot applies further customization by using CodecCustomizer instances. For example, spring.jackson.* configuration keys are applied to the Jackson codec.

If you need to add or customize codecs, you can create a custom CodecCustomizer component, as shown in the following example:

import org.springframework.boot.web.codec.CodecCustomizer;

@Configuration(proxyBeanMethods = false)
public class MyConfiguration {

    @Bean
    public CodecCustomizer myCodecCustomizer() {
        return codecConfigurer -> {
            // ...
        }
    }

}
Static Content

By default, Spring Boot serves static content from a directory called /static (or /public or /resources or /META-INF/resources) in the classpath. It uses the ResourceWebHandler from Spring WebFlux so that you can modify that behavior by adding your own WebFluxConfigurer and overriding the addResourceHandlers method.

By default, resources are mapped on /**, but you can tune that by setting the spring.webflux.static-path-pattern property. For instance, relocating all resources to /resources/** can be achieved as follows:

spring.webflux.static-path-pattern=/resources/**

You can also customize the static resource locations by using spring.resources.static-locations. Doing so replaces the default values with a list of directory locations. If you do so, the default welcome page detection switches to your custom locations. So, if there is an index.html in any of your locations on startup, it is the home page of the application.

In addition to the “standard” static resource locations listed earlier, a special case is made for Webjars content. Any resources with a path in /webjars/** are served from jar files if they are packaged in the Webjars format.

Spring WebFlux applications do not strictly depend on the Servlet API, so they cannot be deployed as war files and do not use the src/main/webapp directory.
Template Engines

As well as REST web services, you can also use Spring WebFlux to serve dynamic HTML content. Spring WebFlux supports a variety of templating technologies, including Thymeleaf, FreeMarker, and Mustache.

Spring Boot includes auto-configuration support for the following templating engines:

When you use one of these templating engines with the default configuration, your templates are picked up automatically from src/main/resources/templates.

Error Handling

Spring Boot provides a WebExceptionHandler that handles all errors in a sensible way. Its position in the processing order is immediately before the handlers provided by WebFlux, which are considered last. For machine clients, it produces a JSON response with details of the error, the HTTP status, and the exception message. For browser clients, there is a “whitelabel” error handler that renders the same data in HTML format. You can also provide your own HTML templates to display errors (see the next section).

The first step to customizing this feature often involves using the existing mechanism but replacing or augmenting the error contents. For that, you can add a bean of type ErrorAttributes.

To change the error handling behavior, you can implement ErrorWebExceptionHandler and register a bean definition of that type. Because a WebExceptionHandler is quite low-level, Spring Boot also provides a convenient AbstractErrorWebExceptionHandler to let you handle errors in a WebFlux functional way, as shown in the following example:

public class CustomErrorWebExceptionHandler extends AbstractErrorWebExceptionHandler {

    // Define constructor here

    @Override
    protected RouterFunction<ServerResponse> getRoutingFunction(ErrorAttributes errorAttributes) {

        return RouterFunctions
                .route(aPredicate, aHandler)
                .andRoute(anotherPredicate, anotherHandler);
    }

}

For a more complete picture, you can also subclass DefaultErrorWebExceptionHandler directly and override specific methods.

Custom Error Pages

If you want to display a custom HTML error page for a given status code, you can add a file to an /error folder. Error pages can either be static HTML (that is, added under any of the static resource folders) or built with templates. The name of the file should be the exact status code or a series mask.

For example, to map 404 to a static HTML file, your folder structure would be as follows:

src/
 +- main/
     +- java/
     |   + <source code>
     +- resources/
         +- public/
             +- error/
             |   +- 404.html
             +- <other public assets>

To map all 5xx errors by using a Mustache template, your folder structure would be as follows:

src/
 +- main/
     +- java/
     |   + <source code>
     +- resources/
         +- templates/
             +- error/
             |   +- 5xx.mustache
             +- <other templates>
Web Filters

Spring WebFlux provides a WebFilter interface that can be implemented to filter HTTP request-response exchanges. WebFilter beans found in the application context will be automatically used to filter each exchange.

Where the order of the filters is important they can implement Ordered or be annotated with @Order. Spring Boot auto-configuration may configure web filters for you. When it does so, the orders shown in the following table will be used:

Web Filter Order

MetricsWebFilter

Ordered.HIGHEST_PRECEDENCE + 1

WebFilterChainProxy (Spring Security)

-100

HttpTraceWebFilter

Ordered.LOWEST_PRECEDENCE - 10

4.7.3. JAX-RS and Jersey

If you prefer the JAX-RS programming model for REST endpoints, you can use one of the available implementations instead of Spring MVC. Jersey and Apache CXF work quite well out of the box. CXF requires you to register its Servlet or Filter as a @Bean in your application context. Jersey has some native Spring support, so we also provide auto-configuration support for it in Spring Boot, together with a starter.

To get started with Jersey, include the spring-boot-starter-jersey as a dependency and then you need one @Bean of type ResourceConfig in which you register all the endpoints, as shown in the following example:

@Component
public class JerseyConfig extends ResourceConfig {

    public JerseyConfig() {
        register(Endpoint.class);
    }

}
Jersey’s support for scanning executable archives is rather limited. For example, it cannot scan for endpoints in a package found in a fully executable jar file or in WEB-INF/classes when running an executable war file. To avoid this limitation, the packages method should not be used, and endpoints should be registered individually by using the register method, as shown in the preceding example.

For more advanced customizations, you can also register an arbitrary number of beans that implement ResourceConfigCustomizer.

All the registered endpoints should be @Components with HTTP resource annotations (@GET and others), as shown in the following example:

@Component
@Path("/hello")
public class Endpoint {

    @GET
    public String message() {
        return "Hello";
    }

}

Since the Endpoint is a Spring @Component, its lifecycle is managed by Spring and you can use the @Autowired annotation to inject dependencies and use the @Value annotation to inject external configuration. By default, the Jersey servlet is registered and mapped to /*. You can change the mapping by adding @ApplicationPath to your ResourceConfig.

By default, Jersey is set up as a Servlet in a @Bean of type ServletRegistrationBean named jerseyServletRegistration. By default, the servlet is initialized lazily, but you can customize that behavior by setting spring.jersey.servlet.load-on-startup. You can disable or override that bean by creating one of your own with the same name. You can also use a filter instead of a servlet by setting spring.jersey.type=filter (in which case, the @Bean to replace or override is jerseyFilterRegistration). The filter has an @Order, which you can set with spring.jersey.filter.order. Both the servlet and the filter registrations can be given init parameters by using spring.jersey.init.* to specify a map of properties.

There is a Jersey sample so that you can see how to set things up.

4.7.4. Embedded Servlet Container Support

Spring Boot includes support for embedded Tomcat, Jetty, and Undertow servers. Most developers use the appropriate “Starter” to obtain a fully configured instance. By default, the embedded server listens for HTTP requests on port 8080.

Servlets, Filters, and listeners

When using an embedded servlet container, you can register servlets, filters, and all the listeners (such as HttpSessionListener) from the Servlet spec, either by using Spring beans or by scanning for Servlet components.

Registering Servlets, Filters, and Listeners as Spring Beans

Any Servlet, Filter, or servlet *Listener instance that is a Spring bean is registered with the embedded container. This can be particularly convenient if you want to refer to a value from your application.properties during configuration.

By default, if the context contains only a single Servlet, it is mapped to /. In the case of multiple servlet beans, the bean name is used as a path prefix. Filters map to /*.

If convention-based mapping is not flexible enough, you can use the ServletRegistrationBean, FilterRegistrationBean, and ServletListenerRegistrationBean classes for complete control.

Spring Boot ships with many auto-configurations that may define Filter beans. Here are a few examples of Filters and their respective order (lower order value means higher precedence):

Servlet Filter Order

OrderedCharacterEncodingFilter

Ordered.HIGHEST_PRECEDENCE

WebMvcMetricsFilter

Ordered.HIGHEST_PRECEDENCE + 1

ErrorPageFilter

Ordered.HIGHEST_PRECEDENCE + 1

HttpTraceFilter

Ordered.LOWEST_PRECEDENCE - 10

It is usually safe to leave Filter beans unordered.

If a specific order is required, you should avoid configuring a Filter that reads the request body at Ordered.HIGHEST_PRECEDENCE, since it might go against the character encoding configuration of your application. If a Servlet filter wraps the request, it should be configured with an order that is less than or equal to OrderedFilter.REQUEST_WRAPPER_FILTER_MAX_ORDER.

Servlet Context Initialization

Embedded servlet containers do not directly execute the Servlet 3.0+ javax.servlet.ServletContainerInitializer interface or Spring’s org.springframework.web.WebApplicationInitializer interface. This is an intentional design decision intended to reduce the risk that third party libraries designed to run inside a war may break Spring Boot applications.

If you need to perform servlet context initialization in a Spring Boot application, you should register a bean that implements the org.springframework.boot.web.servlet.ServletContextInitializer interface. The single onStartup method provides access to the ServletContext and, if necessary, can easily be used as an adapter to an existing WebApplicationInitializer.

Scanning for Servlets, Filters, and listeners

When using an embedded container, automatic registration of classes annotated with @WebServlet, @WebFilter, and @WebListener can be enabled by using @ServletComponentScan.

@ServletComponentScan has no effect in a standalone container, where the container’s built-in discovery mechanisms are used instead.
The ServletWebServerApplicationContext

Under the hood, Spring Boot uses a different type of ApplicationContext for embedded servlet container support. The ServletWebServerApplicationContext is a special type of WebApplicationContext that bootstraps itself by searching for a single ServletWebServerFactory bean. Usually a TomcatServletWebServerFactory, JettyServletWebServerFactory, or UndertowServletWebServerFactory has been auto-configured.

You usually do not need to be aware of these implementation classes. Most applications are auto-configured, and the appropriate ApplicationContext and ServletWebServerFactory are created on your behalf.
Customizing Embedded Servlet Containers

Common servlet container settings can be configured by using Spring Environment properties. Usually, you would define the properties in your application.properties file.

Common server settings include:

  • Network settings: Listen port for incoming HTTP requests (server.port), interface address to bind to server.address, and so on.

  • Session settings: Whether the session is persistent (server.servlet.session.persistence), session timeout (server.servlet.session.timeout), location of session data (server.servlet.session.store-dir), and session-cookie configuration (server.servlet.session.cookie.*).

  • Error management: Location of the error page (server.error.path) and so on.

  • SSL

  • HTTP compression

Spring Boot tries as much as possible to expose common settings, but this is not always possible. For those cases, dedicated namespaces offer server-specific customizations (see server.tomcat and server.undertow). For instance, access logs can be configured with specific features of the embedded servlet container.

See the ServerProperties class for a complete list.
Programmatic Customization

If you need to programmatically configure your embedded servlet container, you can register a Spring bean that implements the WebServerFactoryCustomizer interface. WebServerFactoryCustomizer provides access to the ConfigurableServletWebServerFactory, which includes numerous customization setter methods. The following example shows programmatically setting the port:

import org.springframework.boot.web.server.WebServerFactoryCustomizer;
import org.springframework.boot.web.servlet.server.ConfigurableServletWebServerFactory;
import org.springframework.stereotype.Component;

@Component
public class CustomizationBean implements WebServerFactoryCustomizer<ConfigurableServletWebServerFactory> {

    @Override
    public void customize(ConfigurableServletWebServerFactory server) {
        server.setPort(9000);
    }

}
TomcatServletWebServerFactory, JettyServletWebServerFactory and UndertowServletWebServerFactory are dedicated variants of ConfigurableServletWebServerFactory that have additional customization setter methods for Tomcat, Jetty and Undertow respectively.
Customizing ConfigurableServletWebServerFactory Directly

If the preceding customization techniques are too limited, you can register the TomcatServletWebServerFactory, JettyServletWebServerFactory, or UndertowServletWebServerFactory bean yourself.

@Bean
public ConfigurableServletWebServerFactory webServerFactory() {
    TomcatServletWebServerFactory factory = new TomcatServletWebServerFactory();
    factory.setPort(9000);
    factory.setSessionTimeout(10, TimeUnit.MINUTES);
    factory.addErrorPages(new ErrorPage(HttpStatus.NOT_FOUND, "/notfound.html"));
    return factory;
}

Setters are provided for many configuration options. Several protected method “hooks” are also provided should you need to do something more exotic. See the source code documentation for details.

JSP Limitations

When running a Spring Boot application that uses an embedded servlet container (and is packaged as an executable archive), there are some limitations in the JSP support.

  • With Jetty and Tomcat, it should work if you use war packaging. An executable war will work when launched with java -jar, and will also be deployable to any standard container. JSPs are not supported when using an executable jar.

  • Undertow does not support JSPs.

  • Creating a custom error.jsp page does not override the default view for error handling. Custom error pages should be used instead.

There is a JSP sample so that you can see how to set things up.

4.7.5. Embedded Reactive Server Support

Spring Boot includes support for the following embedded reactive web servers: Reactor Netty, Tomcat, Jetty, and Undertow. Most developers use the appropriate “Starter” to obtain a fully configured instance. By default, the embedded server listens for HTTP requests on port 8080.

4.7.6. Reactive Server Resources Configuration

When auto-configuring a Reactor Netty or Jetty server, Spring Boot will create specific beans that will provide HTTP resources to the server instance: ReactorResourceFactory or JettyResourceFactory.

By default, those resources will be also shared with the Reactor Netty and Jetty clients for optimal performances, given:

  • the same technology is used for server and client

  • the client instance is built using the WebClient.Builder bean auto-configured by Spring Boot

Developers can override the resource configuration for Jetty and Reactor Netty by providing a custom ReactorResourceFactory or JettyResourceFactory bean - this will be applied to both clients and servers.

You can learn more about the resource configuration on the client side in the WebClient Runtime section.

4.8. RSocket

RSocket is a binary protocol for use on byte stream transports. It enables symmetric interaction models via async message passing over a single connection.

Spring Framework, with the Spring Messaging module supports RSocket both on the server and the client side. On the server side, it lets you create special @Controller beans to handle incoming RSocket messages. Methods in your controller are mapped to RSocket routes by using @MessageMapping annotations.

The following code shows a typical @Controller:

@Controller
public class MyRSocketController {

    @MessageMapping("chat.room.{name}")
    public Flux<ChatMessages> enterChatRoom(@DestinationVariable String chatRoom,
            Flux<ChatMessages> messages) {
        // ...
    }

    @MessageMapping("users.{user}.info")
    Mono<ChatUserInfo> getUserInfo(@DestinationVariable String user) {
        // ...
    }

}

4.8.1. RSocket Strategies Auto-configuration

Spring Boot auto-configures an RSocketStrategies bean that provides all the required infrastructure for encoding and decoding RSocket payloads. By default, the auto-configuration will try to configure the following (in order):

  1. CBOR codecs with Jackson

  2. JSON codecs with Jackson

The spring-boot-starter-rsocket Starter provides both dependencies.

Developers can customize the RSocketStrategies component by creating beans that implement the RSocketStrategiesCustomizer interface. Note that their @Order is important, as it determines the order of codecs.

4.8.2. RSocket server Auto-configuration

Spring Boot provides auto-configuration for RSocket servers. The required dependencies are provided by the spring-boot-starter-rsocket.

Spring Boot will start an RSocket server as a new embedded server in your application, or will plug the RSocket infrastructure into an existing reactive Web server. This depends on the type of application and its configuration.

In case of a WebFlux application (i.e. of type WebApplicationType.REACTIVE), the RSocket server will be plugged into the existing Web Server only if the following properties match:

spring.rsocket.server.mapping-path=/rsocket # a mapping path is defined
spring.rsocket.server.transport=websocket # websocket is chosen as a transport
#spring.rsocket.server.port= # no port is defined

The only other way to create an RSocket server is to start an independent, embedded RSocket server. Besides the dependency requirements, the only required configuration is to define a port for that server:

spring.rsocket.server.port=9898 # the only required configuration
spring.rsocket.server.transport=tcp # you're free to configure other properties

4.8.3. Spring Messaging RSocket support

Spring Boot will auto-configure the Spring Messaging infrastructure for RSocket.

An RSocketStrategies bean is created to provide encoding and decoding support for RSocket messages. By default, Spring Boot will try to auto-configure JSON support with Jackson for application/json and "application/*+json" media types. Check out the Jackson support section to know more about customization possibilities.

Developers can create RSocketStrategiesCustomizer beans to add other strategies, assuming there are Encoder and Decoder implementations available.

4.8.4. Calling RSocket Services with RSocketRequester

Once the RSocket channel is established between server and client, any party can send or receive requests to the other.

As a server, you can get injected an RSocketRequester instance on any handler method of an RSocket @Controller. As a client, you need to configure and establish an RSocket connection first. Spring Boot auto-configures an RSocketRequester.Builder for such cases with the expected codecs.

The RSocketRequester.Builder instance is a prototype bean, meaning each injection point will provide you with a new instance - this is done on purpose since this builder is stateful and you shouldn’t create requesters with different setups using the same instance.

The following code shows a typical example:

@Service
public class MyService {

    private final RSocketRequester rsocketRequester;

    public MyService(RSocketRequester.Builder rsocketRequesterBuilder) {
        this.rsocketRequester = rsocketRequesterBuilder
            .connectTcp("example.org", 9090).block();
    }

    public Mono<User> someRSocketCall(String name) {
        return this.requester.route("user").data(payload)
                            .retrieveMono(User.class);
    }

}

4.9. Security

If Spring Security is on the classpath, then web applications are secured by default. Spring Boot relies on Spring Security’s content-negotiation strategy to determine whether to use httpBasic or formLogin. To add method-level security to a web application, you can also add @EnableGlobalMethodSecurity with your desired settings. Additional information can be found in the Spring Security Reference Guide.

The default UserDetailsService has a single user. The user name is user, and the password is random and is printed at INFO level when the application starts, as shown in the following example:

Using generated security password: 78fa095d-3f4c-48b1-ad50-e24c31d5cf35
If you fine-tune your logging configuration, ensure that the org.springframework.boot.autoconfigure.security category is set to log INFO-level messages. Otherwise, the default password is not printed.

You can change the username and password by providing a spring.security.user.name and spring.security.user.password.

The basic features you get by default in a web application are:

  • A UserDetailsService (or ReactiveUserDetailsService in case of a WebFlux application) bean with in-memory store and a single user with a generated password (see SecurityProperties.User for the properties of the user).

  • Form-based login or HTTP Basic security (depending on the Accept header in the request) for the entire application (including actuator endpoints if actuator is on the classpath).

  • A DefaultAuthenticationEventPublisher for publishing authentication events.

You can provide a different AuthenticationEventPublisher by adding a bean for it.

4.9.1. MVC Security

The default security configuration is implemented in SecurityAutoConfiguration and UserDetailsServiceAutoConfiguration. SecurityAutoConfiguration imports SpringBootWebSecurityConfiguration for web security and UserDetailsServiceAutoConfiguration configures authentication, which is also relevant in non-web applications. To switch off the default web application security configuration completely, you can add a bean of type WebSecurityConfigurerAdapter (doing so does not disable the UserDetailsService configuration or Actuator’s security).

To also switch off the UserDetailsService configuration, you can add a bean of type UserDetailsService, AuthenticationProvider, or AuthenticationManager. There are several secure applications in the Spring Boot samples to get you started with common use cases.

Access rules can be overridden by adding a custom WebSecurityConfigurerAdapter. Spring Boot provides convenience methods that can be used to override access rules for actuator endpoints and static resources. EndpointRequest can be used to create a RequestMatcher that is based on the management.endpoints.web.base-path property. PathRequest can be used to create a RequestMatcher for resources in commonly used locations.

4.9.2. WebFlux Security

Similar to Spring MVC applications, you can secure your WebFlux applications by adding the spring-boot-starter-security dependency. The default security configuration is implemented in ReactiveSecurityAutoConfiguration and UserDetailsServiceAutoConfiguration. ReactiveSecurityAutoConfiguration imports WebFluxSecurityConfiguration for web security and UserDetailsServiceAutoConfiguration configures authentication, which is also relevant in non-web applications. To switch off the default web application security configuration completely, you can add a bean of type WebFilterChainProxy (doing so does not disable the UserDetailsService configuration or Actuator’s security).

To also switch off the UserDetailsService configuration, you can add a bean of type ReactiveUserDetailsService or ReactiveAuthenticationManager.

Access rules can be configured by adding a custom SecurityWebFilterChain. Spring Boot provides convenience methods that can be used to override access rules for actuator endpoints and static resources. EndpointRequest can be used to create a ServerWebExchangeMatcher that is based on the management.endpoints.web.base-path property.

PathRequest can be used to create a ServerWebExchangeMatcher for resources in commonly used locations.

For example, you can customize your security configuration by adding something like:

@Bean
public SecurityWebFilterChain springSecurityFilterChain(ServerHttpSecurity http) {
    return http
        .authorizeExchange()
            .matchers(PathRequest.toStaticResources().atCommonLocations()).permitAll()
            .pathMatchers("/foo", "/bar")
                .authenticated().and()
            .formLogin().and()
        .build();
}

4.9.3. OAuth2

OAuth2 is a widely used authorization framework that is supported by Spring.

Client

If you have spring-security-oauth2-client on your classpath, you can take advantage of some auto-configuration to make it easy to set up an OAuth2/Open ID Connect clients. This configuration makes use of the properties under OAuth2ClientProperties. The same properties are applicable to both servlet and reactive applications.

You can register multiple OAuth2 clients and providers under the spring.security.oauth2.client prefix, as shown in the following example:

spring.security.oauth2.client.registration.my-client-1.client-id=abcd
spring.security.oauth2.client.registration.my-client-1.client-secret=password
spring.security.oauth2.client.registration.my-client-1.client-name=Client for user scope
spring.security.oauth2.client.registration.my-client-1.provider=my-oauth-provider
spring.security.oauth2.client.registration.my-client-1.scope=user
spring.security.oauth2.client.registration.my-client-1.redirect-uri-template=https://my-redirect-uri.com
spring.security.oauth2.client.registration.my-client-1.client-authentication-method=basic
spring.security.oauth2.client.registration.my-client-1.authorization-grant-type=authorization_code

spring.security.oauth2.client.registration.my-client-2.client-id=abcd
spring.security.oauth2.client.registration.my-client-2.client-secret=password
spring.security.oauth2.client.registration.my-client-2.client-name=Client for email scope
spring.security.oauth2.client.registration.my-client-2.provider=my-oauth-provider
spring.security.oauth2.client.registration.my-client-2.scope=email
spring.security.oauth2.client.registration.my-client-2.redirect-uri-template=https://my-redirect-uri.com
spring.security.oauth2.client.registration.my-client-2.client-authentication-method=basic
spring.security.oauth2.client.registration.my-client-2.authorization-grant-type=authorization_code

spring.security.oauth2.client.provider.my-oauth-provider.authorization-uri=https://my-auth-server/oauth/authorize
spring.security.oauth2.client.provider.my-oauth-provider.token-uri=https://my-auth-server/oauth/token
spring.security.oauth2.client.provider.my-oauth-provider.user-info-uri=https://my-auth-server/userinfo
spring.security.oauth2.client.provider.my-oauth-provider.user-info-authentication-method=header
spring.security.oauth2.client.provider.my-oauth-provider.jwk-set-uri=https://my-auth-server/token_keys
spring.security.oauth2.client.provider.my-oauth-provider.user-name-attribute=name

For OpenID Connect providers that support OpenID Connect discovery, the configuration can be further simplified. The provider needs to be configured with an issuer-uri which is the URI that the it asserts as its Issuer Identifier. For example, if the issuer-uri provided is "https://example.com", then an OpenID Provider Configuration Request will be made to "https://example.com/.well-known/openid-configuration". The result is expected to be an OpenID Provider Configuration Response. The following example shows how an OpenID Connect Provider can be configured with the issuer-uri:

spring.security.oauth2.client.provider.oidc-provider.issuer-uri=https://dev-123456.oktapreview.com/oauth2/default/

By default, Spring Security’s OAuth2LoginAuthenticationFilter only processes URLs matching /login/oauth2/code/*. If you want to customize the redirect-uri to use a different pattern, you need to provide configuration to process that custom pattern. For example, for servlet applications, you can add your own WebSecurityConfigurerAdapter that resembles the following:

public class OAuth2LoginSecurityConfig extends WebSecurityConfigurerAdapter {

    @Override
    protected void configure(HttpSecurity http) throws Exception {
        http
            .authorizeRequests()
                .anyRequest().authenticated()
                .and()
            .oauth2Login()
                .redirectionEndpoint()
                    .baseUri("/custom-callback");
    }
}
OAuth2 client registration for common providers

For common OAuth2 and OpenID providers, including Google, Github, Facebook, and Okta, we provide a set of provider defaults (google, github, facebook, and okta, respectively).

If you do not need to customize these providers, you can set the provider attribute to the one for which you need to infer defaults. Also, if the key for the client registration matches a default supported provider, Spring Boot infers that as well.

In other words, the two configurations in the following example use the Google provider:

spring.security.oauth2.client.registration.my-client.client-id=abcd
spring.security.oauth2.client.registration.my-client.client-secret=password
spring.security.oauth2.client.registration.my-client.provider=google

spring.security.oauth2.client.registration.google.client-id=abcd
spring.security.oauth2.client.registration.google.client-secret=password
Resource Server

If you have spring-security-oauth2-resource-server on your classpath, Spring Boot can set up an OAuth2 Resource Server. For JWT configuration, a JWK Set URI or OIDC Issuer URI needs to be specified, as shown in the following examples:

spring.security.oauth2.resourceserver.jwt.jwk-set-uri=https://example.com/oauth2/default/v1/keys
spring.security.oauth2.resourceserver.jwt.issuer-uri=https://dev-123456.oktapreview.com/oauth2/default/
If the authorization server does not support a JWK Set URI, you can configure the resource server with the Public Key used for verifying the signature of the JWT. This can be done using the spring.security.oauth2.resourceserver.jwt.public-key-location property, where the value needs to point to a file containing the public key in the PEM-encoded x509 format.

The same properties are applicable for both servlet and reactive applications.

Alternatively, you can define your own JwtDecoder bean for servlet applications or a ReactiveJwtDecoder for reactive applications.

In cases where opaque tokens are used instead of JWTs, you can configure the following properties to validate tokens via introspection:

spring.security.oauth2.resourceserver.opaque-token.introspection-uri=https://example.com/check-token
spring.security.oauth2.resourceserver.opaque-token.client-id=my-client-id
spring.security.oauth2.resourceserver.opaque-token.client-secret-my-client-secret

Again, the same properties are applicable for both servlet and reactive applications.

Alternatively, you can define your own OAuth2TokenIntrospectionClient bean for servlet applications or a ReactiveOAuth2TokenIntrospectionClient for reactive applications.

Authorization Server

Currently, Spring Security does not provide support for implementing an OAuth 2.0 Authorization Server. However, this functionality is available from the Spring Security OAuth project, which will eventually be superseded by Spring Security completely. Until then, you can use the spring-security-oauth2-autoconfigure module to easily set up an OAuth 2.0 authorization server; see its documentation for instructions.

4.9.4. Actuator Security

For security purposes, all actuators other than /health and /info are disabled by default. The management.endpoints.web.exposure.include property can be used to enable the actuators.

If Spring Security is on the classpath and no other WebSecurityConfigurerAdapter is present, all actuators other than /health and /info are secured by Spring Boot auto-configuration. If you define a custom WebSecurityConfigurerAdapter, Spring Boot auto-configuration will back off and you will be in full control of actuator access rules.

Before setting the management.endpoints.web.exposure.include, ensure that the exposed actuators do not contain sensitive information and/or are secured by placing them behind a firewall or by something like Spring Security.
Cross Site Request Forgery Protection

Since Spring Boot relies on Spring Security’s defaults, CSRF protection is turned on by default. This means that the actuator endpoints that require a POST (shutdown and loggers endpoints), PUT or DELETE will get a 403 forbidden error when the default security configuration is in use.

We recommend disabling CSRF protection completely only if you are creating a service that is used by non-browser clients.

Additional information about CSRF protection can be found in the Spring Security Reference Guide.

4.10. Working with SQL Databases

The Spring Framework provides extensive support for working with SQL databases, from direct JDBC access using JdbcTemplate to complete “object relational mapping” technologies such as Hibernate. Spring Data provides an additional level of functionality: creating Repository implementations directly from interfaces and using conventions to generate queries from your method names.

4.10.1. Configure a DataSource

Java’s javax.sql.DataSource interface provides a standard method of working with database connections. Traditionally, a 'DataSource' uses a URL along with some credentials to establish a database connection.

See the “How-to” section for more advanced examples, typically to take full control over the configuration of the DataSource.
Embedded Database Support

It is often convenient to develop applications by using an in-memory embedded database. Obviously, in-memory databases do not provide persistent storage. You need to populate your database when your application starts and be prepared to throw away data when your application ends.

The “How-to” section includes a section on how to initialize a database.

Spring Boot can auto-configure embedded H2, HSQL, and Derby databases. You need not provide any connection URLs. You need only include a build dependency to the embedded database that you want to use.

If you are using this feature in your tests, you may notice that the same database is reused by your whole test suite regardless of the number of application contexts that you use. If you want to make sure that each context has a separate embedded database, you should set spring.datasource.generate-unique-name to true.

For example, the typical POM dependencies would be as follows:

<dependency>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-starter-data-jpa</artifactId>
</dependency>
<dependency>
    <groupId>org.hsqldb</groupId>
    <artifactId>hsqldb</artifactId>
    <scope>runtime</scope>
</dependency>
You need a dependency on spring-jdbc for an embedded database to be auto-configured. In this example, it is pulled in transitively through spring-boot-starter-data-jpa.
If, for whatever reason, you do configure the connection URL for an embedded database, take care to ensure that the database’s automatic shutdown is disabled. If you use H2, you should use DB_CLOSE_ON_EXIT=FALSE to do so. If you use HSQLDB, you should ensure that shutdown=true is not used. Disabling the database’s automatic shutdown lets Spring Boot control when the database is closed, thereby ensuring that it happens once access to the database is no longer needed.
Connection to a Production Database

Production database connections can also be auto-configured by using a pooling DataSource. Spring Boot uses the following algorithm for choosing a specific implementation:

  1. We prefer HikariCP for its performance and concurrency. If HikariCP is available, we always choose it.

  2. Otherwise, if the Tomcat pooling DataSource is available, we use it.

  3. If neither HikariCP nor the Tomcat pooling datasource are available and if Commons DBCP2 is available, we use it.

If you use the spring-boot-starter-jdbc or spring-boot-starter-data-jpa “starters”, you automatically get a dependency to HikariCP.

You can bypass that algorithm completely and specify the connection pool to use by setting the spring.datasource.type property. This is especially important if you run your application in a Tomcat container, as tomcat-jdbc is provided by default.
Additional connection pools can always be configured manually. If you define your own DataSource bean, auto-configuration does not occur.

DataSource configuration is controlled by external configuration properties in spring.datasource.*. For example, you might declare the following section in application.properties:

spring.datasource.url=jdbc:mysql://localhost/test
spring.datasource.username=dbuser
spring.datasource.password=dbpass
spring.datasource.driver-class-name=com.mysql.jdbc.Driver
You should at least specify the URL by setting the spring.datasource.url property. Otherwise, Spring Boot tries to auto-configure an embedded database.
You often do not need to specify the driver-class-name, since Spring Boot can deduce it for most databases from the url.
For a pooling DataSource to be created, we need to be able to verify that a valid Driver class is available, so we check for that before doing anything. In other words, if you set spring.datasource.driver-class-name=com.mysql.jdbc.Driver, then that class has to be loadable.

See DataSourceProperties for more of the supported options. These are the standard options that work regardless of the actual implementation. It is also possible to fine-tune implementation-specific settings by using their respective prefix (spring.datasource.hikari.*, spring.datasource.tomcat.*, and spring.datasource.dbcp2.*). Refer to the documentation of the connection pool implementation you are using for more details.

For instance, if you use the Tomcat connection pool, you could customize many additional settings, as shown in the following example:

# Number of ms to wait before throwing an exception if no connection is available.
spring.datasource.tomcat.max-wait=10000

# Maximum number of active connections that can be allocated from this pool at the same time.
spring.datasource.tomcat.max-active=50

# Validate the connection before borrowing it from the pool.
spring.datasource.tomcat.test-on-borrow=true
Connection to a JNDI DataSource

If you deploy your Spring Boot application to an Application Server, you might want to configure and manage your DataSource by using your Application Server’s built-in features and access it by using JNDI.

The spring.datasource.jndi-name property can be used as an alternative to the spring.datasource.url, spring.datasource.username, and spring.datasource.password properties to access the DataSource from a specific JNDI location. For example, the following section in application.properties shows how you can access a JBoss AS defined DataSource:

spring.datasource.jndi-name=java:jboss/datasources/customers

4.10.2. Using JdbcTemplate

Spring’s JdbcTemplate and NamedParameterJdbcTemplate classes are auto-configured, and you can @Autowire them directly into your own beans, as shown in the following example:

import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.jdbc.core.JdbcTemplate;
import org.springframework.stereotype.Component;

@Component
public class MyBean {

    private final JdbcTemplate jdbcTemplate;

    @Autowired
    public MyBean(JdbcTemplate jdbcTemplate) {
        this.jdbcTemplate = jdbcTemplate;
    }

    // ...

}

You can customize some properties of the template by using the spring.jdbc.template.* properties, as shown in the following example:

spring.jdbc.template.max-rows=500
The NamedParameterJdbcTemplate reuses the same JdbcTemplate instance behind the scenes. If more than one JdbcTemplate is defined and no primary candidate exists, the NamedParameterJdbcTemplate is not auto-configured.

4.10.3. JPA and Spring Data JPA

The Java Persistence API is a standard technology that lets you “map” objects to relational databases. The spring-boot-starter-data-jpa POM provides a quick way to get started. It provides the following key dependencies:

  • Hibernate: One of the most popular JPA implementations.

  • Spring Data JPA: Makes it easy to implement JPA-based repositories.

  • Spring ORMs: Core ORM support from the Spring Framework.

We do not go into too many details of JPA or Spring Data here. You can follow the “Accessing Data with JPA” guide from spring.io and read the Spring Data JPA and Hibernate reference documentation.
Entity Classes

Traditionally, JPA “Entity” classes are specified in a persistence.xml file. With Spring Boot, this file is not necessary and “Entity Scanning” is used instead. By default, all packages below your main configuration class (the one annotated with @EnableAutoConfiguration or @SpringBootApplication) are searched.

Any classes annotated with @Entity, @Embeddable, or @MappedSuperclass are considered. A typical entity class resembles the following example:

package com.example.myapp.domain;

import java.io.Serializable;
import javax.persistence.*;

@Entity
public class City implements Serializable {

    @Id
    @GeneratedValue
    private Long id;

    @Column(nullable = false)
    private String name;

    @Column(nullable = false)
    private String state;

    // ... additional members, often include @OneToMany mappings

    protected City() {
        // no-args constructor required by JPA spec
        // this one is protected since it shouldn't be used directly
    }

    public City(String name, String state) {
        this.name = name;
        this.state = state;
    }

    public String getName() {
        return this.name;
    }

    public String getState() {
        return this.state;
    }

    // ... etc

}
You can customize entity scanning locations by using the @EntityScan annotation. See the “Separate @Entity Definitions from Spring Configuration” how-to.
Spring Data JPA Repositories

Spring Data JPA repositories are interfaces that you can define to access data. JPA queries are created automatically from your method names. For example, a CityRepository interface might declare a findAllByState(String state) method to find all the cities in a given state.

For more complex queries, you can annotate your method with Spring Data’s Query annotation.

Spring Data repositories usually extend from the Repository or CrudRepository interfaces. If you use auto-configuration, repositories are searched from the package containing your main configuration class (the one annotated with @EnableAutoConfiguration or @SpringBootApplication) down.

The following example shows a typical Spring Data repository interface definition:

package com.example.myapp.domain;

import org.springframework.data.domain.*;
import org.springframework.data.repository.*;

public interface CityRepository extends Repository<City, Long> {

    Page<City> findAll(Pageable pageable);

    City findByNameAndStateAllIgnoringCase(String name, String state);

}

Spring Data JPA repositories support three different modes of bootstrapping: default, deferred, and lazy. To enable deferred or lazy bootstrapping, set the spring.data.jpa.repositories.bootstrap-mode to deferred or lazy respectively. When using deferred or lazy bootstrapping, the auto-configured EntityManagerFactoryBuilder will use the context’s AsyncTaskExecutor, if any, as the bootstrap executor. If more than one exists, the one named applicationTaskExecutor will be used.

