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Developer Tools

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

Maven
<dependencies>
	<dependency>
		<groupId>org.springframework.boot</groupId>
		<artifactId>spring-boot-devtools</artifactId>
		<optional>true</optional>
	</dependency>
</dependencies>
Gradle
dependencies {
	developmentOnly("org.springframework.boot:spring-boot-devtools")
}
Note
 Developer tools are automatically disabled when running a fully packaged application. If your application is launched from java -jar or if it is started from a special classloader, then it is considered a “production application”. You can control this behavior by using the spring.devtools.restart.enabled system property. To enable devtools, irrespective of the classloader used to launch your application, set the -Dspring.devtools.restart.enabled=true system property. This must not be done in a production environment where running devtools is a security risk. To disable devtools, exclude the dependency or set the -Dspring.devtools.restart.enabled=false system property.
Tip
 Flagging the dependency as optional in Maven or using the developmentOnly configuration in Gradle (as shown above) prevents devtools from being transitively applied to other modules that use your project.
Tip
 Repackaged archives do not contain devtools by default. If you want to use a certain remote devtools feature, you need to include it. When using the Maven plugin, set the excludeDevtools property to false. When using the Gradle plugin, {spring-boot-gradle-plugin-docs}#packaging-executable-configuring-including-development-only-dependencies[configure the task’s classpath to include the developmentOnly configuration].

Property Defaults

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

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

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

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

Note
 If you don’t want property defaults to be applied you can set configprop:spring.devtools.add-properties[] to false in your application.properties.
Tip
 For a complete list of the properties that are applied by the devtools, see {spring-boot-devtools-module-code}/env/DevToolsPropertyDefaultsPostProcessor.java[DevToolsPropertyDefaultsPostProcessor].

Automatic Restart

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

Triggering a restart

As DevTools monitors classpath resources, the only way to trigger a restart is to update the classpath. The way in which you cause the classpath to be updated depends on the IDE that you are using:

  • In Eclipse, saving a modified file causes the classpath to be updated and triggers a restart.

  • In IntelliJ IDEA, building the project (Build -> Build Project) has the same effect.

  • If using a build plugin, running mvn compile for Maven or gradle build for Gradle will trigger a restart.

Note
 If you are restarting with Maven or Gradle using the build plugin you must leave the forking set to enabled. If you disable forking, the isolated application classloader used by devtools will not be created and restarts will not operate properly.
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
 DevTools needs to customize the ResourceLoader used by the ApplicationContext. If your application provides one already, it is going to be wrapped. Direct override of the getResource method on the ApplicationContext is not supported.
Restart vs Reload

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

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

Logging changes in condition evaluation

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

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

spring:
  devtools:
    restart:
      log-condition-evaluation-delta: false

Excluding Resources

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

Watching Additional Paths

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

Disabling Restart

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

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

link:{docs-java}/devtools/restart/disable/MyApplication.java[role=include]

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.

Note
 Any update to the file will trigger a check, but restart only actually occurs if Devtools has detected it has something to do.

To use a trigger file, set the configprop:spring.devtools.restart.trigger-file[] property to the name (excluding any path) of your trigger file. The trigger file must appear somewhere on your classpath.

For example, if you have a project with the following structure:

src
+- main
   +- resources
      +- .reloadtrigger

Then your trigger-file property would be:

spring:
  devtools:
    restart:
      trigger-file: ".reloadtrigger"

Restarts will now only happen when the src/main/resources/.reloadtrigger is updated.

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.

Some IDEs have features that save you from needing to update your trigger file manually. Spring Tools for Eclipse and IntelliJ IDEA (Ultimate Edition) both have such support. With Spring Tools, you can use the “reload” button from the console view (as long as your trigger-file is named .reloadtrigger). For IntelliJ IDEA, you can follow the instructions in their documentation.

