20. 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, simply add the module dependency to your build:

Maven. 

<dependencies>
    <dependency>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-devtools</artifactId>
        <optional>true</optional>
    </dependency>
</dependencies>

Gradle. 

dependencies {
    compile("org.springframework.boot:spring-boot-devtools")
}

[Note]Note

Developer tools are automatically disabled when running a fully packaged application. If your application is launched using java -jar or if it’s started using a special classloader, then it is considered a “production application”. Flagging the dependency as optional is a best practice that prevents devtools from being transitively applied to other modules using your project. Gradle does not support optional dependencies out-of-the-box so you may want to have a look to the propdeps-plugin in the meantime.

[Tip]Tip

repackaged archives do not contain devtools by default. If you want to use certain remote devtools feature, you’ll need to disable the excludeDevtools build property to include it. The property is supported with both the Maven and Gradle plugins.

20.1 Property defaults

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

Whilst 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 will disable those 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 will automatically apply sensible development-time configuration.

[Tip]Tip

For a complete list of the properties that are applied see DevToolsPropertyDefaultsPostProcessor.

20.2 Automatic restart

Applications that use spring-boot-devtools will 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 will be monitored for changes. Note that certain resources such as static assets and view templates do not need to restart the application.

[Note]Note

You can also start your application via the supported build plugins (i.e. Maven and Gradle) as long as forking is enabled since DevTools need an isolated application classloader to operate properly. Gradle and Maven do that by default when they detect DevTools on the classpath.

[Tip]Tip

Automatic restart works very well when used with LiveReload. See below for details. If you use JRebel automatic restarts will be disabled in favor of dynamic class reloading. Other devtools features (such as LiveReload and property overrides) can still be used.

[Note]Note

DevTools relies on the application context’s shutdown hook to close it during a restart. It will not work correctly if you have disabled the shutdown hook ( SpringApplication.setRegisterShutdownHook(false)).

[Note]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]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.

20.2.1 Excluding resources

Certain resources don’t necessarily need to trigger a restart when they are changed. For example, Thymeleaf templates can just be edited in-place. By default changing resources in /META-INF/maven, /META-INF/resources ,/resources ,/static ,/public or /templates will not trigger a restart but will 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:

spring.devtools.restart.exclude=static/**,public/**
[Tip]Tip

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

20.2.2 Watching additional paths

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

20.2.3 Disabling restart

If you don’t want to use the restart feature you can disable it using the spring.devtools.restart.enabled property. In most cases you can set this in your application.properties (this will still initialize the restart classloader but it won’t watch for file changes).

If you need to completely disable restart support, for example, because it doesn’t work with a specific library, you need to set a System property before calling SpringApplication.run(…​). For example:

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

20.2.4 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 this 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 will only occur if Devtools has detected it has to do something. The trigger file could be updated manually, or via an IDE plugin.

To use a trigger file use the spring.devtools.restart.trigger-file property.

[Tip]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.

20.2.5 Customizing the restart classloader

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

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

The spring-devtools.properties file can contain restart.exclude. and restart.include. prefixed properties. 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 will be applied to the classpath.

For example:

restart.exclude.companycommonlibs=/mycorp-common-[\\w-]+\.jar
restart.include.projectcommon=/mycorp-myproj-[\\w-]+\.jar
[Note]Note

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

[Tip]Tip

All META-INF/spring-devtools.properties from the classpath will be loaded. You can package files inside your project, or in the libraries that the project consumes.

20.2.6 Known limitations

Restart functionality does not work well with objects that are deserialized 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 will need to request a fix with the original authors.

20.3 LiveReload

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

If you don’t want to start the LiveReload server when your application runs you can set the spring.devtools.livereload.enabled property to false.

[Note]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 will have LiveReload support.

20.4 Global settings

You can configure global devtools settings by adding a file named .spring-boot-devtools.properties to your $HOME folder (note that the filename starts with “.”). Any properties added to this file will 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:

~/.spring-boot-devtools.properties. 

spring.devtools.reload.trigger-file=.reloadtrigger

20.5 Remote applications

The Spring Boot developer tools are not just 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:

<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, for example:

spring.devtools.remote.secret=mysecret
[Warning]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; there is 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 using the same classpath as the remote project that you’re connecting to. The non-option argument passed to the application should be the remote URL that you are connecting to.

For example, if you are using Eclipse or STS, and you have a project named my-app that you’ve 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 will look like this:

  .   ____          _                                              __ _ _
 /\\ / ___'_ __ _ _(_)_ __  __ _          ___               _      \ \ \ \
( ( )\___ | '_ | '_| | '_ \/ _` |        | _ \___ _ __  ___| |_ ___ \ \ \ \
 \\/  ___)| |_)| | | | | || (_| []::::::[]   / -_) '  \/ _ \  _/ -_) ) ) ) )
  '  |____| .__|_| |_|_| |_\__, |        |_|_\___|_|_|_\___/\__\___|/ / / /
 =========|_|==============|___/===================================/_/_/_/
 :: Spring Boot Remote :: 1.5.4.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.AnnotationConfigApplicatio[email protected]: 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]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]Tip

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

[Tip]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 will monitor your application classpath for changes in the same way as the local restart. Any updated resource will be pushed to the remote application and (if required) trigger a restart. This can be quite helpful if you are iterating on a feature that uses a cloud service that you don’t have locally. Generally remote updates and restarts are much quicker than a full rebuild and deploy cycle.

[Note]Note

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

20.5.3 Remote debug tunnel

Java remote debugging is useful when diagnosing issues on a remote application. Unfortunately, it’s not always possible to enable remote debugging when your application is deployed outside of your data center. Remote debugging can also be tricky to setup if you are using a container based technology such as Docker.

To help work around these limitations, devtools supports tunneling of remote debug traffic over HTTP. The remote client provides a local server on port 8000 that you can attach a remote debugger to. Once a connection is established, debug traffic is sent over HTTP to the remote application. You can use the spring.devtools.remote.debug.local-port property if you want to use a different port.

You’ll need to ensure that your remote application is started with remote debugging enabled. Often this can be achieved by configuring JAVA_OPTS. For example, with Cloud Foundry you can add the following to your manifest.yml:

---
    env:
        JAVA_OPTS: "-Xdebug -Xrunjdwp:server=y,transport=dt_socket,suspend=n"
[Tip]Tip

Notice that you don’t need to pass an address=NNNN option to -Xrunjdwp. If omitted Java will simply pick a random free port.

[Note]Note

Debugging a remote service over the Internet can be slow and you might need to increase timeouts in your IDE. For example, in Eclipse you can select JavaDebug from Preferences…​ and change the Debugger timeout (ms) to a more suitable value (60000 works well in most situations).

[Warning]Warning

When using the remote debug tunnel with IntelliJ IDEA, all breakpoints must be configured to suspend the thread rather than the VM. By default, breakpoints in IntelliJ IDEA suspend the entire VM rather than only suspending the thread that hit the breakpoint. This has the unwanted side-effect of suspending the thread that manages the remote debug tunnel, causing your debugging session to freeze. When using the remote debug tunnel with IntelliJ IDEA, all breakpoints should be configured to suspend the thread rather than the VM. Please see IDEA-165769 for further details.