We have barely scratched the surface of Spring Data JPA. For complete details, see the Spring Data JPA reference documentation.
Creating and Dropping JPA Databases

By default, JPA databases are automatically created only if you use an embedded database (H2, HSQL, or Derby). You can explicitly configure JPA settings by using spring.jpa.* properties. For example, to create and drop tables you can add the following line to your application.properties:

spring.jpa.hibernate.ddl-auto=create-drop
Hibernate’s own internal property name for this (if you happen to remember it better) is hibernate.hbm2ddl.auto. You can set it, along with other Hibernate native properties, by using spring.jpa.properties.* (the prefix is stripped before adding them to the entity manager). The following line shows an example of setting JPA properties for Hibernate:
spring.jpa.properties.hibernate.globally_quoted_identifiers=true

The line in the preceding example passes a value of true for the hibernate.globally_quoted_identifiers property to the Hibernate entity manager.

By default, the DDL execution (or validation) is deferred until the ApplicationContext has started. There is also a spring.jpa.generate-ddl flag, but it is not used if Hibernate auto-configuration is active, because the ddl-auto settings are more fine-grained.

Open EntityManager in View

If you are running a web application, Spring Boot by default registers OpenEntityManagerInViewInterceptor to apply the “Open EntityManager in View” pattern, to allow for lazy loading in web views. If you do not want this behavior, you should set spring.jpa.open-in-view to false in your application.properties.

4.10.4. Spring Data JDBC

Spring Data includes repository support for JDBC and will automatically generate SQL for the methods on CrudRepository. For more advanced queries, a @Query annotation is provided.

Spring Boot will auto-configure Spring Data’s JDBC repositories when the necessary dependencies are on the classpath. They can be added to your project with a single dependency on spring-boot-starter-data-jdbc. If necessary, you can take control of Spring Data JDBC’s configuration by adding the @EnableJdbcRepositories annotation or a JdbcConfiguration subclass to your application.

For complete details of Spring Data JDBC, please refer to the reference documentation.

4.10.5. Using H2’s Web Console

The H2 database provides a browser-based console that Spring Boot can auto-configure for you. The console is auto-configured when the following conditions are met:

If you are not using Spring Boot’s developer tools but would still like to make use of H2’s console, you can configure the spring.h2.console.enabled property with a value of true.
The H2 console is only intended for use during development, so you should take care to ensure that spring.h2.console.enabled is not set to true in production.
Changing the H2 Console’s Path

By default, the console is available at /h2-console. You can customize the console’s path by using the spring.h2.console.path property.

4.10.6. Using jOOQ

jOOQ Object Oriented Querying (jOOQ) is a popular product from Data Geekery which generates Java code from your database and lets you build type-safe SQL queries through its fluent API. Both the commercial and open source editions can be used with Spring Boot.

Code Generation

In order to use jOOQ type-safe queries, you need to generate Java classes from your database schema. You can follow the instructions in the jOOQ user manual. If you use the jooq-codegen-maven plugin and you also use the spring-boot-starter-parent “parent POM”, you can safely omit the plugin’s <version> tag. You can also use Spring Boot-defined version variables (such as h2.version) to declare the plugin’s database dependency. The following listing shows an example:

<plugin>
    <groupId>org.jooq</groupId>
    <artifactId>jooq-codegen-maven</artifactId>
    <executions>
        ...
    </executions>
    <dependencies>
        <dependency>
            <groupId>com.h2database</groupId>
            <artifactId>h2</artifactId>
            <version>${h2.version}</version>
        </dependency>
    </dependencies>
    <configuration>
        <jdbc>
            <driver>org.h2.Driver</driver>
            <url>jdbc:h2:~/yourdatabase</url>
        </jdbc>
        <generator>
            ...
        </generator>
    </configuration>
</plugin>
Using DSLContext

The fluent API offered by jOOQ is initiated through the org.jooq.DSLContext interface. Spring Boot auto-configures a DSLContext as a Spring Bean and connects it to your application DataSource. To use the DSLContext, you can @Autowire it, as shown in the following example:

@Component
public class JooqExample implements CommandLineRunner {

    private final DSLContext create;

    @Autowired
    public JooqExample(DSLContext dslContext) {
        this.create = dslContext;
    }

}
The jOOQ manual tends to use a variable named create to hold the DSLContext.

You can then use the DSLContext to construct your queries, as shown in the following example:

public List<GregorianCalendar> authorsBornAfter1980() {
    return this.create.selectFrom(AUTHOR)
        .where(AUTHOR.DATE_OF_BIRTH.greaterThan(new GregorianCalendar(1980, 0, 1)))
        .fetch(AUTHOR.DATE_OF_BIRTH);
}
jOOQ SQL Dialect

Unless the spring.jooq.sql-dialect property has been configured, Spring Boot determines the SQL dialect to use for your datasource. If Spring Boot could not detect the dialect, it uses DEFAULT.

Spring Boot can only auto-configure dialects supported by the open source version of jOOQ.
Customizing jOOQ

More advanced customizations can be achieved by defining your own @Bean definitions, which is used when the jOOQ Configuration is created. You can define beans for the following jOOQ Types:

  • ConnectionProvider

  • ExecutorProvider

  • TransactionProvider

  • RecordMapperProvider

  • RecordUnmapperProvider

  • RecordListenerProvider

  • ExecuteListenerProvider

  • VisitListenerProvider

  • TransactionListenerProvider

You can also create your own org.jooq.Configuration @Bean if you want to take complete control of the jOOQ configuration.

4.11. Working with NoSQL Technologies

Spring Data provides additional projects that help you access a variety of NoSQL technologies, including: MongoDB, Neo4J, Elasticsearch, Solr, Redis, Gemfire, Cassandra, Couchbase and LDAP. Spring Boot provides auto-configuration for Redis, MongoDB, Neo4j, Elasticsearch, Solr Cassandra, Couchbase, and LDAP. You can make use of the other projects, but you must configure them yourself. Refer to the appropriate reference documentation at projects.spring.io/spring-data.

4.11.1. Redis

Redis is a cache, message broker, and richly-featured key-value store. Spring Boot offers basic auto-configuration for the Lettuce and Jedis client libraries and the abstractions on top of them provided by Spring Data Redis.

There is a spring-boot-starter-data-redis “Starter” for collecting the dependencies in a convenient way. By default, it uses Lettuce. That starter handles both traditional and reactive applications.

we also provide a spring-boot-starter-data-redis-reactive “Starter” for consistency with the other stores with reactive support.
Connecting to Redis

You can inject an auto-configured RedisConnectionFactory, StringRedisTemplate, or vanilla RedisTemplate instance as you would any other Spring Bean. By default, the instance tries to connect to a Redis server at localhost:6379. The following listing shows an example of such a bean:

@Component
public class MyBean {

    private StringRedisTemplate template;

    @Autowired
    public MyBean(StringRedisTemplate template) {
        this.template = template;
    }

    // ...

}
You can also register an arbitrary number of beans that implement LettuceClientConfigurationBuilderCustomizer for more advanced customizations. If you use Jedis, JedisClientConfigurationBuilderCustomizer is also available.

If you add your own @Bean of any of the auto-configured types, it replaces the default (except in the case of RedisTemplate, when the exclusion is based on the bean name, redisTemplate, not its type). By default, if commons-pool2 is on the classpath, you get a pooled connection factory.

4.11.2. MongoDB

MongoDB is an open-source NoSQL document database that uses a JSON-like schema instead of traditional table-based relational data. Spring Boot offers several conveniences for working with MongoDB, including the spring-boot-starter-data-mongodb and spring-boot-starter-data-mongodb-reactive “Starters”.

Connecting to a MongoDB Database

To access Mongo databases, you can inject an auto-configured org.springframework.data.mongodb.MongoDbFactory. By default, the instance tries to connect to a MongoDB server at mongodb://localhost/test The following example shows how to connect to a MongoDB database:

import org.springframework.data.mongodb.MongoDbFactory;
import com.mongodb.DB;

@Component
public class MyBean {

    private final MongoDbFactory mongo;

    @Autowired
    public MyBean(MongoDbFactory mongo) {
        this.mongo = mongo;
    }

    // ...

    public void example() {
        DB db = mongo.getDb();
        // ...
    }

}

You can set the spring.data.mongodb.uri property to change the URL and configure additional settings such as the replica set, as shown in the following example:

spring.data.mongodb.uri=mongodb://user:[email protected]:12345,mongo2.example.com:23456/test

Alternatively, as long as you use Mongo 2.x, you can specify a host/port. For example, you might declare the following settings in your application.properties:

spring.data.mongodb.host=mongoserver
spring.data.mongodb.port=27017

If you have defined your own MongoClient, it will be used to auto-configure a suitable MongoDbFactory. Both com.mongodb.MongoClient and com.mongodb.client.MongoClient are supported.

If you use the Mongo 3.0 Java driver, spring.data.mongodb.host and spring.data.mongodb.port are not supported. In such cases, spring.data.mongodb.uri should be used to provide all of the configuration.
If spring.data.mongodb.port is not specified, the default of 27017 is used. You could delete this line from the example shown earlier.
If you do not use Spring Data Mongo, you can inject com.mongodb.MongoClient beans instead of using MongoDbFactory. If you want to take complete control of establishing the MongoDB connection, you can also declare your own MongoDbFactory or MongoClient bean.
If you are using the reactive driver, Netty is required for SSL. The auto-configuration configures this factory automatically if Netty is available and the factory to use hasn’t been customized already.
MongoTemplate

Spring Data MongoDB provides a MongoTemplate class that is very similar in its design to Spring’s JdbcTemplate. As with JdbcTemplate, Spring Boot auto-configures a bean for you to inject the template, as follows:

import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.data.mongodb.core.MongoTemplate;
import org.springframework.stereotype.Component;

@Component
public class MyBean {

    private final MongoTemplate mongoTemplate;

    @Autowired
    public MyBean(MongoTemplate mongoTemplate) {
        this.mongoTemplate = mongoTemplate;
    }

    // ...

}

See the MongoOperations Javadoc for complete details.

Spring Data MongoDB Repositories

Spring Data includes repository support for MongoDB. As with the JPA repositories discussed earlier, the basic principle is that queries are constructed automatically, based on method names.

In fact, both Spring Data JPA and Spring Data MongoDB share the same common infrastructure. You could take the JPA example from earlier and, assuming that City is now a Mongo data class rather than a JPA @Entity, it works in the same way, as shown in the following example:

package com.example.myapp.domain;

import org.springframework.data.domain.*;
import org.springframework.data.repository.*;

public interface CityRepository extends Repository<City, Long> {

    Page<City> findAll(Pageable pageable);

    City findByNameAndStateAllIgnoringCase(String name, String state);

}
You can customize document scanning locations by using the @EntityScan annotation.
For complete details of Spring Data MongoDB, including its rich object mapping technologies, refer to its reference documentation.
Embedded Mongo

Spring Boot offers auto-configuration for Embedded Mongo. To use it in your Spring Boot application, add a dependency on de.flapdoodle.embed:de.flapdoodle.embed.mongo.

The port that Mongo listens on can be configured by setting the spring.data.mongodb.port property. To use a randomly allocated free port, use a value of 0. The MongoClient created by MongoAutoConfiguration is automatically configured to use the randomly allocated port.

If you do not configure a custom port, the embedded support uses a random port (rather than 27017) by default.

If you have SLF4J on the classpath, the output produced by Mongo is automatically routed to a logger named org.springframework.boot.autoconfigure.mongo.embedded.EmbeddedMongo.

You can declare your own IMongodConfig and IRuntimeConfig beans to take control of the Mongo instance’s configuration and logging routing. The download configuration can be customized by declaring a DownloadConfigBuilderCustomizer bean.

4.11.3. Neo4j

Neo4j is an open-source NoSQL graph database that uses a rich data model of nodes connected by first class relationships, which is better suited for connected big data than traditional RDBMS approaches. Spring Boot offers several conveniences for working with Neo4j, including the spring-boot-starter-data-neo4j “Starter”.

Connecting to a Neo4j Database

To access a Neo4j server, you can inject an auto-configured org.neo4j.ogm.session.Session. By default, the instance tries to connect to a Neo4j server at localhost:7687 using the Bolt protocol. The following example shows how to inject a Neo4j Session:

@Component
public class MyBean {

    private final Session session;

    @Autowired
    public MyBean(Session session) {
        this.session = session;
    }

    // ...

}

You can configure the uri and credentials to use by setting the spring.data.neo4j.* properties, as shown in the following example:

spring.data.neo4j.uri=bolt://my-server:7687
spring.data.neo4j.username=neo4j
spring.data.neo4j.password=secret

You can take full control over the session creation by adding a org.neo4j.ogm.config.Configuration @Bean. Also, adding a @Bean of type SessionFactory disables the auto-configuration and gives you full control.

Using the Embedded Mode

If you add org.neo4j:neo4j-ogm-embedded-driver to the dependencies of your application, Spring Boot automatically configures an in-process embedded instance of Neo4j that does not persist any data when your application shuts down.

As the embedded Neo4j OGM driver does not provide the Neo4j kernel itself, you have to declare org.neo4j:neo4j as dependency yourself. Refer to the Neo4j OGM documentation for a list of compatible versions.

The embedded driver takes precedence over the other drivers when there are multiple drivers on the classpath. You can explicitly disable the embedded mode by setting spring.data.neo4j.embedded.enabled=false.

Data Neo4j Tests automatically make use of an embedded Neo4j instance if the embedded driver and Neo4j kernel are on the classpath as described above.

You can enable persistence for the embedded mode by providing a path to a database file in your configuration, e.g. spring.data.neo4j.uri=file://var/tmp/graph.db.

Using Native Types

Neo4j-OGM can map some types, like those in java.time.*, to String-based properties or to one of the native types that Neo4j provides. For backwards compatibility reasons the default for Neo4j-OGM is to use a String-based representation. To use native types, add a dependency on either org.neo4j:neo4j-ogm-bolt-native-types or org.neo4j:neo4j-ogm-embedded-native-types, and configure the spring.data.neo4j.use-native-types property as shown in the following example:

spring.data.neo4j.use-native-types=true
Neo4jSession

By default, if you are running a web application, the session is bound to the thread for the entire processing of the request (that is, it uses the "Open Session in View" pattern). If you do not want this behavior, add the following line to your application.properties file:

spring.data.neo4j.open-in-view=false
Spring Data Neo4j Repositories

Spring Data includes repository support for Neo4j.

Spring Data Neo4j shares the common infrastructure with Spring Data JPA as many other Spring Data modules do. You could take the JPA example from earlier and define City as Neo4j OGM @NodeEntity rather than JPA @Entity and the repository abstraction works in the same way, as shown in the following example:

package com.example.myapp.domain;

import java.util.Optional;

import org.springframework.data.neo4j.repository.*;

public interface CityRepository extends Neo4jRepository<City, Long> {

    Optional<City> findOneByNameAndState(String name, String state);

}

The spring-boot-starter-data-neo4j “Starter” enables the repository support as well as transaction management. You can customize the locations to look for repositories and entities by using @EnableNeo4jRepositories and @EntityScan respectively on a @Configuration-bean.

For complete details of Spring Data Neo4j, including its object mapping technologies, refer to the reference documentation.

4.11.4. Gemfire

Spring Data Gemfire provides convenient Spring-friendly tools for accessing the Pivotal Gemfire data management platform. There is a spring-boot-starter-data-gemfire “Starter” for collecting the dependencies in a convenient way. There is currently no auto-configuration support for Gemfire, but you can enable Spring Data Repositories with a single annotation: @EnableGemfireRepositories.

4.11.5. Solr

Apache Solr is a search engine. Spring Boot offers basic auto-configuration for the Solr 5 client library and the abstractions on top of it provided by Spring Data Solr. There is a spring-boot-starter-data-solr “Starter” for collecting the dependencies in a convenient way.

Connecting to Solr

You can inject an auto-configured SolrClient instance as you would any other Spring bean. By default, the instance tries to connect to a server at localhost:8983/solr. The following example shows how to inject a Solr bean:

@Component
public class MyBean {

    private SolrClient solr;

    @Autowired
    public MyBean(SolrClient solr) {
        this.solr = solr;
    }

    // ...

}

If you add your own @Bean of type SolrClient, it replaces the default.

Spring Data Solr Repositories

Spring Data includes repository support for Apache Solr. As with the JPA repositories discussed earlier, the basic principle is that queries are automatically constructed for \ you based on method names.

In fact, both Spring Data JPA and Spring Data Solr share the same common infrastructure. You could take the JPA example from earlier and, assuming that City is now a @SolrDocument class rather than a JPA @Entity, it works in the same way.

For complete details of Spring Data Solr, refer to the reference documentation.

4.11.6. Elasticsearch

Elasticsearch is an open source, distributed, RESTful search and analytics engine. Spring Boot offers basic auto-configuration for Elasticsearch.

Spring Boot supports several clients:

  • The official Java "Low Level" and "High Level" REST clients

  • The ReactiveElasticsearchClient provided by Spring Data Elasticsearch

The transport client is still available but its support has been deprecated in Spring Data Elasticsearch and Elasticsearch itself. It will be removed in a future release. Spring Boot provides a dedicated “Starter”, spring-boot-starter-data-elasticsearch.

The Jest client has been deprecated as well, since both Elasticsearch and Spring Data Elasticsearch provide official support for REST clients.

Connecting to Elasticsearch using REST clients

Elasticsearch ships two different REST clients that you can use to query a cluster: the "Low Level" client and the "High Level" client.

If you have the org.elasticsearch.client:elasticsearch-rest-client dependency on the classpath, Spring Boot will auto-configure and register a RestClient bean that by default targets localhost:9200. You can further tune how RestClient is configured, as shown in the following example:

spring.elasticsearch.rest.uris=https://search.example.com:9200
spring.elasticsearch.rest.read-timeout=10s
spring.elasticsearch.rest.username=user
spring.elasticsearch.rest.password=secret

You can also register an arbitrary number of beans that implement RestClientBuilderCustomizer for more advanced customizations. To take full control over the registration, define a RestClient bean.

If you have the org.elasticsearch.client:elasticsearch-rest-high-level-client dependency on the classpath, Spring Boot will auto-configure a RestHighLevelClient, which wraps any existing RestClient bean, reusing its HTTP configuration.

Connecting to Elasticsearch using Reactive REST clients

Spring Data Elasticsearch ships ReactiveElasticsearchClient for querying Elasticsearch instances in a reactive fashion. It is built on top of WebFlux’s WebClient, so both spring-boot-starter-elasticsearch and spring-boot-starter-webflux dependencies are useful to enable this support.

By default, Spring Boot will auto-configure and register a ReactiveElasticsearchClient bean that targets localhost:9200. You can further tune how it is configured, as shown in the following example:

spring.elasticsearch.reactive.endpoints=search.example.com:9200
spring.elasticsearch.reactive.use-ssl=true
spring.elasticsearch.reactive.socket-timeout=10s
spring.elasticsearch.reactive.username=user
spring.elasticsearch.reactive.password=secret

If the configuration properties are not enough and you’d like to fully control the client configuration, you can register a custom ClientConfiguration bean.

Connecting to Elasticsearch using Jest

Now that Spring Boot supports the official RestHighLevelClient, Jest support is deprecated.

If you have Jest on the classpath, you can inject an auto-configured JestClient that by default targets localhost:9200. You can further tune how the client is configured, as shown in the following example:

spring.elasticsearch.jest.uris=https://search.example.com:9200
spring.elasticsearch.jest.read-timeout=10000
spring.elasticsearch.jest.username=user
spring.elasticsearch.jest.password=secret

You can also register an arbitrary number of beans that implement HttpClientConfigBuilderCustomizer for more advanced customizations. The following example tunes additional HTTP settings:

static class HttpSettingsCustomizer implements HttpClientConfigBuilderCustomizer {

    @Override
    public void customize(HttpClientConfig.Builder builder) {
        builder.maxTotalConnection(100).defaultMaxTotalConnectionPerRoute(5);
    }

}

To take full control over the registration, define a JestClient bean.

Connecting to Elasticsearch by Using Spring Data

To connect to Elasticsearch, a RestHighLevelClient bean must be defined, auto-configured by Spring Boot or manually provided by the application (see previous sections). With this configuration in place, an ElasticsearchRestTemplate can be injected like any other Spring bean, as shown in the following example:

@Component
public class MyBean {

    private final ElasticsearchRestTemplate template;

    public MyBean(ElasticsearchRestTemplate template) {
        this.template = template;
    }

    // ...

}

In the presence of spring-data-elasticsearch and the required dependencies for using a WebClient (typically spring-boot-starter-webflux), Spring Boot can also auto-configure a ReactiveElasticsearchClient and a ReactiveElasticsearchTemplate as beans. They are the reactive equivalent of the other REST clients.

Spring Data Elasticsearch Repositories

Spring Data includes repository support for Elasticsearch. As with the JPA repositories discussed earlier, the basic principle is that queries are constructed for you automatically based on method names.

In fact, both Spring Data JPA and Spring Data Elasticsearch share the same common infrastructure. You could take the JPA example from earlier and, assuming that City is now an Elasticsearch @Document class rather than a JPA @Entity, it works in the same way.

For complete details of Spring Data Elasticsearch, refer to the reference documentation.

Spring Boot supports both classic and reactive Elasticsearch repositories, using the ElasticsearchRestTemplate or ReactiveElasticsearchTemplate beans. Most likely those beans are auto-configured by Spring Boot given the required dependencies are present.

If you wish to use your own template for backing the Elasticsearch repositories, you can add your own ElasticsearchRestTemplate or ElasticsearchOperations @Bean, as long as it is named "elasticsearchTemplate". Same applies to ReactiveElasticsearchTemplate and ReactiveElasticsearchOperations, with the bean name "reactiveElasticsearchTemplate".

You can choose to disable the repositories support with the following property:

spring.data.elasticsearch.repositories.enabled=false

4.11.7. Cassandra

Cassandra is an open source, distributed database management system designed to handle large amounts of data across many commodity servers. Spring Boot offers auto-configuration for Cassandra and the abstractions on top of it provided by Spring Data Cassandra. There is a spring-boot-starter-data-cassandra “Starter” for collecting the dependencies in a convenient way.

Connecting to Cassandra

You can inject an auto-configured CassandraTemplate or a Cassandra Session instance as you would with any other Spring Bean. The spring.data.cassandra.* properties can be used to customize the connection. Generally, you provide keyspace-name and contact-points properties, as shown in the following example:

spring.data.cassandra.keyspace-name=mykeyspace
spring.data.cassandra.contact-points=cassandrahost1,cassandrahost2

You can also register an arbitrary number of beans that implement ClusterBuilderCustomizer for more advanced customizations.

The following code listing shows how to inject a Cassandra bean:

@Component
public class MyBean {

    private CassandraTemplate template;

    @Autowired
    public MyBean(CassandraTemplate template) {
        this.template = template;
    }

    // ...

}

If you add your own @Bean of type CassandraTemplate, it replaces the default.

Spring Data Cassandra Repositories

Spring Data includes basic repository support for Cassandra. Currently, this is more limited than the JPA repositories discussed earlier and needs to annotate finder methods with @Query.

For complete details of Spring Data Cassandra, refer to the reference documentation.

4.11.8. Couchbase

Couchbase is an open-source, distributed, multi-model NoSQL document-oriented database that is optimized for interactive applications. Spring Boot offers auto-configuration for Couchbase and the abstractions on top of it provided by Spring Data Couchbase. There are spring-boot-starter-data-couchbase and spring-boot-starter-data-couchbase-reactive “Starters” for collecting the dependencies in a convenient way.

Connecting to Couchbase

You can get a Bucket and Cluster by adding the Couchbase SDK and some configuration. The spring.couchbase.* properties can be used to customize the connection. Generally, you provide the bootstrap hosts, bucket name, and password, as shown in the following example:

spring.couchbase.bootstrap-hosts=my-host-1,192.168.1.123
spring.couchbase.bucket.name=my-bucket
spring.couchbase.bucket.password=secret

You need to provide at least the bootstrap host(s), in which case the bucket name is default and the password is an empty String. Alternatively, you can define your own org.springframework.data.couchbase.config.CouchbaseConfigurer @Bean to take control over the whole configuration.

It is also possible to customize some of the CouchbaseEnvironment settings. For instance, the following configuration changes the timeout to use to open a new Bucket and enables SSL support:

spring.couchbase.env.timeouts.connect=3000
spring.couchbase.env.ssl.key-store=/location/of/keystore.jks
spring.couchbase.env.ssl.key-store-password=secret

Check the spring.couchbase.env.* properties for more details.

Spring Data Couchbase Repositories

Spring Data includes repository support for Couchbase. For complete details of Spring Data Couchbase, refer to the reference documentation.

You can inject an auto-configured CouchbaseTemplate instance as you would with any other Spring Bean, provided a default CouchbaseConfigurer is available (which happens when you enable Couchbase support, as explained earlier).

The following examples shows how to inject a Couchbase bean:

@Component
public class MyBean {

    private final CouchbaseTemplate template;

    @Autowired
    public MyBean(CouchbaseTemplate template) {
        this.template = template;
    }

    // ...

}

There are a few beans that you can define in your own configuration to override those provided by the auto-configuration:

  • A CouchbaseTemplate @Bean with a name of couchbaseTemplate.

  • An IndexManager @Bean with a name of couchbaseIndexManager.

  • A CustomConversions @Bean with a name of couchbaseCustomConversions.

To avoid hard-coding those names in your own config, you can reuse BeanNames provided by Spring Data Couchbase. For instance, you can customize the converters to use, as follows:

@Configuration(proxyBeanMethods = false)
public class SomeConfiguration {

    @Bean(BeanNames.COUCHBASE_CUSTOM_CONVERSIONS)
    public CustomConversions myCustomConversions() {
        return new CustomConversions(...);
    }

    // ...

}
If you want to fully bypass the auto-configuration for Spring Data Couchbase, provide your own implementation of org.springframework.data.couchbase.config.AbstractCouchbaseDataConfiguration.

4.11.9. LDAP

LDAP (Lightweight Directory Access Protocol) is an open, vendor-neutral, industry standard application protocol for accessing and maintaining distributed directory information services over an IP network. Spring Boot offers auto-configuration for any compliant LDAP server as well as support for the embedded in-memory LDAP server from UnboundID.

LDAP abstractions are provided by Spring Data LDAP. There is a spring-boot-starter-data-ldap “Starter” for collecting the dependencies in a convenient way.

Connecting to an LDAP Server

To connect to an LDAP server, make sure you declare a dependency on the spring-boot-starter-data-ldap “Starter” or spring-ldap-core and then declare the URLs of your server in your application.properties, as shown in the following example:

spring.ldap.urls=ldap://myserver:1235
spring.ldap.username=admin
spring.ldap.password=secret

If you need to customize connection settings, you can use the spring.ldap.base and spring.ldap.base-environment properties.

An LdapContextSource is auto-configured based on these settings. If you need to customize it, for instance to use a PooledContextSource, you can still inject the auto-configured LdapContextSource. Make sure to flag your customized ContextSource as @Primary so that the auto-configured LdapTemplate uses it.

Spring Data LDAP Repositories

Spring Data includes repository support for LDAP. For complete details of Spring Data LDAP, refer to the reference documentation.

You can also inject an auto-configured LdapTemplate instance as you would with any other Spring Bean, as shown in the following example:

@Component
public class MyBean {

    private final LdapTemplate template;

    @Autowired
    public MyBean(LdapTemplate template) {
        this.template = template;
    }

    // ...

}
Embedded In-memory LDAP Server

For testing purposes, Spring Boot supports auto-configuration of an in-memory LDAP server from UnboundID. To configure the server, add a dependency to com.unboundid:unboundid-ldapsdk and declare a base-dn property, as follows:

spring.ldap.embedded.base-dn=dc=spring,dc=io

It is possible to define multiple base-dn values, however, since distinguished names usually contain commas, they must be defined using the correct notation.

In yaml files, you can use the yaml list notation:

spring.ldap.embedded.base-dn:
  - dc=spring,dc=io
  - dc=pivotal,dc=io

In properties files, you must include the index as part of the property name:

spring.ldap.embedded.base-dn[0]=dc=spring,dc=io
spring.ldap.embedded.base-dn[1]=dc=pivotal,dc=io

By default, the server starts on a random port and triggers the regular LDAP support. There is no need to specify a spring.ldap.urls property.

If there is a schema.ldif file on your classpath, it is used to initialize the server. If you want to load the initialization script from a different resource, you can also use the spring.ldap.embedded.ldif property.

By default, a standard schema is used to validate LDIF files. You can turn off validation altogether by setting the spring.ldap.embedded.validation.enabled property. If you have custom attributes, you can use spring.ldap.embedded.validation.schema to define your custom attribute types or object classes.

4.11.10. InfluxDB

InfluxDB is an open-source time series database optimized for fast, high-availability storage and retrieval of time series data in fields such as operations monitoring, application metrics, Internet-of-Things sensor data, and real-time analytics.

Connecting to InfluxDB

Spring Boot auto-configures an InfluxDB instance, provided the influxdb-java client is on the classpath and the URL of the database is set, as shown in the following example:

spring.influx.url=https://172.0.0.1:8086

If the connection to InfluxDB requires a user and password, you can set the spring.influx.user and spring.influx.password properties accordingly.

InfluxDB relies on OkHttp. If you need to tune the http client InfluxDB uses behind the scenes, you can register an InfluxDbOkHttpClientBuilderProvider bean.

4.12. Caching

The Spring Framework provides support for transparently adding caching to an application. At its core, the abstraction applies caching to methods, thus reducing the number of executions based on the information available in the cache. The caching logic is applied transparently, without any interference to the invoker. Spring Boot auto-configures the cache infrastructure as long as caching support is enabled via the @EnableCaching annotation.

Check the relevant section of the Spring Framework reference for more details.

In a nutshell, adding caching to an operation of your service is as easy as adding the relevant annotation to its method, as shown in the following example:

import org.springframework.cache.annotation.Cacheable;
import org.springframework.stereotype.Component;

@Component
public class MathService {

    @Cacheable("piDecimals")
    public int computePiDecimal(int i) {
        // ...
    }

}

This example demonstrates the use of caching on a potentially costly operation. Before invoking computePiDecimal, the abstraction looks for an entry in the piDecimals cache that matches the i argument. If an entry is found, the content in the cache is immediately returned to the caller, and the method is not invoked. Otherwise, the method is invoked, and the cache is updated before returning the value.

You can also use the standard JSR-107 (JCache) annotations (such as @CacheResult) transparently. However, we strongly advise you to not mix and match the Spring Cache and JCache annotations.

If you do not add any specific cache library, Spring Boot auto-configures a simple provider that uses concurrent maps in memory. When a cache is required (such as piDecimals in the preceding example), this provider creates it for you. The simple provider is not really recommended for production usage, but it is great for getting started and making sure that you understand the features. When you have made up your mind about the cache provider to use, please make sure to read its documentation to figure out how to configure the caches that your application uses. Nearly all providers require you to explicitly configure every cache that you use in the application. Some offer a way to customize the default caches defined by the spring.cache.cache-names property.

It is also possible to transparently update or evict data from the cache.

4.12.1. Supported Cache Providers

The cache abstraction does not provide an actual store and relies on abstraction materialized by the org.springframework.cache.Cache and org.springframework.cache.CacheManager interfaces.

If you have not defined a bean of type CacheManager or a CacheResolver named cacheResolver (see CachingConfigurer), Spring Boot tries to detect the following providers (in the indicated order):

  1. Generic

  2. JCache (JSR-107) (EhCache 3, Hazelcast, Infinispan, and others)

  3. EhCache 2.x

  4. Hazelcast

  5. Infinispan

  6. Couchbase

  7. Redis

  8. Caffeine

  9. Simple

It is also possible to force a particular cache provider by setting the spring.cache.type property. Use this property if you need to disable caching altogether in certain environment (such as tests).
Use the spring-boot-starter-cache “Starter” to quickly add basic caching dependencies. The starter brings in spring-context-support. If you add dependencies manually, you must include spring-context-support in order to use the JCache, EhCache 2.x, or Guava support.

If the CacheManager is auto-configured by Spring Boot, you can further tune its configuration before it is fully initialized by exposing a bean that implements the CacheManagerCustomizer interface. The following example sets a flag to say that null values should be passed down to the underlying map:

@Bean
public CacheManagerCustomizer<ConcurrentMapCacheManager> cacheManagerCustomizer() {
    return new CacheManagerCustomizer<ConcurrentMapCacheManager>() {
        @Override
        public void customize(ConcurrentMapCacheManager cacheManager) {
            cacheManager.setAllowNullValues(false);
        }
    };
}

In the preceding example, an auto-configured ConcurrentMapCacheManager is expected. If that is not the case (either you provided your own config or a different cache provider was auto-configured), the customizer is not invoked at all. You can have as many customizers as you want, and you can also order them by using @Order or Ordered.

Generic

Generic caching is used if the context defines at least one org.springframework.cache.Cache bean. A CacheManager wrapping all beans of that type is created.

JCache (JSR-107)

JCache is bootstrapped through the presence of a javax.cache.spi.CachingProvider on the classpath (that is, a JSR-107 compliant caching library exists on the classpath), and the JCacheCacheManager is provided by the spring-boot-starter-cache “Starter”. Various compliant libraries are available, and Spring Boot provides dependency management for Ehcache 3, Hazelcast, and Infinispan. Any other compliant library can be added as well.

It might happen that more than one provider is present, in which case the provider must be explicitly specified. Even if the JSR-107 standard does not enforce a standardized way to define the location of the configuration file, Spring Boot does its best to accommodate setting a cache with implementation details, as shown in the following example:

# Only necessary if more than one provider is present
spring.cache.jcache.provider=com.acme.MyCachingProvider
spring.cache.jcache.config=classpath:acme.xml
When a cache library offers both a native implementation and JSR-107 support, Spring Boot prefers the JSR-107 support, so that the same features are available if you switch to a different JSR-107 implementation.
Spring Boot has general support for Hazelcast. If a single HazelcastInstance is available, it is automatically reused for the CacheManager as well, unless the spring.cache.jcache.config property is specified.

There are two ways to customize the underlying javax.cache.cacheManager:

  • Caches can be created on startup by setting the spring.cache.cache-names property. If a custom javax.cache.configuration.Configuration bean is defined, it is used to customize them.

  • org.springframework.boot.autoconfigure.cache.JCacheManagerCustomizer beans are invoked with the reference of the CacheManager for full customization.

If a standard javax.cache.CacheManager bean is defined, it is wrapped automatically in an org.springframework.cache.CacheManager implementation that the abstraction expects. No further customization is applied to it.
EhCache 2.x

EhCache 2.x is used if a file named ehcache.xml can be found at the root of the classpath. If EhCache 2.x is found, the EhCacheCacheManager provided by the spring-boot-starter-cache “Starter” is used to bootstrap the cache manager. An alternate configuration file can be provided as well, as shown in the following example:

spring.cache.ehcache.config=classpath:config/another-config.xml
Hazelcast

Spring Boot has general support for Hazelcast. If a HazelcastInstance has been auto-configured, it is automatically wrapped in a CacheManager.

Infinispan

Infinispan has no default configuration file location, so it must be specified explicitly. Otherwise, the default bootstrap is used.

spring.cache.infinispan.config=infinispan.xml

Caches can be created on startup by setting the spring.cache.cache-names property. If a custom ConfigurationBuilder bean is defined, it is used to customize the caches.

The support of Infinispan in Spring Boot is restricted to the embedded mode and is quite basic. If you want more options, you should use the official Infinispan Spring Boot starter instead. See Infinispan’s documentation for more details.

Couchbase

If the Couchbase Java client and the couchbase-spring-cache implementation are available and Couchbase is configured, a CouchbaseCacheManager is auto-configured. It is also possible to create additional caches on startup by setting the spring.cache.cache-names property. These caches operate on the Bucket that was auto-configured. You can also create additional caches on another Bucket by using the customizer. Assume you need two caches (cache1 and cache2) on the "main" Bucket and one (cache3) cache with a custom time to live of 2 seconds on the “another” Bucket. You can create the first two caches through configuration, as follows:

spring.cache.cache-names=cache1,cache2

Then you can define a @Configuration class to configure the extra Bucket and the cache3 cache, as follows:

@Configuration(proxyBeanMethods = false)
public class CouchbaseCacheConfiguration {

    private final Cluster cluster;

    public CouchbaseCacheConfiguration(Cluster cluster) {
        this.cluster = cluster;
    }

    @Bean
    public Bucket anotherBucket() {
        return this.cluster.openBucket("another", "secret");
    }

    @Bean
    public CacheManagerCustomizer<CouchbaseCacheManager> cacheManagerCustomizer() {
        return c -> {
            c.prepareCache("cache3", CacheBuilder.newInstance(anotherBucket())
                    .withExpiration(2));
        };
    }

}

This sample configuration reuses the Cluster that was created through auto-configuration.

