Spring Boot Reference Documentation

Spring Boot supports the following embedded servlet containers:

You can also deploy Spring Boot applications to any Servlet 3.1 or 4.0 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).

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 a quick way to get a Spring application off the ground without an IDE.

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:

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).

Spring Boot provides a number of “Starters” that let you add jars to your classpath. Our applications for smoke tests use the 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:

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:

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 directory structure and then add a file named src/main/java/MyApplication.java to contain the following code:

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

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:

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

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:

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

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:

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:

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.

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.

To learn about using Spring Boot with Maven, see the documentation for Spring Boot’s Maven plugin:

To learn about using Spring Boot with Gradle, see 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:

A typical build.xml looks like the following example:

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:

To learn how to swap technical facets, please see the how-to documentation for swapping web server and logging system.

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

We 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.

The following listing shows a typical layout:

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

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 @SpringBootApplication to disable them, as shown in the following example:

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

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

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

If you 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:

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:

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:

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

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:

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

Since Spring Boot applications are 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 Hot swapping “How-to” for details.

As described 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, in particular in multi-module projects.

To diagnose whether the classloading issues are indeed caused by devtools and its two classloaders, try disabling restart. If this solves your problems, customize the restart classloader to include your entire project.

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.

The following table lists all the properties that are applied:

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

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 directory is monitored for changes. Note that certain resources, such as static assets and view templates, do not need to restart the application.

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

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

You can configure global devtools settings by adding any of the following files to the $HOME/.config/spring-boot directory:

Any properties added to these files 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 to your spring-boot-devtools file:

By default, $HOME is the user’s home directory. To customize this location, set the SPRING_DEVTOOLS_HOME environment variable or the spring.devtools.home system property.

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 as enabling it can be a security risk. It should only be enabled when running on a trusted network or when secured with SSL. If neither of these options is available to you, you should not use DevTools' remote support. You should never enable support on a production deployment.

To enable it, you need to make sure that devtools is included in the repackaged archive, as shown in the following listing:

Then you need to set the spring.devtools.remote.secret property. Like any important password or secret, the value should be unique and strong such that it cannot be guessed or brute-forced.

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.

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:

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:

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

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

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

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 key available in the Environment as well as any of the following placeholders:

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.

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:

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

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

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:

When deployed on platforms, applications can provide information about their availability to the platform using infrastructure such as Kubernetes Probes. Spring Boot includes out-of-the box support for the commonly used “liveness” and “readiness” availability states. If you are using Spring Boot’s “actuator” support then these states are exposed as health endpoint groups.

In addition, you can also obtain availability states by injecting the ApplicationAvailability interface into your own beans.

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

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

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

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 the following:

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 setApplicationContextFactory(…​).

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:

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 string array, whereas the ApplicationRunner uses the ApplicationArguments interface discussed earlier. The following example shows a CommandLineRunner with a run method:

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:

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.

If there is more than one ExitCodeGenerator, the first non-zero exit code that is generated is used. To control the order in which the generators are called, additionally implement the org.springframework.core.Ordered interface or use the org.springframework.core.annotation.Order annotation.

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.

During the application startup, the SpringApplication and the ApplicationContext perform many tasks related to the application lifecycle, the beans lifecycle or even processing application events. With ApplicationStartup, Spring Framework allows you to track the application startup sequence with StartupStep objects. This data can be collected for profiling purposes, or just to have a better understanding of an application startup process.

You can choose an ApplicationStartup implementation when setting up the SpringApplication instance. For example, to use the BufferingApplicationStartup, you could write:

The first available implementation, FlightRecorderApplicationStartup is provided by Spring Framework. It adds Spring-specific startup events to a Java Flight Recorder session and is meant for profiling applications and correlating their Spring context lifecycle with JVM events (such as allocations, GCs, class loading…​). Once configured, you can record data by running the application with the Flight Recorder enabled:

Spring Boot ships with the BufferingApplicationStartup variant; this implementation is meant for buffering the startup steps and draining them into an external metrics system. Applications can ask for the bean of type BufferingApplicationStartup in any component.

Spring Boot can also be configured to expose a startup endpoint that provides this information as a JSON document.

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 file-based 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).

Environment variables and system properties often have restrictions that mean some property names cannot be used. To help with this, Spring Boot allows you to encode a block of properties into a single JSON structure.

