Awesome

Hot Chai (tea) with Java coffee (ch.vorburger.hotea)

Hotea is a "kept it simple and stupid" (KISS) Java Mod-ules/Plug-In mini framework with HOT Class reloading to "support dynamic script like source code" (through isolated ClassLoader/s). This may be of interest and useful to you to build things ranging from e.g. plugins for game engines to perhaps some sort of runtime changeable coded out "business rule" stuff.

https://github.com/vorburger/HoTea/blob/master/ch.vorburger.hoteajava/ch/vorburger/hotea/examples/swing/HotSwingExampleMain.java

This video illustrates the HotSwingExampleMain; it lives "live updates" JLabel(" hello, world ") changes made in ExampleSwingDrawer.
This video shows a very cool :space_invader: Minecraft demo :video_game: built with this. (Picked up here with this.)
The tests may also be of interest

Caveat emptor: This is intended for and best works with simple plugin-like scenarios, where the classpath of each such plugin does not overlap nor need to share instances among different plugins. In more interesting use cases, by experience, Very Weired Things (VWT) may happen if you don't fully understand what you are actually doing when several class loaders are involved in Java. You have been warned.

See also follow-up projects ch.vorburger.osgi.gradle and ch.vorburger.minecraft.osgi.

Features

Features, what this project does do:

Load Java byte code for additional *.classes from directory/-ies and/or JARs, through a child ClassLoader
Hot reload such extensions/modules/plugins/bundles whenever any files in the watched classpath directory are updated
Notify your code whenever it hot reloaded, so that you can do what you need to do in your code (say cleanly shut down previous instances of your classes, and re-instantiate objects using your new class definitions)
No need to follow a particular "Plugin API" at all (of Hotea, there is none) - you can hot (re)load any class implementing any of your own "API" interface

Non-Features, what it does NOT do, not today and not planned:

Hot-reload classes on your (already loaded) "main" primary classpath - no JVM Agent
Dependencies between modules/plugins/bundles (perhaps you want to look at OSGi?)
Versioning of modules/plugins/bundles (perhaps you want to look at OSGi?)
Repositories, remote provisioning, etc.
Remote API export etc.

Other Projects

Approaches which hot-reload classes on your (already loaded) "main" primary classpath typically through a custom JVM Agent (with more or less limitations)

http://hotswapagent.org
http://dcevm.github.io
https://github.com/spring-projects/spring-loaded
http://zeroturnaround.com/software/jrebel/
https://code.google.com/p/jreloader/
Standard JVM Hot Deploy in Debug mode
https://github.com/jmarranz/relproxy/ : Seems to be intended to (re)load one class, only, e.g. for a single Web View

Other extensions/modules/plugins/bundles frameworks

OSGi (eclipse.org/rt: eclipse.org/equinox, eclipse.org/virgo, eclipse.org/gemini; felix.apache.org & karaf.apache.org; bndtools.org BND Tools; Spring Dynamic Modulees DM)
JBoss Modules (and Doc)
https://pf4j.org
NucleusPowered/QuickStartModuleLoader
http://jpf.sourceforge.net
https://code.google.com/p/jspf/ : No hot unload
https://code.google.com/p/impala/

Scripting

JSR 223 javax.script
http://commons.apache.org/proper/commons-jci
http://docs.codehaus.org/display/JANINO/Home

History

The watchdir utility is now a separate project, see https://github.com/vorburger/ch.vorburger.fswatch.

The Minecraft specific part was in minecraft/ module. This project now focuses on a general library that is not Minecraft related. (Which the Minecraft module used this in 2017. Since changes made when picking up this project in 2022, the original Minecraft demo is broken; however work has now started to add the equivalent funcationality into https://github.com/OASIS-learn-study/minecraft-storeys-maker, see its issue #58.

Use of Java 8 is recommended to avoid PermGen memory issues due to Class reloading.