Redis

If Redis is available and configured, a RedisCacheManager is auto-configured. It is possible to create additional caches on startup by setting the spring.cache.cache-names property and cache defaults can be configured by using spring.cache.redis.* properties. For instance, the following configuration creates cache1 and cache2 caches with a time to live of 10 minutes:

spring.cache.cache-names=cache1,cache2
spring.cache.redis.time-to-live=600000

By default, a key prefix is added so that, if two separate caches use the same key, Redis does not have overlapping keys and cannot return invalid values. We strongly recommend keeping this setting enabled if you create your own RedisCacheManager.

You can take full control of the configuration by adding a RedisCacheConfiguration @Bean of your own. This can be useful if you’re looking for customizing the serialization strategy.
Caffeine

Caffeine is a Java 8 rewrite of Guava’s cache that supersedes support for Guava. If Caffeine is present, a CaffeineCacheManager (provided by the spring-boot-starter-cache “Starter”) is auto-configured. Caches can be created on startup by setting the spring.cache.cache-names property and can be customized by one of the following (in the indicated order):

  1. A cache spec defined by spring.cache.caffeine.spec

  2. A com.github.benmanes.caffeine.cache.CaffeineSpec bean is defined

  3. A com.github.benmanes.caffeine.cache.Caffeine bean is defined

For instance, the following configuration creates cache1 and cache2 caches with a maximum size of 500 and a time to live of 10 minutes

spring.cache.cache-names=cache1,cache2
spring.cache.caffeine.spec=maximumSize=500,expireAfterAccess=600s

If a com.github.benmanes.caffeine.cache.CacheLoader bean is defined, it is automatically associated to the CaffeineCacheManager. Since the CacheLoader is going to be associated with all caches managed by the cache manager, it must be defined as CacheLoader<Object, Object>. The auto-configuration ignores any other generic type.

Simple

If none of the other providers can be found, a simple implementation using a ConcurrentHashMap as the cache store is configured. This is the default if no caching library is present in your application. By default, caches are created as needed, but you can restrict the list of available caches by setting the cache-names property. For instance, if you want only cache1 and cache2 caches, set the cache-names property as follows:

spring.cache.cache-names=cache1,cache2

If you do so and your application uses a cache not listed, then it fails at runtime when the cache is needed, but not on startup. This is similar to the way the "real" cache providers behave if you use an undeclared cache.

None

When @EnableCaching is present in your configuration, a suitable cache configuration is expected as well. If you need to disable caching altogether in certain environments, force the cache type to none to use a no-op implementation, as shown in the following example:

spring.cache.type=none

4.13. Messaging

The Spring Framework provides extensive support for integrating with messaging systems, from simplified use of the JMS API using JmsTemplate to a complete infrastructure to receive messages asynchronously. Spring AMQP provides a similar feature set for the Advanced Message Queuing Protocol. Spring Boot also provides auto-configuration options for RabbitTemplate and RabbitMQ. Spring WebSocket natively includes support for STOMP messaging, and Spring Boot has support for that through starters and a small amount of auto-configuration. Spring Boot also has support for Apache Kafka.

4.13.1. JMS

The javax.jms.ConnectionFactory interface provides a standard method of creating a javax.jms.Connection for interacting with a JMS broker. Although Spring needs a ConnectionFactory to work with JMS, you generally need not use it directly yourself and can instead rely on higher level messaging abstractions. (See the relevant section of the Spring Framework reference documentation for details.) Spring Boot also auto-configures the necessary infrastructure to send and receive messages.

ActiveMQ Support

When ActiveMQ is available on the classpath, Spring Boot can also configure a ConnectionFactory. If the broker is present, an embedded broker is automatically started and configured (provided no broker URL is specified through configuration).

If you use spring-boot-starter-activemq, the necessary dependencies to connect or embed an ActiveMQ instance are provided, as is the Spring infrastructure to integrate with JMS.

ActiveMQ configuration is controlled by external configuration properties in spring.activemq.*. For example, you might declare the following section in application.properties:

spring.activemq.broker-url=tcp://192.168.1.210:9876
spring.activemq.user=admin
spring.activemq.password=secret

By default, a CachingConnectionFactory wraps the native ConnectionFactory with sensible settings that you can control by external configuration properties in spring.jms.*:

spring.jms.cache.session-cache-size=5

If you’d rather use native pooling, you can do so by adding a dependency to org.messaginghub:pooled-jms and configuring the JmsPoolConnectionFactory accordingly, as shown in the following example:

spring.activemq.pool.enabled=true
spring.activemq.pool.max-connections=50
See ActiveMQProperties for more of the supported options. You can also register an arbitrary number of beans that implement ActiveMQConnectionFactoryCustomizer for more advanced customizations.

By default, ActiveMQ creates a destination if it does not yet exist so that destinations are resolved against their provided names.

Artemis Support

Spring Boot can auto-configure a ConnectionFactory when it detects that Artemis is available on the classpath. If the broker is present, an embedded broker is automatically started and configured (unless the mode property has been explicitly set). The supported modes are embedded (to make explicit that an embedded broker is required and that an error should occur if the broker is not available on the classpath) and native (to connect to a broker using the netty transport protocol). When the latter is configured, Spring Boot configures a ConnectionFactory that connects to a broker running on the local machine with the default settings.

If you use spring-boot-starter-artemis, the necessary dependencies to connect to an existing Artemis instance are provided, as well as the Spring infrastructure to integrate with JMS. Adding org.apache.activemq:artemis-jms-server to your application lets you use embedded mode.

Artemis configuration is controlled by external configuration properties in spring.artemis.*. For example, you might declare the following section in application.properties:

spring.artemis.mode=native
spring.artemis.host=192.168.1.210
spring.artemis.port=9876
spring.artemis.user=admin
spring.artemis.password=secret

When embedding the broker, you can choose if you want to enable persistence and list the destinations that should be made available. These can be specified as a comma-separated list to create them with the default options, or you can define bean(s) of type org.apache.activemq.artemis.jms.server.config.JMSQueueConfiguration or org.apache.activemq.artemis.jms.server.config.TopicConfiguration, for advanced queue and topic configurations, respectively.

By default, a CachingConnectionFactory wraps the native ConnectionFactory with sensible settings that you can control by external configuration properties in spring.jms.*:

spring.jms.cache.session-cache-size=5

If you’d rather use native pooling, you can do so by adding a dependency to org.messaginghub:pooled-jms and configuring the JmsPoolConnectionFactory accordingly, as shown in the following example:

spring.artemis.pool.enabled=true
spring.artemis.pool.max-connections=50

See ArtemisProperties for more supported options.

No JNDI lookup is involved, and destinations are resolved against their names, using either the name attribute in the Artemis configuration or the names provided through configuration.

Using a JNDI ConnectionFactory

If you are running your application in an application server, Spring Boot tries to locate a JMS ConnectionFactory by using JNDI. By default, the java:/JmsXA and java:/XAConnectionFactory location are checked. You can use the spring.jms.jndi-name property if you need to specify an alternative location, as shown in the following example:

spring.jms.jndi-name=java:/MyConnectionFactory
Sending a Message

Spring’s JmsTemplate is auto-configured, and you can autowire it directly into your own beans, as shown in the following example:

import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.jms.core.JmsTemplate;
import org.springframework.stereotype.Component;

@Component
public class MyBean {

    private final JmsTemplate jmsTemplate;

    @Autowired
    public MyBean(JmsTemplate jmsTemplate) {
        this.jmsTemplate = jmsTemplate;
    }

    // ...

}
JmsMessagingTemplate can be injected in a similar manner. If a DestinationResolver or a MessageConverter bean is defined, it is associated automatically to the auto-configured JmsTemplate.
Receiving a Message

When the JMS infrastructure is present, any bean can be annotated with @JmsListener to create a listener endpoint. If no JmsListenerContainerFactory has been defined, a default one is configured automatically. If a DestinationResolver or a MessageConverter beans is defined, it is associated automatically to the default factory.

By default, the default factory is transactional. If you run in an infrastructure where a JtaTransactionManager is present, it is associated to the listener container by default. If not, the sessionTransacted flag is enabled. In that latter scenario, you can associate your local data store transaction to the processing of an incoming message by adding @Transactional on your listener method (or a delegate thereof). This ensures that the incoming message is acknowledged, once the local transaction has completed. This also includes sending response messages that have been performed on the same JMS session.

The following component creates a listener endpoint on the someQueue destination:

@Component
public class MyBean {

    @JmsListener(destination = "someQueue")
    public void processMessage(String content) {
        // ...
    }

}
See the Javadoc of @EnableJms for more details.

If you need to create more JmsListenerContainerFactory instances or if you want to override the default, Spring Boot provides a DefaultJmsListenerContainerFactoryConfigurer that you can use to initialize a DefaultJmsListenerContainerFactory with the same settings as the one that is auto-configured.

For instance, the following example exposes another factory that uses a specific MessageConverter:

@Configuration(proxyBeanMethods = false)
static class JmsConfiguration {

    @Bean
    public DefaultJmsListenerContainerFactory myFactory(
            DefaultJmsListenerContainerFactoryConfigurer configurer) {
        DefaultJmsListenerContainerFactory factory =
                new DefaultJmsListenerContainerFactory();
        configurer.configure(factory, connectionFactory());
        factory.setMessageConverter(myMessageConverter());
        return factory;
    }

}

Then you can use the factory in any @JmsListener-annotated method as follows:

@Component
public class MyBean {

    @JmsListener(destination = "someQueue", containerFactory="myFactory")
    public void processMessage(String content) {
        // ...
    }

}

4.13.2. AMQP

The Advanced Message Queuing Protocol (AMQP) is a platform-neutral, wire-level protocol for message-oriented middleware. The Spring AMQP project applies core Spring concepts to the development of AMQP-based messaging solutions. Spring Boot offers several conveniences for working with AMQP through RabbitMQ, including the spring-boot-starter-amqp “Starter”.

RabbitMQ support

RabbitMQ is a lightweight, reliable, scalable, and portable message broker based on the AMQP protocol. Spring uses RabbitMQ to communicate through the AMQP protocol.

RabbitMQ configuration is controlled by external configuration properties in spring.rabbitmq.*. For example, you might declare the following section in application.properties:

spring.rabbitmq.host=localhost
spring.rabbitmq.port=5672
spring.rabbitmq.username=admin
spring.rabbitmq.password=secret

If a ConnectionNameStrategy bean exists in the context, it will be automatically used to name connections created by the auto-configured ConnectionFactory. See RabbitProperties for more of the supported options.

Sending a Message

Spring’s AmqpTemplate and AmqpAdmin are auto-configured, and you can autowire them directly into your own beans, as shown in the following example:

import org.springframework.amqp.core.AmqpAdmin;
import org.springframework.amqp.core.AmqpTemplate;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.stereotype.Component;

@Component
public class MyBean {

    private final AmqpAdmin amqpAdmin;
    private final AmqpTemplate amqpTemplate;

    @Autowired
    public MyBean(AmqpAdmin amqpAdmin, AmqpTemplate amqpTemplate) {
        this.amqpAdmin = amqpAdmin;
        this.amqpTemplate = amqpTemplate;
    }

    // ...

}
RabbitMessagingTemplate can be injected in a similar manner. If a MessageConverter bean is defined, it is associated automatically to the auto-configured AmqpTemplate.

If necessary, any org.springframework.amqp.core.Queue that is defined as a bean is automatically used to declare a corresponding queue on the RabbitMQ instance.

To retry operations, you can enable retries on the AmqpTemplate (for example, in the event that the broker connection is lost):

spring.rabbitmq.template.retry.enabled=true
spring.rabbitmq.template.retry.initial-interval=2s

Retries are disabled by default. You can also customize the RetryTemplate programmatically by declaring a RabbitRetryTemplateCustomizer bean.

Receiving a Message

When the Rabbit infrastructure is present, any bean can be annotated with @RabbitListener to create a listener endpoint. If no RabbitListenerContainerFactory has been defined, a default SimpleRabbitListenerContainerFactory is automatically configured and you can switch to a direct container using the spring.rabbitmq.listener.type property. If a MessageConverter or a MessageRecoverer bean is defined, it is automatically associated with the default factory.

The following sample component creates a listener endpoint on the someQueue queue:

@Component
public class MyBean {

    @RabbitListener(queues = "someQueue")
    public void processMessage(String content) {
        // ...
    }

}
See the Javadoc of @EnableRabbit for more details.

If you need to create more RabbitListenerContainerFactory instances or if you want to override the default, Spring Boot provides a SimpleRabbitListenerContainerFactoryConfigurer and a DirectRabbitListenerContainerFactoryConfigurer that you can use to initialize a SimpleRabbitListenerContainerFactory and a DirectRabbitListenerContainerFactory with the same settings as the factories used by the auto-configuration.

It does not matter which container type you chose. Those two beans are exposed by the auto-configuration.

For instance, the following configuration class exposes another factory that uses a specific MessageConverter:

@Configuration(proxyBeanMethods = false)
static class RabbitConfiguration {

    @Bean
    public SimpleRabbitListenerContainerFactory myFactory(
            SimpleRabbitListenerContainerFactoryConfigurer configurer) {
        SimpleRabbitListenerContainerFactory factory =
                new SimpleRabbitListenerContainerFactory();
        configurer.configure(factory, connectionFactory);
        factory.setMessageConverter(myMessageConverter());
        return factory;
    }

}

Then you can use the factory in any @RabbitListener-annotated method, as follows:

@Component
public class MyBean {

    @RabbitListener(queues = "someQueue", containerFactory="myFactory")
    public void processMessage(String content) {
        // ...
    }

}

You can enable retries to handle situations where your listener throws an exception. By default, RejectAndDontRequeueRecoverer is used, but you can define a MessageRecoverer of your own. When retries are exhausted, the message is rejected and either dropped or routed to a dead-letter exchange if the broker is configured to do so. By default, retries are disabled. You can also customize the RetryTemplate programmatically by declaring a RabbitRetryTemplateCustomizer bean.

By default, if retries are disabled and the listener throws an exception, the delivery is retried indefinitely. You can modify this behavior in two ways: Set the defaultRequeueRejected property to false so that zero re-deliveries are attempted or throw an AmqpRejectAndDontRequeueException to signal the message should be rejected. The latter is the mechanism used when retries are enabled and the maximum number of delivery attempts is reached.

4.13.3. Apache Kafka Support

Apache Kafka is supported by providing auto-configuration of the spring-kafka project.

Kafka configuration is controlled by external configuration properties in spring.kafka.*. For example, you might declare the following section in application.properties:

spring.kafka.bootstrap-servers=localhost:9092
spring.kafka.consumer.group-id=myGroup
To create a topic on startup, add a bean of type NewTopic. If the topic already exists, the bean is ignored.

See KafkaProperties for more supported options.

Sending a Message

Spring’s KafkaTemplate is auto-configured, and you can autowire it directly in your own beans, as shown in the following example:

@Component
public class MyBean {

    private final KafkaTemplate kafkaTemplate;

    @Autowired
    public MyBean(KafkaTemplate kafkaTemplate) {
        this.kafkaTemplate = kafkaTemplate;
    }

    // ...

}
If the property spring.kafka.producer.transaction-id-prefix is defined, a KafkaTransactionManager is automatically configured. Also, if a RecordMessageConverter bean is defined, it is automatically associated to the auto-configured KafkaTemplate.
Receiving a Message

When the Apache Kafka infrastructure is present, any bean can be annotated with @KafkaListener to create a listener endpoint. If no KafkaListenerContainerFactory has been defined, a default one is automatically configured with keys defined in spring.kafka.listener.*.

The following component creates a listener endpoint on the someTopic topic:

@Component
public class MyBean {

    @KafkaListener(topics = "someTopic")
    public void processMessage(String content) {
        // ...
    }

}

If a KafkaTransactionManager bean is defined, it is automatically associated to the container factory. Similarly, if a ErrorHandler, AfterRollbackProcessor or ConsumerAwareRebalanceListener bean is defined, it is automatically associated to the default factory.

Depending on the listener type, a RecordMessageConverter or BatchMessageConverter bean is associated to the default factory. If only a RecordMessageConverter bean is present for a batch listener, it is wrapped in a BatchMessageConverter.

A custom ChainedKafkaTransactionManager must be marked @Primary as it usually references the auto-configured KafkaTransactionManager bean.
Kafka Streams

Spring for Apache Kafka provides a factory bean to create a StreamsBuilder object and manage the lifecycle of its streams. Spring Boot auto-configures the required KafkaStreamsConfiguration bean as long as kafka-streams is on the classpath and Kafka Streams is enabled via the @EnableKafkaStreams annotation.

Enabling Kafka Streams means that the application id and bootstrap servers must be set. The former can be configured using spring.kafka.streams.application-id, defaulting to spring.application.name if not set. The latter can be set globally or specifically overridden just for streams.

Several additional properties are available using dedicated properties; other arbitrary Kafka properties can be set using the spring.kafka.streams.properties namespace. See also Additional Kafka Properties for more information.

To use the factory bean, simply wire StreamsBuilder into your @Bean as shown in the following example:

@Configuration(proxyBeanMethods = false)
@EnableKafkaStreams
static class KafkaStreamsExampleConfiguration {

    @Bean
    public KStream<Integer, String> kStream(StreamsBuilder streamsBuilder) {
        KStream<Integer, String> stream = streamsBuilder.stream("ks1In");
        stream.map((k, v) -> new KeyValue<>(k, v.toUpperCase())).to("ks1Out",
                Produced.with(Serdes.Integer(), new JsonSerde<>()));
        return stream;
    }

}

By default, the streams managed by the StreamBuilder object it creates are started automatically. You can customize this behaviour using the spring.kafka.streams.auto-startup property.

Additional Kafka Properties

The properties supported by auto configuration are shown in Common application properties. Note that, for the most part, these properties (hyphenated or camelCase) map directly to the Apache Kafka dotted properties. Refer to the Apache Kafka documentation for details.

The first few of these properties apply to all components (producers, consumers, admins, and streams) but can be specified at the component level if you wish to use different values. Apache Kafka designates properties with an importance of HIGH, MEDIUM, or LOW. Spring Boot auto-configuration supports all HIGH importance properties, some selected MEDIUM and LOW properties, and any properties that do not have a default value.

Only a subset of the properties supported by Kafka are available directly through the KafkaProperties class. If you wish to configure the producer or consumer with additional properties that are not directly supported, use the following properties:

spring.kafka.properties.prop.one=first
spring.kafka.admin.properties.prop.two=second
spring.kafka.consumer.properties.prop.three=third
spring.kafka.producer.properties.prop.four=fourth
spring.kafka.streams.properties.prop.five=fifth

This sets the common prop.one Kafka property to first (applies to producers, consumers and admins), the prop.two admin property to second, the prop.three consumer property to third, the prop.four producer property to fourth and the prop.five streams property to fifth.

You can also configure the Spring Kafka JsonDeserializer as follows:

spring.kafka.consumer.value-deserializer=org.springframework.kafka.support.serializer.JsonDeserializer
spring.kafka.consumer.properties.spring.json.value.default.type=com.example.Invoice
spring.kafka.consumer.properties.spring.json.trusted.packages=com.example,org.acme

Similarly, you can disable the JsonSerializer default behavior of sending type information in headers:

spring.kafka.producer.value-serializer=org.springframework.kafka.support.serializer.JsonSerializer
spring.kafka.producer.properties.spring.json.add.type.headers=false
Properties set in this way override any configuration item that Spring Boot explicitly supports.

4.14. Calling REST Services with RestTemplate

If you need to call remote REST services from your application, you can use the Spring Framework’s RestTemplate class. Since RestTemplate instances often need to be customized before being used, Spring Boot does not provide any single auto-configured RestTemplate bean. It does, however, auto-configure a RestTemplateBuilder, which can be used to create RestTemplate instances when needed. The auto-configured RestTemplateBuilder ensures that sensible HttpMessageConverters are applied to RestTemplate instances.

The following code shows a typical example:

@Service
public class MyService {

    private final RestTemplate restTemplate;

    public MyService(RestTemplateBuilder restTemplateBuilder) {
        this.restTemplate = restTemplateBuilder.build();
    }

    public Details someRestCall(String name) {
        return this.restTemplate.getForObject("/{name}/details", Details.class, name);
    }

}
RestTemplateBuilder includes a number of useful methods that can be used to quickly configure a RestTemplate. For example, to add BASIC auth support, you can use builder.basicAuthentication("user", "password").build().

4.14.1. RestTemplate Customization

There are three main approaches to RestTemplate customization, depending on how broadly you want the customizations to apply.

To make the scope of any customizations as narrow as possible, inject the auto-configured RestTemplateBuilder and then call its methods as required. Each method call returns a new RestTemplateBuilder instance, so the customizations only affect this use of the builder.

To make an application-wide, additive customization, use a RestTemplateCustomizer bean. All such beans are automatically registered with the auto-configured RestTemplateBuilder and are applied to any templates that are built with it.

The following example shows a customizer that configures the use of a proxy for all hosts except 192.168.0.5:

static class ProxyCustomizer implements RestTemplateCustomizer {

    @Override
    public void customize(RestTemplate restTemplate) {
        HttpHost proxy = new HttpHost("proxy.example.com");
        HttpClient httpClient = HttpClientBuilder.create().setRoutePlanner(new DefaultProxyRoutePlanner(proxy) {

            @Override
            public HttpHost determineProxy(HttpHost target, HttpRequest request, HttpContext context)
                    throws HttpException {
                if (target.getHostName().equals("192.168.0.5")) {
                    return null;
                }
                return super.determineProxy(target, request, context);
            }

        }).build();
        restTemplate.setRequestFactory(new HttpComponentsClientHttpRequestFactory(httpClient));
    }

}

Finally, the most extreme (and rarely used) option is to create your own RestTemplateBuilder bean. Doing so switches off the auto-configuration of a RestTemplateBuilder and prevents any RestTemplateCustomizer beans from being used.

4.15. Calling REST Services with WebClient

If you have Spring WebFlux on your classpath, you can also choose to use WebClient to call remote REST services. Compared to RestTemplate, this client has a more functional feel and is fully reactive. You can learn more about the WebClient in the dedicated section in the Spring Framework docs.

Spring Boot creates and pre-configures a WebClient.Builder for you; it is strongly advised to inject it in your components and use it to create WebClient instances. Spring Boot is configuring that builder to share HTTP resources, reflect codecs setup in the same fashion as the server ones (see WebFlux HTTP codecs auto-configuration), and more.

The following code shows a typical example:

@Service
public class MyService {

    private final WebClient webClient;

    public MyService(WebClient.Builder webClientBuilder) {
        this.webClient = webClientBuilder.baseUrl("https://example.org").build();
    }

    public Mono<Details> someRestCall(String name) {
        return this.webClient.get().uri("/{name}/details", name)
                        .retrieve().bodyToMono(Details.class);
    }

}

4.15.1. WebClient Runtime

Spring Boot will auto-detect which ClientHttpConnector to use to drive WebClient, depending on the libraries available on the application classpath. For now, Reactor Netty and Jetty RS client are supported.

The spring-boot-starter-webflux starter depends on io.projectreactor.netty:reactor-netty by default, which brings both server and client implementations. If you choose to use Jetty as a reactive server instead, you should add a dependency on the Jetty Reactive HTTP client library, org.eclipse.jetty:jetty-reactive-httpclient. Using the same technology for server and client has it advantages, as it will automatically share HTTP resources between client and server.

Developers can override the resource configuration for Jetty and Reactor Netty by providing a custom ReactorResourceFactory or JettyResourceFactory bean - this will be applied to both clients and servers.

If you wish to override that choice for the client, you can define your own ClientHttpConnector bean and have full control over the client configuration.

4.15.2. WebClient Customization

There are three main approaches to WebClient customization, depending on how broadly you want the customizations to apply.

To make the scope of any customizations as narrow as possible, inject the auto-configured WebClient.Builder and then call its methods as required. WebClient.Builder instances are stateful: Any change on the builder is reflected in all clients subsequently created with it. If you want to create several clients with the same builder, you can also consider cloning the builder with WebClient.Builder other = builder.clone();.

To make an application-wide, additive customization to all WebClient.Builder instances, you can declare WebClientCustomizer beans and change the WebClient.Builder locally at the point of injection.

Finally, you can fall back to the original API and use WebClient.create(). In that case, no auto-configuration or WebClientCustomizer is applied.

4.16. Validation

The method validation feature supported by Bean Validation 1.1 is automatically enabled as long as a JSR-303 implementation (such as Hibernate validator) is on the classpath. This lets bean methods be annotated with javax.validation constraints on their parameters and/or on their return value. Target classes with such annotated methods need to be annotated with the @Validated annotation at the type level for their methods to be searched for inline constraint annotations.

For instance, the following service triggers the validation of the first argument, making sure its size is between 8 and 10:

@Service
@Validated
public class MyBean {

    public Archive findByCodeAndAuthor(@Size(min = 8, max = 10) String code,
            Author author) {
        ...
    }

}

4.17. Sending Email

The Spring Framework provides an easy abstraction for sending email by using the JavaMailSender interface, and Spring Boot provides auto-configuration for it as well as a starter module.

See the reference documentation for a detailed explanation of how you can use JavaMailSender.

If spring.mail.host and the relevant libraries (as defined by spring-boot-starter-mail) are available, a default JavaMailSender is created if none exists. The sender can be further customized by configuration items from the spring.mail namespace. See MailProperties for more details.

In particular, certain default timeout values are infinite, and you may want to change that to avoid having a thread blocked by an unresponsive mail server, as shown in the following example:

spring.mail.properties.mail.smtp.connectiontimeout=5000
spring.mail.properties.mail.smtp.timeout=3000
spring.mail.properties.mail.smtp.writetimeout=5000

It is also possible to configure a JavaMailSender with an existing Session from JNDI:

spring.mail.jndi-name=mail/Session

When a jndi-name is set, it takes precedence over all other Session-related settings.

4.18. Distributed Transactions with JTA

Spring Boot supports distributed JTA transactions across multiple XA resources by using either an Atomikos or Bitronix embedded transaction manager. JTA transactions are also supported when deploying to a suitable Java EE Application Server.

When a JTA environment is detected, Spring’s JtaTransactionManager is used to manage transactions. Auto-configured JMS, DataSource, and JPA beans are upgraded to support XA transactions. You can use standard Spring idioms, such as @Transactional, to participate in a distributed transaction. If you are within a JTA environment and still want to use local transactions, you can set the spring.jta.enabled property to false to disable the JTA auto-configuration.

4.18.1. Using an Atomikos Transaction Manager

Atomikos is a popular open source transaction manager which can be embedded into your Spring Boot application. You can use the spring-boot-starter-jta-atomikos Starter to pull in the appropriate Atomikos libraries. Spring Boot auto-configures Atomikos and ensures that appropriate depends-on settings are applied to your Spring beans for correct startup and shutdown ordering.

By default, Atomikos transaction logs are written to a transaction-logs directory in your application’s home directory (the directory in which your application jar file resides). You can customize the location of this directory by setting a spring.jta.log-dir property in your application.properties file. Properties starting with spring.jta.atomikos.properties can also be used to customize the Atomikos UserTransactionServiceImp. See the AtomikosProperties Javadoc for complete details.

To ensure that multiple transaction managers can safely coordinate the same resource managers, each Atomikos instance must be configured with a unique ID. By default, this ID is the IP address of the machine on which Atomikos is running. To ensure uniqueness in production, you should configure the spring.jta.transaction-manager-id property with a different value for each instance of your application.

4.18.2. Using a Bitronix Transaction Manager

Bitronix is a popular open-source JTA transaction manager implementation. You can use the spring-boot-starter-jta-bitronix starter to add the appropriate Bitronix dependencies to your project. As with Atomikos, Spring Boot automatically configures Bitronix and post-processes your beans to ensure that startup and shutdown ordering is correct.

By default, Bitronix transaction log files (part1.btm and part2.btm) are written to a transaction-logs directory in your application home directory. You can customize the location of this directory by setting the spring.jta.log-dir property. Properties starting with spring.jta.bitronix.properties are also bound to the bitronix.tm.Configuration bean, allowing for complete customization. See the Bitronix documentation for details.

To ensure that multiple transaction managers can safely coordinate the same resource managers, each Bitronix instance must be configured with a unique ID. By default, this ID is the IP address of the machine on which Bitronix is running. To ensure uniqueness in production, you should configure the spring.jta.transaction-manager-id property with a different value for each instance of your application.

4.18.3. Using a Java EE Managed Transaction Manager

If you package your Spring Boot application as a war or ear file and deploy it to a Java EE application server, you can use your application server’s built-in transaction manager. Spring Boot tries to auto-configure a transaction manager by looking at common JNDI locations (java:comp/UserTransaction, java:comp/TransactionManager, and so on). If you use a transaction service provided by your application server, you generally also want to ensure that all resources are managed by the server and exposed over JNDI. Spring Boot tries to auto-configure JMS by looking for a ConnectionFactory at the JNDI path (java:/JmsXA or java:/XAConnectionFactory), and you can use the spring.datasource.jndi-name property to configure your DataSource.

4.18.4. Mixing XA and Non-XA JMS Connections

When using JTA, the primary JMS ConnectionFactory bean is XA-aware and participates in distributed transactions. In some situations, you might want to process certain JMS messages by using a non-XA ConnectionFactory. For example, your JMS processing logic might take longer than the XA timeout.

If you want to use a non-XA ConnectionFactory, you can inject the nonXaJmsConnectionFactory bean rather than the @Primary jmsConnectionFactory bean. For consistency, the jmsConnectionFactory bean is also provided by using the bean alias xaJmsConnectionFactory.

The following example shows how to inject ConnectionFactory instances:

// Inject the primary (XA aware) ConnectionFactory
@Autowired
private ConnectionFactory defaultConnectionFactory;

// Inject the XA aware ConnectionFactory (uses the alias and injects the same as above)
@Autowired
@Qualifier("xaJmsConnectionFactory")
private ConnectionFactory xaConnectionFactory;

// Inject the non-XA aware ConnectionFactory
@Autowired
@Qualifier("nonXaJmsConnectionFactory")
private ConnectionFactory nonXaConnectionFactory;

4.18.5. Supporting an Alternative Embedded Transaction Manager

The XAConnectionFactoryWrapper and XADataSourceWrapper interfaces can be used to support alternative embedded transaction managers. The interfaces are responsible for wrapping XAConnectionFactory and XADataSource beans and exposing them as regular ConnectionFactory and DataSource beans, which transparently enroll in the distributed transaction. DataSource and JMS auto-configuration use JTA variants, provided you have a JtaTransactionManager bean and appropriate XA wrapper beans registered within your ApplicationContext.

The BitronixXAConnectionFactoryWrapper and BitronixXADataSourceWrapper provide good examples of how to write XA wrappers.

4.19. Hazelcast

If Hazelcast is on the classpath and a suitable configuration is found, Spring Boot auto-configures a HazelcastInstance that you can inject in your application.

If you define a com.hazelcast.config.Config bean, Spring Boot uses that. If your configuration defines an instance name, Spring Boot tries to locate an existing instance rather than creating a new one.

You could also specify the Hazelcast configuration file to use through configuration, as shown in the following example:

spring.hazelcast.config=classpath:config/my-hazelcast.xml

Otherwise, Spring Boot tries to find the Hazelcast configuration from the default locations: hazelcast.xml in the working directory or at the root of the classpath, or a .yaml counterpart in the same locations. We also check if the hazelcast.config system property is set. See the Hazelcast documentation for more details.

If hazelcast-client is present on the classpath, Spring Boot first attempts to create a client by checking the following configuration options:

  • The presence of a com.hazelcast.client.config.ClientConfig bean.

  • A configuration file defined by the spring.hazelcast.config property.

  • The presence of the hazelcast.client.config system property.

  • A hazelcast-client.xml in the working directory or at the root of the classpath.

  • A hazelcast-client.yaml in the working directory or at the root of the classpath.

Spring Boot also has explicit caching support for Hazelcast. If caching is enabled, the HazelcastInstance is automatically wrapped in a CacheManager implementation.

4.20. Quartz Scheduler

Spring Boot offers several conveniences for working with the Quartz scheduler, including the spring-boot-starter-quartz “Starter”. If Quartz is available, a Scheduler is auto-configured (through the SchedulerFactoryBean abstraction).

Beans of the following types are automatically picked up and associated with the Scheduler:

  • JobDetail: defines a particular Job. JobDetail instances can be built with the JobBuilder API.

  • Calendar.

  • Trigger: defines when a particular job is triggered.

By default, an in-memory JobStore is used. However, it is possible to configure a JDBC-based store if a DataSource bean is available in your application and if the spring.quartz.job-store-type property is configured accordingly, as shown in the following example:

spring.quartz.job-store-type=jdbc

When the JDBC store is used, the schema can be initialized on startup, as shown in the following example:

spring.quartz.jdbc.initialize-schema=always
By default, the database is detected and initialized by using the standard scripts provided with the Quartz library. These scripts drop existing tables, deleting all triggers on every restart. It is also possible to provide a custom script by setting the spring.quartz.jdbc.schema property.

To have Quartz use a DataSource other than the application’s main DataSource, declare a DataSource bean, annotating its @Bean method with @QuartzDataSource. Doing so ensures that the Quartz-specific DataSource is used by both the SchedulerFactoryBean and for schema initialization.

By default, jobs created by configuration will not overwrite already registered jobs that have been read from a persistent job store. To enable overwriting existing job definitions set the spring.quartz.overwrite-existing-jobs property.

Quartz Scheduler configuration can be customized using spring.quartz properties and SchedulerFactoryBeanCustomizer beans, which allow programmatic SchedulerFactoryBean customization. Advanced Quartz configuration properties can be customized using spring.quartz.properties.*.

In particular, an Executor bean is not associated with the scheduler as Quartz offers a way to configure the scheduler via spring.quartz.properties. If you need to customize the task executor, consider implementing SchedulerFactoryBeanCustomizer.

Jobs can define setters to inject data map properties. Regular beans can also be injected in a similar manner, as shown in the following example:

public class SampleJob extends QuartzJobBean {

    private MyService myService;

    private String name;

    // Inject "MyService" bean
    public void setMyService(MyService myService) { ... }

    // Inject the "name" job data property
    public void setName(String name) { ... }

    @Override
    protected void executeInternal(JobExecutionContext context)
            throws JobExecutionException {
        ...
    }

}

4.21. Task Execution and Scheduling

In the absence of an Executor bean in the context, Spring Boot auto-configures a ThreadPoolTaskExecutor with sensible defaults that can be automatically associated to asynchronous task execution (@EnableAsync) and Spring MVC asynchronous request processing.

If you have defined a custom Executor in the context, regular task execution (i.e. @EnableAsync) will use it transparently but the Spring MVC support will not be configured as it requires an AsyncTaskExecutor implementation (named applicationTaskExecutor). Depending on your target arrangement, you could change your Executor into a ThreadPoolTaskExecutor or define both a ThreadPoolTaskExecutor and an AsyncConfigurer wrapping your custom Executor.

The auto-configured TaskExecutorBuilder allows you to easily create instances that reproduce what the auto-configuration does by default.

The thread pool uses 8 core threads that can grow and shrink according to the load. Those default settings can be fine-tuned using the spring.task.execution namespace as shown in the following example:

spring.task.execution.pool.max-threads=16
spring.task.execution.pool.queue-capacity=100
spring.task.execution.pool.keep-alive=10s

This changes the thread pool to use a bounded queue so that when the queue is full (100 tasks), the thread pool increases to maximum 16 threads. Shrinking of the pool is more aggressive as threads are reclaimed when they are idle for 10 seconds (rather than 60 seconds by default).

A ThreadPoolTaskScheduler can also be auto-configured if need to be associated to scheduled task execution (@EnableScheduling). The thread pool uses one thread by default and those settings can be fine-tuned using the spring.task.scheduling namespace.

