A Java agent enabling unlimited runtime class and resource redefinition for faster development cycles.
Hotswap Agent is a Java agent that enables unlimited runtime class and resource redefinition, allowing developers to see code changes immediately without restarting the application. It works with DCEVM to support structural changes like adding methods or fields and integrates with frameworks via plugins to reload configurations automatically.
Java developers working on applications with frameworks like Spring, Hibernate, or servlet containers who want to accelerate development by eliminating restarts during debugging and testing.
It provides a free, open-source alternative to commercial tools like JRebel, with minimal configuration, support for Java 8 through 25, and a plugin ecosystem that keeps framework configurations in sync after class changes.
Java unlimited redefinition of classes at runtime.
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Allows runtime changes like adding methods and fields beyond standard Java hotswap, using DCEVM for almost unlimited redefinition except superclass changes.
Preconfigured plugins for Spring, Hibernate, Tomcat, and others automatically reload framework configurations after code changes, reducing manual restarts.
Watches local classes and resources on the filesystem for changes and reloads them automatically, streamlining the development feedback loop.
Supports fatjar, core, and external modes to balance plugin availability and startup performance, with extraClasspath for watching JAR-internal changes.
Requires installing and configuring DCEVM, a patched JVM, which varies by Java version (e.g., TravaJDK for Java 11, JBR for Java 17+), adding setup overhead.
The README admits JRebel is more mature with more plugins; unsupported or newer frameworks may lack reliable reloading mechanisms.
Fatjar mode includes all plugins by default and can slow down application startup, as noted in the runtime overhead measurements.
HotswapAgent is an open-source alternative to the following products: