How to change methods in .class file without recompiling

Question

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?

But if you don't have dependencies of that class how would you run that class after all? — Andrey Adamovich, Mar 04 '11 at 09:29

score 2 · Accepted Answer · answered Mar 04 '11 at 09:30

2

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.

answered Mar 04 '11 at 09:30

Jon Skeet

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>> how are you expecting to use this code? This class is a part of a big application that we need to change. But all dependencies are located on the customer side and we cannot get them – Mikhail Mar 04 '11 at 09:32
@Mikhail: So why don't you have the dependencies, or why can't you get hold of them? Trying to go forward with only part of a working environment really isn't a good idea. – Jon Skeet Mar 04 '11 at 09:33
3

A further question, how will you test your changes before giving the code to the customer? – Peter Lawrey Mar 04 '11 at 09:39

score 1 · Answer 2 · answered Mar 04 '11 at 09:32

1

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.

answered Mar 04 '11 at 09:32

score 1 · Answer 3 · edited May 23 '17 at 11:47

1

Try this question on Java Bytecode editors.

java bytecode editor?

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

edited May 23 '17 at 11:47

Community

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answered Mar 04 '11 at 09:33

JeremyP

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score 0 · Answer 4 · answered Mar 08 '13 at 18:58

0

You can use a java decompiler for opening the class file and edit it and save it.

Here's one good decomp --> http://java.decompiler.free.fr/

answered Mar 08 '13 at 18:58

user2149600

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score 0 · Answer 5 · answered Mar 04 '11 at 09:29

0

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.

answered Mar 04 '11 at 09:29

adarshr

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I cannot do this because this class is a part of application that I cannot re-write to use my new class. – Mikhail Mar 04 '11 at 09:30

score 0 · Answer 6 · answered Mar 04 '11 at 09:30

0

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.

answered Mar 04 '11 at 09:30

Boris Pavlović

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This is a good idea but I cannot add aspects to the application that I need to change. Anyway thanks! – Mikhail Mar 04 '11 at 09:32
Do not change the app at all. Manipulate it with aspects. If you are not allowed to use aspects then it's a completely other thing. – Boris Pavlović Mar 04 '11 at 09:33

score 0 · Answer 7 · answered Mar 04 '11 at 09:31

0

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

answered Mar 04 '11 at 09:31

Dean Barnes

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score 0 · Answer 8 · answered Mar 04 '11 at 09:33

0

You can use Javaasist library to modify existing class file.

answered Mar 04 '11 at 09:33

Andrey Adamovich

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score 0 · Answer 9 · answered Mar 04 '11 at 21:19

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.

How to change methods in .class file without recompiling

9 Answers9

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