Both a TaskExecutorBuilder bean and a TaskSchedulerBuilder bean are made available in the context if a custom executor or scheduler needs to be created.

4.22. Spring Integration

Spring Boot offers several conveniences for working with Spring Integration, including the spring-boot-starter-integration “Starter”. Spring Integration provides abstractions over messaging and also other transports such as HTTP, TCP, and others. If Spring Integration is available on your classpath, it is initialized through the @EnableIntegration annotation.

Spring Boot also configures some features that are triggered by the presence of additional Spring Integration modules. If spring-integration-jmx is also on the classpath, message processing statistics are published over JMX . If spring-integration-jdbc is available, the default database schema can be created on startup, as shown in the following line:

spring.integration.jdbc.initialize-schema=always

See the IntegrationAutoConfiguration and IntegrationProperties classes for more details.

By default, if a Micrometer meterRegistry bean is present, Spring Integration metrics will be managed by Micrometer. If you wish to use legacy Spring Integration metrics, add a DefaultMetricsFactory bean to the application context.

4.23. Spring Session

Spring Boot provides Spring Session auto-configuration for a wide range of data stores. When building a Servlet web application, the following stores can be auto-configured:

  • JDBC

  • Redis

  • Hazelcast

  • MongoDB

When building a reactive web application, the following stores can be auto-configured:

  • Redis

  • MongoDB

If a single Spring Session module is present on the classpath, Spring Boot uses that store implementation automatically. If you have more than one implementation, you must choose the StoreType that you wish to use to store the sessions. For instance, to use JDBC as the back-end store, you can configure your application as follows:

spring.session.store-type=jdbc
You can disable Spring Session by setting the store-type to none.

Each store has specific additional settings. For instance, it is possible to customize the name of the table for the JDBC store, as shown in the following example:

spring.session.jdbc.table-name=SESSIONS

For setting the timeout of the session you can use the spring.session.timeout property. If that property is not set, the auto-configuration falls back to the value of server.servlet.session.timeout.

4.24. Monitoring and Management over JMX

Java Management Extensions (JMX) provide a standard mechanism to monitor and manage applications. Spring Boot exposes the most suitable MBeanServer as a bean with an ID of mbeanServer. Any of your beans that are annotated with Spring JMX annotations ( @ManagedResource, @ManagedAttribute, or @ManagedOperation) are exposed to it.

If your platform provides a standard MBeanServer, Spring Boot will use that and default to the VM MBeanServer if necessary. If all that fails, a new MBeanServer will be created.

See the JmxAutoConfiguration class for more details.

4.25. Testing

Spring Boot provides a number of utilities and annotations to help when testing your application. Test support is provided by two modules: spring-boot-test contains core items, and spring-boot-test-autoconfigure supports auto-configuration for tests.

Most developers use the spring-boot-starter-test “Starter”, which imports both Spring Boot test modules as well as JUnit Jupiter, AssertJ, Hamcrest, and a number of other useful libraries.

The starter also brings the vintage engine and junit:junit so that you can run both JUnit 4 and Junit 5 tests. If you have migrated your tests to JUnit 5, you should exclude JUnit 4 support, as shown in the following example:

<dependency>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-starter-test</artifactId>
    <scope>test</scope>
    <exclusions>
        <exclusion>
            <groupId>org.junit.vintage</groupId>
            <artifactId>junit-vintage-engine</artifactId>
        </exclusion>
        <exclusion>
            <groupId>junit</groupId>
            <artifactId>junit</artifactId>
        </exclusion>
    </exclusions>
</dependency>

4.25.1. Test Scope Dependencies

The spring-boot-starter-test “Starter” (in the test scope) contains the following provided libraries:

  • JUnit 5 (including the vintage engine for backward compatibility with JUnit 4: The de-facto standard for unit testing Java applications.

  • Spring Test & Spring Boot Test: Utilities and integration test support for Spring Boot applications.

  • AssertJ: A fluent assertion library.

  • Hamcrest: A library of matcher objects (also known as constraints or predicates).

  • Mockito: A Java mocking framework.

  • JSONassert: An assertion library for JSON.

  • JsonPath: XPath for JSON.

We generally find these common libraries to be useful when writing tests. If these libraries do not suit your needs, you can add additional test dependencies of your own.

4.25.2. Testing Spring Applications

One of the major advantages of dependency injection is that it should make your code easier to unit test. You can instantiate objects by using the new operator without even involving Spring. You can also use mock objects instead of real dependencies.

Often, you need to move beyond unit testing and start integration testing (with a Spring ApplicationContext). It is useful to be able to perform integration testing without requiring deployment of your application or needing to connect to other infrastructure.

The Spring Framework includes a dedicated test module for such integration testing. You can declare a dependency directly to org.springframework:spring-test or use the spring-boot-starter-test “Starter” to pull it in transitively.

If you have not used the spring-test module before, you should start by reading the relevant section of the Spring Framework reference documentation.

4.25.3. Testing Spring Boot Applications

A Spring Boot application is a Spring ApplicationContext, so nothing very special has to be done to test it beyond what you would normally do with a vanilla Spring context.

External properties, logging, and other features of Spring Boot are installed in the context by default only if you use SpringApplication to create it.

Spring Boot provides a @SpringBootTest annotation, which can be used as an alternative to the standard spring-test @ContextConfiguration annotation when you need Spring Boot features. The annotation works by creating the ApplicationContext used in your tests through SpringApplication. In addition to @SpringBootTest a number of other annotations are also provided for testing more specific slices of an application.

If you are using JUnit 4, don’t forget to also add @RunWith(SpringRunner.class) to your test, otherwise the annotations will be ignored. If you are using JUnit 5, there’s no need to add the equivalent @RunWith(SpringExtension.class) as @SpringBootTest and the other @…Test annotations are already annotated with it.

By default, @SpringBootTest will not start a server. You can use the webEnvironment attribute of @SpringBootTest to further refine how your tests run:

  • MOCK(Default) : Loads a web ApplicationContext and provides a mock web environment. Embedded servers are not started when using this annotation. If a web environment is not available on your classpath, this mode transparently falls back to creating a regular non-web ApplicationContext. It can be used in conjunction with @AutoConfigureMockMvc or @AutoConfigureWebTestClient for mock-based testing of your web application.

  • RANDOM_PORT: Loads a WebServerApplicationContext and provides a real web environment. Embedded servers are started and listen on a random port.

  • DEFINED_PORT: Loads a WebServerApplicationContext and provides a real web environment. Embedded servers are started and listen on a defined port (from your application.properties) or on the default port of 8080.

  • NONE: Loads an ApplicationContext by using SpringApplication but does not provide any web environment (mock or otherwise).

If your test is @Transactional, it rolls back the transaction at the end of each test method by default. However, as using this arrangement with either RANDOM_PORT or DEFINED_PORT implicitly provides a real servlet environment, the HTTP client and server run in separate threads and, thus, in separate transactions. Any transaction initiated on the server does not roll back in this case.
@SpringBootTest with webEnvironment = WebEnvironment.RANDOM_PORT will also start the management server on a separate random port if your application uses a different port for the management server.
Detecting Web Application Type

If Spring MVC is available, a regular MVC-based application context is configured. If you have only Spring WebFlux, we’ll detect that and configure a WebFlux-based application context instead.

If both are present, Spring MVC takes precedence. If you want to test a reactive web application in this scenario, you must set the spring.main.web-application-type property:

@RunWith(SpringRunner.class)
@SpringBootTest(properties = "spring.main.web-application-type=reactive")
public class MyWebFluxTests { ... }
Detecting Test Configuration

If you are familiar with the Spring Test Framework, you may be used to using @ContextConfiguration(classes=…​) in order to specify which Spring @Configuration to load. Alternatively, you might have often used nested @Configuration classes within your test.

When testing Spring Boot applications, this is often not required. Spring Boot’s @*Test annotations search for your primary configuration automatically whenever you do not explicitly define one.

The search algorithm works up from the package that contains the test until it finds a class annotated with @SpringBootApplication or @SpringBootConfiguration. As long as you structured your code in a sensible way, your main configuration is usually found.

If you use a test annotation to test a more specific slice of your application, you should avoid adding configuration settings that are specific to a particular area on the main method’s application class.

The underlying component scan configuration of @SpringBootApplication defines exclude filters that are used to make sure slicing works as expected. If you are using an explicit @ComponentScan directive on your @SpringBootApplication-annotated class, be aware that those filters will be disabled. If you are using slicing, you should define them again.

If you want to customize the primary configuration, you can use a nested @TestConfiguration class. Unlike a nested @Configuration class, which would be used instead of your application’s primary configuration, a nested @TestConfiguration class is used in addition to your application’s primary configuration.

Spring’s test framework caches application contexts between tests. Therefore, as long as your tests share the same configuration (no matter how it is discovered), the potentially time-consuming process of loading the context happens only once.
Excluding Test Configuration

If your application uses component scanning (for example, if you use @SpringBootApplication or @ComponentScan), you may find top-level configuration classes that you created only for specific tests accidentally get picked up everywhere.

As we have seen earlier, @TestConfiguration can be used on an inner class of a test to customize the primary configuration. When placed on a top-level class, @TestConfiguration indicates that classes in src/test/java should not be picked up by scanning. You can then import that class explicitly where it is required, as shown in the following example:

@SpringBootTest
@Import(MyTestsConfiguration.class)
public class MyTests {

    @Test
    public void exampleTest() {
        ...
    }

}
If you directly use @ComponentScan (that is, not through @SpringBootApplication) you need to register the TypeExcludeFilter with it. See the Javadoc for details.
Using Application Arguments

If your application expects arguments, you can have @SpringBootTest inject them using the args attribute.

@SpringBootTest(args = "--app.test=one")
class ApplicationArgumentsExampleTests {

    @Test
    void applicationArgumentsPopulated(@Autowired ApplicationArguments args) {
        assertThat(args.getOptionNames()).containsOnly("app.test");
        assertThat(args.getOptionValues("app.test")).containsOnly("one");
    }

}
Testing with a mock environment

By default, @SpringBootTest does not start the server. If you have web endpoints that you want to test against this mock environment, you can additionally configure MockMvc as shown in the following example:

import org.junit.jupiter.api.Test;

import org.springframework.boot.test.autoconfigure.web.servlet.AutoConfigureMockMvc;
import org.springframework.boot.test.context.SpringBootTest;
import org.springframework.test.web.servlet.MockMvc;

import static org.springframework.test.web.servlet.request.MockMvcRequestBuilders.get;
import static org.springframework.test.web.servlet.result.MockMvcResultMatchers.content;
import static org.springframework.test.web.servlet.result.MockMvcResultMatchers.status;

@SpringBootTest
@AutoConfigureMockMvc
class MockMvcExampleTests {

    @Test
    void exampleTest(MockMvc mvc) throws Exception {
        mvc.perform(get("/")).andExpect(status().isOk()).andExpect(content().string("Hello World"));
    }

}
If you want to focus only on the web layer and not start a complete ApplicationContext, consider using @WebMvcTest instead.

Alternatively, you can configure a WebTestClient as shown in the following example:

import org.junit.jupiter.api.Test;

import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.boot.test.autoconfigure.web.reactive.AutoConfigureWebTestClient;
import org.springframework.boot.test.context.SpringBootTest;
import org.springframework.test.web.reactive.server.WebTestClient;

@SpringBootTest
@AutoConfigureWebTestClient
class MockWebTestClientExampleTests {

    @Test
    void exampleTest(@Autowired WebTestClient webClient) {
        webClient.get().uri("/").exchange().expectStatus().isOk().expectBody(String.class).isEqualTo("Hello World");
    }

}
Testing with a running server

If you need to start a full running server, we recommend that you use random ports. If you use @SpringBootTest(webEnvironment=WebEnvironment.RANDOM_PORT), an available port is picked at random each time your test runs.

The @LocalServerPort annotation can be used to inject the actual port used into your test. For convenience, tests that need to make REST calls to the started server can additionally @Autowire a WebTestClient, which resolves relative links to the running server and comes with a dedicated API for verifying responses, as shown in the following example:

import org.junit.jupiter.api.Test;

import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.boot.test.context.SpringBootTest;
import org.springframework.boot.test.context.SpringBootTest.WebEnvironment;
import org.springframework.test.web.reactive.server.WebTestClient;

@SpringBootTest(webEnvironment = WebEnvironment.RANDOM_PORT)
public class RandomPortWebTestClientExampleTests {

    @Test
    void exampleTest(@Autowired WebTestClient webClient) {
        webClient.get().uri("/").exchange().expectStatus().isOk().expectBody(String.class).isEqualTo("Hello World");
    }

}

This setup requires spring-webflux on the classpath. If you can’t or won’t add webflux, Spring Boot also provides a TestRestTemplate facility:

import org.junit.jupiter.api.Test;

import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.boot.test.context.SpringBootTest;
import org.springframework.boot.test.context.SpringBootTest.WebEnvironment;
import org.springframework.boot.test.web.client.TestRestTemplate;

import static org.assertj.core.api.Assertions.assertThat;

@SpringBootTest(webEnvironment = WebEnvironment.RANDOM_PORT)
class RandomPortTestRestTemplateExampleTests {

    @Test
    void exampleTest(@Autowired TestRestTemplate restTemplate) {
        String body = restTemplate.getForObject("/", String.class);
        assertThat(body).isEqualTo("Hello World");
    }

}
Using JMX

As the test context framework caches context, JMX is disabled by default to prevent identical components to register on the same domain. If such test needs access to an MBeanServer, consider marking it dirty as well:

@ExtendWith(SpringExtension.class)
@SpringBootTest(properties = "spring.jmx.enabled=true")
@DirtiesContext
class SampleJmxTests {

    @Autowired
    private MBeanServer mBeanServer;

    @Test
    void exampleTest() {
        // ...
    }

}
Mocking and Spying Beans

When running tests, it is sometimes necessary to mock certain components within your application context. For example, you may have a facade over some remote service that is unavailable during development. Mocking can also be useful when you want to simulate failures that might be hard to trigger in a real environment.

Spring Boot includes a @MockBean annotation that can be used to define a Mockito mock for a bean inside your ApplicationContext. You can use the annotation to add new beans or replace a single existing bean definition. The annotation can be used directly on test classes, on fields within your test, or on @Configuration classes and fields. When used on a field, the instance of the created mock is also injected. Mock beans are automatically reset after each test method.

If your test uses one of Spring Boot’s test annotations (such as @SpringBootTest), this feature is automatically enabled. To use this feature with a different arrangement, a listener must be explicitly added, as shown in the following example:

@TestExecutionListeners(MockitoTestExecutionListener.class)

The following example replaces an existing RemoteService bean with a mock implementation:

import org.junit.jupiter.api.Test;
import org.springframework.beans.factory.annotation.*;
import org.springframework.boot.test.context.*;
import org.springframework.boot.test.mock.mockito.*;

import static org.assertj.core.api.Assertions.*;
import static org.mockito.BDDMockito.*;

@SpringBootTest
class MyTests {

    @MockBean
    private RemoteService remoteService;

    @Autowired
    private Reverser reverser;

    @Test
    void exampleTest() {
        // RemoteService has been injected into the reverser bean
        given(this.remoteService.someCall()).willReturn("mock");
        String reverse = reverser.reverseSomeCall();
        assertThat(reverse).isEqualTo("kcom");
    }

}
@MockBean cannot be used to mock the behavior of a bean that’s exercised during application context refresh. By the time the test is executed, the application context refresh has completed and it is too late to configure the mocked behavior. We recommend using a @Bean method to create and configure the mock in this situation.

Additionally, you can use @SpyBean to wrap any existing bean with a Mockito spy. See the Javadoc for full details.

While Spring’s test framework caches application contexts between tests and reuses a context for tests sharing the same configuration, the use of @MockBean or @SpyBean influences the cache key, which will most likely increase the number of contexts.
If you are using @SpyBean to spy on a bean with @Cacheable methods that refer to parameters by name, your application must be compiled with -parameters. This ensures that the parameter names are available to the caching infrastructure once the bean has been spied upon.
Auto-configured Tests

Spring Boot’s auto-configuration system works well for applications but can sometimes be a little too much for tests. It often helps to load only the parts of the configuration that are required to test a “slice” of your application. For example, you might want to test that Spring MVC controllers are mapping URLs correctly, and you do not want to involve database calls in those tests, or you might want to test JPA entities, and you are not interested in the web layer when those tests run.

The spring-boot-test-autoconfigure module includes a number of annotations that can be used to automatically configure such “slices”. Each of them works in a similar way, providing a @…​Test annotation that loads the ApplicationContext and one or more @AutoConfigure…​ annotations that can be used to customize auto-configuration settings.

Each slice restricts component scan to appropriate components and loads a very restricted set of auto-configuration classes. If you need to exclude one of them, most @…​Test annotations provide an excludeAutoConfiguration attribute. Alternatively, you can use @ImportAutoConfiguration#exclude.
Including multiple “slices” by using several @…​Test annotations in one test is not supported. If you need multiple “slices”, pick one of the @…​Test annotations and include the @AutoConfigure…​ annotations of the other “slices” by hand.
It is also possible to use the @AutoConfigure…​ annotations with the standard @SpringBootTest annotation. You can use this combination if you are not interested in “slicing” your application but you want some of the auto-configured test beans.
Auto-configured JSON Tests

To test that object JSON serialization and deserialization is working as expected, you can use the @JsonTest annotation. @JsonTest auto-configures the available supported JSON mapper, which can be one of the following libraries:

  • Jackson ObjectMapper, any @JsonComponent beans and any Jackson Modules

  • Gson

  • Jsonb

A list of the auto-configurations that are enabled by @JsonTest can be found in the appendix.

If you need to configure elements of the auto-configuration, you can use the @AutoConfigureJsonTesters annotation.

Spring Boot includes AssertJ-based helpers that work with the JSONAssert and JsonPath libraries to check that JSON appears as expected. The JacksonTester, GsonTester, JsonbTester, and BasicJsonTester classes can be used for Jackson, Gson, Jsonb, and Strings respectively. Any helper fields on the test class can be @Autowired when using @JsonTest. The following example shows a test class for Jackson:

import org.junit.jupiter.api.Test;
import org.springframework.beans.factory.annotation.*;
import org.springframework.boot.test.autoconfigure.json.*;
import org.springframework.boot.test.context.*;
import org.springframework.boot.test.json.*;

import static org.assertj.core.api.Assertions.*;

@JsonTest
class MyJsonTests {

    @Autowired
    private JacksonTester<VehicleDetails> json;

    @Test
    void testSerialize() throws Exception {
        VehicleDetails details = new VehicleDetails("Honda", "Civic");
        // Assert against a `.json` file in the same package as the test
        assertThat(this.json.write(details)).isEqualToJson("expected.json");
        // Or use JSON path based assertions
        assertThat(this.json.write(details)).hasJsonPathStringValue("@.make");
        assertThat(this.json.write(details)).extractingJsonPathStringValue("@.make")
                .isEqualTo("Honda");
    }

    @Test
    public void testDeserialize() throws Exception {
        String content = "{\"make\":\"Ford\",\"model\":\"Focus\"}";
        assertThat(this.json.parse(content))
                .isEqualTo(new VehicleDetails("Ford", "Focus"));
        assertThat(this.json.parseObject(content).getMake()).isEqualTo("Ford");
    }

}
JSON helper classes can also be used directly in standard unit tests. To do so, call the initFields method of the helper in your @Before method if you do not use @JsonTest.

If you’re using Spring Boot’s AssertJ-based helpers to assert on a number value at a given JSON path, you might not be able to use isEqualTo depending on the type. Instead, you can use AssertJ’s satisfies to assert that the value matches the given condition. For instance, the following example asserts that the actual number is a float value close to 0.15 within an offset of 0.01.

assertThat(json.write(message))
    .extractingJsonPathNumberValue("@.test.numberValue")
    .satisfies((number) -> assertThat(number.floatValue()).isCloseTo(0.15f, within(0.01f)));
Auto-configured Spring MVC Tests

To test whether Spring MVC controllers are working as expected, use the @WebMvcTest annotation. @WebMvcTest auto-configures the Spring MVC infrastructure and limits scanned beans to @Controller, @ControllerAdvice, @JsonComponent, Converter, GenericConverter, Filter, WebMvcConfigurer, and HandlerMethodArgumentResolver. Regular @Component beans are not scanned when using this annotation.

A list of the auto-configuration settings that are enabled by @WebMvcTest can be found in the appendix.
If you need to register extra components, such as the Jackson Module, you can import additional configuration classes by using @Import on your test.

Often, @WebMvcTest is limited to a single controller and is used in combination with @MockBean to provide mock implementations for required collaborators.

@WebMvcTest also auto-configures MockMvc. Mock MVC offers a powerful way to quickly test MVC controllers without needing to start a full HTTP server.

You can also auto-configure MockMvc in a non-@WebMvcTest (such as @SpringBootTest) by annotating it with @AutoConfigureMockMvc. The following example uses MockMvc:
import org.junit.jupiter.api.*;
import org.springframework.beans.factory.annotation.*;
import org.springframework.boot.test.autoconfigure.web.servlet.*;
import org.springframework.boot.test.mock.mockito.*;

import static org.assertj.core.api.Assertions.*;
import static org.mockito.BDDMockito.*;
import static org.springframework.test.web.servlet.request.MockMvcRequestBuilders.*;
import static org.springframework.test.web.servlet.result.MockMvcResultMatchers.*;

@WebMvcTest(UserVehicleController.class)
class MyControllerTests {

    @Autowired
    private MockMvc mvc;

    @MockBean
    private UserVehicleService userVehicleService;

    @Test
    void testExample() throws Exception {
        given(this.userVehicleService.getVehicleDetails("sboot"))
                .willReturn(new VehicleDetails("Honda", "Civic"));
        this.mvc.perform(get("/sboot/vehicle").accept(MediaType.TEXT_PLAIN))
                .andExpect(status().isOk()).andExpect(content().string("Honda Civic"));
    }

}
If you need to configure elements of the auto-configuration (for example, when servlet filters should be applied) you can use attributes in the @AutoConfigureMockMvc annotation.

If you use HtmlUnit or Selenium, auto-configuration also provides an HTMLUnit WebClient bean and/or a WebDriver bean. The following example uses HtmlUnit:

import com.gargoylesoftware.htmlunit.*;
import org.junit.jupiter.api.*;
import org.springframework.beans.factory.annotation.*;
import org.springframework.boot.test.autoconfigure.web.servlet.*;
import org.springframework.boot.test.mock.mockito.*;

import static org.assertj.core.api.Assertions.*;
import static org.mockito.BDDMockito.*;

@WebMvcTest(UserVehicleController.class)
class MyHtmlUnitTests {

    @Autowired
    private WebClient webClient;

    @MockBean
    private UserVehicleService userVehicleService;

    @Test
    void testExample() throws Exception {
        given(this.userVehicleService.getVehicleDetails("sboot"))
                .willReturn(new VehicleDetails("Honda", "Civic"));
        HtmlPage page = this.webClient.getPage("/sboot/vehicle.html");
        assertThat(page.getBody().getTextContent()).isEqualTo("Honda Civic");
    }

}
By default, Spring Boot puts WebDriver beans in a special “scope” to ensure that the driver exits after each test and that a new instance is injected. If you do not want this behavior, you can add @Scope("singleton") to your WebDriver @Bean definition.
The webDriver scope created by Spring Boot will replace any user defined scope of the same name. If you define your own webDriver scope you may find it stops working when you use @WebMvcTest.

If you have Spring Security on the classpath, @WebMvcTest will also scan WebSecurityConfigurer beans. Instead of disabling security completely for such tests, you can use Spring Security’s test support. More details on how to use Spring Security’s MockMvc support can be found in this Testing With Spring Security how-to section.

Sometimes writing Spring MVC tests is not enough; Spring Boot can help you run full end-to-end tests with an actual server.
Auto-configured Spring WebFlux Tests

To test that Spring WebFlux controllers are working as expected, you can use the @WebFluxTest annotation. @WebFluxTest auto-configures the Spring WebFlux infrastructure and limits scanned beans to @Controller, @ControllerAdvice, @JsonComponent, Converter, GenericConverter, and WebFluxConfigurer. Regular @Component beans are not scanned when the @WebFluxTest annotation is used.

A list of the auto-configurations that are enabled by @WebFluxTest can be found in the appendix.
If you need to register extra components, such as Jackson Module, you can import additional configuration classes using @Import on your test.

Often, @WebFluxTest is limited to a single controller and used in combination with the @MockBean annotation to provide mock implementations for required collaborators.

@WebFluxTest also auto-configures WebTestClient, which offers a powerful way to quickly test WebFlux controllers without needing to start a full HTTP server.

You can also auto-configure WebTestClient in a non-@WebFluxTest (such as @SpringBootTest) by annotating it with @AutoConfigureWebTestClient. The following example shows a class that uses both @WebFluxTest and a WebTestClient:
import org.junit.jupiter.api.Test;

import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.boot.test.autoconfigure.web.reactive.WebFluxTest;
import org.springframework.http.MediaType;
import org.springframework.test.web.reactive.server.WebTestClient;

@WebFluxTest(UserVehicleController.class)
class MyControllerTests {

    @Autowired
    private WebTestClient webClient;

    @MockBean
    private UserVehicleService userVehicleService;

    @Test
    void testExample() throws Exception {
        given(this.userVehicleService.getVehicleDetails("sboot"))
                .willReturn(new VehicleDetails("Honda", "Civic"));
        this.webClient.get().uri("/sboot/vehicle").accept(MediaType.TEXT_PLAIN)
                .exchange()
                .expectStatus().isOk()
                .expectBody(String.class).isEqualTo("Honda Civic");
    }

}
This setup is only supported by WebFlux applications as using WebTestClient in a mocked web application only works with WebFlux at the moment.
@WebFluxTest cannot detect routes registered via the functional web framework. For testing RouterFunction beans in the context, consider importing your RouterFunction yourself via @Import or using @SpringBootTest.
@WebFluxTest cannot detect custom security configuration registered via a @Bean of type SecurityWebFilterChain. To include that in your test, you will need to import the configuration that registers the bean via @Import or use @SpringBootTest.
Sometimes writing Spring WebFlux tests is not enough; Spring Boot can help you run full end-to-end tests with an actual server.
Auto-configured Data JPA Tests

You can use the @DataJpaTest annotation to test JPA applications. By default, it scans for @Entity classes and configures Spring Data JPA repositories. If an embedded database is available on the classpath, it configures one as well. Regular @Component beans are not loaded into the ApplicationContext.

A list of the auto-configuration settings that are enabled by @DataJpaTest can be found in the appendix.

By default, data JPA tests are transactional and roll back at the end of each test. See the relevant section in the Spring Framework Reference Documentation for more details. If that is not what you want, you can disable transaction management for a test or for the whole class as follows:

import org.junit.jupiter.api.Test;
import org.springframework.boot.test.autoconfigure.orm.jpa.DataJpaTest;
import org.springframework.transaction.annotation.Propagation;
import org.springframework.transaction.annotation.Transactional;

@DataJpaTest
@Transactional(propagation = Propagation.NOT_SUPPORTED)
class ExampleNonTransactionalTests {

}

Data JPA tests may also inject a TestEntityManager bean, which provides an alternative to the standard JPA EntityManager that is specifically designed for tests. If you want to use TestEntityManager outside of @DataJpaTest instances, you can also use the @AutoConfigureTestEntityManager annotation. A JdbcTemplate is also available if you need that. The following example shows the @DataJpaTest annotation in use:

import org.junit.jupiter.api.Test;
import org.springframework.boot.test.autoconfigure.orm.jpa.*;

import static org.assertj.core.api.Assertions.*;

@DataJpaTest
class ExampleRepositoryTests {

    @Autowired
    private TestEntityManager entityManager;

    @Autowired
    private UserRepository repository;

    @Test
    void testExample() throws Exception {
        this.entityManager.persist(new User("sboot", "1234"));
        User user = this.repository.findByUsername("sboot");
        assertThat(user.getUsername()).isEqualTo("sboot");
        assertThat(user.getVin()).isEqualTo("1234");
    }

}

In-memory embedded databases generally work well for tests, since they are fast and do not require any installation. If, however, you prefer to run tests against a real database you can use the @AutoConfigureTestDatabase annotation, as shown in the following example:

@RunWith(SpringRunner.class)
@DataJpaTest
@AutoConfigureTestDatabase(replace=Replace.NONE)
public class ExampleRepositoryTests {

    // ...

}
Auto-configured JDBC Tests

@JdbcTest is similar to @DataJpaTest but is for tests that only require a DataSource and do not use Spring Data JDBC. By default, it configures an in-memory embedded database and a JdbcTemplate. Regular @Component beans are not loaded into the ApplicationContext.

A list of the auto-configurations that are enabled by @JdbcTest can be found in the appendix.

By default, JDBC tests are transactional and roll back at the end of each test. See the relevant section in the Spring Framework Reference Documentation for more details. If that is not what you want, you can disable transaction management for a test or for the whole class, as follows:

import org.junit.jupiter.api.Test;
import org.springframework.boot.test.autoconfigure.jdbc.JdbcTest;
import org.springframework.transaction.annotation.Propagation;
import org.springframework.transaction.annotation.Transactional;

@JdbcTest
@Transactional(propagation = Propagation.NOT_SUPPORTED)
class ExampleNonTransactionalTests {

}

If you prefer your test to run against a real database, you can use the @AutoConfigureTestDatabase annotation in the same way as for DataJpaTest. (See "Auto-configured Data JPA Tests".)

Auto-configured Data JDBC Tests

@DataJdbcTest is similar to @JdbcTest but is for tests that use Spring Data JDBC repositories. By default, it configures an in-memory embedded database, a JdbcTemplate, and Spring Data JDBC repositories. Regular @Component beans are not loaded into the ApplicationContext.

A list of the auto-configurations that are enabled by @DataJdbcTest can be found in the appendix.

By default, Data JDBC tests are transactional and roll back at the end of each test. See the relevant section in the Spring Framework Reference Documentation for more details. If that is not what you want, you can disable transaction management for a test or for the whole test class as shown in the JDBC example.

If you prefer your test to run against a real database, you can use the @AutoConfigureTestDatabase annotation in the same way as for DataJpaTest. (See "Auto-configured Data JPA Tests".)

Auto-configured jOOQ Tests

You can use @JooqTest in a similar fashion as @JdbcTest but for jOOQ-related tests. As jOOQ relies heavily on a Java-based schema that corresponds with the database schema, the existing DataSource is used. If you want to replace it with an in-memory database, you can use @AutoConfigureTestDatabase to override those settings. (For more about using jOOQ with Spring Boot, see "Using jOOQ", earlier in this chapter.) Regular @Component beans are not loaded into the ApplicationContext.

A list of the auto-configurations that are enabled by @JooqTest can be found in the appendix.

@JooqTest configures a DSLContext. Regular @Component beans are not loaded into the ApplicationContext. The following example shows the @JooqTest annotation in use:

import org.jooq.DSLContext;
import org.junit.jupiter.api.Test;
import org.springframework.boot.test.autoconfigure.jooq.JooqTest;

@JooqTest
class ExampleJooqTests {

    @Autowired
    private DSLContext dslContext;
}

JOOQ tests are transactional and roll back at the end of each test by default. If that is not what you want, you can disable transaction management for a test or for the whole test class as shown in the JDBC example.

Auto-configured Data MongoDB Tests

You can use @DataMongoTest to test MongoDB applications. By default, it configures an in-memory embedded MongoDB (if available), configures a MongoTemplate, scans for @Document classes, and configures Spring Data MongoDB repositories. Regular @Component beans are not loaded into the ApplicationContext. (For more about using MongoDB with Spring Boot, see "MongoDB", earlier in this chapter.)

A list of the auto-configuration settings that are enabled by @DataMongoTest can be found in the appendix.

The following class shows the @DataMongoTest annotation in use:

import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.boot.test.autoconfigure.data.mongo.DataMongoTest;
import org.springframework.data.mongodb.core.MongoTemplate;

@DataMongoTest
public class ExampleDataMongoTests {

    @Autowired
    private MongoTemplate mongoTemplate;

    //
}

In-memory embedded MongoDB generally works well for tests, since it is fast and does not require any developer installation. If, however, you prefer to run tests against a real MongoDB server, you should exclude the embedded MongoDB auto-configuration, as shown in the following example:

import org.springframework.boot.autoconfigure.mongo.embedded.EmbeddedMongoAutoConfiguration;
import org.springframework.boot.test.autoconfigure.data.mongo.DataMongoTest;

@DataMongoTest(excludeAutoConfiguration = EmbeddedMongoAutoConfiguration.class)
public class ExampleDataMongoNonEmbeddedTests {

}
Auto-configured Data Neo4j Tests

You can use @DataNeo4jTest to test Neo4j applications. By default, it uses an in-memory embedded Neo4j (if the embedded driver is available), scans for @NodeEntity classes, and configures Spring Data Neo4j repositories. Regular @Component beans are not loaded into the ApplicationContext. (For more about using Neo4J with Spring Boot, see "Neo4j", earlier in this chapter.)

A list of the auto-configuration settings that are enabled by @DataNeo4jTest can be found in the appendix.

The following example shows a typical setup for using Neo4J tests in Spring Boot:

import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.boot.test.autoconfigure.data.neo4j.DataNeo4jTest;

@DataNeo4jTest
public class ExampleDataNeo4jTests {

    @Autowired
    private YourRepository repository;

    //
}

By default, Data Neo4j tests are transactional and roll back at the end of each test. See the relevant section in the Spring Framework Reference Documentation for more details. If that is not what you want, you can disable transaction management for a test or for the whole class, as follows:

import org.springframework.boot.test.autoconfigure.data.neo4j.DataNeo4jTest;
import org.springframework.transaction.annotation.Propagation;
import org.springframework.transaction.annotation.Transactional;

@DataNeo4jTest
@Transactional(propagation = Propagation.NOT_SUPPORTED)
public class ExampleNonTransactionalTests {

}
Auto-configured Data Redis Tests

You can use @DataRedisTest to test Redis applications. By default, it scans for @RedisHash classes and configures Spring Data Redis repositories. Regular @Component beans are not loaded into the ApplicationContext. (For more about using Redis with Spring Boot, see "Redis", earlier in this chapter.)

A list of the auto-configuration settings that are enabled by @DataRedisTest can be found in the appendix.

The following example shows the @DataRedisTest annotation in use:

import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.boot.test.autoconfigure.data.redis.DataRedisTest;

@DataRedisTest
public class ExampleDataRedisTests {

    @Autowired
    private YourRepository repository;

    //
}
Auto-configured Data LDAP Tests

You can use @DataLdapTest to test LDAP applications. By default, it configures an in-memory embedded LDAP (if available), configures an LdapTemplate, scans for @Entry classes, and configures Spring Data LDAP repositories. Regular @Component beans are not loaded into the ApplicationContext. (For more about using LDAP with Spring Boot, see "LDAP", earlier in this chapter.)

A list of the auto-configuration settings that are enabled by @DataLdapTest can be found in the appendix.

The following example shows the @DataLdapTest annotation in use:

import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.boot.test.autoconfigure.data.ldap.DataLdapTest;
import org.springframework.ldap.core.LdapTemplate;

@DataLdapTest
public class ExampleDataLdapTests {

    @Autowired
    private LdapTemplate ldapTemplate;

    //
}

In-memory embedded LDAP generally works well for tests, since it is fast and does not require any developer installation. If, however, you prefer to run tests against a real LDAP server, you should exclude the embedded LDAP auto-configuration, as shown in the following example:

import org.springframework.boot.autoconfigure.ldap.embedded.EmbeddedLdapAutoConfiguration;
import org.springframework.boot.test.autoconfigure.data.ldap.DataLdapTest;

@DataLdapTest(excludeAutoConfiguration = EmbeddedLdapAutoConfiguration.class)
public class ExampleDataLdapNonEmbeddedTests {

}
Auto-configured REST Clients

You can use the @RestClientTest annotation to test REST clients. By default, it auto-configures Jackson, GSON, and Jsonb support, configures a RestTemplateBuilder, and adds support for MockRestServiceServer. Regular @Component beans are not loaded into the ApplicationContext.

A list of the auto-configuration settings that are enabled by @RestClientTest can be found in the appendix.

