Manually add a method call to a class file?

Question

I need to manually add a method call to a class file without decompiling and recompiling the code because It depends on thousands of other classes and I don't wan't have to do more than is nessescary. I know java but not how class files are made.

Any help is appreciated.

EDIT:

I am not the owner of the source and I need this to work on any computer, which means I cannot redistribute the sources and have them compiled realtime while my patcher is working.

Answer 1

I got it now! I Created fake source files with the same names/methods but didn't add anything else except for class and method names. That way I only needed to pack the ones that are directly linked to my Class file. But now compiling takes a few milliseconds whereas it used to take around 124s, Lol. Works great!

Answer 2

You have the source code, and you have all other classes compiled. So you can recompile just that source file, passing compiled classes as parameters to java compiler with -classpath option.

Answer 3

You should use ASM or Javaassist to manipulate the bytecode. ASM is a little bit more complex and requires you to understand more about the JVM, but it's faster. Javaassist doesn't require you to know much about the JVM's internals.

However, I don't see why you can't just recompile that single sourcefile? If you only need to add this method once, it's very inefficient to learn ASM or Javaassist.

Answer 4

How about subclassing? Then you don't need to touch the sources.

Answer 5

So if you have the source code and want to add some methods into only one class. Then you don't have to worry about other classes even they are dependent on your current modified class. Re-compiling a file doesn't affect other classes. Since the output will be produced at run-time.

Answer 6

If your class is not declared final and the method you are interested is not final, you can extend the class and override just that method.

Answer 7

Just change the source code, recompile ! Everything will work fine. Subclassing won't work .Because Already existing classes won't know about the new subclass until you change their code to use the new subclass instead of old superclass.

Answer 8

For manual editing of classfiles, I'd recommend Krakatau. (Disclosure, I wrote it). It lets you disassemble a classfile, edit it, and reassemble. There are other assemblers out there, but AFAIK, Krakatau is the only one that supports all the weird edge cases in the classfile format.

The main caveat is that Krakatau by default does not preserve certain optional debugging attributes (specifically LineNumberTable, LocalVariableTable, and LocalVariableTypeTable), since there is no simple way to represent them in a human editable format, and failing to edit them when the bytecode changes will result in a verification error. Most likely you don't actually need this though so it shouldn't matter.

The other caveat of course is that you have to understand bytecode. But if you don't, you won't be able to manually edit classfiles anyway.