Spring Boot supports the following embedded servlet containers:

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

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).

10.1.1 Maven Installation

Spring Boot is compatible with Apache Maven 3.2 or above. If you do not already have Maven installed, you can follow the instructions at maven.apache.org.

	Tip
	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 http://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>2.0.5.RELEASE</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>

</project>

	Tip
	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 Section 13.2.2, “Using Spring Boot without the Parent POM” for an alternative solution that uses an import scope.

plugins {
	id 'org.springframework.boot' version '2.0.5.RELEASE'
	id 'java'
}


jar {
	baseName = 'myproject'
	version =  '0.0.1-SNAPSHOT'
}

repositories {
	jcenter()
}

dependencies {
	compile("org.springframework.boot:spring-boot-starter-web")
	testCompile("org.springframework.boot:spring-boot-starter-test")
}

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.

$ sdk install springboot
$ spring --version
Spring Boot v2.0.5.RELEASE

$ sdk install springboot dev /path/to/spring-boot/spring-boot-cli/target/spring-boot-cli-2.0.5.RELEASE-bin/spring-2.0.5.RELEASE/
$ sdk default springboot dev
$ spring --version
Spring CLI v2.0.5.RELEASE

$ sdk ls springboot

================================================================================
Available Springboot Versions
================================================================================
> + dev
* 2.0.5.RELEASE

================================================================================
+ - local version
* - installed
> - currently in use
================================================================================

10.2.3 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.

	Note
	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.

$ sudo port install spring-boot-cli

10.2.5 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

	Note
	If you install the Spring Boot CLI by using Homebrew or MacPorts, the command-line completion scripts are automatically registered with your shell.

10.2.6 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.

	Note
	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.

10.2.7 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

	Note
	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!

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>

	Warning
	Properties that are added late to the environment, such as when using @PropertySource, will not be taken into account.

	Note
	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.

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 http://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>2.0.5.RELEASE</version>
	</parent>

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

</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).

	Note
	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.

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.

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) throws Exception {
		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.

11.3.1 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.

	Tip
	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.

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 ::  (v2.0.5.RELEASE)
....... . . .
....... . . . (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.

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.

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>

	Note
	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:2.0.5.RELEASE: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 ::  (v2.0.5.RELEASE)
....... . . .
....... . . . (log output here)
....... . . .
........ Started Example in 2.536 seconds (JVM running for 2.864)

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

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.

	Note
	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.

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

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

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.)

13.2.1 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>2.0.5.RELEASE</version>
</parent>

	Note
	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>

	Tip
	Check the spring-boot-dependencies pom for a list of supported properties.

13.2.2 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>2.0.5.RELEASE</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>2.0.5.RELEASE</version>
			<type>pom</type>
			<scope>import</scope>
		</dependency>
	</dependencies>
</dependencyManagement>

	Note
	In the preceding example, we specify a BOM, but any dependency type can be overridden in the same way.

13.2.3 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>

	Note
	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.

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

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="2.0.5.RELEASE" />

	<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>

	Tip
	If you do not want to use the spring-boot-antlib module, see the Section 87.9, “Build an Executable Archive from Ant without Using spring-boot-antlib” “How-to” .

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.

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

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

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

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

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, @EntityScan, or @SpringBootApplication annotations, since every class from every jar is read.

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

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.

	Tip
	If you don’t want to use @SpringBootApplication, the @EnableAutoConfiguration and @ComponentScan annotations that it imports defines that behaviour so you can also use that 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);
	}

}

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.

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.

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.

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
@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.

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

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.

	Tip
	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.

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

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

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

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 Chapter 20, Developer Tools section later in this chapter and the Hot swapping “How-to” for details.

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.

	Tip
	For a complete list of the properties that are applied by the devtools, see DevToolsPropertyDefaultsPostProcessor.

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.

	Note
	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, Gradle and Maven do that when they detect DevTools on the classpath.

	Tip
	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.

	Note
	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)).

	Note
	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.

	Note
	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.

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

20.2.2 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/**

	Tip
	If you want to keep those defaults and add additional exclusions, use the spring.devtools.restart.additional-exclude property instead.

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

20.2.5 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.

