HotswapAgent
2.0.0 (2024-09-03)
This is an overview page, please visit hotswapagent.org for more information.
Java unlimited runtime class and resource redefinition.
The primary goal of this project was to eliminate the need for the traditional "change code -> restart and wait... -> check" development cycle. Over time, this concept has evolved into a new paradigm within the Java ecosystem, allowing for real-time software development within a running application. This approach is even feasible in restricted environments, such as Docker containers.
If you're an IntelliJ user, you can simplify setup of HA and DCEVM by using the IntelliJ HotSwapHelper plugin.
Download and Install:
For Java 17/21: Download the latest JBR17 or JBR21. Since these versions do not include a built-in
Hotswap Agent, you will need to manually copy hotswap-agent.jar to the lib/hotswap folder. You can find the latest Hotswap Agent here.
Ensure that the file in the lib/hotswap folder is named hotswap-agent.jar without any version numbers in the filename.
For Java 11: Use TravaJDK, which has an integrated HotswapAgent, and install it as an alternative JDK. Alternatively, TravaJDK includes an embedded HotswapAgent.
For Java 8: Use jdk8-dcevm along with the HotswapAgent.
HotswapAgent Modes:
Starting with dcevm-11.0.9, the HotswapAgent is disabled by default. You can enable support for HotswapAgent using JVM options in one of three modes:
The HotswapAgent=core mode operates without additional plugins, except for core JVM plugins, resulting in faster performance due to reduced scanning and class copying tasks.
To use additional plugins, you need to configure them as Maven dependencies in your pom.xml file. On the other hand, the HotswapAgent=fatjar mode includes all plugins by default, which may slightly slow down application startup.
-XX:HotswapAgent=fatjar activates the internal fatjar HotswapAgent.
-XX:HotswapAgent=core activates the internal core HotswapAgent.
-XX:HotswapAgent=external configures JVM support for HotswapAgent and allows the user to supply an external hotswap-agent.jar using the -javaagent: option.
3.Launching:
Java17/21: launch your application with the options -XX:+AllowEnhancedClassRedefinition -XX:HotswapAgent=fatjar to turn
on advanced hotswap (dcevm) and use Hotswap Agent fatjar release. As an alternative core or external modes can be used insted of fatjar.
Java11: launch your application with the options -XX:HotswapAgent=fatjar to use Hotswap Agent fatjar release.
Java8: launch your application with the options -XXaltjvm=dcevm -javaagent:hotswap-agent.jar to get a basic setup. Optionally you can add hotswap-agent.properties to your application to configure plugins and agent's behavior.
3.Run your application:
Start the application in debug mode, check that the agent and plugins are initialized correctly:
HOTSWAP AGENT: 9:49:29.548 INFO (org.hotswap.agent.HotswapAgent) - Loading Hotswap agent - unlimited runtime class redefinition. HOTSWAP AGENT: 9:49:29.725 INFO (org.hotswap.agent.config.PluginRegistry) - Discovered plugins: [org.hotswap.agent.plugin.hotswapper.HotswapperPlugin, org.hotswap.agent.plugin.jvm.AnonymousClassPatchPlugin, org.hotswap.agent.plugin.hibernate.HibernatePlugin, org.hotswap.agent.plugin.spring.SpringPlugin, org.hotswap.agent.plugin.jetty.JettyPlugin, org.hotswap.agent.plugin.tomcat.TomcatPlugin, org.hotswap.agent.plugin.zk.ZkPlugin, org.hotswap.agent.plugin.logback.LogbackPlugin] ... HOTSWAP AGENT: 9:49:38.700 INFO (org.hotswap.agent.plugin.spring.SpringPlugin) - Spring plugin initialized - Spring core version '3.2.3.RELEASE'
4.Check redefinition
Save a changed resource and/or use the HotSwap feature of your IDE to reload changes
Each application framework (Spring, Hibernate, Logback, ...) needs a special reloading mechanism to keep up-to-date after class redefinition (e.g. Hibernate configuration reload after new entity class is introduced). Hotswap agent works as a plugin system and is shipped preconfigured with all major framework plugins. It is easy to write your custom plugin even as part of your application.
This project is very complex due to a lot of supported frameworks and various versions. Community contribution is mandatory to keep it alive. You can start by creating a plugin inside your application or by writing an example/integration test. There is always a need for documentation improvement :-). Thank you for any help!
Enhanced Java Hotswap - change method body, add/rename a method, field, ...The only unsupported operation is changing the superclass.
You can use standard Java Hotswap from IDE in debug mode to reload changed class
or set autoHotswap property -XXaltjvm=dcevm -javaagent:PATH_TO_AGENThotswap-agent.jar=autoHotswap=true to reload
changed classes after compilation. This setup allows even reload on a production system without a restart.
Automatic configuration - all local classes and resources, known to the running Java application, are automatically discovered and watched for the reload (all files on the local filesystem, not inside any JAR file).
