JRebel

JRebel, formerly known as JavaRebel, is a plug-in for the Java Virtual Machine (JVM) that enables instant reloading of changes made to a Java class file. JRebel was developed by Jevgeni Kabanov and Toomas Römer who went on to found ZeroTurnaround, a software company based in Tartu, Estonia. In June 2011, JRebel was recognized as "Most Innovative Java Technology" by the JAX Innovation Awards. At JavaOne / Oracle Develop 2011 in San Francisco, JRebel was awarded the Duke's Choice Award for "Innovative Compiler for Java Code".

Starting from version 1.4, the Java Virtual Machine includes a hot swapping feature that allows developers to update the code on-the-fly during debugging. However hot swapping was limited to updating method bodies only, and trying to add methods and fields to classes would not succeed. Since a Java compiler generates synthetic fields and methods for features like class literals and inner classes the feature was even less useful than could be expected.

JRebel is an alternative solution to updating classes which does not require a debugging session to be started. Instead it monitors the file system for changes and updates the classes in-memory. This means that only classes compiled to ".class" files will be updated and changes to classes in JAR files will be ignored. JRebel imposes a performance overhead on the application and should not be used in production or performance tests. It is meant to be a development tool only.

JRebel is Java-based and usable on any operating system that supports Java. JRebel is IDE-agnostic and designed for integration with various Java EE standards and Java application servers. Although JRebel is subscription-based commercial software, it is freely available to open source software projects and developers using the Scala programming language.

JRebel supports major IDEs, including Eclipse, NetBeans and IntelliJ IDEA. In July, 2011, they announced a partnership with Genuitec in launching JRebel for MyEclipse. In February 2012, ZeroTurnaround launched a product partnership with Vaadin, makers of the popular open source web framework.