The specific beans that you want to test should be specified by using the value or components attribute of @RestClientTest, as shown in the following example:

@RestClientTest(RemoteVehicleDetailsService.class)
public class ExampleRestClientTest {

    @Autowired
    private RemoteVehicleDetailsService service;

    @Autowired
    private MockRestServiceServer server;

    @Test
    public void getVehicleDetailsWhenResultIsSuccessShouldReturnDetails()
            throws Exception {
        this.server.expect(requestTo("/greet/details"))
                .andRespond(withSuccess("hello", MediaType.TEXT_PLAIN));
        String greeting = this.service.callRestService();
        assertThat(greeting).isEqualTo("hello");
    }

}
Auto-configured Spring REST Docs Tests

You can use the @AutoConfigureRestDocs annotation to use Spring REST Docs in your tests with Mock MVC, REST Assured, or WebTestClient. It removes the need for the JUnit extension in Spring REST Docs.

@AutoConfigureRestDocs can be used to override the default output directory (target/generated-snippets if you are using Maven or build/generated-snippets if you are using Gradle). It can also be used to configure the host, scheme, and port that appears in any documented URIs.

Auto-configured Spring REST Docs Tests with Mock MVC

@AutoConfigureRestDocs customizes the MockMvc bean to use Spring REST Docs. You can inject it by using @Autowired and use it in your tests as you normally would when using Mock MVC and Spring REST Docs, as shown in the following example:

import org.junit.jupiter.api.Test;

import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.boot.test.autoconfigure.web.servlet.WebMvcTest;
import org.springframework.http.MediaType;
import org.springframework.test.web.servlet.MockMvc;

import static org.springframework.restdocs.mockmvc.MockMvcRestDocumentation.document;
import static org.springframework.test.web.servlet.request.MockMvcRequestBuilders.get;
import static org.springframework.test.web.servlet.result.MockMvcResultMatchers.*;

@WebMvcTest(UserController.class)
@AutoConfigureRestDocs
class UserDocumentationTests {

    @Autowired
    private MockMvc mvc;

    @Test
    void listUsers() throws Exception {
        this.mvc.perform(get("/users").accept(MediaType.TEXT_PLAIN))
                .andExpect(status().isOk())
                .andDo(document("list-users"));
    }

}

If you require more control over Spring REST Docs configuration than offered by the attributes of @AutoConfigureRestDocs, you can use a RestDocsMockMvcConfigurationCustomizer bean, as shown in the following example:

@TestConfiguration
static class CustomizationConfiguration
        implements RestDocsMockMvcConfigurationCustomizer {

    @Override
    public void customize(MockMvcRestDocumentationConfigurer configurer) {
        configurer.snippets().withTemplateFormat(TemplateFormats.markdown());
    }

}

If you want to make use of Spring REST Docs support for a parameterized output directory, you can create a RestDocumentationResultHandler bean. The auto-configuration calls alwaysDo with this result handler, thereby causing each MockMvc call to automatically generate the default snippets. The following example shows a RestDocumentationResultHandler being defined:

@TestConfiguration
static class ResultHandlerConfiguration {

    @Bean
    public RestDocumentationResultHandler restDocumentation() {
        return MockMvcRestDocumentation.document("{method-name}");
    }

}
Auto-configured Spring REST Docs Tests with WebTestClient

@AutoConfigureRestDocs can also be used with WebTestClient. You can inject it by using @Autowired and use it in your tests as you normally would when using @WebFluxTest and Spring REST Docs, as shown in the following example:

import org.junit.jupiter.api.Test;
import org.junit.jupiter.api.extension.ExtendWith;

import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.boot.test.autoconfigure.restdocs.AutoConfigureRestDocs;
import org.springframework.boot.test.autoconfigure.web.reactive.WebFluxTest;
import org.springframework.test.context.junit.jupiter.SpringExtension;
import org.springframework.test.web.reactive.server.WebTestClient;

import static org.springframework.restdocs.webtestclient.WebTestClientRestDocumentation.document;

@ExtendWith(SpringExtension.class)
@WebFluxTest
@AutoConfigureRestDocs
public class UsersDocumentationTests {

    @Autowired
    private WebTestClient webTestClient;

    @Test
    void listUsers() {
        this.webTestClient.get().uri("/").exchange().expectStatus().isOk().expectBody()
                .consumeWith(document("list-users"));
    }

}

If you require more control over Spring REST Docs configuration than offered by the attributes of @AutoConfigureRestDocs, you can use a RestDocsWebTestClientConfigurationCustomizer bean, as shown in the following example:

@TestConfiguration
public static class CustomizationConfiguration implements RestDocsWebTestClientConfigurationCustomizer {

    @Override
    public void customize(WebTestClientRestDocumentationConfigurer configurer) {
        configurer.snippets().withEncoding("UTF-8");
    }

}
Auto-configured Spring REST Docs Tests with REST Assured

@AutoConfigureRestDocs makes a RequestSpecification bean, preconfigured to use Spring REST Docs, available to your tests. You can inject it by using @Autowired and use it in your tests as you normally would when using REST Assured and Spring REST Docs, as shown in the following example:

import io.restassured.specification.RequestSpecification;
import org.junit.jupiter.api.Test;

import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.boot.test.autoconfigure.restdocs.AutoConfigureRestDocs;
import org.springframework.boot.test.context.SpringBootTest;
import org.springframework.boot.test.context.SpringBootTest.WebEnvironment;
import org.springframework.boot.web.server.LocalServerPort;

import static io.restassured.RestAssured.given;
import static org.hamcrest.Matchers.is;
import static org.springframework.restdocs.restassured3.RestAssuredRestDocumentation.document;

@SpringBootTest(webEnvironment = WebEnvironment.RANDOM_PORT)
@AutoConfigureRestDocs
class UserDocumentationTests {

    @Test
    void listUsers(@Autowired RequestSpecification documentationSpec, @LocalServerPort int port) {
        given(documentationSpec).filter(document("list-users")).when().port(port).get("/").then().assertThat()
                .statusCode(is(200));
    }

}

If you require more control over Spring REST Docs configuration than offered by the attributes of @AutoConfigureRestDocs, a RestDocsRestAssuredConfigurationCustomizer bean can be used, as shown in the following example:

@TestConfiguration
public static class CustomizationConfiguration implements RestDocsRestAssuredConfigurationCustomizer {

    @Override
    public void customize(RestAssuredRestDocumentationConfigurer configurer) {
        configurer.snippets().withTemplateFormat(TemplateFormats.markdown());
    }

}
Additional Auto-configuration and Slicing

Each slice provides one or more @AutoConfigure…​ annotations that namely defines the auto-configurations that should be included as part of a slice. Additional auto-configurations can be added by creating a custom @AutoConfigure…​ annotation or simply by adding @ImportAutoConfiguration to the test as shown in the following example:

@RunWith(SpringRunner.class)
@JdbcTest
@ImportAutoConfiguration(IntegrationAutoConfiguration.class)
public class ExampleJdbcTests {

}
Make sure to not use the regular @Import annotation to import auto-configurations as they are handled in a specific way by Spring Boot.
User Configuration and Slicing

If you structure your code in a sensible way, your @SpringBootApplication class is used by default as the configuration of your tests.

It then becomes important not to litter the application’s main class with configuration settings that are specific to a particular area of its functionality.

Assume that you are using Spring Batch and you rely on the auto-configuration for it. You could define your @SpringBootApplication as follows:

@SpringBootApplication
@EnableBatchProcessing
public class SampleApplication { ... }

Because this class is the source configuration for the test, any slice test actually tries to start Spring Batch, which is definitely not what you want to do. A recommended approach is to move that area-specific configuration to a separate @Configuration class at the same level as your application, as shown in the following example:

@Configuration(proxyBeanMethods = false)
@EnableBatchProcessing
public class BatchConfiguration { ... }
Depending on the complexity of your application, you may either have a single @Configuration class for your customizations or one class per domain area. The latter approach lets you enable it in one of your tests, if necessary, with the @Import annotation.

Test slices exclude @Configuration classes from scanning. For example, for a @WebMvcTest, the following configuration will not include the given WebMvcConfigurer bean in the application context loaded by the test slice:

@Configuration
public class WebConfiguration {
    @Bean
    public WebMvcConfigurer testConfigurer() {
        return new WebMvcConfigurer() {
            ...
        };
    }
}

The configuration below will, however, cause the custom WebMvcConfigurer to be loaded by the test slice.

@Component
public class TestWebMvcConfigurer implements WebMvcConfigurer {
    ...
}

Another source of confusion is classpath scanning. Assume that, while you structured your code in a sensible way, you need to scan an additional package. Your application may resemble the following code:

@SpringBootApplication
@ComponentScan({ "com.example.app", "org.acme.another" })
public class SampleApplication { ... }

Doing so effectively overrides the default component scan directive with the side effect of scanning those two packages regardless of the slice that you chose. For instance, a @DataJpaTest seems to suddenly scan components and user configurations of your application. Again, moving the custom directive to a separate class is a good way to fix this issue.

If this is not an option for you, you can create a @SpringBootConfiguration somewhere in the hierarchy of your test so that it is used instead. Alternatively, you can specify a source for your test, which disables the behavior of finding a default one.
Using Spock to Test Spring Boot Applications

If you wish to use Spock to test a Spring Boot application, you should add a dependency on Spock’s spock-spring module to your application’s build. spock-spring integrates Spring’s test framework into Spock. It is recommended that you use Spock 1.2 or later to benefit from a number of improvements to Spock’s Spring Framework and Spring Boot integration. See the documentation for Spock’s Spring module for further details.

4.25.4. Test Utilities

A few test utility classes that are generally useful when testing your application are packaged as part of spring-boot.

ConfigFileApplicationContextInitializer

ConfigFileApplicationContextInitializer is an ApplicationContextInitializer that you can apply to your tests to load Spring Boot application.properties files. You can use it when you do not need the full set of features provided by @SpringBootTest, as shown in the following example:

@ContextConfiguration(classes = Config.class,
    initializers = ConfigFileApplicationContextInitializer.class)
Using ConfigFileApplicationContextInitializer alone does not provide support for @Value("${…​}") injection. Its only job is to ensure that application.properties files are loaded into Spring’s Environment. For @Value support, you need to either additionally configure a PropertySourcesPlaceholderConfigurer or use @SpringBootTest, which auto-configures one for you.
TestPropertyValues

TestPropertyValues lets you quickly add properties to a ConfigurableEnvironment or ConfigurableApplicationContext. You can call it with key=value strings, as follows:

TestPropertyValues.of("org=Spring", "name=Boot").applyTo(env);
OutputCapture

OutputCapture is a JUnit Extension that you can use to capture System.out and System.err output. To use with {@link ExtendWith @ExtendWith}, you can inject CapturedOutput as an argument to your test class constructor or test method as follows:

import org.junit.jupiter.api.Test;
import org.junit.jupiter.api.extension.ExtendWith;

import org.springframework.boot.test.system.CapturedOutput;
import org.springframework.boot.test.system.OutputCaptureExtension;

import static org.assertj.core.api.Assertions.assertThat;

@ExtendWith(OutputCaptureExtension.class)
class MyTest {

    @Test
    void testName(CapturedOutput output) {
        System.out.println("Hello World!");
        assertThat(output).contains("World"));
    }

}
TestRestTemplate
Spring Framework 5.0 provides a new WebTestClient that works for WebFlux integration tests and both WebFlux and MVC end-to-end testing. It provides a fluent API for assertions, unlike TestRestTemplate.

TestRestTemplate is a convenience alternative to Spring’s RestTemplate that is useful in integration tests. You can get a vanilla template or one that sends Basic HTTP authentication (with a username and password). In either case, the template behaves in a test-friendly way by not throwing exceptions on server-side errors. It is recommended, but not mandatory, to use the Apache HTTP Client (version 4.3.2 or better). If you have that on your classpath, the TestRestTemplate responds by configuring the client appropriately. If you do use Apache’s HTTP client, some additional test-friendly features are enabled:

  • Redirects are not followed (so you can assert the response location).

  • Cookies are ignored (so the template is stateless).

TestRestTemplate can be instantiated directly in your integration tests, as shown in the following example:

public class MyTest {

    private TestRestTemplate template = new TestRestTemplate();

    @Test
    public void testRequest() throws Exception {
        HttpHeaders headers = this.template.getForEntity(
                "https://myhost.example.com/example", String.class).getHeaders();
        assertThat(headers.getLocation()).hasHost("other.example.com");
    }

}

Alternatively, if you use the @SpringBootTest annotation with WebEnvironment.RANDOM_PORT or WebEnvironment.DEFINED_PORT, you can inject a fully configured TestRestTemplate and start using it. If necessary, additional customizations can be applied through the RestTemplateBuilder bean. Any URLs that do not specify a host and port automatically connect to the embedded server, as shown in the following example:

@ExtendWith(SpringExtension.class)
@SpringBootTest(webEnvironment = WebEnvironment.RANDOM_PORT)
class SampleWebClientTests {

    @Autowired
    private TestRestTemplate template;

    @Test
    void testRequest() {
        HttpHeaders headers = this.template.getForEntity("/example", String.class).getHeaders();
        assertThat(headers.getLocation()).hasHost("other.example.com");
    }

    @TestConfiguration
    static class Config {

        @Bean
        public RestTemplateBuilder restTemplateBuilder() {
            return new RestTemplateBuilder().setConnectTimeout(Duration.ofSeconds(1))
                    .setReadTimeout(Duration.ofSeconds(1));
        }

    }

}

4.26. WebSockets

Spring Boot provides WebSockets auto-configuration for embedded Tomcat, Jetty, and Undertow. If you deploy a war file to a standalone container, Spring Boot assumes that the container is responsible for the configuration of its WebSocket support.

Spring Framework provides rich WebSocket support for MVC web applications that can be easily accessed through the spring-boot-starter-websocket module.

WebSocket support is also available for reactive web applications and requires to include the WebSocket API alongside spring-boot-starter-webflux:

<dependency>
    <groupId>javax.websocket</groupId>
    <artifactId>javax.websocket-api</artifactId>
</dependency>

4.27. Web Services

Spring Boot provides Web Services auto-configuration so that all you must do is define your Endpoints.

The Spring Web Services features can be easily accessed with the spring-boot-starter-webservices module.

SimpleWsdl11Definition and SimpleXsdSchema beans can be automatically created for your WSDLs and XSDs respectively. To do so, configure their location, as shown in the following example:

spring.webservices.wsdl-locations=classpath:/wsdl

4.27.1. Calling Web Services with WebServiceTemplate

If you need to call remote Web services from your application, you can use the WebServiceTemplate class. Since WebServiceTemplate instances often need to be customized before being used, Spring Boot does not provide any single auto-configured WebServiceTemplate bean. It does, however, auto-configure a WebServiceTemplateBuilder, which can be used to create WebServiceTemplate instances when needed.

The following code shows a typical example:

@Service
public class MyService {

    private final WebServiceTemplate webServiceTemplate;

    public MyService(WebServiceTemplateBuilder webServiceTemplateBuilder) {
        this.webServiceTemplate = webServiceTemplateBuilder.build();
    }

    public DetailsResp someWsCall(DetailsReq detailsReq) {
         return (DetailsResp) this.webServiceTemplate.marshalSendAndReceive(detailsReq, new SoapActionCallback(ACTION));

    }

}

By default, WebServiceTemplateBuilder detects a suitable HTTP-based WebServiceMessageSender using the available HTTP client libraries on the classpath. You can also customize read and connection timeouts as follows:

@Bean
public WebServiceTemplate webServiceTemplate(WebServiceTemplateBuilder builder) {
    return builder.messageSenders(new HttpWebServiceMessageSenderBuilder()
            .setConnectTimeout(5000).setReadTimeout(2000).build()).build();
}

4.28. Creating Your Own Auto-configuration

If you work in a company that develops shared libraries, or if you work on an open-source or commercial library, you might want to develop your own auto-configuration. Auto-configuration classes can be bundled in external jars and still be picked-up by Spring Boot.

Auto-configuration can be associated to a “starter” that provides the auto-configuration code as well as the typical libraries that you would use with it. We first cover what you need to know to build your own auto-configuration and then we move on to the typical steps required to create a custom starter.

A demo project is available to showcase how you can create a starter step-by-step.

4.28.1. Understanding Auto-configured Beans

Under the hood, auto-configuration is implemented with standard @Configuration classes. Additional @Conditional annotations are used to constrain when the auto-configuration should apply. Usually, auto-configuration classes use @ConditionalOnClass and @ConditionalOnMissingBean annotations. This ensures that auto-configuration applies only when relevant classes are found and when you have not declared your own @Configuration.

You can browse the source code of spring-boot-autoconfigure to see the @Configuration classes that Spring provides (see the META-INF/spring.factories file).

4.28.2. Locating Auto-configuration Candidates

Spring Boot checks for the presence of a META-INF/spring.factories file within your published jar. The file should list your configuration classes under the EnableAutoConfiguration key, as shown in the following example:

org.springframework.boot.autoconfigure.EnableAutoConfiguration=\
com.mycorp.libx.autoconfigure.LibXAutoConfiguration,\
com.mycorp.libx.autoconfigure.LibXWebAutoConfiguration

Auto-configurations must be loaded that way only. Make sure that they are defined in a specific package space and that they are never the target of component scanning. Furthermore, auto-configuration classes should not enable component scanning to find additional components. Specific @Imports should be used instead.

You can use the @AutoConfigureAfter or @AutoConfigureBefore annotations if your configuration needs to be applied in a specific order. For example, if you provide web-specific configuration, your class may need to be applied after WebMvcAutoConfiguration.

If you want to order certain auto-configurations that should not have any direct knowledge of each other, you can also use @AutoConfigureOrder. That annotation has the same semantic as the regular @Order annotation but provides a dedicated order for auto-configuration classes.

4.28.3. Condition Annotations

You almost always want to include one or more @Conditional annotations on your auto-configuration class. The @ConditionalOnMissingBean annotation is one common example that is used to allow developers to override auto-configuration if they are not happy with your defaults.

Spring Boot includes a number of @Conditional annotations that you can reuse in your own code by annotating @Configuration classes or individual @Bean methods. These annotations include:

Class Conditions

The @ConditionalOnClass and @ConditionalOnMissingClass annotations let @Configuration classes be included based on the presence or absence of specific classes. Due to the fact that annotation metadata is parsed by using ASM, you can use the value attribute to refer to the real class, even though that class might not actually appear on the running application classpath. You can also use the name attribute if you prefer to specify the class name by using a String value.

This mechanism does not apply the same way to @Bean methods where typically the return type is the target of the condition: before the condition on the method applies, the JVM will have loaded the class and potentially processed method references which will fail if the class is not present.

To handle this scenario, a separate @Configuration class can be used to isolate the condition, as shown in the following example:

@Configuration(proxyBeanMethods = false)
// Some conditions
public class MyAutoConfiguration {

    // Auto-configured beans

    @Configuration(proxyBeanMethods = false)
    @ConditionalOnClass(EmbeddedAcmeService.class)
    static class EmbeddedConfiguration {

        @Bean
        @ConditionalOnMissingBean
        public EmbeddedAcmeService embeddedAcmeService() { ... }

    }

}

If you use @ConditionalOnClass or @ConditionalOnMissingClass as a part of a meta-annotation to compose your own composed annotations, you must use name as referring to the class in such a case is not handled.

Bean Conditions

The @ConditionalOnBean and @ConditionalOnMissingBean annotations let a bean be included based on the presence or absence of specific beans. You can use the value attribute to specify beans by type or name to specify beans by name. The search attribute lets you limit the ApplicationContext hierarchy that should be considered when searching for beans.

When placed on a @Bean method, the target type defaults to the return type of the method, as shown in the following example:

@Configuration(proxyBeanMethods = false)
public class MyAutoConfiguration {

    @Bean
    @ConditionalOnMissingBean
    public MyService myService() { ... }

}

In the preceding example, the myService bean is going to be created if no bean of type MyService is already contained in the ApplicationContext.

You need to be very careful about the order in which bean definitions are added, as these conditions are evaluated based on what has been processed so far. For this reason, we recommend using only @ConditionalOnBean and @ConditionalOnMissingBean annotations on auto-configuration classes (since these are guaranteed to load after any user-defined bean definitions have been added).
@ConditionalOnBean and @ConditionalOnMissingBean do not prevent @Configuration classes from being created. The only difference between using these conditions at the class level and marking each contained @Bean method with the annotation is that the former prevents registration of the @Configuration class as a bean if the condition does not match.
Property Conditions

The @ConditionalOnProperty annotation lets configuration be included based on a Spring Environment property. Use the prefix and name attributes to specify the property that should be checked. By default, any property that exists and is not equal to false is matched. You can also create more advanced checks by using the havingValue and matchIfMissing attributes.

Resource Conditions

The @ConditionalOnResource annotation lets configuration be included only when a specific resource is present. Resources can be specified by using the usual Spring conventions, as shown in the following example: file:/home/user/test.dat.

Web Application Conditions

The @ConditionalOnWebApplication and @ConditionalOnNotWebApplication annotations let configuration be included depending on whether the application is a “web application”. A web application is any application that uses a Spring WebApplicationContext, defines a session scope, or has a StandardServletEnvironment.

SpEL Expression Conditions

The @ConditionalOnExpression annotation lets configuration be included based on the result of a SpEL expression.

4.28.4. Testing your Auto-configuration

An auto-configuration can be affected by many factors: user configuration (@Bean definition and Environment customization), condition evaluation (presence of a particular library), and others. Concretely, each test should create a well defined ApplicationContext that represents a combination of those customizations. ApplicationContextRunner provides a great way to achieve that.

ApplicationContextRunner is usually defined as a field of the test class to gather the base, common configuration. The following example makes sure that UserServiceAutoConfiguration is always invoked:

private final ApplicationContextRunner contextRunner = new ApplicationContextRunner()
        .withConfiguration(AutoConfigurations.of(UserServiceAutoConfiguration.class));
If multiple auto-configurations have to be defined, there is no need to order their declarations as they are invoked in the exact same order as when running the application.

Each test can use the runner to represent a particular use case. For instance, the sample below invokes a user configuration (UserConfiguration) and checks that the auto-configuration backs off properly. Invoking run provides a callback context that can be used with Assert4J.

@Test
void defaultServiceBacksOff() {
    this.contextRunner.withUserConfiguration(UserConfiguration.class).run((context) -> {
        assertThat(context).hasSingleBean(UserService.class);
        assertThat(context).getBean("myUserService").isSameAs(context.getBean(UserService.class));
    });
}

@Configuration(proxyBeanMethods = false)
static class UserConfiguration {

    @Bean
    public UserService myUserService() {
        return new UserService("mine");
    }

}

It is also possible to easily customize the Environment, as shown in the following example:

@Test
void serviceNameCanBeConfigured() {
    this.contextRunner.withPropertyValues("user.name=test123").run((context) -> {
        assertThat(context).hasSingleBean(UserService.class);
        assertThat(context.getBean(UserService.class).getName()).isEqualTo("test123");
    });
}

The runner can also be used to display the ConditionEvaluationReport. The report can be printed at INFO or DEBUG level. The following example shows how to use the ConditionEvaluationReportLoggingListener to print the report in auto-configuration tests.

@Test
public void autoConfigTest {
    ConditionEvaluationReportLoggingListener initializer = new ConditionEvaluationReportLoggingListener(
            LogLevel.INFO);
    ApplicationContextRunner contextRunner = new ApplicationContextRunner()
            .withInitializer(initializer).run((context) -> {
                    // Do something...
            });
}
Simulating a Web Context

If you need to test an auto-configuration that only operates in a Servlet or Reactive web application context, use the WebApplicationContextRunner or ReactiveWebApplicationContextRunner respectively.

Overriding the Classpath

It is also possible to test what happens when a particular class and/or package is not present at runtime. Spring Boot ships with a FilteredClassLoader that can easily be used by the runner. In the following example, we assert that if UserService is not present, the auto-configuration is properly disabled:

@Test
void serviceIsIgnoredIfLibraryIsNotPresent() {
    this.contextRunner.withClassLoader(new FilteredClassLoader(UserService.class))
            .run((context) -> assertThat(context).doesNotHaveBean("userService"));
}

4.28.5. Creating Your Own Starter

A full Spring Boot starter for a library may contain the following components:

  • The autoconfigure module that contains the auto-configuration code.

  • The starter module that provides a dependency to the autoconfigure module as well as the library and any additional dependencies that are typically useful. In a nutshell, adding the starter should provide everything needed to start using that library.

You may combine the auto-configuration code and the dependency management in a single module if you do not need to separate those two concerns.
Naming

You should make sure to provide a proper namespace for your starter. Do not start your module names with spring-boot, even if you use a different Maven groupId. We may offer official support for the thing you auto-configure in the future.

As a rule of thumb, you should name a combined module after the starter. For example, assume that you are creating a starter for "acme" and that you name the auto-configure module acme-spring-boot-autoconfigure and the starter acme-spring-boot-starter. If you only have one module that combines the two, name it acme-spring-boot-starter.

Also, if your starter provides configuration keys, use a unique namespace for them. In particular, do not include your keys in the namespaces that Spring Boot uses (such as server, management, spring, and so on). If you use the same namespace, we may modify these namespaces in the future in ways that break your modules.

Make sure to trigger meta-data generation so that IDE assistance is available for your keys as well. You may want to review the generated meta-data (META-INF/spring-configuration-metadata.json) to make sure your keys are properly documented.

autoconfigure Module

The autoconfigure module contains everything that is necessary to get started with the library. It may also contain configuration key definitions (such as @ConfigurationProperties) and any callback interface that can be used to further customize how the components are initialized.

You should mark the dependencies to the library as optional so that you can include the autoconfigure module in your projects more easily. If you do it that way, the library is not provided and, by default, Spring Boot backs off.

Spring Boot uses an annotation processor to collect the conditions on auto-configurations in a metadata file (META-INF/spring-autoconfigure-metadata.properties). If that file is present, it is used to eagerly filter auto-configurations that do not match, which will improve startup time. It is recommended to add the following dependency in a module that contains auto-configurations:

<dependency>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-autoconfigure-processor</artifactId>
    <optional>true</optional>
</dependency>

With Gradle 4.5 and earlier, the dependency should be declared in the compileOnly configuration, as shown in the following example:

dependencies {
    compileOnly "org.springframework.boot:spring-boot-autoconfigure-processor"
}

With Gradle 4.6 and later, the dependency should be declared in the annotationProcessor configuration, as shown in the following example:

dependencies {
    annotationProcessor "org.springframework.boot:spring-boot-autoconfigure-processor"
}
Starter Module

The starter is really an empty jar. Its only purpose is to provide the necessary dependencies to work with the library. You can think of it as an opinionated view of what is required to get started.

Do not make assumptions about the project in which your starter is added. If the library you are auto-configuring typically requires other starters, mention them as well. Providing a proper set of default dependencies may be hard if the number of optional dependencies is high, as you should avoid including dependencies that are unnecessary for a typical usage of the library. In other words, you should not include optional dependencies.

Either way, your starter must reference the core Spring Boot starter (spring-boot-starter) directly or indirectly (i.e. no need to add it if your starter relies on another starter). If a project is created with only your custom starter, Spring Boot’s core features will be honoured by the presence of the core starter.

4.29. Kotlin support

Kotlin is a statically-typed language targeting the JVM (and other platforms) which allows writing concise and elegant code while providing interoperability with existing libraries written in Java.

Spring Boot provides Kotlin support by leveraging the support in other Spring projects such as Spring Framework, Spring Data, and Reactor. See the Spring Framework Kotlin support documentation for more information.

The easiest way to start with Spring Boot and Kotlin is to follow this comprehensive tutorial. You can create new Kotlin projects via start.spring.io. Feel free to join the #spring channel of Kotlin Slack or ask a question with the spring and kotlin tags on Stack Overflow if you need support.

4.29.1. Requirements

Spring Boot supports Kotlin 1.3.x. To use Kotlin, org.jetbrains.kotlin:kotlin-stdlib and org.jetbrains.kotlin:kotlin-reflect must be present on the classpath. The kotlin-stdlib variants kotlin-stdlib-jdk7 and kotlin-stdlib-jdk8 can also be used.

Since Kotlin classes are final by default, you are likely to want to configure kotlin-spring plugin in order to automatically open Spring-annotated classes so that they can be proxied.

Jackson’s Kotlin module is required for serializing / deserializing JSON data in Kotlin. It is automatically registered when found on the classpath. A warning message is logged if Jackson and Kotlin are present but the Jackson Kotlin module is not.

These dependencies and plugins are provided by default if one bootstraps a Kotlin project on start.spring.io.

4.29.2. Null-safety

One of Kotlin’s key features is null-safety. It deals with null values at compile time rather than deferring the problem to runtime and encountering a NullPointerException. This helps to eliminate a common source of bugs without paying the cost of wrappers like Optional. Kotlin also allows using functional constructs with nullable values as described in this comprehensive guide to null-safety in Kotlin.

Although Java does not allow one to express null-safety in its type system, Spring Framework, Spring Data, and Reactor now provide null-safety of their API via tooling-friendly annotations. By default, types from Java APIs used in Kotlin are recognized as platform types for which null-checks are relaxed. Kotlin’s support for JSR 305 annotations combined with nullability annotations provide null-safety for the related Spring API in Kotlin.

The JSR 305 checks can be configured by adding the -Xjsr305 compiler flag with the following options: -Xjsr305={strict|warn|ignore}. The default behavior is the same as -Xjsr305=warn. The strict value is required to have null-safety taken in account in Kotlin types inferred from Spring API but should be used with the knowledge that Spring API nullability declaration could evolve even between minor releases and more checks may be added in the future).

Generic type arguments, varargs and array elements nullability are not yet supported. See SPR-15942 for up-to-date information. Also be aware that Spring Boot’s own API is not yet annotated.

4.29.3. Kotlin API

runApplication

Spring Boot provides an idiomatic way to run an application with runApplication<MyApplication>(*args) as shown in the following example:

import org.springframework.boot.autoconfigure.SpringBootApplication
import org.springframework.boot.runApplication

@SpringBootApplication
class MyApplication

fun main(args: Array<String>) {
    runApplication<MyApplication>(*args)
}

This is a drop-in replacement for SpringApplication.run(MyApplication::class.java, *args). It also allows customization of the application as shown in the following example:

runApplication<MyApplication>(*args) {
    setBannerMode(OFF)
}
Extensions

Kotlin extensions provide the ability to extend existing classes with additional functionality. The Spring Boot Kotlin API makes use of these extensions to add new Kotlin specific conveniences to existing APIs.

TestRestTemplate extensions, similar to those provided by Spring Framework for RestOperations in Spring Framework, are provided. Among other things, the extensions make it possible to take advantage of Kotlin reified type parameters.

4.29.4. Dependency management

In order to avoid mixing different versions of Kotlin dependencies on the classpath, Spring Boot imports the Kotlin BOM.

With Maven, the Kotlin version can be customized via the kotlin.version property and plugin management is provided for kotlin-maven-plugin. With Gradle, the Spring Boot plugin automatically aligns the kotlin.version with the version of the Kotlin plugin.

4.29.5. @ConfigurationProperties

@ConfigurationProperties supports classes with immutable val properties as shown in the following example:

@ConfigurationProperties("example.kotlin")
data class KotlinExampleProperties(
        val name: String,
        val description: String,
        val myService: MyService
)

data class MyService(
    val apiToken: String,
    val uri: URI
)
To generate your own metadata using the annotation processor, kapt should be configured with the spring-boot-configuration-processor dependency. Note that some features (such as detecting the default value or deperecated items) are not working due to limitations in the model kapt provides.

4.29.6. Testing

While it is possible to use JUnit 4 to test Kotlin code, JUnit 5 is recommended. JUnit 5 enables a test class to be instantiated once and reused for all of the class’s tests. This makes it possible to use @BeforeClass and @AfterClass annotations on non-static methods, which is a good fit for Kotlin.

JUnit 5 is the default and the vintage engine is provided for backward compatibility with JUnit 4. If you don’t use it, exclude org.junit.vintange:junit-vintage-engine and junit:junit. See the JUnit 5 documentation for more details. You also need to switch test instance lifecycle to "per-class".

To mock Kotlin classes, MockK is recommended. If you need the Mockk equivalent of the Mockito specific @MockBean and @SpyBean annotations, you can use SpringMockK which provides similar @MockkBean and @SpykBean annotations.

4.29.7. Resources

Examples

4.30. What to Read Next

If you want to learn more about any of the classes discussed in this section, you can check out the Spring Boot API documentation or you can browse the source code directly. If you have specific questions, take a look at the how-to section.

If you are comfortable with Spring Boot’s core features, you can continue on and read about production-ready features.

5. Spring Boot Actuator: Production-ready Features

5.1. Enabling Production-ready Features

The spring-boot-actuator module provides all of Spring Boot’s production-ready features. The simplest way to enable the features is to add a dependency to the spring-boot-starter-actuator ‘Starter’.

Definition of Actuator

An actuator is a manufacturing term that refers to a mechanical device for moving or controlling something. Actuators can generate a large amount of motion from a small change.

To add the actuator to a Maven based project, add the following ‘Starter’ dependency:

<dependencies>
    <dependency>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-starter-actuator</artifactId>
    </dependency>
</dependencies>

For Gradle, use the following declaration:

dependencies {
    compile("org.springframework.boot:spring-boot-starter-actuator")
}

5.2. Endpoints

Actuator endpoints let you monitor and interact with your application. Spring Boot includes a number of built-in endpoints and lets you add your own. For example, the health endpoint provides basic application health information.

Each individual endpoint can be enabled or disabled. This controls whether or not the endpoint is created and its bean exists in the application context. To be remotely accessible an endpoint also has to be exposed via JMX or HTTP. Most applications choose HTTP, where the ID of the endpoint along with a prefix of /actuator is mapped to a URL. For example, by default, the health endpoint is mapped to /actuator/health.

The following technology-agnostic endpoints are available:

ID Description Enabled by default

auditevents

Exposes audit events information for the current application.

Yes

beans

Displays a complete list of all the Spring beans in your application.

Yes

caches

Exposes available caches.

Yes

conditions

Shows the conditions that were evaluated on configuration and auto-configuration classes and the reasons why they did or did not match.

Yes

configprops

Displays a collated list of all @ConfigurationProperties.

Yes

env

Exposes properties from Spring’s ConfigurableEnvironment.

Yes

flyway

Shows any Flyway database migrations that have been applied.

Yes

health

Shows application health information.

Yes

httptrace

Displays HTTP trace information (by default, the last 100 HTTP request-response exchanges).

Yes

info

Displays arbitrary application info.

Yes

integrationgraph

Shows the Spring Integration graph.

Yes

loggers

Shows and modifies the configuration of loggers in the application.

Yes

liquibase

Shows any Liquibase database migrations that have been applied.

Yes

metrics

Shows ‘metrics’ information for the current application.

Yes

mappings

Displays a collated list of all @RequestMapping paths.

Yes

scheduledtasks

Displays the scheduled tasks in your application.

Yes

sessions

Allows retrieval and deletion of user sessions from a Spring Session-backed session store. Not available when using Spring Session’s support for reactive web applications.

Yes

shutdown

Lets the application be gracefully shutdown.

No

threaddump

Performs a thread dump.

Yes

If your application is a web application (Spring MVC, Spring WebFlux, or Jersey), you can use the following additional endpoints:

ID Description Enabled by default

heapdump

Returns an hprof heap dump file.

Yes

jolokia

Exposes JMX beans over HTTP (when Jolokia is on the classpath, not available for WebFlux).

Yes

logfile

Returns the contents of the logfile (if logging.file.name or logging.file.path properties have been set). Supports the use of the HTTP Range header to retrieve part of the log file’s content.

Yes

prometheus

Exposes metrics in a format that can be scraped by a Prometheus server.

Yes

To learn more about the Actuator’s endpoints and their request and response formats, please refer to the separate API documentation (HTML or PDF).

5.2.1. Enabling Endpoints

By default, all endpoints except for shutdown are enabled. To configure the enablement of an endpoint, use its management.endpoint.<id>.enabled property. The following example enables the shutdown endpoint:

management.endpoint.shutdown.enabled=true

If you prefer endpoint enablement to be opt-in rather than opt-out, set the management.endpoints.enabled-by-default property to false and use individual endpoint enabled properties to opt back in. The following example enables the info endpoint and disables all other endpoints:

management.endpoints.enabled-by-default=false
management.endpoint.info.enabled=true
Disabled endpoints are removed entirely from the application context. If you want to change only the technologies over which an endpoint is exposed, use the include and exclude properties instead.

