编译或不编译.class文件

Question

There is a web application, that is being ran on Windows, inside a Tomcat browser. Application users number of .jar files as a storage for it's logic.

When I open a jar file, that I need to alter a logic of, I see a bunch of .class files. (Java compiled classes, I assume). When I try to open .class file in a text editor, I see a semi-readable gibberish.

When I feed this compiled .class to a service like showmycode.com, I can see the actual java code. At this point I alter the code according to my need and want to put the updated .class file for application to use (put it back inside a jar).

Is it mandatory that I compile the .class file to a "semi-readable gibberish", or I can put it there as a source code file (probably not, but thought I would ask just in case). If I have to compile the atered code, can I do it without bringing up a project in some Java IDE, and just compile it into .class via command line or something?

Answer 1

There are two main methods for editing an application you don't have the source for.

The first one, as you've discovered is decompile, edit, recompile. The advantage is that you don't have to know anything about bytecode since it's just decompiled to Java. The main disadvantage is that even the best decompilers can't guarantee producing recompileable code. And if the application has been obfuscated at all, you can forget about recompilation.

The alternative is disassemble, edit, reassemble. I'd recommend Krakatau for this (disclosure, I wrote it). One alternative is ASM, but it's designed for programatic modification, not hand editing, and it's clunkier.

The main advantage of bytecode editing is that it always works, even if the application has been obfuscated. The downside is that it obviously requires you to understand and be able to program directly in bytecode.