Java to Python (j2p)

This tool can translate java programs into python programs. This is a complex mechanism and still experimental/in development. This documentation is very incomplete. This code is not yet published on the artifactory.

It can translate both single file java programs as multi-file projects. It supports translating calls to external libraries through a pluggable system of translators, so that translation support for new libraries can be easily added without need for recompilation of the tool itself. It generates a zip file containing the translated files plus installation directions, ready to be used with the "pip install" command; We also have a library supporting running these zip files from java.

There are a number of packages inside this project, because this translator has a modular setup:

package name= artifactid	description	current state
core	translation of java programs + library containing translators for a few basic java classes	general translation somewhat complete; library translators very partial
jackson-t	library with translators for jackson->pyson translation	quite complete
tudutils-t	library with translators for tudelft utilities package.	very partial
junit-t	library, translators for junit	very partial

These modules offer translation support for external java libraries. For instance jackson is an external library to translate json to java; the jackson-t library translates jackson-based code and annotations into pyson-based python code.

You can easily extend the standard core translator with these plugin translators by just importing these plugins in your java project. Just make sure that they are all based on the same core version.

Internal mechanism

There are two main components in the core:

1. The translator that parses java code and creates equivalent python code
2. per-java-class translators that know how to translate a specific call to a java class function or field into equivalent python code.

The first is generic and used for all java programs. It is currently pretty complete but details will be filled in over time as the need arises to support more java syntax.

The second is currently very partial. The reason is that there are a huge number of java classes, and every field and function in it will need a specialized translator. This will grow slowly over time as needed.

Usage

Overriding the translation

Comments can contain python code to override the automatic code, if the block starts with #PY. This code replaces the entire object (if/case/while block; statement) that follows the comment.

Python has strict requirements regarding indentation. To make this possible, we need to be strict about indentation as well. In a single line python comment, the code must look exactly like

 //#PY single_python_line 

Note the single whitespace after the #PY. Your code starts after this single whitespace.

In a multi line comment the code must look exactly like

/*#PY
 * codeline1
 * codeline2
 * ...
 */

Your code lines each start with "* ", note the whitespace after the star. You are free to indent before the "*".

Your code is automatically indented to the level needed at the insertion place in the code.

Code must be placed in either a standard block comment or a single line comment. Starting a javadoc with #PY is not allowed. This is to encourage proper use of javadoc.

A comment block overrides also annotations.

If the code block contains no code at all, it is translated as pass, to ensure that the code is a proper statement.

Translation modules

J2P has a modular translation mechanism. A translation module can be created separately and plugged in as needed, to add support for translating libraries. Also this allows to change the translation process, for instance to use another python library for the translation.

A number of translation modules are already available

module	what it translates	details	limitations
jackson-t	jackson serialization annotations	translates jackson to pyson annotations that look very similar to jackson	TBD
junit-t	junit calls assertTrue,assertEquals		no support yet for junit annotations
tudutils-t	translates calls to utilities package	currently mainly to support immutablelist translation

Resulting file

FAQs

Question	Explanation
I'm getting "No translator found for X". But X is a class that I'm trying to translate	The java files you are trying to translate are probably not compiled by the java compiler. When the translator finds a method call, it needs the compiled java to determine the proper signature (function name and arguments).