Extra classpath - Need change a runtime class inside dependent JAR? Use extraClasspath property to add any directory as a classpath to watch for class files.
Reload resource after a change - resources from the webapp directory are usually reloaded by the application server. But what about other resources like src/main/resources? Use watchResources property to add any directory to watch for a resource change.
Framework support - through plugin-system, many frameworks are supported. New plugins can be easily added.
Fast - until the plugin is initialized, it does not consume any resources or slow down the application (see Runtime overhead for more information)
Should you have any problems or questions, ask at HotswapAgent forum.
This project is similar to JRebel. The main differences are:
HotswapAgent (DCEVM) supports Java8, Java11 and Java17!
HotswapAgent does not need any additional configuration for basic project setup.
JRebel is currently more mature and contains more plugins.
JRebel is neither open source nor free.
JRebel modifies bytecode of all classes on reload. You need a special IDE plugin to fix debugging.
HotswapAgent extraClasspath is similar to JRebel configuration
HotswapAgent adds watchResources configuration
See HotswapAgentExamples GitHub project. The purpose of an example application is:
complex automate integration tests (check various configurations before a release, see run-tests.sh script)
to check "real world" plugin usage during plugin development (i.e. inside a container)
to provide a working solution for typical application setups
sandbox to simulate issues for existing or new setups
Feel free to fork/branch and create an application for your setup (functional, but as simple as possible). General setups will be merged into the master.
None needed :) Really! All changes are transparent and all you need to do is to download patch+agent and setup your application/application server. Because we use standard java hotswap behaviour, your IDE will work as expected. However, we work on IDE plugins to help with download & configuration.
Some plugins are already available:
Add two action next to the "Debug" button in intellij, Run with hotswap, Debug with hotswap.
When click the action,will set vm parameters for you,no need to set vm parameters manually.
Source code and documentation: https://github.com/gejun123456/HotSwapHelper.
The basic configuration is set to reload classes and resources from the classpath known to the running application
(classloader). If you need a different configuration, add the hotswap-agent.properties file to the classpath root
(e.g. src/main/resources/hotswap-agent.properties).
Detail documentation of available properties and default values can be found in the agent properties file
Full syntax of command line options is:
-javaagent:[yourpath/]hotswap-agent.jar=[option1]=[value1],[option2]=[value2]
Hotswap agent accepts the following options:
autoHotswap=true - watch all .class files for change and automatically Hotswap the class in the running application (instead of running Hotswap from your IDE debugging session)
disablePlugin=[pluginName] - disable a plugin. Note that this will completely forbid the plugin to load (opposite to disablePlugin option in hotswap-agent.properties, which will only disable the plugin for a classloader. You can repeat this option for every plugin to disable.
Add vm option -Dhotswapagent.disablePlugin=Spring,SpringBoot to disable plugins, works same as agent option disablePlugin in previous section.
Hotswap agent does the work of reloading resources and framework configuration (Spring, Hibernate, ...), but it depends on the standard Java hotswap mechanism to reload classes. Standard Java hotswap allows only method body change, which makes it practically unusable. DCEVM is a JVM (Hotspot) patch that allows almost any structural class change on hotswap (with an exception to a hierarchy change). Although hotswap agent works even with standard java, we recommend using DCEVM (and all tutorials use DCEVM as target JVM).
Hotswap agent is a plugin container with plugin manager, plugin registry, and several agent services (e.g. to watch for class/resource change). It helps with common tasks and classloading issues. It scans the classpath for class annotated with @Plugin annotation, injects agent services, and registers reloading hooks. Runtime bytecode modification is provided by javaasist library.
Plugins administered by Hotswap Agent are usually focused on a specific framework. For example, Spring plugin uses HA services to:
Modify root Spring classes to get Spring contexts and registered scan path
Watch for any resource change on a scan path
Watch for a hotswap of a class file within a scan path package
Reload bean definition after a change
... and many others
CXF-JAXRS (3.x) - redefine JAXRS resource after resource class redefinition, reinject instance if integrated with Spring and CDI (Weld/OWB).
Deltaspike (1.x,2.x) - messages, ViewConfig, repository, proxy reloading. Deltaspike scoped CDI beans reinjection.
ELResolver (2.x-5.x) (JuelEL, Appache Commons EL, Oracle EL 3.0)- clear ELResolver cache on class change. Support hotswap for #{...} expressions.
FreeMarker - clear the Apache Freemarker beans class-introspection cache on class definition change.
Hibernate (3.x-6.x) - Reload Hibernate configuration after entity create/change.
iBatis - iBatis configuration reload.
IDEA - support for IntelliJ IDEA development in IDEA
Jackson - clears jackson internal caches when class redefined.
Jersey1 - reload Jersey1 container after root resource or provider class definition or redefinition.
Jersey2 - reload Jersey2 container after root resource or provider class definition or redefinition.
Logback - Logback configuration reload.
Log4j2 - Log4j2 configuration reload.
Mojarra (2.x) - support for application resource bundle changes (properties file). Support for ViewScoped beans reinjection/reloading.