5.2.2. Exposing Endpoints

Since Endpoints may contain sensitive information, careful consideration should be given about when to expose them. The following table shows the default exposure for the built-in endpoints:

ID JMX Web

auditevents

Yes

No

beans

Yes

No

caches

Yes

No

conditions

Yes

No

configprops

Yes

No

env

Yes

No

flyway

Yes

No

health

Yes

Yes

heapdump

N/A

No

httptrace

Yes

No

info

Yes

Yes

integrationgraph

Yes

No

jolokia

N/A

No

logfile

N/A

No

loggers

Yes

No

liquibase

Yes

No

metrics

Yes

No

mappings

Yes

No

prometheus

N/A

No

scheduledtasks

Yes

No

sessions

Yes

No

shutdown

Yes

No

threaddump

Yes

No

To change which endpoints are exposed, use the following technology-specific include and exclude properties:

Property Default

management.endpoints.jmx.exposure.exclude

management.endpoints.jmx.exposure.include

*

management.endpoints.web.exposure.exclude

management.endpoints.web.exposure.include

info, health

The include property lists the IDs of the endpoints that are exposed. The exclude property lists the IDs of the endpoints that should not be exposed. The exclude property takes precedence over the include property. Both include and exclude properties can be configured with a list of endpoint IDs.

For example, to stop exposing all endpoints over JMX and only expose the health and info endpoints, use the following property:

management.endpoints.jmx.exposure.include=health,info

* can be used to select all endpoints. For example, to expose everything over HTTP except the env and beans endpoints, use the following properties:

management.endpoints.web.exposure.include=*
management.endpoints.web.exposure.exclude=env,beans

* has a special meaning in YAML, so be sure to add quotes if you want to include (or exclude) all endpoints, as shown in the following example:

management:
  endpoints:
    web:
      exposure:
        include: "*"
If your application is exposed publicly, we strongly recommend that you also secure your endpoints.
If you want to implement your own strategy for when endpoints are exposed, you can register an EndpointFilter bean.

5.2.3. Securing HTTP Endpoints

You should take care to secure HTTP endpoints in the same way that you would any other sensitive URL. If Spring Security is present, endpoints are secured by default using Spring Security’s content-negotiation strategy. If you wish to configure custom security for HTTP endpoints, for example, only allow users with a certain role to access them, Spring Boot provides some convenient RequestMatcher objects that can be used in combination with Spring Security.

A typical Spring Security configuration might look something like the following example:

@Configuration(proxyBeanMethods = false)
public class ActuatorSecurity extends WebSecurityConfigurerAdapter {

    @Override
    protected void configure(HttpSecurity http) throws Exception {
        http.requestMatcher(EndpointRequest.toAnyEndpoint()).authorizeRequests()
                .anyRequest().hasRole("ENDPOINT_ADMIN")
                .and()
            .httpBasic();
    }

}

The preceding example uses EndpointRequest.toAnyEndpoint() to match a request to any endpoint and then ensures that all have the ENDPOINT_ADMIN role. Several other matcher methods are also available on EndpointRequest. See the API documentation (HTML or PDF) for details.

If you deploy applications behind a firewall, you may prefer that all your actuator endpoints can be accessed without requiring authentication. You can do so by changing the management.endpoints.web.exposure.include property, as follows:

application.properties
management.endpoints.web.exposure.include=*

Additionally, if Spring Security is present, you would need to add custom security configuration that allows unauthenticated access to the endpoints as shown in the following example:

@Configuration(proxyBeanMethods = false)
public class ActuatorSecurity extends WebSecurityConfigurerAdapter {

    @Override
    protected void configure(HttpSecurity http) throws Exception {
        http.requestMatcher(EndpointRequest.toAnyEndpoint()).authorizeRequests()
            .anyRequest().permitAll();
    }

}

5.2.4. Configuring Endpoints

Endpoints automatically cache responses to read operations that do not take any parameters. To configure the amount of time for which an endpoint will cache a response, use its cache.time-to-live property. The following example sets the time-to-live of the beans endpoint’s cache to 10 seconds:

application.properties
management.endpoint.beans.cache.time-to-live=10s
The prefix management.endpoint.<name> is used to uniquely identify the endpoint that is being configured.
When making an authenticated HTTP request, the Principal is considered as input to the endpoint and, therefore, the response will not be cached.

5.2.5. Hypermedia for Actuator Web Endpoints

A “discovery page” is added with links to all the endpoints. The “discovery page” is available on /actuator by default.

When a custom management context path is configured, the “discovery page” automatically moves from /actuator to the root of the management context. For example, if the management context path is /management, then the discovery page is available from /management. When the management context path is set to /, the discovery page is disabled to prevent the possibility of a clash with other mappings.

5.2.6. CORS Support

Cross-origin resource sharing (CORS) is a W3C specification that lets you specify in a flexible way what kind of cross-domain requests are authorized. If you use Spring MVC or Spring WebFlux, Actuator’s web endpoints can be configured to support such scenarios.

CORS support is disabled by default and is only enabled once the management.endpoints.web.cors.allowed-origins property has been set. The following configuration permits GET and POST calls from the example.com domain:

management.endpoints.web.cors.allowed-origins=https://example.com
management.endpoints.web.cors.allowed-methods=GET,POST
See CorsEndpointProperties for a complete list of options.

5.2.7. Implementing Custom Endpoints

If you add a @Bean annotated with @Endpoint, any methods annotated with @ReadOperation, @WriteOperation, or @DeleteOperation are automatically exposed over JMX and, in a web application, over HTTP as well. Endpoints can be exposed over HTTP using Jersey, Spring MVC, or Spring WebFlux.

You can also write technology-specific endpoints by using @JmxEndpoint or @WebEndpoint. These endpoints are restricted to their respective technologies. For example, @WebEndpoint is exposed only over HTTP and not over JMX.

You can write technology-specific extensions by using @EndpointWebExtension and @EndpointJmxExtension. These annotations let you provide technology-specific operations to augment an existing endpoint.

Finally, if you need access to web-framework-specific functionality, you can implement Servlet or Spring @Controller and @RestController endpoints at the cost of them not being available over JMX or when using a different web framework.

Receiving Input

Operations on an endpoint receive input via their parameters. When exposed via the web, the values for these parameters are taken from the URL’s query parameters and from the JSON request body. When exposed via JMX, the parameters are mapped to the parameters of the MBean’s operations. Parameters are required by default. They can be made optional by annotating them with @org.springframework.lang.Nullable.

Each root property in the JSON request body can be mapped to a parameter of the endpoint. Consider the following JSON request body:

{
    "name": "test",
    "counter": 42
}

This can be used to invoke a write operation that takes String name and int counter parameters.

Because endpoints are technology agnostic, only simple types can be specified in the method signature. In particular declaring a single parameter with a custom type defining a name and counter properties is not supported.
To allow the input to be mapped to the operation method’s parameters, Java code implementing an endpoint should be compiled with -parameters, and Kotlin code implementing an endpoint should be compiled with -java-parameters. This will happen automatically if you are using Spring Boot’s Gradle plugin or if you are using Maven and spring-boot-starter-parent.
Input type conversion

The parameters passed to endpoint operation methods are, if necessary, automatically converted to the required type. Before calling an operation method, the input received via JMX or an HTTP request is converted to the required types using an instance of ApplicationConversionService.

Custom Web Endpoints

Operations on an @Endpoint, @WebEndpoint, or @EndpointWebExtension are automatically exposed over HTTP using Jersey, Spring MVC, or Spring WebFlux.

Web Endpoint Request Predicates

A request predicate is automatically generated for each operation on a web-exposed endpoint.

Path

The path of the predicate is determined by the ID of the endpoint and the base path of web-exposed endpoints. The default base path is /actuator. For example, an endpoint with the ID sessions will use /actuator/sessions as its path in the predicate.

The path can be further customized by annotating one or more parameters of the operation method with @Selector. Such a parameter is added to the path predicate as a path variable. The variable’s value is passed into the operation method when the endpoint operation is invoked.

HTTP method

The HTTP method of the predicate is determined by the operation type, as shown in the following table:

Operation HTTP method

@ReadOperation

GET

@WriteOperation

POST

@DeleteOperation

DELETE

Consumes

For a @WriteOperation (HTTP POST) that uses the request body, the consumes clause of the predicate is application/vnd.spring-boot.actuator.v2+json, application/json. For all other operations the consumes clause is empty.

Produces

The produces clause of the predicate can be determined by the produces attribute of the @DeleteOperation, @ReadOperation, and @WriteOperation annotations. The attribute is optional. If it is not used, the produces clause is determined automatically.

If the operation method returns void or Void the produces clause is empty. If the operation method returns a org.springframework.core.io.Resource, the produces clause is application/octet-stream. For all other operations the produces clause is application/vnd.spring-boot.actuator.v2+json, application/json.

Web Endpoint Response Status

The default response status for an endpoint operation depends on the operation type (read, write, or delete) and what, if anything, the operation returns.

A @ReadOperation returns a value, the response status will be 200 (OK). If it does not return a value, the response status will be 404 (Not Found).

If a @WriteOperation or @DeleteOperation returns a value, the response status will be 200 (OK). If it does not return a value the response status will be 204 (No Content).

If an operation is invoked without a required parameter, or with a parameter that cannot be converted to the required type, the operation method will not be called and the response status will be 400 (Bad Request).

Web Endpoint Range Requests

An HTTP range request can be used to request part of an HTTP resource. When using Spring MVC or Spring Web Flux, operations that return a org.springframework.core.io.Resource automatically support range requests.

Range requests are not supported when using Jersey.
Web Endpoint Security

An operation on a web endpoint or a web-specific endpoint extension can receive the current java.security.Principal or org.springframework.boot.actuate.endpoint.SecurityContext as a method parameter. The former is typically used in conjunction with @Nullable to provide different behaviour for authenticated and unauthenticated users. The latter is typically used to perform authorization checks using its isUserInRole(String) method.

Servlet endpoints

A Servlet can be exposed as an endpoint by implementing a class annotated with @ServletEndpoint that also implements Supplier<EndpointServlet>. Servlet endpoints provide deeper integration with the Servlet container but at the expense of portability. They are intended to be used to expose an existing Servlet as an endpoint. For new endpoints, the @Endpoint and @WebEndpoint annotations should be preferred whenever possible.

Controller endpoints

@ControllerEndpoint and @RestControllerEndpoint can be used to implement an endpoint that is only exposed by Spring MVC or Spring WebFlux. Methods are mapped using the standard annotations for Spring MVC and Spring WebFlux such as @RequestMapping and @GetMapping, with the endpoint’s ID being used as a prefix for the path. Controller endpoints provide deeper integration with Spring’s web frameworks but at the expense of portability. The @Endpoint and @WebEndpoint annotations should be preferred whenever possible.

5.2.8. Health Information

You can use health information to check the status of your running application. It is often used by monitoring software to alert someone when a production system goes down. The information exposed by the health endpoint depends on the management.endpoint.health.show-details property which can be configured with one of the following values:

Name Description

never

Details are never shown.

when-authorized

Details are only shown to authorized users. Authorized roles can be configured using management.endpoint.health.roles.

always

Details are shown to all users.

The default value is never. A user is considered to be authorized when they are in one or more of the endpoint’s roles. If the endpoint has no configured roles (the default) all authenticated users are considered to be authorized. The roles can be configured using the management.endpoint.health.roles property.

If you have secured your application and wish to use always, your security configuration must permit access to the health endpoint for both authenticated and unauthenticated users.

Health information is collected from the content of a HealthIndicatorRegistry (by default all HealthIndicator instances defined in your ApplicationContext. Spring Boot includes a number of auto-configured HealthIndicators and you can also write your own. By default, the final system state is derived by the HealthAggregator which sorts the statuses from each HealthIndicator based on an ordered list of statuses. The first status in the sorted list is used as the overall health status. If no HealthIndicator returns a status that is known to the HealthAggregator, an UNKNOWN status is used.

The HealthIndicatorRegistry can be used to register and unregister health indicators at runtime.
Auto-configured HealthIndicators

The following HealthIndicators are auto-configured by Spring Boot when appropriate:

Name Description

CassandraHealthIndicator

Checks that a Cassandra database is up.

CouchbaseHealthIndicator

Checks that a Couchbase cluster is up.

DiskSpaceHealthIndicator

Checks for low disk space.

DataSourceHealthIndicator

Checks that a connection to DataSource can be obtained.

ElasticsearchHealthIndicator

Checks that an Elasticsearch cluster is up.

InfluxDbHealthIndicator

Checks that an InfluxDB server is up.

JmsHealthIndicator

Checks that a JMS broker is up.

MailHealthIndicator

Checks that a mail server is up.

MongoHealthIndicator

Checks that a Mongo database is up.

Neo4jHealthIndicator

Checks that a Neo4j server is up.

RabbitHealthIndicator

Checks that a Rabbit server is up.

RedisHealthIndicator

Checks that a Redis server is up.

SolrHealthIndicator

Checks that a Solr server is up.

You can disable them all by setting the management.health.defaults.enabled property.
Writing Custom HealthIndicators

To provide custom health information, you can register Spring beans that implement the HealthIndicator interface. You need to provide an implementation of the health() method and return a Health response. The Health response should include a status and can optionally include additional details to be displayed. The following code shows a sample HealthIndicator implementation:

import org.springframework.boot.actuate.health.Health;
import org.springframework.boot.actuate.health.HealthIndicator;
import org.springframework.stereotype.Component;

@Component
public class MyHealthIndicator implements HealthIndicator {

    @Override
    public Health health() {
        int errorCode = check(); // perform some specific health check
        if (errorCode != 0) {
            return Health.down().withDetail("Error Code", errorCode).build();
        }
        return Health.up().build();
    }

}
The identifier for a given HealthIndicator is the name of the bean without the HealthIndicator suffix, if it exists. In the preceding example, the health information is available in an entry named my.

In addition to Spring Boot’s predefined Status types, it is also possible for Health to return a custom Status that represents a new system state. In such cases, a custom implementation of the HealthAggregator interface also needs to be provided, or the default implementation has to be configured by using the management.health.status.order configuration property.

For example, assume a new Status with code FATAL is being used in one of your HealthIndicator implementations. To configure the severity order, add the following property to your application properties:

management.health.status.order=FATAL, DOWN, OUT_OF_SERVICE, UNKNOWN, UP

The HTTP status code in the response reflects the overall health status (for example, UP maps to 200, while OUT_OF_SERVICE and DOWN map to 503). You might also want to register custom status mappings if you access the health endpoint over HTTP. For example, the following property maps FATAL to 503 (service unavailable):

management.health.status.http-mapping.FATAL=503
If you need more control, you can define your own HealthStatusHttpMapper bean.

The following table shows the default status mappings for the built-in statuses:

Status Mapping

DOWN

SERVICE_UNAVAILABLE (503)

OUT_OF_SERVICE

SERVICE_UNAVAILABLE (503)

UP

No mapping by default, so http status is 200

UNKNOWN

No mapping by default, so http status is 200

Reactive Health Indicators

For reactive applications, such as those using Spring WebFlux, ReactiveHealthIndicator provides a non-blocking contract for getting application health. Similar to a traditional HealthIndicator, health information is collected from the content of a ReactiveHealthIndicatorRegistry (by default all HealthIndicator and ReactiveHealthIndicator instances defined in your ApplicationContext. Regular HealthIndicator that do not check against a reactive API are executed on the elastic scheduler.

In a reactive application, The ReactiveHealthIndicatorRegistry can be used to register and unregister health indicators at runtime.

To provide custom health information from a reactive API, you can register Spring beans that implement the ReactiveHealthIndicator interface. The following code shows a sample ReactiveHealthIndicator implementation:

@Component
public class MyReactiveHealthIndicator implements ReactiveHealthIndicator {

    @Override
    public Mono<Health> health() {
        return doHealthCheck() //perform some specific health check that returns a Mono<Health>
            .onErrorResume(ex -> Mono.just(new Health.Builder().down(ex).build())));
    }

}
To handle the error automatically, consider extending from AbstractReactiveHealthIndicator.
Auto-configured ReactiveHealthIndicators

The following ReactiveHealthIndicators are auto-configured by Spring Boot when appropriate:

Name Description

CassandraReactiveHealthIndicator

Checks that a Cassandra database is up.

CouchbaseReactiveHealthIndicator

Checks that a Couchbase cluster is up.

MongoReactiveHealthIndicator

Checks that a Mongo database is up.

RedisReactiveHealthIndicator

Checks that a Redis server is up.

If necessary, reactive indicators replace the regular ones. Also, any HealthIndicator that is not handled explicitly is wrapped automatically.

5.2.9. Application Information

Application information exposes various information collected from all InfoContributor beans defined in your ApplicationContext. Spring Boot includes a number of auto-configured InfoContributor beans, and you can write your own.

Auto-configured InfoContributors

The following InfoContributor beans are auto-configured by Spring Boot, when appropriate:

Name Description

EnvironmentInfoContributor

Exposes any key from the Environment under the info key.

GitInfoContributor

Exposes git information if a git.properties file is available.

BuildInfoContributor

Exposes build information if a META-INF/build-info.properties file is available.

It is possible to disable them all by setting the management.info.defaults.enabled property.
Custom Application Information

You can customize the data exposed by the info endpoint by setting info.* Spring properties. All Environment properties under the info key are automatically exposed. For example, you could add the following settings to your application.properties file:

info.app.encoding=UTF-8
info.app.java.source=1.8
info.app.java.target=1.8

Rather than hardcoding those values, you could also expand info properties at build time.

Assuming you use Maven, you could rewrite the preceding example as follows:

Git Commit Information

Another useful feature of the info endpoint is its ability to publish information about the state of your git source code repository when the project was built. If a GitProperties bean is available, the git.branch, git.commit.id, and git.commit.time properties are exposed.

A GitProperties bean is auto-configured if a git.properties file is available at the root of the classpath. See "Generate git information" for more details.

If you want to display the full git information (that is, the full content of git.properties), use the management.info.git.mode property, as follows:

management.info.git.mode=full
Build Information

If a BuildProperties bean is available, the info endpoint can also publish information about your build. This happens if a META-INF/build-info.properties file is available in the classpath.

The Maven and Gradle plugins can both generate that file. See "Generate build information" for more details.
Writing Custom InfoContributors

To provide custom application information, you can register Spring beans that implement the InfoContributor interface.

The following example contributes an example entry with a single value:

import java.util.Collections;

import org.springframework.boot.actuate.info.Info;
import org.springframework.boot.actuate.info.InfoContributor;
import org.springframework.stereotype.Component;

@Component
public class ExampleInfoContributor implements InfoContributor {

    @Override
    public void contribute(Info.Builder builder) {
        builder.withDetail("example",
                Collections.singletonMap("key", "value"));
    }

}

If you reach the info endpoint, you should see a response that contains the following additional entry:

{
    "example": {
        "key" : "value"
    }
}

5.3. Monitoring and Management over HTTP

If you are developing a web application, Spring Boot Actuator auto-configures all enabled endpoints to be exposed over HTTP. The default convention is to use the id of the endpoint with a prefix of /actuator as the URL path. For example, health is exposed as /actuator/health. TIP: Actuator is supported natively with Spring MVC, Spring WebFlux, and Jersey.

5.3.1. Customizing the Management Endpoint Paths

Sometimes, it is useful to customize the prefix for the management endpoints. For example, your application might already use /actuator for another purpose. You can use the management.endpoints.web.base-path property to change the prefix for your management endpoint, as shown in the following example:

management.endpoints.web.base-path=/manage

The preceding application.properties example changes the endpoint from /actuator/{id} to /manage/{id} (for example, /manage/info).

Unless the management port has been configured to expose endpoints by using a different HTTP port, management.endpoints.web.base-path is relative to server.servlet.context-path. If management.server.port is configured, management.endpoints.web.base-path is relative to management.server.servlet.context-path.

If you want to map endpoints to a different path, you can use the management.endpoints.web.path-mapping property.

The following example remaps /actuator/health to /healthcheck:

application.properties
management.endpoints.web.base-path=/
management.endpoints.web.path-mapping.health=healthcheck

5.3.2. Customizing the Management Server Port

Exposing management endpoints by using the default HTTP port is a sensible choice for cloud-based deployments. If, however, your application runs inside your own data center, you may prefer to expose endpoints by using a different HTTP port.

You can set the management.server.port property to change the HTTP port, as shown in the following example:

management.server.port=8081
On Cloud Foundry, applications only receive requests on port 8080 for both HTTP and TCP routing, by default. If you want to use a custom management port on Cloud Foundry, you will need to explicitly set up the application’s routes to forward traffic to the custom port.

5.3.3. Configuring Management-specific SSL

When configured to use a custom port, the management server can also be configured with its own SSL by using the various management.server.ssl.* properties. For example, doing so lets a management server be available over HTTP while the main application uses HTTPS, as shown in the following property settings:

server.port=8443
server.ssl.enabled=true
server.ssl.key-store=classpath:store.jks
server.ssl.key-password=secret
management.server.port=8080
management.server.ssl.enabled=false

Alternatively, both the main server and the management server can use SSL but with different key stores, as follows:

server.port=8443
server.ssl.enabled=true
server.ssl.key-store=classpath:main.jks
server.ssl.key-password=secret
management.server.port=8080
management.server.ssl.enabled=true
management.server.ssl.key-store=classpath:management.jks
management.server.ssl.key-password=secret

5.3.4. Customizing the Management Server Address

You can customize the address that the management endpoints are available on by setting the management.server.address property. Doing so can be useful if you want to listen only on an internal or ops-facing network or to listen only for connections from localhost.

You can listen on a different address only when the port differs from the main server port.

The following example application.properties does not allow remote management connections:

management.server.port=8081
management.server.address=127.0.0.1

5.3.5. Disabling HTTP Endpoints

If you do not want to expose endpoints over HTTP, you can set the management port to -1, as shown in the following example:

management.server.port=-1

This can be achieved using the management.endpoints.web.exposure.exclude property as well, as shown in following example:

management.endpoints.web.exposure.exclude=*

5.4. Monitoring and Management over JMX

Java Management Extensions (JMX) provide a standard mechanism to monitor and manage applications. By default, this feature is not enabled and can be turned on with the configuration property spring.jmx.enabled=true. Spring Boot exposes management endpoints as JMX MBeans under the org.springframework.boot domain by default.

5.4.1. Customizing MBean Names

The name of the MBean is usually generated from the id of the endpoint. For example, the health endpoint is exposed as org.springframework.boot:type=Endpoint,name=Health.

If your application contains more than one Spring ApplicationContext, you may find that names clash. To solve this problem, you can set the spring.jmx.unique-names property to true so that MBean names are always unique.

You can also customize the JMX domain under which endpoints are exposed. The following settings show an example of doing so in application.properties:

spring.jmx.unique-names=true
management.endpoints.jmx.domain=com.example.myapp

5.4.2. Disabling JMX Endpoints

If you do not want to expose endpoints over JMX, you can set the management.endpoints.jmx.exposure.exclude property to *, as shown in the following example:

management.endpoints.jmx.exposure.exclude=*

5.4.3. Using Jolokia for JMX over HTTP

Jolokia is a JMX-HTTP bridge that provides an alternative method of accessing JMX beans. To use Jolokia, include a dependency to org.jolokia:jolokia-core. For example, with Maven, you would add the following dependency:

<dependency>
    <groupId>org.jolokia</groupId>
    <artifactId>jolokia-core</artifactId>
</dependency>

The Jolokia endpoint can then be exposed by adding jolokia or * to the management.endpoints.web.exposure.include property. You can then access it by using /actuator/jolokia on your management HTTP server.

Customizing Jolokia

Jolokia has a number of settings that you would traditionally configure by setting servlet parameters. With Spring Boot, you can use your application.properties file. To do so, prefix the parameter with management.endpoint.jolokia.config., as shown in the following example:

management.endpoint.jolokia.config.debug=true
Disabling Jolokia

If you use Jolokia but do not want Spring Boot to configure it, set the management.endpoint.jolokia.enabled property to false, as follows:

management.endpoint.jolokia.enabled=false

5.5. Loggers

Spring Boot Actuator includes the ability to view and configure the log levels of your application at runtime. You can view either the entire list or an individual logger’s configuration, which is made up of both the explicitly configured logging level as well as the effective logging level given to it by the logging framework. These levels can be one of:

  • TRACE

  • DEBUG

  • INFO

  • WARN

  • ERROR

  • FATAL

  • OFF

  • null

null indicates that there is no explicit configuration.

5.5.1. Configure a Logger

To configure a given logger, POST a partial entity to the resource’s URI, as shown in the following example:

{
    "configuredLevel": "DEBUG"
}
To “reset” the specific level of the logger (and use the default configuration instead), you can pass a value of null as the configuredLevel.

5.6. Metrics

Spring Boot Actuator provides dependency management and auto-configuration for Micrometer, an application metrics facade that supports numerous monitoring systems, including:

To learn more about Micrometer’s capabilities, please refer to its reference documentation, in particular the concepts section.

5.6.1. Getting started

Spring Boot auto-configures a composite MeterRegistry and adds a registry to the composite for each of the supported implementations that it finds on the classpath. Having a dependency on micrometer-registry-{system} in your runtime classpath is enough for Spring Boot to configure the registry.

Most registries share common features. For instance, you can disable a particular registry even if the Micrometer registry implementation is on the classpath. For instance, to disable Datadog:

management.metrics.export.datadog.enabled=false

Spring Boot will also add any auto-configured registries to the global static composite registry on the Metrics class unless you explicitly tell it not to:

management.metrics.use-global-registry=false

You can register any number of MeterRegistryCustomizer beans to further configure the registry, such as applying common tags, before any meters are registered with the registry:

@Bean
MeterRegistryCustomizer<MeterRegistry> metricsCommonTags() {
    return registry -> registry.config().commonTags("region", "us-east-1");
}

You can apply customizations to particular registry implementations by being more specific about the generic type:

@Bean
MeterRegistryCustomizer<GraphiteMeterRegistry> graphiteMetricsNamingConvention() {
    return registry -> registry.config().namingConvention(MY_CUSTOM_CONVENTION);
}

With that setup in place you can inject MeterRegistry in your components and register metrics:

@Component
public class SampleBean {

    private final Counter counter;

    public SampleBean(MeterRegistry registry) {
        this.counter = registry.counter("received.messages");
    }

    public void handleMessage(String message) {
        this.counter.increment();
        // handle message implementation
    }

}

Spring Boot also configures built-in instrumentation (i.e. MeterBinder implementations) that you can control via configuration or dedicated annotation markers.

5.6.2. Supported monitoring systems

AppOptics

By default, the AppOptics registry pushes metrics to api.appoptics.com/v1/measurements periodically. To export metrics to SaaS AppOptics, your API token must be provided:

management.metrics.export.appoptics.api-token=YOUR_TOKEN
Atlas

By default, metrics are exported to Atlas running on your local machine. The location of the Atlas server to use can be provided using:

management.metrics.export.atlas.uri=https://atlas.example.com:7101/api/v1/publish
Datadog

Datadog registry pushes metrics to datadoghq periodically. To export metrics to Datadog, your API key must be provided:

management.metrics.export.datadog.api-key=YOUR_KEY

You can also change the interval at which metrics are sent to Datadog:

management.metrics.export.datadog.step=30s
Dynatrace

Dynatrace registry pushes metrics to the configured URI periodically. To export metrics to Dynatrace, your API token, device ID, and URI must be provided:

management.metrics.export.dynatrace.api-token=YOUR_TOKEN
management.metrics.export.dynatrace.device-id=YOUR_DEVICE_ID
management.metrics.export.dynatrace.uri=YOUR_URI

You can also change the interval at which metrics are sent to Dynatrace:

management.metrics.export.dynatrace.step=30s
Elastic

By default, metrics are exported to Elastic running on your local machine. The location of the Elastic server to use can be provided using the following property:

management.metrics.export.elastic.host=https://elastic.example.com:8086
Ganglia

By default, metrics are exported to Ganglia running on your local machine. The Ganglia server host and port to use can be provided using:

management.metrics.export.ganglia.host=ganglia.example.com
management.metrics.export.ganglia.port=9649
Graphite

By default, metrics are exported to Graphite running on your local machine. The Graphite server host and port to use can be provided using:

management.metrics.export.graphite.host=graphite.example.com
management.metrics.export.graphite.port=9004

Micrometer provides a default HierarchicalNameMapper that governs how a dimensional meter id is mapped to flat hierarchical names.

To take control over this behaviour, define your GraphiteMeterRegistry and supply your own HierarchicalNameMapper. An auto-configured GraphiteConfig and Clock beans are provided unless you define your own:
@Bean
public GraphiteMeterRegistry graphiteMeterRegistry(GraphiteConfig config, Clock clock) {
    return new GraphiteMeterRegistry(config, clock, MY_HIERARCHICAL_MAPPER);
}
Humio

By default, the Humio registry pushes metrics to cloud.humio.com periodically. To export metrics to SaaS Humio, your API token must be provided:

management.metrics.export.humio.api-token=YOUR_TOKEN

You should also configure one or more tags to identify the data source to which metrics will be pushed:

management.metrics.export.humio.tags.alpha=a
management.metrics.export.humio.tags.bravo=b
Influx

By default, metrics are exported to Influx running on your local machine. The location of the Influx server to use can be provided using:

management.metrics.export.influx.uri=https://influx.example.com:8086
JMX

Micrometer provides a hierarchical mapping to JMX, primarily as a cheap and portable way to view metrics locally. By default, metrics are exported to the metrics JMX domain. The domain to use can be provided using:

management.metrics.export.jmx.domain=com.example.app.metrics

Micrometer provides a default HierarchicalNameMapper that governs how a dimensional meter id is mapped to flat hierarchical names.

To take control over this behaviour, define your JmxMeterRegistry and supply your own HierarchicalNameMapper. An auto-configured JmxConfig and Clock beans are provided unless you define your own:
@Bean
public JmxMeterRegistry jmxMeterRegistry(JmxConfig config, Clock clock) {
    return new JmxMeterRegistry(config, clock, MY_HIERARCHICAL_MAPPER);
}
KairosDB

By default, metrics are exported to KairosDB running on your local machine. The location of the KairosDB server to use can be provided using:

management.metrics.export.kairos.uri=https://kairosdb.example.com:8080/api/v1/datapoints
New Relic

New Relic registry pushes metrics to New Relic periodically. To export metrics to New Relic, your API key and account id must be provided:

management.metrics.export.newrelic.api-key=YOUR_KEY
management.metrics.export.newrelic.account-id=YOUR_ACCOUNT_ID

You can also change the interval at which metrics are sent to New Relic:

management.metrics.export.newrelic.step=30s
Prometheus

Prometheus expects to scrape or poll individual app instances for metrics. Spring Boot provides an actuator endpoint available at /actuator/prometheus to present a Prometheus scrape with the appropriate format.

The endpoint is not available by default and must be exposed, see exposing endpoints for more details.

Here is an example scrape_config to add to prometheus.yml:

scrape_configs:
  - job_name: 'spring'
    metrics_path: '/actuator/prometheus'
    static_configs:
      - targets: ['HOST:PORT']
SignalFx

SignalFx registry pushes metrics to SignalFx periodically. To export metrics to SignalFx, your access token must be provided:

management.metrics.export.signalfx.access-token=YOUR_ACCESS_TOKEN

You can also change the interval at which metrics are sent to SignalFx:

management.metrics.export.signalfx.step=30s
Simple

Micrometer ships with a simple, in-memory backend that is automatically used as a fallback if no other registry is configured. This allows you to see what metrics are collected in the metrics endpoint.

The in-memory backend disables itself as soon as you’re using any of the other available backend. You can also disable it explicitly:

management.metrics.export.simple.enabled=false
StatsD

The StatsD registry pushes metrics over UDP to a StatsD agent eagerly. By default, metrics are exported to a StatsD agent running on your local machine. The StatsD agent host and port to use can be provided using:

management.metrics.export.statsd.host=statsd.example.com
management.metrics.export.statsd.port=9125

You can also change the StatsD line protocol to use (default to Datadog):

management.metrics.export.statsd.flavor=etsy
Wavefront

Wavefront registry pushes metrics to Wavefront periodically. If you are exporting metrics to Wavefront directly, your API token must be provided:

management.metrics.export.wavefront.api-token=YOUR_API_TOKEN

Alternatively, you may use a Wavefront sidecar or an internal proxy set up in your environment that forwards metrics data to the Wavefront API host:

management.metrics.export.wavefront.uri=proxy://localhost:2878
If publishing metrics to a Wavefront proxy (as described in the documentation), the host must be in the proxy://HOST:PORT format.

You can also change the interval at which metrics are sent to Wavefront:

management.metrics.export.wavefront.step=30s

5.6.3. Supported Metrics

Spring Boot registers the following core metrics when applicable:

  • JVM metrics, report utilization of:

    • Various memory and buffer pools

    • Statistics related to garbage collection

    • Threads utilization

    • Number of classes loaded/unloaded

  • CPU metrics

  • File descriptor metrics

  • Kafka consumer metrics

  • Log4j2 metrics: record the number of events logged to Log4j2 at each level

  • Logback metrics: record the number of events logged to Logback at each level

  • Uptime metrics: report a gauge for uptime and a fixed gauge representing the application’s absolute start time

  • Tomcat metrics (server.tomcat.mbeanregistry.enabled must be set to true for all Tomcat metrics to be registered)

  • Spring Integration metrics

Spring MVC Metrics

Auto-configuration enables the instrumentation of requests handled by Spring MVC. When management.metrics.web.server.request.autotime.enabled is true, this instrumentation occurs for all requests. Alternatively, when set to false, you can enable instrumentation by adding @Timed to a request-handling method:

@RestController
@Timed (1)
public class MyController {

    @GetMapping("/api/people")
    @Timed(extraTags = { "region", "us-east-1" }) (2)
    @Timed(value = "all.people", longTask = true) (3)
    public List<Person> listPeople() { ... }

}
1 A controller class to enable timings on every request handler in the controller.
2 A method to enable for an individual endpoint. This is not necessary if you have it on the class, but can be used to further customize the timer for this particular endpoint.
3 A method with longTask = true to enable a long task timer for the method. Long task timers require a separate metric name, and can be stacked with a short task timer.

By default, metrics are generated with the name, http.server.requests. The name can be customized by setting the management.metrics.web.server.requests-metric-name property.

By default, Spring MVC-related metrics are tagged with the following information:

Tag Description

exception

Simple class name of any exception that was thrown while handling the request.

method

Request’s method (for example, GET or POST)

outcome

Request’s outcome based on the status code of the response. 1xx is INFORMATIONAL, 2xx is SUCCESS, 3xx is REDIRECTION, 4xx CLIENT_ERROR, and 5xx is SERVER_ERROR

status

Response’s HTTP status code (for example, 200 or 500)

uri

Request’s URI template prior to variable substitution, if possible (for example, /api/person/{id})

To customize the tags, provide a @Bean that implements WebMvcTagsProvider.

Spring WebFlux Metrics

Auto-configuration enables the instrumentation of all requests handled by WebFlux controllers and functional handlers.

By default, metrics are generated with the name http.server.requests. You can customize the name by setting the management.metrics.web.server.requests-metric-name property.

By default, WebFlux-related metrics are tagged with the following information:

Tag Description

exception

Simple class name of any exception that was thrown while handling the request.

method

Request’s method (for example, GET or POST)

outcome

Request’s outcome based on the status code of the response. 1xx is INFORMATIONAL, 2xx is SUCCESS, 3xx is REDIRECTION, 4xx CLIENT_ERROR, and 5xx is SERVER_ERROR

status

Response’s HTTP status code (for example, 200 or 500)

uri

Request’s URI template prior to variable substitution, if possible (for example, /api/person/{id})

To customize the tags, provide a @Bean that implements WebFluxTagsProvider.

Jersey Server Metrics

Auto-configuration enables the instrumentation of requests handled by the Jersey JAX-RS implementation. When management.metrics.web.server.request.autotime.enabled is true, this instrumentation occurs for all requests. Alternatively, when set to false, you can enable instrumentation by adding @Timed to a request-handling method:

@Component
@Path("/api/people")
@Timed (1)
public class Endpoint {
    @GET
    @Timed(extraTags = { "region", "us-east-1" }) (2)
    @Timed(value = "all.people", longTask = true) (3)
    public List<Person> listPeople() { ... }
}
1 On a resource class to enable timings on every request handler in the resource.
2 On a method to enable for an individual endpoint. This is not necessary if you have it on the class, but can be used to further customize the timer for this particular endpoint.
3 On a method with longTask = true to enable a long task timer for the method. Long task timers require a separate metric name, and can be stacked with a short task timer.

