Documentation

Documentation versions (currently viewingVaadin 24)

Hot Deploy & Live Reload

Live reload with JetBrains Runtime and HotswapAgent.

JetBrains Runtime offers hotswapping of any type of Java changes into a running JVM through the debug connection,typically established by the IDE, to the running JVM. With other JDKs there’s limited support for only some types of Java changes.

In addition to getting the Java changes into the running JVM, most frameworks have some global data that needs to be updated when a class changes — or some caches that need to be cleared for the new class to take effect. HotswapAgent is a collection of plugins for various frameworks, which offers these kinds of global updates and cache clearing when needed.

Setting up hotswapping consists of three steps:

  • Install JetBrains Runtime;

  • Download HotswapAgent and install it into JetBrains Runtime; and

  • Run the application using JetBrains Runtime with additional parameters.

JetBrains Runtime

You can download the latest version from the JetBrains GitHub release page. Be sure to select the correct architecture. Use JetBrains Runtime to execute your application, not necessarily your IDE.

HotswapAgent

You can download HotSwapAgent from the HotswapAgent GitHub release page. You’ll need version 1.4.2 or later.

Download the JAR file and place it inside the JetBrains Runtime installation directory as lib/hotswap/hotswap-agent.jar. Use that exact file name — you’ll need to create the hotswap folder — for JetBrains Runtime to get it.

If you want to know more about the features of HotswapAgent, the documentation in the HotswapAgent webpage is a good resource.

Running Application

To enable hotswapping when launching the application, you need to pass -XX:+AllowEnhancedClassRedefinition -XX:HotswapAgent=fatjar as JVM arguments — not program arguments.

If you have a Spring Boot application, you would launch the application by running the main() method of your application class. After you’ve launched it once, you’ll have a run configuration that you can edit in your IDE so that you can select the proper JDK and give the necessary parameters.

To do that in IntelliJ IDEA, select Run  Edit configurations…​. With your application run configuration chosen, you can select the JDK to use in the first drop-down. If JetBrains Runtime isn’t shown, you can select it through Select alternative JRE…​.

In the same dialog you need to click Modify options  Add VM options to be able to see the field where the JVM arguments can be defined. Then you can add -XX:+AllowEnhancedClassRedefinition -XX:HotswapAgent=fatjar.

If you have another type of application, you’ll need to pass the JVM options in the appropriate way.

Verifying It Works

With the settings in place, when you start the application in debug mode, you’ll see the following type of log output:

HOTSWAP AGENT: 10:19:01.802 INFO (org.hotswap.agent.HotswapAgent) - Loading Hotswap agent {1.4.2-SNAPSHOT} - unlimited runtime class redefinition.
HOTSWAP AGENT: 10:19:02.065 INFO (org.hotswap.agent.config.PluginRegistry) - Discovered plugins: [JdkPlugin, ClassInitPlugin, AnonymousClassPatch, WatchResources, Hotswapper, Hibernate, Hibernate3JPA, Hibernate3, Spring, Jersey1, Jersey2, Jetty, Tomcat, ZK, Logback, Log4j2, MyFaces, Mojarra, Omnifaces, ELResolver, WildFlyELResolver, OsgiEquinox, Owb, OwbJakarta, Proxy, WebObjects, Weld, WeldJakarta, JBossModules, ResteasyRegistry, Deltaspike, GlassFish, Weblogic, Vaadin, Wicket, CxfJAXRS, FreeMarker, Undertow, MyBatis, IBatis, JacksonPlugin, Idea]

This indicates that HotswapAgent is loaded. Now, when you navigate to a view in your application, open the corresponding Java file in your project and make some changes — remember to compile and not only save the class if using IntelliJ IDEA. When you make changes, your view is reloaded in the browser and the changes are shown.

Caution
Use "debug" & Not spring-boot:run
Don’t start the application in "run" mode. The debug connection to the JVM running the application is needed for hotswapping to work.

Don’t launch the Maven spring-boot:run target as that forks another JVM process. The debug connection is then established only to the Maven process and not the actual application. Therefore, hotswapping won’t work.

Working Efficiently

By default, any change in the Java code causes the application to reload in the browser. This is convenient when you make changes to the initial layout, constructors, post construct methods or similar. However, if you’re modifying the logic inside a listener, there’s no need to do a page reload for the change to take effect.

You can turn off reloading in the Vaadin Developer Tools under the Info tab. It can be more efficient to disable live reload and manually reload the browser when needed, especially when editing code related to a dialog or some other UI part which isn’t directly mapped to a URL.

IntelliJ doesn’t compile by default a Java file when you save it. If your habit is to press the compile shortcut, hotswapping works fine. Otherwise, you might want to set IntelliJ to build the project when you save. You can do that through Settings  Build, Execution, Deployment  Compiler  Build project automatically. You’ll also need to ensure that the build takes place when the application is running in debug mode. This is done through Advanced Settings  Allow auto-make to start even if developed application is currently running. Finally, through Settings  Build, Execution, Deployment  Debugger  HotSwap, Reload classes after compilation should be set to Always.

Additional Considerations

You should disable other reloading or hot swapping solutions. For example, disable the automatic restart in the Jetty Maven plugin if you’re using that. Omit or set <scanIntervalSeconds> to a value of 0 or less.

The live reload “quiet time” — which is in milliseconds since last Java change before refreshing the browser — can be adjusted with the parameter vaadin.liveReloadQuietTime in the hotswap-agent.properties file. Increase this value if you notice the browser refreshing before modified Java files have been fully compiled. The default is 1000 ms.

Changes to some parts of the Java code, such application startup listeners, won’t have any effect until the server is restarted and the startup listeners run again. For these you need to restart the server.

If you’re using the @PreserveOnRefresh annotation, the same view instance is reused when the browser is reloaded. That means changes aren’t necessarily visible until the view is opened in another browser tab.

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