MyBatis (5.3) - reload configuration after mapper file changes
MyFaces (2.x-4.x) - support for application resource bundle changes (properties files). Support for ViewScoped beans reinjection/reloading.
OmniFaces - support for ViewScoped beans reinjection/reloading.
OpenWebBeans - (CDI) (1.x-4.x) - reload bean class definition after class definition/change. Beans can be reloaded according strategy defined in property file.
OsgiEquinox - Hotswap support for Eclipse plugin or Eclipse platform development.
RestEasy (2.x, 3.x) - Cleanups and registers class redefinitions.
Spring (3.2.x+, 4.x, 5.x) - Reload Spring configuration after class definition/change.
Spring Boot (1.5.x+, 2.0.x) - Dynamic reloading of Spring Boot configuration files in real-time.
Vaadin (23.x, 24.x) - Update routes, template models and in practice, anything on the fly.
WebObjects - Clear key value coding, component, action and validation caches after class change.
Weld (CDI) (2.x-5.x) - reload bean class definition after class definition/change. Beans can be reloaded according strategy defined in property file.
Wicket - clear wicket caches if property files are changed
WildFlyELResolver - Clear BeanELResolver after any class redefinition.
ZK (5x-7x) - ZK Framework (http://www.zkoss.org/). Change library properties default values to disable caches, maintains Label cache and bean resolver cache.
JBossModules - add extra class path to JBoss's module class loader. (Wildfly)
Jetty - add extra classpath to the app classloader. All versions supporting WebAppContext.getExtraClasspath should be supported.
Tomcat (7.x,8.x,9.x,10.x) configure Apache Tomcat with extraClasspath and webApp property. Supports also GlassFish, Payara and Tomee7.
Undertow - add extra classpath, watchResources and webappDir to the undertow's resource manager.
Weblogic - add extra classpath to the app classloader.
AnonymousClassPatch - Swap anonymous inner class names to avoid not compatible changes.
ClassInit - initializes new static members/enum values after class/enum redefinition and keeps surviving static values. (Fix of known DCEVM limitation)
Hotswapper - Watch for any class file change and reload (hotswap) it on the fly via Java Platform Debugger Architecture (JPDA)
Proxy (supported com.sun.proxy, CGlib) - redefines proxy classes that implement or extend changed interfaces or classes.
Find detailed documentation of each plugin in the plugin project main README.md file.
It depends on how many frameworks you use and which caches are disabled. Example measurements for a large, real-world enterprise application based on Spring + Hibernate, run on Jetty.
Setup | Startup time -----------------------------|------------- Run (plain Java) | 23s Debug (plain Java) | 24s Debug (plain DCEVM) | 28s Agent - disabled all plugins | 31s Agent - all plugins | 35s
You can write plugin directly as a part of your application. Set pluginPackages=your.plugin.package inside
your hotswap-agent.properties configuration to discover @Plugin annotated classes. You will also need
agent JAR dependency to compile, but be careful NOT to add the JAR to your application; it must be loaded only
as a javaagent. Maven dependency:
org.hotswapagent HotswapAgent ${project.version} provided
See ExamplePlugin (part of TestApplication) to go through a commented simple plugin. Read agent readme to understand agent concepts. Check existing plugins source code for more examples.
Launch run-tests.sh script in the main directory. Currently, you have to set up JAVA_HOME location directory manually.
At least Java 11 with DCEVM should be checked before a release. All automatic tests are set to fail the whole script in case of any single test failure.
Go to the directory representing repository root. In case DCEVM is named dcevm
mvn release:prepare mvn release:perform
Hotswap agent:
Jiri Bubnik - project coordinator, initial implementation
Alexandros Papadakis - Maven Versioning, Weld, JSF, Hibernate3, RestEasy, WildFly plugins
Erki Ehtla - Spring plugin, Proxy plugin
Vladimir Dvorak - ELResolver, OsgiEquinox, Weld, Owb, Deltaspike, Jvm, Jdk, JBossModules, ClassInit, JSF, Mybatis
Sergey Lysenko - Weld plugin
Samuel Pelletier - WebObjects plugin
Jan Tecl - web design
@liuzhengyang - jackson plugin
Lukasz Warzecha - Log4j2 plugin
@muwaiwai - iBatis plugin
Thomas Heigl - Wicket plugin
AJ Banck - FreeMarker plugin
Sinan Yumak - Mojarra, MyFaces plugins
smallfour - Mybatis plugin
@cvictory - Spring plugin, Spring Boot plugin
@homejim - MyBatis plugin, MyBatisPlus plugin
DCEVM:
Thomas Würthinger - initial implementation.
Ivan Dubrov - former project coordinator, update to Java7+Java8, patches, build system (Gradle)
Kerstin Breitender - contributor.
Christoph Wimberger - contributor.
Vladimir Dvorak - java9,java11,jbr17,jbr21 migration, contributor
Jiri Bubnik - java9,java11 migration