By default, metrics are generated with the name, http.server.requests. The name can be customized by setting the management.metrics.web.server.requests-metric-name property.

By default, Jersey server metrics are tagged with the following information:

Tag Description

exception

Simple class name of any exception that was thrown while handling the request.

method

Request’s method (for example, GET or POST)

outcome

Request’s outcome based on the status code of the response. 1xx is INFORMATIONAL, 2xx is SUCCESS, 3xx is REDIRECTION, 4xx CLIENT_ERROR, and 5xx is SERVER_ERROR

status

Response’s HTTP status code (for example, 200 or 500)

uri

Request’s URI template prior to variable substitution, if possible (for example, /api/person/{id})

To customize the tags, provide a @Bean that implements JerseyTagsProvider.

HTTP Client Metrics

Spring Boot Actuator manages the instrumentation of both RestTemplate and WebClient. For that, you have to get injected with an auto-configured builder and use it to create instances:

  • RestTemplateBuilder for RestTemplate

  • WebClient.Builder for WebClient

It is also possible to apply manually the customizers responsible for this instrumentation, namely MetricsRestTemplateCustomizer and MetricsWebClientCustomizer.

By default, metrics are generated with the name, http.client.requests. The name can be customized by setting the management.metrics.web.client.requests-metric-name property.

By default, metrics generated by an instrumented client are tagged with the following information:

Tag Description

clientName

Host portion of the URI

method

Request’s method (for example, GET or POST)

outcome

Request’s outcome based on the status code of the response. 1xx is INFORMATIONAL, 2xx is SUCCESS, 3xx is REDIRECTION, 4xx CLIENT_ERROR, and 5xx is SERVER_ERROR, UNKNOWN otherwise

status

Response’s HTTP status code if available (for example, 200 or 500), or IO_ERROR in case or I/O issues, CLIENT_ERROR otherwise

uri

Request’s URI template prior to variable substitution, if possible (for example, /api/person/{id})

To customize the tags, and depending on your choice of client, you can provide a @Bean that implements RestTemplateExchangeTagsProvider or WebClientExchangeTagsProvider. There are convenience static functions in RestTemplateExchangeTags and WebClientExchangeTags.

Cache Metrics

Auto-configuration enables the instrumentation of all available Caches on startup with metrics prefixed with cache. Cache instrumentation is standardized for a basic set of metrics. Additional, cache-specific metrics are also available.

The following cache libraries are supported:

  • Caffeine

  • EhCache 2

  • Hazelcast

  • Any compliant JCache (JSR-107) implementation

Metrics are tagged by the name of the cache and by the name of the CacheManager that is derived from the bean name.

Only caches that are available on startup are bound to the registry. For caches created on-the-fly or programmatically after the startup phase, an explicit registration is required. A CacheMetricsRegistrar bean is made available to make that process easier.
DataSource Metrics

Auto-configuration enables the instrumentation of all available DataSource objects with a metric named jdbc. Data source instrumentation results in gauges representing the currently active, maximum allowed, and minimum allowed connections in the pool. Each of these gauges has a name that is prefixed by jdbc.

Metrics are also tagged by the name of the DataSource computed based on the bean name.

By default, Spring Boot provides metadata for all supported data sources; you can add additional DataSourcePoolMetadataProvider beans if your favorite data source isn’t supported out of the box. See DataSourcePoolMetadataProvidersConfiguration for examples.

Also, Hikari-specific metrics are exposed with a hikaricp prefix. Each metric is tagged by the name of the Pool (can be controlled with spring.datasource.name).

Hibernate Metrics

Auto-configuration enables the instrumentation of all available Hibernate EntityManagerFactory instances that have statistics enabled with a metric named hibernate.

Metrics are also tagged by the name of the EntityManagerFactory that is derived from the bean name.

To enable statistics, the standard JPA property hibernate.generate_statistics must be set to true. You can enable that on the auto-configured EntityManagerFactory as shown in the following example:

spring.jpa.properties.hibernate.generate_statistics=true
RabbitMQ Metrics

Auto-configuration will enable the instrumentation of all available RabbitMQ connection factories with a metric named rabbitmq.

5.6.4. Registering custom metrics

To register custom metrics, inject MeterRegistry into your component, as shown in the following example:

class Dictionary {

    private final List<String> words = new CopyOnWriteArrayList<>();

    Dictionary(MeterRegistry registry) {
        registry.gaugeCollectionSize("dictionary.size", Tags.empty(), this.words);
    }

    // …

}

If you find that you repeatedly instrument a suite of metrics across components or applications, you may encapsulate this suite in a MeterBinder implementation. By default, metrics from all MeterBinder beans will be automatically bound to the Spring-managed MeterRegistry.

5.6.5. Customizing individual metrics

If you need to apply customizations to specific Meter instances you can use the io.micrometer.core.instrument.config.MeterFilter interface. By default, all MeterFilter beans will be automatically applied to the micrometer MeterRegistry.Config.

For example, if you want to rename the mytag.region tag to mytag.area for all meter IDs beginning with com.example, you can do the following:

@Bean
public MeterFilter renameRegionTagMeterFilter() {
    return MeterFilter.renameTag("com.example", "mytag.region", "mytag.area");
}
Common tags

Common tags are generally used for dimensional drill-down on the operating environment like host, instance, region, stack, etc. Commons tags are applied to all meters and can be configured as shown in the following example:

management.metrics.tags.region=us-east-1
management.metrics.tags.stack=prod

The example above adds region and stack tags to all meters with a value of us-east-1 and prod respectively.

The order of common tags is important if you are using Graphite. As the order of common tags cannot be guaranteed using this approach, Graphite users are advised to define a custom MeterFilter instead.
Per-meter properties

In addition to MeterFilter beans, it’s also possible to apply a limited set of customization on a per-meter basis using properties. Per-meter customizations apply to any all meter IDs that start with the given name. For example, the following will disable any meters that have an ID starting with example.remote

management.metrics.enable.example.remote=false

The following properties allow per-meter customization:

Table 8. Per-meter customizations
Property Description

management.metrics.enable

Whether to deny meters from emitting any metrics.

management.metrics.distribution.percentiles-histogram

Whether to publish a histogram suitable for computing aggregable (across dimension) percentile approximations.

management.metrics.distribution.minimum-expected-value, management.metrics.distribution.maximum-expected-value

Publish less histogram buckets by clamping the range of expected values.

management.metrics.distribution.percentiles

Publish percentile values computed in your application

management.metrics.distribution.sla

Publish a cumulative histogram with buckets defined by your SLAs.

For more details on concepts behind percentiles-histogram, percentiles and sla refer to the "Histograms and percentiles" section of the micrometer documentation.

5.6.6. Metrics endpoint

Spring Boot provides a metrics endpoint that can be used diagnostically to examine the metrics collected by an application. The endpoint is not available by default and must be exposed, see exposing endpoints for more details.

Navigating to /actuator/metrics displays a list of available meter names. You can drill down to view information about a particular meter by providing its name as a selector, e.g. /actuator/metrics/jvm.memory.max.

The name you use here should match the name used in the code, not the name after it has been naming-convention normalized for a monitoring system it is shipped to. In other words, if jvm.memory.max appears as jvm_memory_max in Prometheus because of its snake case naming convention, you should still use jvm.memory.max as the selector when inspecting the meter in the metrics endpoint.

You can also add any number of tag=KEY:VALUE query parameters to the end of the URL to dimensionally drill down on a meter, e.g. /actuator/metrics/jvm.memory.max?tag=area:nonheap.

The reported measurements are the sum of the statistics of all meters matching the meter name and any tags that have been applied. So in the example above, the returned "Value" statistic is the sum of the maximum memory footprints of "Code Cache", "Compressed Class Space", and "Metaspace" areas of the heap. If you just wanted to see the maximum size for the "Metaspace", you could add an additional tag=id:Metaspace, i.e. /actuator/metrics/jvm.memory.max?tag=area:nonheap&tag=id:Metaspace.

5.7. Auditing

Once Spring Security is in play, Spring Boot Actuator has a flexible audit framework that publishes events (by default, “authentication success”, “failure” and “access denied” exceptions). This feature can be very useful for reporting and for implementing a lock-out policy based on authentication failures.

Auditing can be enabled by providing a bean of type AuditEventRepository in your application’s configuration. For convenience, Spring Boot offers an InMemoryAuditEventRepository. InMemoryAuditEventRepository has limited capabilities and we recommend using it only for development environments. For production environments, consider creating your own alternative AuditEventRepository implementation.

5.7.1. Custom Auditing

To customize published security events, you can provide your own implementations of AbstractAuthenticationAuditListener and AbstractAuthorizationAuditListener.

You can also use the audit services for your own business events. To do so, either inject the AuditEventRepository bean into your own components and use that directly or publish an AuditApplicationEvent with the Spring ApplicationEventPublisher (by implementing ApplicationEventPublisherAware).

5.8. HTTP Tracing

HTTP Tracing can be enabled by providing a bean of type HttpTraceRepository in your application’s configuration. For convenience, Spring Boot offers an InMemoryHttpTraceRepository that stores traces for the last 100 request-response exchanges, by default. InMemoryHttpTraceRepository is limited compared to other tracing solutions and we recommend using it only for development environments. For production environments, consider creating your own alternative HttpTraceRepository implementation. You can view the httptrace endpoint and obtain information about the request-response exchanges.

5.8.1. Custom HTTP tracing

To customize the items that are included in each trace, use the management.trace.http.include configuration property. For advanced customization, consider registering your own HttpExchangeTracer implementation.

5.9. Process Monitoring

In the spring-boot module, you can find two classes to create files that are often useful for process monitoring:

  • ApplicationPidFileWriter creates a file containing the application PID (by default, in the application directory with a file name of application.pid).

  • WebServerPortFileWriter creates a file (or files) containing the ports of the running web server (by default, in the application directory with a file name of application.port).

By default, these writers are not activated, but you can enable:

5.9.1. Extending Configuration

In the META-INF/spring.factories file, you can activate the listener(s) that writes a PID file, as shown in the following example:

org.springframework.context.ApplicationListener=\
org.springframework.boot.context.ApplicationPidFileWriter,\
org.springframework.boot.web.context.WebServerPortFileWriter

5.9.2. Programmatically

You can also activate a listener by invoking the SpringApplication.addListeners(…​) method and passing the appropriate Writer object. This method also lets you customize the file name and path in the Writer constructor.

5.10. Cloud Foundry Support

Spring Boot’s actuator module includes additional support that is activated when you deploy to a compatible Cloud Foundry instance. The /cloudfoundryapplication path provides an alternative secured route to all @Endpoint beans.

The extended support lets Cloud Foundry management UIs (such as the web application that you can use to view deployed applications) be augmented with Spring Boot actuator information. For example, an application status page may include full health information instead of the typical “running” or “stopped” status.

The /cloudfoundryapplication path is not directly accessible to regular users. In order to use the endpoint, a valid UAA token must be passed with the request.

5.10.1. Disabling Extended Cloud Foundry Actuator Support

If you want to fully disable the /cloudfoundryapplication endpoints, you can add the following setting to your application.properties file:

application.properties
management.cloudfoundry.enabled=false

5.10.2. Cloud Foundry Self-signed Certificates

By default, the security verification for /cloudfoundryapplication endpoints makes SSL calls to various Cloud Foundry services. If your Cloud Foundry UAA or Cloud Controller services use self-signed certificates, you need to set the following property:

application.properties
management.cloudfoundry.skip-ssl-validation=true

5.10.3. Custom context path

If the server’s context-path has been configured to anything other than /, the Cloud Foundry endpoints will not be available at the root of the application. For example, if server.servlet.context-path=/app, Cloud Foundry endpoints will be available at /app/cloudfoundryapplication/*.

If you expect the Cloud Foundry endpoints to always be available at /cloudfoundryapplication/*, regardless of the server’s context-path, you will need to explicitly configure that in your application. The configuration will differ depending on the web server in use. For Tomcat, the following configuration can be added:

@Bean
public TomcatServletWebServerFactory servletWebServerFactory() {
    return new TomcatServletWebServerFactory() {

        @Override
        protected void prepareContext(Host host, ServletContextInitializer[] initializers) {
            super.prepareContext(host, initializers);
            StandardContext child = new StandardContext();
            child.addLifecycleListener(new Tomcat.FixContextListener());
            child.setPath("/cloudfoundryapplication");
            ServletContainerInitializer initializer = getServletContextInitializer(getContextPath());
            child.addServletContainerInitializer(initializer, Collections.emptySet());
            child.setCrossContext(true);
            host.addChild(child);
        }

    };
}

private ServletContainerInitializer getServletContextInitializer(String contextPath) {
    return (c, context) -> {
        Servlet servlet = new GenericServlet() {

            @Override
            public void service(ServletRequest req, ServletResponse res) throws ServletException, IOException {
                ServletContext context = req.getServletContext().getContext(contextPath);
                context.getRequestDispatcher("/cloudfoundryapplication").forward(req, res);
            }

        };
        context.addServlet("cloudfoundry", servlet).addMapping("/*");
    };
}

5.11. What to Read Next

If you want to explore some of the concepts discussed in this chapter, you can take a look at the actuator sample applications. You also might want to read about graphing tools such as Graphite.

Otherwise, you can continue on, to read about ‘deployment options’ or jump ahead for some in-depth information about Spring Boot’s build tool plugins.

6. Deploying Spring Boot Applications

6.1. Deploying to the Cloud

Spring Boot’s executable jars are ready-made for most popular cloud PaaS (Platform-as-a-Service) providers. These providers tend to require that you “bring your own container”. They manage application processes (not Java applications specifically), so they need an intermediary layer that adapts your application to the cloud’s notion of a running process.

Two popular cloud providers, Heroku and Cloud Foundry, employ a “buildpack” approach. The buildpack wraps your deployed code in whatever is needed to start your application. It might be a JDK and a call to java, an embedded web server, or a full-fledged application server. A buildpack is pluggable, but ideally you should be able to get by with as few customizations to it as possible. This reduces the footprint of functionality that is not under your control. It minimizes divergence between development and production environments.

Ideally, your application, like a Spring Boot executable jar, has everything that it needs to run packaged within it.

In this section, we look at what it takes to get the simple application that we developed in the “Getting Started” section up and running in the Cloud.

6.1.1. Cloud Foundry

Cloud Foundry provides default buildpacks that come into play if no other buildpack is specified. The Cloud Foundry Java buildpack has excellent support for Spring applications, including Spring Boot. You can deploy stand-alone executable jar applications as well as traditional .war packaged applications.

Once you have built your application (by using, for example, mvn clean package) and have installed the cf command line tool, deploy your application by using the cf push command, substituting the path to your compiled .jar. Be sure to have logged in with your cf command line client before pushing an application. The following line shows using the cf push command to deploy an application:

$ cf push acloudyspringtime -p target/demo-0.0.1-SNAPSHOT.jar
In the preceding example, we substitute acloudyspringtime for whatever value you give cf as the name of your application.

See the cf push documentation for more options. If there is a Cloud Foundry manifest.yml file present in the same directory, it is considered.

At this point, cf starts uploading your application, producing output similar to the following example:

Uploading acloudyspringtime... OK
Preparing to start acloudyspringtime... OK
-----> Downloaded app package (8.9M)
-----> Java Buildpack Version: v3.12 (offline) | https://github.com/cloudfoundry/java-buildpack.git#6f25b7e
-----> Downloading Open Jdk JRE 1.8.0_121 from https://java-buildpack.cloudfoundry.org/openjdk/trusty/x86_64/openjdk-1.8.0_121.tar.gz (found in cache)
       Expanding Open Jdk JRE to .java-buildpack/open_jdk_jre (1.6s)
-----> Downloading Open JDK Like Memory Calculator 2.0.2_RELEASE from https://java-buildpack.cloudfoundry.org/memory-calculator/trusty/x86_64/memory-calculator-2.0.2_RELEASE.tar.gz (found in cache)
       Memory Settings: -Xss349K -Xmx681574K -XX:MaxMetaspaceSize=104857K -Xms681574K -XX:MetaspaceSize=104857K
-----> Downloading Container Certificate Trust Store 1.0.0_RELEASE from https://java-buildpack.cloudfoundry.org/container-certificate-trust-store/container-certificate-trust-store-1.0.0_RELEASE.jar (found in cache)
       Adding certificates to .java-buildpack/container_certificate_trust_store/truststore.jks (0.6s)
-----> Downloading Spring Auto Reconfiguration 1.10.0_RELEASE from https://java-buildpack.cloudfoundry.org/auto-reconfiguration/auto-reconfiguration-1.10.0_RELEASE.jar (found in cache)
Checking status of app 'acloudyspringtime'...
  0 of 1 instances running (1 starting)
  ...
  0 of 1 instances running (1 starting)
  ...
  0 of 1 instances running (1 starting)
  ...
  1 of 1 instances running (1 running)

App started

Congratulations! The application is now live!

Once your application is live, you can verify the status of the deployed application by using the cf apps command, as shown in the following example:

$ cf apps
Getting applications in ...
OK

name                 requested state   instances   memory   disk   urls
...
acloudyspringtime    started           1/1         512M     1G     acloudyspringtime.cfapps.io
...

Once Cloud Foundry acknowledges that your application has been deployed, you should be able to find the application at the URI given. In the preceding example, you could find it at https://acloudyspringtime.cfapps.io/.

Binding to Services

By default, metadata about the running application as well as service connection information is exposed to the application as environment variables (for example: $VCAP_SERVICES). This architecture decision is due to Cloud Foundry’s polyglot (any language and platform can be supported as a buildpack) nature. Process-scoped environment variables are language agnostic.

Environment variables do not always make for the easiest API, so Spring Boot automatically extracts them and flattens the data into properties that can be accessed through Spring’s Environment abstraction, as shown in the following example:

@Component
class MyBean implements EnvironmentAware {

    private String instanceId;

    @Override
    public void setEnvironment(Environment environment) {
        this.instanceId = environment.getProperty("vcap.application.instance_id");
    }

    // ...

}

All Cloud Foundry properties are prefixed with vcap. You can use vcap properties to access application information (such as the public URL of the application) and service information (such as database credentials). See the ‘CloudFoundryVcapEnvironmentPostProcessor’ Javadoc for complete details.

The Spring Cloud Connectors project is a better fit for tasks such as configuring a DataSource. Spring Boot includes auto-configuration support and a spring-boot-starter-cloud-connectors starter.

6.1.2. Heroku

Heroku is another popular PaaS platform. To customize Heroku builds, you provide a Procfile, which provides the incantation required to deploy an application. Heroku assigns a port for the Java application to use and then ensures that routing to the external URI works.

You must configure your application to listen on the correct port. The following example shows the Procfile for our starter REST application:

web: java -Dserver.port=$PORT -jar target/demo-0.0.1-SNAPSHOT.jar

Spring Boot makes -D arguments available as properties accessible from a Spring Environment instance. The server.port configuration property is fed to the embedded Tomcat, Jetty, or Undertow instance, which then uses the port when it starts up. The $PORT environment variable is assigned to us by the Heroku PaaS.

This should be everything you need. The most common deployment workflow for Heroku deployments is to git push the code to production, as shown in the following example:

$ git push heroku master

Initializing repository, done.
Counting objects: 95, done.
Delta compression using up to 8 threads.
Compressing objects: 100% (78/78), done.
Writing objects: 100% (95/95), 8.66 MiB | 606.00 KiB/s, done.
Total 95 (delta 31), reused 0 (delta 0)

-----> Java app detected
-----> Installing OpenJDK 1.8... done
-----> Installing Maven 3.3.1... done
-----> Installing settings.xml... done
-----> Executing: mvn -B -DskipTests=true clean install

       [INFO] Scanning for projects...
       Downloading: https://repo.spring.io/...
       Downloaded: https://repo.spring.io/... (818 B at 1.8 KB/sec)
        ....
       Downloaded: https://s3pository.heroku.com/jvm/... (152 KB at 595.3 KB/sec)
       [INFO] Installing /tmp/build_0c35a5d2-a067-4abc-a232-14b1fb7a8229/target/...
       [INFO] Installing /tmp/build_0c35a5d2-a067-4abc-a232-14b1fb7a8229/pom.xml ...
       [INFO] ------------------------------------------------------------------------
       [INFO] BUILD SUCCESS
       [INFO] ------------------------------------------------------------------------
       [INFO] Total time: 59.358s
       [INFO] Finished at: Fri Mar 07 07:28:25 UTC 2014
       [INFO] Final Memory: 20M/493M
       [INFO] ------------------------------------------------------------------------

-----> Discovering process types
       Procfile declares types -> web

-----> Compressing... done, 70.4MB
-----> Launching... done, v6
       https://agile-sierra-1405.herokuapp.com/ deployed to Heroku

To [email protected]:agile-sierra-1405.git
 * [new branch]      master -> master

Your application should now be up and running on Heroku. For more details, refer to Deploying Spring Boot Applications to Heroku.

6.1.3. OpenShift

OpenShift is the Red Hat public (and enterprise) extension of the Kubernetes container orchestration platform. Similarly to Kubernetes, OpenShift has many options for installing Spring Boot based applications.

OpenShift has many resources describing how to deploy Spring Boot applications, including:

6.1.4. Amazon Web Services (AWS)

Amazon Web Services offers multiple ways to install Spring Boot-based applications, either as traditional web applications (war) or as executable jar files with an embedded web server. The options include:

  • AWS Elastic Beanstalk

  • AWS Code Deploy

  • AWS OPS Works

  • AWS Cloud Formation

  • AWS Container Registry

Each has different features and pricing models. In this document, we describe only the simplest option: AWS Elastic Beanstalk.

AWS Elastic Beanstalk

As described in the official Elastic Beanstalk Java guide, there are two main options to deploy a Java application. You can either use the “Tomcat Platform” or the “Java SE platform”.

Using the Tomcat Platform

This option applies to Spring Boot projects that produce a war file. No special configuration is required. You need only follow the official guide.

Using the Java SE Platform

This option applies to Spring Boot projects that produce a jar file and run an embedded web container. Elastic Beanstalk environments run an nginx instance on port 80 to proxy the actual application, running on port 5000. To configure it, add the following line to your application.properties file:

server.port=5000
Upload binaries instead of sources

By default, Elastic Beanstalk uploads sources and compiles them in AWS. However, it is best to upload the binaries instead. To do so, add lines similar to the following to your .elasticbeanstalk/config.yml file:

deploy:
    artifact: target/demo-0.0.1-SNAPSHOT.jar
Reduce costs by setting the environment type

By default an Elastic Beanstalk environment is load balanced. The load balancer has a significant cost. To avoid that cost, set the environment type to “Single instance”, as described in the Amazon documentation. You can also create single instance environments by using the CLI and the following command:

eb create -s
Summary

This is one of the easiest ways to get to AWS, but there are more things to cover, such as how to integrate Elastic Beanstalk into any CI / CD tool, use the Elastic Beanstalk Maven plugin instead of the CLI, and others. There is a blog post covering these topics more in detail.

6.1.5. Boxfuse and Amazon Web Services

Boxfuse works by turning your Spring Boot executable jar or war into a minimal VM image that can be deployed unchanged either on VirtualBox or on AWS. Boxfuse comes with deep integration for Spring Boot and uses the information from your Spring Boot configuration file to automatically configure ports and health check URLs. Boxfuse leverages this information both for the images it produces as well as for all the resources it provisions (instances, security groups, elastic load balancers, and so on).

Once you have created a Boxfuse account, connected it to your AWS account, installed the latest version of the Boxfuse Client, and ensured that the application has been built by Maven or Gradle (by using, for example, mvn clean package), you can deploy your Spring Boot application to AWS with a command similar to the following:

$ boxfuse run myapp-1.0.jar -env=prod

See the boxfuse run documentation for more options. If there is a boxfuse.conf file present in the current directory, it is considered.

By default, Boxfuse activates a Spring profile named boxfuse on startup. If your executable jar or war contains an application-boxfuse.properties file, Boxfuse bases its configuration on the properties it contains.

At this point, boxfuse creates an image for your application, uploads it, and configures and starts the necessary resources on AWS, resulting in output similar to the following example:

Fusing Image for myapp-1.0.jar ...
Image fused in 00:06.838s (53937 K) -> axelfontaine/myapp:1.0
Creating axelfontaine/myapp ...
Pushing axelfontaine/myapp:1.0 ...
Verifying axelfontaine/myapp:1.0 ...
Creating Elastic IP ...
Mapping myapp-axelfontaine.boxfuse.io to 52.28.233.167 ...
Waiting for AWS to create an AMI for axelfontaine/myapp:1.0 in eu-central-1 (this may take up to 50 seconds) ...
AMI created in 00:23.557s -> ami-d23f38cf
Creating security group boxfuse-sg_axelfontaine/myapp:1.0 ...
Launching t2.micro instance of axelfontaine/myapp:1.0 (ami-d23f38cf) in eu-central-1 ...
Instance launched in 00:30.306s -> i-92ef9f53
Waiting for AWS to boot Instance i-92ef9f53 and Payload to start at https://52.28.235.61/ ...
Payload started in 00:29.266s -> https://52.28.235.61/
Remapping Elastic IP 52.28.233.167 to i-92ef9f53 ...
Waiting 15s for AWS to complete Elastic IP Zero Downtime transition ...
Deployment completed successfully. axelfontaine/myapp:1.0 is up and running at https://myapp-axelfontaine.boxfuse.io/

Your application should now be up and running on AWS.

See the blog post on deploying Spring Boot apps on EC2 as well as the documentation for the Boxfuse Spring Boot integration to get started with a Maven build to run the app.

6.1.6. Google Cloud

Google Cloud has several options that can be used to launch Spring Boot applications. The easiest to get started with is probably App Engine, but you could also find ways to run Spring Boot in a container with Container Engine or on a virtual machine with Compute Engine.

To run in App Engine, you can create a project in the UI first, which sets up a unique identifier for you and also sets up HTTP routes. Add a Java app to the project and leave it empty and then use the Google Cloud SDK to push your Spring Boot app into that slot from the command line or CI build.

App Engine Standard requires you to use WAR packaging. Follow these steps to deploy App Engine Standard application to Google Cloud.

Alternatively, App Engine Flex requires you to create an app.yaml file to describe the resources your app requires. Normally, you put this file in src/main/appengine, and it should resemble the following file:

service: default

runtime: java
env: flex

runtime_config:
  jdk: openjdk8

handlers:
- url: /.*
  script: this field is required, but ignored

manual_scaling:
  instances: 1

health_check:
  enable_health_check: False

env_variables:
  ENCRYPT_KEY: your_encryption_key_here

You can deploy the app (for example, with a Maven plugin) by adding the project ID to the build configuration, as shown in the following example:

<plugin>
    <groupId>com.google.cloud.tools</groupId>
    <artifactId>appengine-maven-plugin</artifactId>
    <version>1.3.0</version>
    <configuration>
        <project>myproject</project>
    </configuration>
</plugin>

Then deploy with mvn appengine:deploy (if you need to authenticate first, the build fails).

6.2. Installing Spring Boot Applications

In addition to running Spring Boot applications by using java -jar, it is also possible to make fully executable applications for Unix systems. A fully executable jar can be executed like any other executable binary or it can be registered with init.d or systemd. This makes it very easy to install and manage Spring Boot applications in common production environments.

Fully executable jars work by embedding an extra script at the front of the file. Currently, some tools do not accept this format, so you may not always be able to use this technique. For example, jar -xf may silently fail to extract a jar or war that has been made fully executable. It is recommended that you make your jar or war fully executable only if you intend to execute it directly, rather than running it with java -jar or deploying it to a servlet container.

To create a ‘fully executable’ jar with Maven, use the following plugin configuration:

<plugin>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-maven-plugin</artifactId>
    <configuration>
        <executable>true</executable>
    </configuration>
</plugin>

The following example shows the equivalent Gradle configuration:

bootJar {
    launchScript()
}

You can then run your application by typing ./my-application.jar (where my-application is the name of your artifact). The directory containing the jar is used as your application’s working directory.

6.2.1. Supported Operating Systems

The default script supports most Linux distributions and is tested on CentOS and Ubuntu. Other platforms, such as OS X and FreeBSD, require the use of a custom embeddedLaunchScript.

6.2.2. Unix/Linux Services

Spring Boot application can be easily started as Unix/Linux services by using either init.d or systemd.

Installation as an init.d Service (System V)

If you configured Spring Boot’s Maven or Gradle plugin to generate a fully executable jar, and you do not use a custom embeddedLaunchScript, your application can be used as an init.d service. To do so, symlink the jar to init.d to support the standard start, stop, restart, and status commands.

The script supports the following features:

  • Starts the services as the user that owns the jar file

  • Tracks the application’s PID by using /var/run/<appname>/<appname>.pid

  • Writes console logs to /var/log/<appname>.log

Assuming that you have a Spring Boot application installed in /var/myapp, to install a Spring Boot application as an init.d service, create a symlink, as follows:

$ sudo ln -s /var/myapp/myapp.jar /etc/init.d/myapp

Once installed, you can start and stop the service in the usual way. For example, on a Debian-based system, you could start it with the following command:

$ service myapp start
If your application fails to start, check the log file written to /var/log/<appname>.log for errors.

You can also flag the application to start automatically by using your standard operating system tools. For example, on Debian, you could use the following command:

$ update-rc.d myapp defaults <priority>
Securing an init.d Service
The following is a set of guidelines on how to secure a Spring Boot application that runs as an init.d service. It is not intended to be an exhaustive list of everything that should be done to harden an application and the environment in which it runs.

When executed as root, as is the case when root is being used to start an init.d service, the default executable script runs the application as the user who owns the jar file. You should never run a Spring Boot application as root, so your application’s jar file should never be owned by root. Instead, create a specific user to run your application and use chown to make it the owner of the jar file, as shown in the following example:

$ chown bootapp:bootapp your-app.jar

In this case, the default executable script runs the application as the bootapp user.

To reduce the chances of the application’s user account being compromised, you should consider preventing it from using a login shell. For example, you can set the account’s shell to /usr/sbin/nologin.

You should also take steps to prevent the modification of your application’s jar file. Firstly, configure its permissions so that it cannot be written and can only be read or executed by its owner, as shown in the following example:

$ chmod 500 your-app.jar

Second, you should also take steps to limit the damage if your application or the account that’s running it is compromised. If an attacker does gain access, they could make the jar file writable and change its contents. One way to protect against this is to make it immutable by using chattr, as shown in the following example:

$ sudo chattr +i your-app.jar

This will prevent any user, including root, from modifying the jar.

If root is used to control the application’s service and you use a .conf file to customize its startup, the .conf file is read and evaluated by the root user. It should be secured accordingly. Use chmod so that the file can only be read by the owner and use chown to make root the owner, as shown in the following example:

$ chmod 400 your-app.conf
$ sudo chown root:root your-app.conf
Installation as a systemd Service

systemd is the successor of the System V init system and is now being used by many modern Linux distributions. Although you can continue to use init.d scripts with systemd, it is also possible to launch Spring Boot applications by using systemd ‘service’ scripts.

Assuming that you have a Spring Boot application installed in /var/myapp, to install a Spring Boot application as a systemd service, create a script named myapp.service and place it in /etc/systemd/system directory. The following script offers an example:

[Unit]
Description=myapp
After=syslog.target

[Service]
User=myapp
ExecStart=/var/myapp/myapp.jar
SuccessExitStatus=143

[Install]
WantedBy=multi-user.target
Remember to change the Description, User, and ExecStart fields for your application.
The ExecStart field does not declare the script action command, which means that the run command is used by default.

Note that, unlike when running as an init.d service, the user that runs the application, the PID file, and the console log file are managed by systemd itself and therefore must be configured by using appropriate fields in the ‘service’ script. Consult the service unit configuration man page for more details.

To flag the application to start automatically on system boot, use the following command:

$ systemctl enable myapp.service

Refer to man systemctl for more details.

Customizing the Startup Script

The default embedded startup script written by the Maven or Gradle plugin can be customized in a number of ways. For most people, using the default script along with a few customizations is usually enough. If you find you cannot customize something that you need to, use the embeddedLaunchScript option to write your own file entirely.

Customizing the Start Script when It Is Written

It often makes sense to customize elements of the start script as it is written into the jar file. For example, init.d scripts can provide a “description”. Since you know the description up front (and it need not change), you may as well provide it when the jar is generated.

To customize written elements, use the embeddedLaunchScriptProperties option of the Spring Boot Maven plugin or the properties property of the Spring Boot Gradle plugin’s launchScript.

The following property substitutions are supported with the default script:

Name Description Gradle default Maven default

mode

The script mode.

auto

auto

initInfoProvides

The Provides section of “INIT INFO”

${task.baseName}

${project.artifactId}

initInfoRequiredStart

Required-Start section of “INIT INFO”.

$remote_fs $syslog $network

$remote_fs $syslog $network

initInfoRequiredStop

Required-Stop section of “INIT INFO”.

$remote_fs $syslog $network

$remote_fs $syslog $network

initInfoDefaultStart

Default-Start section of “INIT INFO”.

2 3 4 5

2 3 4 5

initInfoDefaultStop

Default-Stop section of “INIT INFO”.

0 1 6

0 1 6

initInfoShortDescription

Short-Description section of “INIT INFO”.

Single-line version of ${project.description} (falling back to ${task.baseName})

${project.name}

initInfoDescription

Description section of “INIT INFO”.

${project.description} (falling back to ${task.baseName})

${project.description} (falling back to ${project.name})

initInfoChkconfig

chkconfig section of “INIT INFO”

2345 99 01

2345 99 01

confFolder

The default value for CONF_FOLDER

Folder containing the jar

Folder containing the jar

inlinedConfScript

Reference to a file script that should be inlined in the default launch script. This can be used to set environmental variables such as JAVA_OPTS before any external config files are loaded

logFolder

Default value for LOG_FOLDER. Only valid for an init.d service

logFilename

Default value for LOG_FILENAME. Only valid for an init.d service

pidFolder

Default value for PID_FOLDER. Only valid for an init.d service

pidFilename

Default value for the name of the PID file in PID_FOLDER. Only valid for an init.d service

useStartStopDaemon

Whether the start-stop-daemon command, when it’s available, should be used to control the process

true

true

stopWaitTime

Default value for STOP_WAIT_TIME in seconds. Only valid for an init.d service

60

60

Customizing a Script When It Runs

For items of the script that need to be customized after the jar has been written, you can use environment variables or a config file.

The following environment properties are supported with the default script:

Variable Description

MODE

The “mode” of operation. The default depends on the way the jar was built but is usually auto (meaning it tries to guess if it is an init script by checking if it is a symlink in a directory called init.d). You can explicitly set it to service so that the stop|start|status|restart commands work or to run if you want to run the script in the foreground.

USE_START_STOP_DAEMON

Whether the start-stop-daemon command, when it’s available, should be used to control the process. Defaults to true.

PID_FOLDER

The root name of the pid folder (/var/run by default).

LOG_FOLDER

The name of the folder in which to put log files (/var/log by default).

CONF_FOLDER

The name of the folder from which to read .conf files (same folder as jar-file by default).

LOG_FILENAME

The name of the log file in the LOG_FOLDER (<appname>.log by default).

APP_NAME

The name of the app. If the jar is run from a symlink, the script guesses the app name. If it is not a symlink or you want to explicitly set the app name, this can be useful.

RUN_ARGS

The arguments to pass to the program (the Spring Boot app).

JAVA_HOME

The location of the java executable is discovered by using the PATH by default, but you can set it explicitly if there is an executable file at $JAVA_HOME/bin/java.

JAVA_OPTS

Options that are passed to the JVM when it is launched.

JARFILE

The explicit location of the jar file, in case the script is being used to launch a jar that it is not actually embedded.

DEBUG

If not empty, sets the -x flag on the shell process, making it easy to see the logic in the script.

STOP_WAIT_TIME

The time in seconds to wait when stopping the application before forcing a shutdown (60 by default).

The PID_FOLDER, LOG_FOLDER, and LOG_FILENAME variables are only valid for an init.d service. For systemd, the equivalent customizations are made by using the ‘service’ script. See the service unit configuration man page for more details.

With the exception of JARFILE and APP_NAME, the settings listed in the preceding section can be configured by using a .conf file. The file is expected to be next to the jar file and have the same name but suffixed with .conf rather than .jar. For example, a jar named /var/myapp/myapp.jar uses the configuration file named /var/myapp/myapp.conf, as shown in the following example:

myapp.conf
JAVA_OPTS=-Xmx1024M
LOG_FOLDER=/custom/log/folder
If you do not like having the config file next to the jar file, you can set a CONF_FOLDER environment variable to customize the location of the config file.

