A java-to-javascript and java-to-C# source to source compiler.
To allow the use of existing java code inside a browser/node.js/.NET environment, java code can be compiled into equivalent javascript (compatible with ECMAScript 5 or later) or C# (compatible with netcoreapp1.1). The compiler supports all language constructs of java 8 (including lambdas), but imposes a few restrictions of its own on the input code. A large subset of the functionality of the java.lang and java.util packages are provided to be used by the program at runtime. The generated code replicates the behaviour of the java original with near perfection, with only some slight incompatibilities.
As all comments are stripped and the generated code is generally not as readable as natively written c# or javascript code, the main purpose of the compiler is to use it as part of a toolchain for building and not for a one-off conversion step to generate maintainable source code.
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Compile to a library: This simplest form would be some java classes that form a self-contained library that provides functionality to a caller. This library can be easily compiled and called from any native javascript or C# code.
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Compile code to use functionality from 'outside' When the java code needs to access stuff not included in the provided libraries (for example file access or some web service calls), you need to provide native implementations to interface with the underlying system (browsers,node.js,.NET runtime) to actually perform the tasks. This is straight-forward for .NET. Creating native javascript classes is documented in native_code_javascript.txt.
The compiler can be used as a command-line tool or as an Ant task to compile a bunch of java source files to the equivalent .js (javascript) or .cs (C#) files. For the case of javascript, a linker step is also needed to put all the small .js files together to a big .js file in the correct loading order. You can later do a minifying step (using Google's closure compiler) that will greatly reduce javascript code size. The compiler is optimized to generate minify-able output. See the getting started document for more details.
The Java-Converter is based on the extendj java compiler that does most of the tricky parts of compilation. Only the backends for javascript and C# and the necessary runtime libraries were actually developed in this project. Nevertheless every release of the Java-Converter is a completely self-contained application (basically a .JAR) that contains everything needed to use the compiler.