So I started playing with parsing Java class files, creating a cross compiler capable of converting Java class files into Objective C files. I even had a sufficient amount of Apache Harmony running so I could use a good part of the java.lang and java.util classes; roughly in parity with the GWT cross compiler that can compile Java class files into Javascript.

Then Apple dropped the “no cross compiling” bombshell.

Now, keep in mind that I’m just me, tinkering on my spare time during weekends. I don’t have the desire or the time to go up against Apple. I’d rather allow the XMLVM project (which has a well established ecosystem, or so it seems) to decide to go (or not go) against Apple’s wishes.

Then time went by, and I sort of lost interest in this thing.

So I’ve taken the liberty to post the source code here: the Java to Objective C Compiler sources, and the J2OC RTL, which contains a subset of the Apache Harmony project, and implementing the java.lang and java.util classes.

It’s been an interesting project, and hopefully in the next few weeks I’ll document how this all works–including the wierdnesses and pitfalls I came across with the Java VM to get Apache Harmony to work. (Nothing like working through a very large collection of class files to find all the fringe cases.) The output code was intended to be human readable–but it really isn’t for some expressions.

But I’ll describe that in the next few weeks.

And at some point I’ll post an example iPhone application which includes Java code.

Note that my approach was different than the XMLVM project. Instead of providing Java bindings of the iOS libraries, my intent was to only allow the compilation of a computational kernel, then have the user provide the UI elements separately for Android, the iPhone, the iPad, and whatever other target the code was to compile for.

So you won’t find a turn-key solution for recompiling Android code and have it run on the iPhone. You should really check out the XMLVM project instead.

All this code, by the way, is being published under a BSD style license: go ahead and use the code, but leave me out of it and don’t blame me if it goes haywire.

While I don’t intend to get into the functioning of the compiler, I will give a taste of how the code works. The bulk of the .class file parser, which reads and loads the .class file data into memory, is contained in the class ClassFile in com.chaosinmotion.j2oc.vm. This class takes in its constructor an input stream opened to the first byte of a .class file, and loads the entire class file into memory.

Once read, the entire class file can be accessed using the getters associated with that class. The bulk of the code contained inside the .vm (and subpackages within .vm) are used to represent the contents of the class file. The .vm.data classes contain the various data types used to store the meta data within a class file (such as the method names, the attributes fields, and the like), and the .vm.code classes contain a code parser to convert the code within the .class files into an array of processed instructions.

Once the instructions are parsed (by the vm.code.Code class), the code in a method is represented as an array of code segments; a run of instructions that starts with an instruction first jumped into by another instruction, and terminates with either the end of the method or with a jump instruction. In other words, a CodeSeg (Code.CodeSeg class) is a section of instructions that always enters at the first instruction and executes sequentially to the last instruction in the segment. Additional information, such as the list of variables that are used when the segment is entered are noted; this is the current state of the Java operator stack as this segment is entered.

Ultimately the code parser and class file reader represents the code in a .class file in memory in an intermediate state that can then be used to write Objective C with the WriteOCMethod class (com.chaosinmotion.j2oc.oc). A class, CodeOptimize (.oc package) provides utilities that determine if code preambles must be written for memory management or for exception handling: memory management preamble does not need to be written if I never invoke another method. (This is the case for simple functions which return a field or does simple math.)

The theory is that in practice, it should be possible to replace the code writer method with a writer method capable of writing a different language, such as C++ or C.

In the future, when I have more time, I’ll write more about the J2OC project. But for now, if there are any segments or parts you want to use or play with, be my guest.