To learn about securing this file appropriately, see the guidelines for securing an init.d service.

6.2.3. Microsoft Windows Services

A Spring Boot application can be started as a Windows service by using winsw.

A (separately maintained sample) describes step-by-step how you can create a Windows service for your Spring Boot application.

6.3. What to Read Next

Check out the Cloud Foundry, Heroku, OpenShift, and Boxfuse web sites for more information about the kinds of features that a PaaS can offer. These are just four of the most popular Java PaaS providers. Since Spring Boot is so amenable to cloud-based deployment, you can freely consider other providers as well.

The next section goes on to cover the Spring Boot CLI, or you can jump ahead to read about build tool plugins.

7. Spring Boot CLI

7.1. Installing the CLI

The Spring Boot CLI (Command-Line Interface) can be installed manually by using SDKMAN! (the SDK Manager) or by using Homebrew or MacPorts if you are an OSX user. See Installing the Spring Boot CLI in the “Getting started” section for comprehensive installation instructions.

7.2. Using the CLI

Once you have installed the CLI, you can run it by typing spring and pressing Enter at the command line. If you run spring without any arguments, a simple help screen is displayed, as follows:

$ spring
usage: spring [--help] [--version]
       <command> [<args>]

Available commands are:

  run [options] <files> [--] [args]
    Run a spring groovy script

  ... more command help is shown here

You can type spring help to get more details about any of the supported commands, as shown in the following example:

$ spring help run
spring run - Run a spring groovy script

usage: spring run [options] <files> [--] [args]

Option                     Description
------                     -----------
--autoconfigure [Boolean]  Add autoconfigure compiler
                             transformations (default: true)
--classpath, -cp           Additional classpath entries
-e, --edit                 Open the file with the default system
                             editor
--no-guess-dependencies    Do not attempt to guess dependencies
--no-guess-imports         Do not attempt to guess imports
-q, --quiet                Quiet logging
-v, --verbose              Verbose logging of dependency
                             resolution
--watch                    Watch the specified file for changes

The version command provides a quick way to check which version of Spring Boot you are using, as follows:

$ spring version
Spring CLI v2.2.0.M4

7.2.1. Running Applications with the CLI

You can compile and run Groovy source code by using the run command. The Spring Boot CLI is completely self-contained, so you do not need any external Groovy installation.

The following example shows a “hello world” web application written in Groovy:

hello.groovy
@RestController
class WebApplication {

    @RequestMapping("/")
    String home() {
        "Hello World!"
    }

}

To compile and run the application, type the following command:

$ spring run hello.groovy

To pass command-line arguments to the application, use -- to separate the commands from the “spring” command arguments, as shown in the following example:

$ spring run hello.groovy -- --server.port=9000

To set JVM command line arguments, you can use the JAVA_OPTS environment variable, as shown in the following example:

$ JAVA_OPTS=-Xmx1024m spring run hello.groovy
When setting JAVA_OPTS on Microsoft Windows, make sure to quote the entire instruction, such as set "JAVA_OPTS=-Xms256m -Xmx2048m". Doing so ensures the values are properly passed to the process.
Deduced “grab” Dependencies

Standard Groovy includes a @Grab annotation, which lets you declare dependencies on third-party libraries. This useful technique lets Groovy download jars in the same way as Maven or Gradle would but without requiring you to use a build tool.

Spring Boot extends this technique further and tries to deduce which libraries to “grab” based on your code. For example, since the WebApplication code shown previously uses @RestController annotations, Spring Boot grabs "Tomcat" and "Spring MVC".

The following items are used as “grab hints”:

Items Grabs

JdbcTemplate, NamedParameterJdbcTemplate, DataSource

JDBC Application.

@EnableJms

JMS Application.

@EnableCaching

Caching abstraction.

@Test

JUnit.

@EnableRabbit

RabbitMQ.

extends Specification

Spock test.

@EnableBatchProcessing

Spring Batch.

@MessageEndpoint @EnableIntegration

Spring Integration.

@Controller @RestController @EnableWebMvc

Spring MVC + Embedded Tomcat.

@EnableWebSecurity

Spring Security.

@EnableTransactionManagement

Spring Transaction Management.

See subclasses of CompilerAutoConfiguration in the Spring Boot CLI source code to understand exactly how customizations are applied.
Deduced “grab” Coordinates

Spring Boot extends Groovy’s standard @Grab support by letting you specify a dependency without a group or version (for example, @Grab('freemarker')). Doing so consults Spring Boot’s default dependency metadata to deduce the artifact’s group and version.

The default metadata is tied to the version of the CLI that you use. it changes only when you move to a new version of the CLI, putting you in control of when the versions of your dependencies may change. A table showing the dependencies and their versions that are included in the default metadata can be found in the appendix.
Default Import Statements

To help reduce the size of your Groovy code, several import statements are automatically included. Notice how the preceding example refers to @Component, @RestController, and @RequestMapping without needing to use fully-qualified names or import statements.

Many Spring annotations work without using import statements. Try running your application to see what fails before adding imports.
Automatic Main Method

Unlike the equivalent Java application, you do not need to include a public static void main(String[] args) method with your Groovy scripts. A SpringApplication is automatically created, with your compiled code acting as the source.

Custom Dependency Management

By default, the CLI uses the dependency management declared in spring-boot-dependencies when resolving @Grab dependencies. Additional dependency management, which overrides the default dependency management, can be configured by using the @DependencyManagementBom annotation. The annotation’s value should specify the coordinates (groupId:artifactId:version) of one or more Maven BOMs.

For example, consider the following declaration:

@DependencyManagementBom("com.example.custom-bom:1.0.0")

The preceding declaration picks up custom-bom-1.0.0.pom in a Maven repository under com/example/custom-versions/1.0.0/.

When you specify multiple BOMs, they are applied in the order in which you declare them, as shown in the following example:

@DependencyManagementBom(["com.example.custom-bom:1.0.0",
        "com.example.another-bom:1.0.0"])

The preceding example indicates that the dependency management in another-bom overrides the dependency management in custom-bom.

You can use @DependencyManagementBom anywhere that you can use @Grab. However, to ensure consistent ordering of the dependency management, you can use @DependencyManagementBom at most once in your application. A useful source of dependency management (which is a superset of Spring Boot’s dependency management) is the Spring IO Platform, which you might include with the following line:

@DependencyManagementBom('io.spring.platform:platform-bom:1.1.2.RELEASE')

7.2.2. Applications with Multiple Source Files

You can use “shell globbing” with all commands that accept file input. Doing so lets you use multiple files from a single directory, as shown in the following example:

$ spring run *.groovy

7.2.3. Packaging Your Application

You can use the jar command to package your application into a self-contained executable jar file, as shown in the following example:

$ spring jar my-app.jar *.groovy

The resulting jar contains the classes produced by compiling the application and all of the application’s dependencies so that it can then be run by using java -jar. The jar file also contains entries from the application’s classpath. You can add and remove explicit paths to the jar by using --include and --exclude. Both are comma-separated, and both accept prefixes, in the form of “+” and “-”, to signify that they should be removed from the defaults. The default includes are as follows:

public/**, resources/**, static/**, templates/**, META-INF/**, *

The default excludes are as follows:

.*, repository/**, build/**, target/**, **/*.jar, **/*.groovy

Type spring help jar on the command line for more information.

7.2.4. Initialize a New Project

The init command lets you create a new project by using start.spring.io without leaving the shell, as shown in the following example:

$ spring init --dependencies=web,data-jpa my-project
Using service at https://start.spring.io
Project extracted to '/Users/developer/example/my-project'

The preceding example creates a my-project directory with a Maven-based project that uses spring-boot-starter-web and spring-boot-starter-data-jpa. You can list the capabilities of the service by using the --list flag, as shown in the following example:

$ spring init --list
=======================================
Capabilities of https://start.spring.io
=======================================

Available dependencies:
-----------------------
actuator - Actuator: Production ready features to help you monitor and manage your application
...
web - Web: Support for full-stack web development, including Tomcat and spring-webmvc
websocket - Websocket: Support for WebSocket development
ws - WS: Support for Spring Web Services

Available project types:
------------------------
gradle-build -  Gradle Config [format:build, build:gradle]
gradle-project -  Gradle Project [format:project, build:gradle]
maven-build -  Maven POM [format:build, build:maven]
maven-project -  Maven Project [format:project, build:maven] (default)

...

The init command supports many options. See the help output for more details. For instance, the following command creates a Gradle project that uses Java 8 and war packaging:

$ spring init --build=gradle --java-version=1.8 --dependencies=websocket --packaging=war sample-app.zip
Using service at https://start.spring.io
Content saved to 'sample-app.zip'

7.2.5. Using the Embedded Shell

Spring Boot includes command-line completion scripts for the BASH and zsh shells. If you do not use either of these shells (perhaps you are a Windows user), you can use the shell command to launch an integrated shell, as shown in the following example:

$ spring shell
Spring Boot (v2.2.0.M4)
Hit TAB to complete. Type \'help' and hit RETURN for help, and \'exit' to quit.

From inside the embedded shell, you can run other commands directly:

$ version
Spring CLI v2.2.0.M4

The embedded shell supports ANSI color output as well as tab completion. If you need to run a native command, you can use the ! prefix. To exit the embedded shell, press ctrl-c.

7.2.6. Adding Extensions to the CLI

You can add extensions to the CLI by using the install command. The command takes one or more sets of artifact coordinates in the format group:artifact:version, as shown in the following example:

$ spring install com.example:spring-boot-cli-extension:1.0.0.RELEASE

In addition to installing the artifacts identified by the coordinates you supply, all of the artifacts' dependencies are also installed.

To uninstall a dependency, use the uninstall command. As with the install command, it takes one or more sets of artifact coordinates in the format of group:artifact:version, as shown in the following example:

$ spring uninstall com.example:spring-boot-cli-extension:1.0.0.RELEASE

It uninstalls the artifacts identified by the coordinates you supply and their dependencies.

To uninstall all additional dependencies, you can use the --all option, as shown in the following example:

$ spring uninstall --all

7.3. Developing Applications with the Groovy Beans DSL

Spring Framework 4.0 has native support for a beans{} “DSL” (borrowed from Grails), and you can embed bean definitions in your Groovy application scripts by using the same format. This is sometimes a good way to include external features like middleware declarations, as shown in the following example:

@Configuration(proxyBeanMethods = false)
class Application implements CommandLineRunner {

    @Autowired
    SharedService service

    @Override
    void run(String... args) {
        println service.message
    }

}

import my.company.SharedService

beans {
    service(SharedService) {
        message = "Hello World"
    }
}

You can mix class declarations with beans{} in the same file as long as they stay at the top level, or, if you prefer, you can put the beans DSL in a separate file.

7.4. Configuring the CLI with settings.xml

The Spring Boot CLI uses Aether, Maven’s dependency resolution engine, to resolve dependencies. The CLI makes use of the Maven configuration found in ~/.m2/settings.xml to configure Aether. The following configuration settings are honored by the CLI:

  • Offline

  • Mirrors

  • Servers

  • Proxies

  • Profiles

    • Activation

    • Repositories

  • Active profiles

See Maven’s settings documentation for further information.

7.5. What to Read Next

There are some sample groovy scripts available from the GitHub repository that you can use to try out the Spring Boot CLI. There is also extensive Javadoc throughout the source code.

If you find that you reach the limit of the CLI tool, you probably want to look at converting your application to a full Gradle or Maven built “Groovy project”. The next section covers Spring Boot’s "Build tool plugins", which you can use with Gradle or Maven.

8. Build Tool Plugins

8.1. Spring Boot Maven Plugin

The Spring Boot Maven Plugin provides Spring Boot support in Maven, letting you package executable jar or war archives and run an application “in-place”. To use it, you must use Maven 3.2 (or later).

See the Spring Boot Maven Plugin Site for complete plugin documentation.

8.1.1. Including the Plugin

To use the Spring Boot Maven Plugin, include the appropriate XML in the plugins section of your pom.xml, as shown in the following example:

<?xml version="1.0" encoding="UTF-8"?>
<project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 https://maven.apache.org/xsd/maven-4.0.0.xsd">
    <modelVersion>4.0.0</modelVersion>
    <!-- ... -->
    <build>
        <plugins>
            <plugin>
                <groupId>org.springframework.boot</groupId>
                <artifactId>spring-boot-maven-plugin</artifactId>
                <version>2.2.0.M4</version>
                <executions>
                    <execution>
                        <goals>
                            <goal>repackage</goal>
                        </goals>
                    </execution>
                </executions>
            </plugin>
        </plugins>
    </build>
</project>

The preceding configuration repackages a jar or war that is built during the package phase of the Maven lifecycle. The following example shows both the repackaged jar as well as the original jar in the target directory:

$ mvn package
$ ls target/*.jar
target/myproject-1.0.0.jar target/myproject-1.0.0.jar.original

If you do not include the <execution/> configuration, as shown in the prior example, you can run the plugin on its own (but only if the package goal is used as well), as shown in the following example:

$ mvn package spring-boot:repackage
$ ls target/*.jar
target/myproject-1.0.0.jar target/myproject-1.0.0.jar.original

If you use a milestone or snapshot release, you also need to add the appropriate pluginRepository elements, as shown in the following listing:

<pluginRepositories>
    <pluginRepository>
        <id>spring-snapshots</id>
        <url>https://repo.spring.io/snapshot</url>
    </pluginRepository>
    <pluginRepository>
        <id>spring-milestones</id>
        <url>https://repo.spring.io/milestone</url>
    </pluginRepository>
</pluginRepositories>

8.1.2. Packaging Executable Jar and War Files

Once spring-boot-maven-plugin has been included in your pom.xml, it automatically tries to rewrite archives to make them executable by using the spring-boot:repackage goal. You should configure your project to build a jar or war (as appropriate) by using the usual packaging element, as shown in the following example:

<?xml version="1.0" encoding="UTF-8"?>
<project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 https://maven.apache.org/xsd/maven-4.0.0.xsd">
    <!-- ... -->
    <packaging>jar</packaging>
    <!-- ... -->
</project>

Your existing archive is enhanced by Spring Boot during the package phase. The main class that you want to launch can be specified either by using a configuration option or by adding a Main-Class attribute to the manifest in the usual way. If you do not specify a main class, the plugin searches for a class with a public static void main(String[] args) method.

To build and run a project artifact, you can type the following:

$ mvn package
$ java -jar target/mymodule-0.0.1-SNAPSHOT.jar

To build a war file that is both executable and deployable into an external container, you need to mark the embedded container dependencies as “provided”, as shown in the following example:

<?xml version="1.0" encoding="UTF-8"?>
<project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 https://maven.apache.org/xsd/maven-4.0.0.xsd">
    <!-- ... -->
    <packaging>war</packaging>
    <!-- ... -->
    <dependencies>
        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter-web</artifactId>
        </dependency>
        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter-tomcat</artifactId>
            <scope>provided</scope>
        </dependency>
        <!-- ... -->
    </dependencies>
</project>
See the “Create a Deployable War File” section for more details on how to create a deployable war file.

Advanced configuration options and examples are available in the plugin info page.

8.2. Spring Boot Gradle Plugin

The Spring Boot Gradle Plugin provides Spring Boot support in Gradle, letting you package executable jar or war archives, run Spring Boot applications, and use the dependency management provided by spring-boot-dependencies. It requires Gradle 5.x (4.10 is also supported but this support is deprecated and will be removed in a future release). Please refer to the plugin’s documentation to learn more:

8.3. Spring Boot AntLib Module

The Spring Boot AntLib module provides basic Spring Boot support for Apache Ant. You can use the module to create executable jars. To use the module, you need to declare an additional spring-boot namespace in your build.xml, as shown in the following example:

<project xmlns:ivy="antlib:org.apache.ivy.ant"
    xmlns:spring-boot="antlib:org.springframework.boot.ant"
    name="myapp" default="build">
    ...
</project>

You need to remember to start Ant using the -lib option, as shown in the following example:

$ ant -lib <folder containing spring-boot-antlib-2.2.0.M4.jar>
The “Using Spring Boot” section includes a more complete example of using Apache Ant with spring-boot-antlib.

8.3.1. Spring Boot Ant Tasks

Once the spring-boot-antlib namespace has been declared, the following additional tasks are available:

spring-boot:exejar

You can use the exejar task to create a Spring Boot executable jar. The following attributes are supported by the task:

Attribute Description Required

destfile

The destination jar file to create

Yes

classes

The root directory of Java class files

Yes

start-class

The main application class to run

No (the default is the first class found that declares a main method)

The following nested elements can be used with the task:

Element Description

resources

One or more Resource Collections describing a set of Resources that should be added to the content of the created jar file.

lib

One or more Resource Collections that should be added to the set of jar libraries that make up the runtime dependency classpath of the application.

Examples

This section shows two examples of Ant tasks.

Specify start-class
<spring-boot:exejar destfile="target/my-application.jar"
        classes="target/classes" start-class="com.example.MyApplication">
    <resources>
        <fileset dir="src/main/resources" />
    </resources>
    <lib>
        <fileset dir="lib" />
    </lib>
</spring-boot:exejar>
Detect start-class
<exejar destfile="target/my-application.jar" classes="target/classes">
    <lib>
        <fileset dir="lib" />
    </lib>
</exejar>

8.3.2. spring-boot:findmainclass

The findmainclass task is used internally by exejar to locate a class declaring a main. If necessary, you can also use this task directly in your build. The following attributes are supported:

Attribute Description Required

classesroot

The root directory of Java class files

Yes (unless mainclass is specified)

mainclass

Can be used to short-circuit the main class search

No

property

The Ant property that should be set with the result

No (result will be logged if unspecified)

Examples

This section contains three examples of using findmainclass.

Find and log
<findmainclass classesroot="target/classes" />
Find and set
<findmainclass classesroot="target/classes" property="main-class" />
Override and set
<findmainclass mainclass="com.example.MainClass" property="main-class" />

8.4. Supporting Other Build Systems

If you want to use a build tool other than Maven, Gradle, or Ant, you likely need to develop your own plugin. Executable jars need to follow a specific format and certain entries need to be written in an uncompressed form (see the “executable jar format” section in the appendix for details).

The Spring Boot Maven and Gradle plugins both make use of spring-boot-loader-tools to actually generate jars. If you need to, you may use this library directly.

8.4.1. Repackaging Archives

To repackage an existing archive so that it becomes a self-contained executable archive, use org.springframework.boot.loader.tools.Repackager. The Repackager class takes a single constructor argument that refers to an existing jar or war archive. Use one of the two available repackage() methods to either replace the original file or write to a new destination. Various settings can also be configured on the repackager before it is run.

8.4.2. Nested Libraries

When repackaging an archive, you can include references to dependency files by using the org.springframework.boot.loader.tools.Libraries interface. We do not provide any concrete implementations of Libraries here as they are usually build-system-specific.

If your archive already includes libraries, you can use Libraries.NONE.

8.4.3. Finding a Main Class

If you do not use Repackager.setMainClass() to specify a main class, the repackager uses ASM to read class files and tries to find a suitable class with a public static void main(String[] args) method. An exception is thrown if more than one candidate is found.

8.4.4. Example Repackage Implementation

The following example shows a typical repackage implementation:

Repackager repackager = new Repackager(sourceJarFile);
repackager.setBackupSource(false);
repackager.repackage(new Libraries() {
            @Override
            public void doWithLibraries(LibraryCallback callback) throws IOException {
                // Build system specific implementation, callback for each dependency
                // callback.library(new Library(nestedFile, LibraryScope.COMPILE));
            }
        });

8.5. What to Read Next

If you are interested in how the build tool plugins work, you can look at the spring-boot-tools module on GitHub. More technical details of the executable jar format are covered in the appendix.

If you have specific build-related questions, you can check out the “how-to” guides.

9. “How-to” Guides

9.1. Spring Boot Application

This section includes topics relating directly to Spring Boot applications.

9.1.1. Create Your Own FailureAnalyzer

FailureAnalyzer is a great way to intercept an exception on startup and turn it into a human-readable message, wrapped in a FailureAnalysis. Spring Boot provides such an analyzer for application-context-related exceptions, JSR-303 validations, and more. You can also create your own.

AbstractFailureAnalyzer is a convenient extension of FailureAnalyzer that checks the presence of a specified exception type in the exception to handle. You can extend from that so that your implementation gets a chance to handle the exception only when it is actually present. If, for whatever reason, you cannot handle the exception, return null to give another implementation a chance to handle the exception.

FailureAnalyzer implementations must be registered in META-INF/spring.factories. The following example registers ProjectConstraintViolationFailureAnalyzer:

org.springframework.boot.diagnostics.FailureAnalyzer=\
com.example.ProjectConstraintViolationFailureAnalyzer
If you need access to the BeanFactory or the Environment, your FailureAnalyzer can simply implement BeanFactoryAware or EnvironmentAware respectively.

9.1.2. Troubleshoot Auto-configuration

The Spring Boot auto-configuration tries its best to “do the right thing”, but sometimes things fail, and it can be hard to tell why.

There is a really useful ConditionEvaluationReport available in any Spring Boot ApplicationContext. You can see it if you enable DEBUG logging output. If you use the spring-boot-actuator (see the Actuator chapter), there is also a conditions endpoint that renders the report in JSON. Use that endpoint to debug the application and see what features have been added (and which have not been added) by Spring Boot at runtime.

Many more questions can be answered by looking at the source code and the Javadoc. When reading the code, remember the following rules of thumb:

  • Look for classes called *AutoConfiguration and read their sources. Pay special attention to the @Conditional* annotations to find out what features they enable and when. Add --debug to the command line or a System property -Ddebug to get a log on the console of all the auto-configuration decisions that were made in your app. In a running Actuator app, look at the conditions endpoint (/actuator/conditions or the JMX equivalent) for the same information.

  • Look for classes that are @ConfigurationProperties (such as ServerProperties) and read from there the available external configuration options. The @ConfigurationProperties annotation has a name attribute that acts as a prefix to external properties. Thus, ServerProperties has prefix="server" and its configuration properties are server.port, server.address, and others. In a running Actuator app, look at the configprops endpoint.

  • Look for uses of the bind method on the Binder to pull configuration values explicitly out of the Environment in a relaxed manner. It is often used with a prefix.

  • Look for @Value annotations that bind directly to the Environment.

  • Look for @ConditionalOnExpression annotations that switch features on and off in response to SpEL expressions, normally evaluated with placeholders resolved from the Environment.

9.1.3. Customize the Environment or ApplicationContext Before It Starts

A SpringApplication has ApplicationListeners and ApplicationContextInitializers that are used to apply customizations to the context or environment. Spring Boot loads a number of such customizations for use internally from META-INF/spring.factories. There is more than one way to register additional customizations:

  • Programmatically, per application, by calling the addListeners and addInitializers methods on SpringApplication before you run it.

  • Declaratively, per application, by setting the context.initializer.classes or context.listener.classes properties.

  • Declaratively, for all applications, by adding a META-INF/spring.factories and packaging a jar file that the applications all use as a library.

The SpringApplication sends some special ApplicationEvents to the listeners (some even before the context is created) and then registers the listeners for events published by the ApplicationContext as well. See “Application Events and Listeners” in the ‘Spring Boot features’ section for a complete list.

It is also possible to customize the Environment before the application context is refreshed by using EnvironmentPostProcessor. Each implementation should be registered in META-INF/spring.factories, as shown in the following example:

org.springframework.boot.env.EnvironmentPostProcessor=com.example.YourEnvironmentPostProcessor

The implementation can load arbitrary files and add them to the Environment. For instance, the following example loads a YAML configuration file from the classpath:

public class EnvironmentPostProcessorExample implements EnvironmentPostProcessor {

    private final YamlPropertySourceLoader loader = new YamlPropertySourceLoader();

    @Override
    public void postProcessEnvironment(ConfigurableEnvironment environment, SpringApplication application) {
        Resource path = new ClassPathResource("com/example/myapp/config.yml");
        PropertySource<?> propertySource = loadYaml(path);
        environment.getPropertySources().addLast(propertySource);
    }

    private PropertySource<?> loadYaml(Resource path) {
        if (!path.exists()) {
            throw new IllegalArgumentException("Resource " + path + " does not exist");
        }
        try {
            return this.loader.load("custom-resource", path).get(0);
        }
        catch (IOException ex) {
            throw new IllegalStateException("Failed to load yaml configuration from " + path, ex);
        }
    }

}
The Environment has already been prepared with all the usual property sources that Spring Boot loads by default. It is therefore possible to get the location of the file from the environment. The preceding example adds the custom-resource property source at the end of the list so that a key defined in any of the usual other locations takes precedence. A custom implementation may define another order.
While using @PropertySource on your @SpringBootApplication may seem to be a convenient and easy way to load a custom resource in the Environment, we do not recommend it, because Spring Boot prepares the Environment before the ApplicationContext is refreshed. Any key defined with @PropertySource is loaded too late to have any effect on auto-configuration.

9.1.4. Build an ApplicationContext Hierarchy (Adding a Parent or Root Context)

You can use the ApplicationBuilder class to create parent/child ApplicationContext hierarchies. See “Fluent Builder API” in the ‘Spring Boot features’ section for more information.

9.1.5. Create a Non-web Application

Not all Spring applications have to be web applications (or web services). If you want to execute some code in a main method but also bootstrap a Spring application to set up the infrastructure to use, you can use the SpringApplication features of Spring Boot. A SpringApplication changes its ApplicationContext class, depending on whether it thinks it needs a web application or not. The first thing you can do to help it is to leave server-related dependencies (e.g. servlet API) off the classpath. If you cannot do that (for example, you run two applications from the same code base) then you can explicitly call setWebApplicationType(WebApplicationType.NONE) on your SpringApplication instance or set the applicationContextClass property (through the Java API or with external properties). Application code that you want to run as your business logic can be implemented as a CommandLineRunner and dropped into the context as a @Bean definition.

9.2. Properties and Configuration

This section includes topics about setting and reading properties and configuration settings and their interaction with Spring Boot applications.

9.2.1. Automatically Expand Properties at Build Time

Rather than hardcoding some properties that are also specified in your project’s build configuration, you can automatically expand them by instead using the existing build configuration. This is possible in both Maven and Gradle.

Automatic Property Expansion Using Maven

You can automatically expand properties from the Maven project by using resource filtering. If you use the spring-boot-starter-parent, you can then refer to your Maven ‘project properties’ with @..@ placeholders, as shown in the following example:

Only production configuration is filtered that way (in other words, no filtering is applied on src/test/resources).
If you enable the addResources flag, the spring-boot:run goal can add src/main/resources directly to the classpath (for hot reloading purposes). Doing so circumvents the resource filtering and this feature. Instead, you can use the exec:java goal or customize the plugin’s configuration. See the plugin usage page for more details.

If you do not use the starter parent, you need to include the following element inside the <build/> element of your pom.xml:

<resources>
    <resource>
        <directory>src/main/resources</directory>
        <filtering>true</filtering>
    </resource>
</resources>

You also need to include the following element inside <plugins/>:

<plugin>
    <groupId>org.apache.maven.plugins</groupId>
    <artifactId>maven-resources-plugin</artifactId>
    <version>2.7</version>
    <configuration>
        <delimiters>
            <delimiter>@</delimiter>
        </delimiters>
        <useDefaultDelimiters>false</useDefaultDelimiters>
    </configuration>
</plugin>
The useDefaultDelimiters property is important if you use standard Spring placeholders (such as ${placeholder}) in your configuration. If that property is not set to false, these may be expanded by the build.
Automatic Property Expansion Using Gradle

You can automatically expand properties from the Gradle project by configuring the Java plugin’s processResources task to do so, as shown in the following example:

processResources {
    expand(project.properties)
}

You can then refer to your Gradle project’s properties by using placeholders, as shown in the following example:

app.name=${name}
app.description=${description}
Gradle’s expand method uses Groovy’s SimpleTemplateEngine, which transforms ${..} tokens. The ${..} style conflicts with Spring’s own property placeholder mechanism. To use Spring property placeholders together with automatic expansion, escape the Spring property placeholders as follows: \${..}.

9.2.2. Externalize the Configuration of SpringApplication

A SpringApplication has bean properties (mainly setters), so you can use its Java API as you create the application to modify its behavior. Alternatively, you can externalize the configuration by setting properties in spring.main.*. For example, in application.properties, you might have the following settings:

spring.main.web-application-type=none
spring.main.banner-mode=off

Then the Spring Boot banner is not printed on startup, and the application is not starting an embedded web server.

Properties defined in external configuration override the values specified with the Java API, with the notable exception of the sources used to create the ApplicationContext. Consider the following application:

new SpringApplicationBuilder()
    .bannerMode(Banner.Mode.OFF)
    .sources(demo.MyApp.class)
    .run(args);

Now consider the following configuration:

spring.main.sources=com.acme.Config,com.acme.ExtraConfig
spring.main.banner-mode=console

The actual application now shows the banner (as overridden by configuration) and uses three sources for the ApplicationContext (in the following order): demo.MyApp, com.acme.Config, and com.acme.ExtraConfig.

9.2.3. Change the Location of External Properties of an Application

By default, properties from different sources are added to the Spring Environment in a defined order (see “Externalized Configuration” in the ‘Spring Boot features’ section for the exact order).

A nice way to augment and modify this ordering is to add @PropertySource annotations to your application sources. Classes passed to the SpringApplication static convenience methods and those added using setSources() are inspected to see if they have @PropertySources. If they do, those properties are added to the Environment early enough to be used in all phases of the ApplicationContext lifecycle. Properties added in this way have lower priority than any added by using the default locations (such as application.properties), system properties, environment variables, or the command line.

You can also provide the following System properties (or environment variables) to change the behavior:

  • spring.config.name (SPRING_CONFIG_NAME): Defaults to application as the root of the file name.

  • spring.config.location (SPRING_CONFIG_LOCATION): The file to load (such as a classpath resource or a URL). A separate Environment property source is set up for this document and it can be overridden by system properties, environment variables, or the command line.

No matter what you set in the environment, Spring Boot always loads application.properties as described above. By default, if YAML is used, then files with the ‘.yml’ extension are also added to the list.

Spring Boot logs the configuration files that are loaded at the DEBUG level and the candidates it has not found at TRACE level.

See ConfigFileApplicationListener for more detail.

9.2.4. Use ‘Short’ Command Line Arguments

Some people like to use (for example) --port=9000 instead of --server.port=9000 to set configuration properties on the command line. You can enable this behavior by using placeholders in application.properties, as shown in the following example:

server.port=${port:8080}
If you inherit from the spring-boot-starter-parent POM, the default filter token of the maven-resources-plugins has been changed from ${*} to @ (that is, @maven.token@ instead of ${maven.token}) to prevent conflicts with Spring-style placeholders. If you have enabled Maven filtering for the application.properties directly, you may want to also change the default filter token to use other delimiters.
In this specific case, the port binding works in a PaaS environment such as Heroku or Cloud Foundry. In those two platforms, the PORT environment variable is set automatically and Spring can bind to capitalized synonyms for Environment properties.

9.2.5. Use YAML for External Properties

YAML is a superset of JSON and, as such, is a convenient syntax for storing external properties in a hierarchical format, as shown in the following example:

spring:
    application:
        name: cruncher
    datasource:
        driverClassName: com.mysql.jdbc.Driver
        url: jdbc:mysql://localhost/test
server:
    port: 9000

Create a file called application.yml and put it in the root of your classpath. Then add snakeyaml to your dependencies (Maven coordinates org.yaml:snakeyaml, already included if you use the spring-boot-starter). A YAML file is parsed to a Java Map<String,Object> (like a JSON object), and Spring Boot flattens the map so that it is one level deep and has period-separated keys, as many people are used to with Properties files in Java.

The preceding example YAML corresponds to the following application.properties file:

spring.application.name=cruncher
spring.datasource.driverClassName=com.mysql.jdbc.Driver
spring.datasource.url=jdbc:mysql://localhost/test
server.port=9000

See “Using YAML Instead of Properties” in the ‘Spring Boot features’ section for more information about YAML.

9.2.6. Set the Active Spring Profiles

The Spring Environment has an API for this, but you would normally set a System property (spring.profiles.active) or an OS environment variable (SPRING_PROFILES_ACTIVE). Also, you can launch your application with a -D argument (remember to put it before the main class or jar archive), as follows:

$ java -jar -Dspring.profiles.active=production demo-0.0.1-SNAPSHOT.jar

In Spring Boot, you can also set the active profile in application.properties, as shown in the following example:

spring.profiles.active=production

A value set this way is replaced by the System property or environment variable setting but not by the SpringApplicationBuilder.profiles() method. Thus, the latter Java API can be used to augment the profiles without changing the defaults.

See “Profiles” in the “Spring Boot features” section for more information.

9.2.7. Change Configuration Depending on the Environment

A YAML file is actually a sequence of documents separated by --- lines, and each document is parsed separately to a flattened map.

If a YAML document contains a spring.profiles key, then the profiles value (a comma-separated list of profiles) is fed into the Spring Environment.acceptsProfiles() method. If any of those profiles is active, that document is included in the final merge (otherwise, it is not), as shown in the following example:

server:
    port: 9000
---

spring:
    profiles: development
server:
    port: 9001

---

spring:
    profiles: production
server:
    port: 0

In the preceding example, the default port is 9000. However, if the Spring profile called ‘development’ is active, then the port is 9001. If ‘production’ is active, then the port is 0.

The YAML documents are merged in the order in which they are encountered. Later values override earlier values.

To do the same thing with properties files, you can use application-${profile}.properties to specify profile-specific values.

9.2.8. Discover Built-in Options for External Properties

Spring Boot binds external properties from application.properties (or .yml files and other places) into an application at runtime. There is not (and technically cannot be) an exhaustive list of all supported properties in a single location, because contributions can come from additional jar files on your classpath.

A running application with the Actuator features has a configprops endpoint that shows all the bound and bindable properties available through @ConfigurationProperties.

The appendix includes an application.properties example with a list of the most common properties supported by Spring Boot. The definitive list comes from searching the source code for @ConfigurationProperties and @Value annotations as well as the occasional use of Binder. For more about the exact ordering of loading properties, see "Externalized Configuration".

9.3. Embedded Web Servers

Each Spring Boot web application includes an embedded web server. This feature leads to a number of how-to questions, including how to change the embedded server and how to configure the embedded server. This section answers those questions.

9.3.1. Use Another Web Server

Many Spring Boot starters include default embedded containers.

  • For servlet stack applications, the spring-boot-starter-web includes Tomcat by including spring-boot-starter-tomcat, but you can use spring-boot-starter-jetty or spring-boot-starter-undertow instead.

  • For reactive stack applications, the spring-boot-starter-webflux includes Reactor Netty by including spring-boot-starter-reactor-netty, but you can use spring-boot-starter-tomcat, spring-boot-starter-jetty, or spring-boot-starter-undertow instead.

When switching to a different HTTP server, you need to exclude the default dependencies in addition to including the one you need. Spring Boot provides separate starters for HTTP servers to help make this process as easy as possible.

The following Maven example shows how to exclude Tomcat and include Jetty for Spring MVC:

<properties>
    <servlet-api.version>3.1.0</servlet-api.version>
</properties>
<dependency>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-starter-web</artifactId>
    <exclusions>
        <!-- Exclude the Tomcat dependency -->
        <exclusion>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter-tomcat</artifactId>
        </exclusion>
    </exclusions>
</dependency>
<!-- Use Jetty instead -->
<dependency>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-starter-jetty</artifactId>
</dependency>
The version of the Servlet API has been overridden as, unlike Tomcat 9 and Undertow 2.0, Jetty 9.4 does not support Servlet 4.0.

The following Gradle example shows how to exclude Netty and include Undertow for Spring WebFlux:

configurations {
    // exclude Reactor Netty
    compile.exclude module: 'spring-boot-starter-reactor-netty'
}

dependencies {
    compile 'org.springframework.boot:spring-boot-starter-webflux'
    // Use Undertow instead
    compile 'org.springframework.boot:spring-boot-starter-undertow'
    // ...
}
spring-boot-starter-reactor-netty is required to use the WebClient class, so you may need to keep a dependency on Netty even when you need to include a different HTTP server.

9.3.2. Disabling the Web Server

If your classpath contains the necessary bits to start a web server, Spring Boot will automatically start it. To disable this behaviour configure the WebApplicationType in your application.properties, as shown in the following example: