How to change methods in .class file without recompiling

问题

I need to change existing compiled .class file. Actually I have even sources of it, but I cannot just change and recompile it because of many dependencies that I don't have.

So I need to change 2 methods. Both them have void return type. The first contains just 2 lines that are calls of another methods of the same class, i.e.

public void a() {
    System.out.println("a");
}

public void b() {
    System.out.println("b");
}

public void ca() {
    a();
    b();

}

And I need to change method ca sp that it calls only a() method.

The second method that I need to change contains some logic, but I want to clear it at all, i.e. to have method with empty body that does nothing.

How can I do this?

回答1:

If you don't have the required dependencies, how are you expecting to use this code? I would strongly recommend that you devote your time to being able to compile this normally, instead of trying to just change the binary. It's likely to be a better bet in the long run.

回答2:

I would take a look at AspectJ and set triggers to every call of ca. Then you can easily block that call and call a instead.

回答3:

Try this question on Java Bytecode editors.

java bytecode editor?

However, I think Jon Skeet's answer is the one that really applies here.

回答4:

I am not sure if you can change it in the first place, but even if you did succeed, I wouldn't recommend that. A cleaner solution would be to extend your class and override the implementation of the ca() method to call only the a() method.

回答5:

If method a is public then the easiest way is to use an aspect (Aspect Oriented Programming, AspectJ) and intercept every call to ca. Instead of invoking ca just invoke a.

回答6:

Have you tried looking into reflection? I have limited experience with it, but I'm not sure what you want to do it possible.

回答7:

You can use Javaasist library to modify existing class file.

回答8:

I had a similar problem.

I had to modify a method in a class file with no source. The method was rethrowing the exceptions badly so I had to tweak it. I have decompiled the class with Classfile Analyzer (http://classfileanalyzer.javaseiten.de/) I have edited then the resulted file and recompiled it with Jasmine assembler.

Using Apache BCEL you can accomplish the same thing but in a more elegant way - at runtime - not as I have described above at compile time.