Customizing the Restart Classloader

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

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

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

restart:
  exclude:
    companycommonlibs: "/mycorp-common-[\\w\\d-\\.]+\\.jar"
  include:
    projectcommon: "/mycorp-myproj-[\\w\\d-\\.]+\\.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.

Known Limitations

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

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

LiveReload

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

If you do not want to start the LiveReload server when your application runs, you can set the configprop: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.
Warning
 To trigger LiveReload when a file changes, using.adoc must be enabled.

Global Settings

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

  1. spring-boot-devtools.properties

  2. spring-boot-devtools.yaml

  3. spring-boot-devtools.yml

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

spring:
  devtools:
    restart:
      trigger-file: ".reloadtrigger"
Note
 If devtools configuration files are not found in $HOME/.config/spring-boot, the root of the $HOME directory is searched for the presence of a .spring-boot-devtools.properties file. This allows you to share the devtools global configuration with applications that are on an older version of Spring Boot that does not support the $HOME/.config/spring-boot location.
Note

Profiles are not supported in devtools properties/yaml files.

Any profiles activated in .spring-boot-devtools.properties will not affect the loading of profile-specific configuration files. Profile specific filenames (of the form spring-boot-devtools-<profile>.properties) and spring.config.activate.on-profile documents in both YAML and Properties files are not supported.

Configuring File System Watcher

{spring-boot-devtools-module-code}/filewatch/FileSystemWatcher.java[FileSystemWatcher] works by polling the class changes with a certain time interval, and then waiting for a predefined quiet period to make sure there are no more changes. Since Spring Boot relies entirely on the IDE to compile and copy files into the location from where Spring Boot can read them, you might find that there are times when certain changes are not reflected when devtools restarts the application. If you observe such problems constantly, try increasing the spring.devtools.restart.poll-interval and spring.devtools.restart.quiet-period parameters to the values that fit your development environment:

spring:
  devtools:
    restart:
      poll-interval: "2s"
      quiet-period: "1s"

The monitored classpath directories are now polled every 2 seconds for changes, and a 1 second quiet period is maintained to make sure there are no additional class changes.

Remote Applications

The Spring Boot developer tools are not limited to local development. You can also use several features when running applications remotely. Remote support is opt-in 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:

<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 the configprop: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 configprop:spring.devtools.remote.secret[] property is set. The client component must be launched manually.

Running the Remote Client Application

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

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

  • Select Run Configurations…​ from the Run menu.

  • Create a new Java Application “launch configuration”.

  • Browse for the my-app project.

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

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

A running remote client might resemble the following listing:

  .   ____          _                                              __ _ _
 /\\ / ___'_ __ _ _(_)_ __  __ _          ___               _      \ \ \ \
( ( )\___ | '_ | '_| | '_ \/ _` |        | _ \___ _ __  ___| |_ ___ \ \ \ \
 \\/  ___)| |_)| | | | | || (_| []::::::[]   / -_) '  \/ _ \  _/ -_) ) ) ) )
  '  |____| .__|_| |_|_| |_\__, |        |_|_\___|_|_|_\___/\__\___|/ / / /
 =========|_|==============|___/===================================/_/_/_/
 :: Spring Boot Remote :: {spring-boot-version}

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-project/spring-boot-devtools/target/classes started by pwebb in /Users/pwebb/projects/spring-boot/code)
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 configprop: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.

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.

On a slower development environment, it may happen that the quiet period is not enough, and the changes in the classes may be split into batches. The server is restarted after the first batch of class changes is uploaded. The next batch can’t be sent to the application, since the server is restarting.

This is typically manifested by a warning in the RemoteSpringApplication logs about failing to upload some of the classes, and a consequent retry. But it may also lead to application code inconsistency and failure to restart after the first batch of changes is uploaded. If you observe such problems constantly, try increasing the spring.devtools.restart.poll-interval and spring.devtools.restart.quiet-period parameters to the values that fit your development environment. See the using.adoc section for configuring these properties.

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.