Please see the project's BountySource page to vote on issues that matter to you. Commenting/voting on issues helps me prioritize the small amount of time I have to maintain this library :)
JBox2d is a Java port of the C++ physics engines LiquidFun and Box2d.
If you're looking for help, see the wiki or come visit us at the Java Box2d subforum. Please post bugs here on the issues page.
If you're planning on maintaining/customizing your own copy of the code, please join our group so we can keep you updated.
If you're looking to deploy on the web, see PlayN, which compiles JBox2d through GWT so it runs in the browser. The JBox2d library has GWT support out of the box. Also, TeaVM support jbox2d in the browser as well.
If you've downloaded this as an archive, you should find the built java jars in the 'target' directories of each project.
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jbox2d-library - this is the main physics library. The only dependency is the SLF4J logging library.
jbox2d-serialization - this adds serialization tools. Requires google's protocol buffer library installed to fully build (http://code.google.com/p/protobuf/), but this is optional, as the generated sources are included.
jbox2d-testbed - A simple framework for creating and running physics tests.
jbox2d-testbed-jogl - The testbed with OpenGL rendering.
jbox2d-jni-broadphase - Experiment with moving parts of the engine to C++. Not faster.