Hot replaces classes in the JVM

Fakereplace Class Reloader

This project provides a javaagent and a client to hot replace classes in the JVM over and above what is provided by standard JDK hotswap.

It does this using bytecode trickery to instrument both the classes to be replaced and the reflection API, which allows it to fake replacements that hotswap would not normally allow (For instance, if you remove a method the agent simply adds back a noop method and then hides it from the reflection API).

Provided Integrations

Being able to hot replace classes is one thing, but it is not very useful unless the framework you are using can also pickup on these changes and re-load its metadata. To this end Fakereplace integrates with the following frameworks:

  • Seam 2
  • Weld (Basic integration, it is still a work in progress)
  • JSF
  • Metawidget
  • Hibernate (This restarts the EMF if an entity is modified, this is still experimental)
  • Resteasy

It also provides JBoss AS7 integration, and a maven plugin to allow maven to automatically replace classes after it has compiled them.

Getting Started

There is a single jar in the distribution, fakereplace.jar to use it you need to set the JVM -javaagent option to point to this jar. If you are not using JBoss AS7 you also need to specify the packages that you want to be able to hot replace.

For example, on JBoss AS7 you would edit standalone.conf and add the following to JAVA_OPTS:


For other containers, you would need to add the following to the JVM options:


Where ${com.mycompany.myclasses} is the top level package of the classes that you want to hot replace. All classes in this package or sub packages are instrumented to allow them to be replaced. If you are using JBoss AS7 this step is not nessesary, as the integration will just mark all user deployed classes as replacable.

To set the JVM options you will probably need to modify your app servers startup script, or if you are using an IDE server plugin set the VM arguments in the launch configuration.

Performing a hot deployment

To actually perform a hot deployment add the following to your projects pom.xml:


And then run

mvn package

to perform the hot replacement.

Supported Options

In addition to the packages option mentioned above, the following options are also supported. They should be specified after the -javaagent command and comma seperated, e.g.


  • packages Hot replacable packages
  • log Supported options are trace,debug,info,error
  • index-file The path to the fakereplace index file. Fakereplace stores this file after the first run to speed up later boots
  • dump-dir Dumps classes to this dir on hot replacement, only useful for developers working on Fakereplace
  • port The port that Fakereplace listens on


If you run into a bug report it at:

For details on how if works see