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A Java Bytecode to JavaScript Cross-Compiler.
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A Java Bytecode to JavaScript Cross-Compiler.


  1. You need to install this and the following projects

  1. Build all projects. An Eclipse project description is included.

  2. Optionally install

Building your project

It is highly recommended that you compile your project (i.e. generate class files) against j2js-jre, and not rt.jar. This way you avoid link errors (missing classes or methods) at the cross-compile stage. See the warning at the bottom of the document.


java -cp <RUNTIME_CLASSPATH> com.j2js.J2JSCompiler <basedir> <CROSS_COMPILE_CLASSPATH> <entryPointClassName> <targetLocation>


This is the standard Java classpath. The cross-compiler needs the project j2js-compiler, bcel and commons-io on its classpath.


All non-absolute paths are relative to the basedir.


The name of the class to cross-compile. This class must have a method public void main(java.lang.String[]).

The compiler cross-compiles the main method and all other methods which are called from the main method.


This classpath must contain all classes whose methods are referenced by the main method. In normal operation, this classpath consists of

  • the j2js-jre classes directory or jar
  • the j2js-agent classes directory or jar
  • the classes directory of your personal project


All cross-compiled code is stored in the target location. It is one or more JavaScript file starting at 0.js. Only this initial file must be included in your web page with <script src='targetLocation/0.js'/>.


Suppose the directory layout is

|- j2js-compiler
|- j2js-jre
|- j2js-agent
|- my-project
     |- target
         |- assemblies
             |- 0.js
             |- 1.js
             |- 2.js ...

and you want to cross-compile class Then

java -cp ../j2js-compiler/libs/commons-io-1.4.jar;^
../j2js-compiler/target/classes ^
com.j2js.J2JSCompiler . ^
target/classes;../j2js-jre/target/classes;../j2js-agent/target/classes ^ target/assemblies

will create the assemblies.


The project j2js-jre contains a hand-crafted version of the Java Runtime Environment. Do not put Java/jreX/lib/rt.jar into the <CROSS_COMPILE_CLASSPATH>.


The cross-compiler is able to translate any legal Java Bytecode. However, naturally, the compiler cannot support the Java Native Interface (JNI).

Cross Compiling Scala

The current Scala version uses JRE classes such as ClassLoader or SecurityContext even in base Scala classes. These JRE classes do not make sense in a web agent context and are therefore not implemented in j2js-jre.

A workaround would be to hand-craft a subset of Scala classes, avoiding dependencies to JRE classes such as ClassLoader. But this is a lot of work.

So, sadly, we cannot support Scala under this circumstance.

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