When your application starts, any spring.application.json or SPRING_APPLICATION_JSON properties will be parsed and added to the Environment.

For example, the SPRING_APPLICATION_JSON property can be supplied on the command line in a UN*X shell as an environment variable:

In the preceding example, you end up with my.name=test in the Spring Environment.

The same JSON can also be provided as a system property:

Or you could supply the JSON by using a command line argument:

If you are deploying to a classic Application Server, you could also use a JNDI variable named java:comp/env/spring.application.json.

Spring Boot will automatically find and load application.properties and application.yaml files from the following locations when your application starts:

The list is ordered by precedence (with values from lower items overriding earlier ones). Documents from the loaded files are added as PropertySources to the Spring Environment.

If you do not like application as the configuration file name, you can switch to another file name by specifying a spring.config.name environment property. For example, to look for myproject.properties and myproject.yaml files you can run your application as follows:

You can also refer to an explicit location by using the spring.config.location environment property. This property accepts a comma-separated list of one or more locations to check.

The following example shows how to specify two distinct files:

If spring.config.location contains directories (as opposed to files), they should end in /. At runtime they will be appended with the names generated from spring.config.name before being loaded. Files specified in spring.config.location are imported directly.

In most situations, each spring.config.location item you add will reference a single file or directory. Locations are processed in the order that they are defined and later ones can override the values of earlier ones.

If you have a complex location setup, and you use profile-specific configuration files, you may need to provide further hints so that Spring Boot knows how they should be grouped. A location group is a collection of locations that are all considered at the same level. For example, you might want to group all classpath locations, then all external locations. Items within a location group should be separated with ;. See the example in the “Profile Specific Files” section for more details.

Locations configured by using spring.config.location replace the default locations. For example, if spring.config.location is configured with the value optional:classpath:/custom-config/,optional:file:./custom-config/, the complete set of locations considered is:

If you prefer to add additional locations, rather than replacing them, you can use spring.config.additional-location. Properties loaded from additional locations can override those in the default locations. For example, if spring.config.additional-location is configured with the value optional:classpath:/custom-config/,optional:file:./custom-config/, the complete set of locations considered is:

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.

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

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

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.

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:

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).

Spring Boot supports setting a prefix for environment properties. This is useful if the system environment is shared by multiple Spring Boot applications with different configuration requirements. The prefix for system environment properties can be set directly on SpringApplication.

For example, if you set the prefix to input, a property such as remote.timeout will also be resolved as input.remote.timeout in the system environment.

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.

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

For example, when an application with the following properties is run, the common and local profiles will be activated even when it runs using the --spring.profiles.active switch:

Profile groups, which are described in the next section can also be used to add active profiles if a given profile is active.

Occasionally the profiles that you define and use in your application are too fine-grained and become cumbersome to use. For example, you might have proddb and prodmq profiles that you use to enable database and messaging features independently.

To help with this, Spring Boot lets you define profile groups. A profile group allows you to define a logical name for a related group of profiles.

For example, we can create a production group that consists of our proddb and prodmq profiles.

Our application can now be started using --spring.profiles.active=production to active the production, proddb and prodmq profiles in one hit.

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 "Profile Specific Files" for details.

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

The following items are output:

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

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).

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

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

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.

If you are using the Logback, it is possible to fine-tune log rotation settings using your application.properties or application.yaml file. For all other logging system, you will need to configure rotation settings directly yourself (for example, if you use Log4J2 then you could add a log4j2.xml or log4j2-spring.xml file).

The following rotation policy properties are supported:

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:

It is also possible to set logging levels using environment variables. For example, LOGGING_LEVEL_ORG_SPRINGFRAMEWORK_WEB=DEBUG will set org.springframework.web to DEBUG.

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

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

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

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

In order to release logging resources when your application terminates, a shutdown hook that will trigger log system cleanup when the JVM exits is provided. This shutdown hook is registered automatically unless your application is deployed as a war file. If your application has complex context hierarchies the shutdown hook may not meet your needs. If it does not, disable the shutdown hook and investigate the options provided directly by the underlying logging system. For example, Logback offers context selectors which allow each Logger to be created in its own context. You can use the logging.register-shutdown-hook property to disable the shutdown hook. Setting it to false will disable the registration. You can set the property in your application.properties or application.yaml file:

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.

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

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