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

20.2.6 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-]+\.jar
restart.include.projectcommon=/mycorp-myproj-[\\w-]+\.jar

	Note
	All property keys must be unique. As long as a property starts with restart.include. or restart.exclude. it is considered.

	Tip
	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.

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.

	Note
	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.

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

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

	Warning
	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.

20.5.1 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 :: 2.0.5.RELEASE

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)

	Note
	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.

	Tip
	It is always advisable to use https:// as the connection protocol, so that traffic is encrypted and passwords cannot be intercepted.

	Tip
	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.

20.5.2 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.

	Note
	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.

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.

	Note
	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

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:

	Tip
	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.

	Note
	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"

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);
}

	Note
	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 Chapter 24, Externalized Configuration for details.

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

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);

	Note
	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.

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

Note

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:

	Tip
	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.

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

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(…​).

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

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"]
	}

}

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

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.

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.

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.

	Tip
	If you want to know on which HTTP port the application is running, get the property with a key of local.server.port.

	Caution
	Take care when enabling this feature, as the MBean exposes a method to shutdown the application.

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).

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).

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

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

	Note
	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

	Warning
	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:

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:

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:

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.

	Note
	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).

	Note
	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.

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.

	Note
	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.

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

	Tip
	You can also use this technique to create “short” variants of existing Spring Boot properties. See the Section 74.4, “Use ‘Short’ Command Line Arguments” how-to for details.

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.

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

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

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

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

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

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

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

	public List<String> getServers() {
		return this.servers;
	}
}

server:
	address: 192.168.1.100
---
spring:
	profiles: development
server:
	address: 127.0.0.1
---
spring:
	profiles: production
server:
	address: 192.168.1.120

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

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

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, 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:

Note

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.

	Tip
	See also the differences between @Value and @ConfigurationProperties.

You also need to list the properties classes to register in the @EnableConfigurationProperties annotation, as shown in the following example:

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

Note

When the @ConfigurationProperties bean is registered that way, 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.

Even if the preceding configuration creates a regular bean for AcmeProperties, we recommend that @ConfigurationProperties only deal with the environment and, in particular, does not inject other beans from the context. Having said that, the @EnableConfigurationProperties annotation is also automatically applied to your project so that any existing bean annotated with @ConfigurationProperties is configured from the Environment. You could shortcut MyConfiguration by making sure AcmeProperties is already a bean, as shown in the following example:

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

	// ... see the preceding example

}

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());
		// ...
	}

}

	Tip
	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 B, Configuration Metadata appendix for details.

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

24.7.2 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 24.1. 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.

	Note
	The prefix value for the annotation must be in kebab case (lowercase and separated by -, such as acme.my-project.person).

Table 24.2. 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

	Tip
	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.

24.7.3 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).

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

24.7.4 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).

Note

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.util.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 unit. 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.

	Tip
	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.

24.7.5 @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

}

	Tip
	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.

	Tip
	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.

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

	Note
	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 Section 74.7, “Change Configuration Depending on the Environment” for more details.

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.

Profile-specific variants of both application.properties (or application.yml) and files referenced through @ConfigurationProperties are considered as files and loaded. See "Section 24.4, “Profile-specific Properties”" for details.

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:

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

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

	Note
	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).

%clr(%5p)

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

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 or logging.path property (for example, in your application.properties).

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

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.

	Note
	The logging system is initialized early in the application lifecycle. Consequently, logging properties are not found in property files loaded through @PropertySource annotations.

	Tip
	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.

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

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.

	Note
	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:

	Note
	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.

	Warning
	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:

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

	Tip
	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 :-.

Tip

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

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.

	Note
	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.

	Warning
	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]]

<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>

26.6.2 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>

	Note
	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.

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.

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

@Configuration
public class MyConfiguration {

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

}

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> {
		// ...
	}

}

27.1.5 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.

	Tip
	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.

	Note
	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=/**

	Note
	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.

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

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

spring.mvc.contentnegotiation.favor-path-extension=true

# You can also restrict that feature to known extensions only
# spring.mvc.pathmatch.use-registered-suffix-pattern=true

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

27.1.10 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:

• FreeMarker
• Groovy
• Thymeleaf
• Mustache

	Tip
	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.

Tip

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/.

27.1.11 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.

Tip

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.ftl
             +- <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"));
	}

}

	Note
	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.