redefine¶

Load the external *.class files to re-define the loaded classes in JVM.

Reference: Instrumentation#redefineClasses

Notes: Re-defined classes cannot be restored. There are chances that redefining may fail due to some reasons, for example: there’s new field introduced in the new version of the class, pls. refer to JDK’s documentation for the limitations.

Options¶

Name	Specification
`[c:]`	hashcode of the class loader
`[p:]`	absolute path of the external `*.class`, multiple paths are separated with 'space'

Usage¶

redefine /tmp/Test.class
redefine -c 327a647b /tmp/Test.class /tmp/Test$Inner.class

Use with the jad/mc command¶

jad --source-only com.example.demo.arthas.user.UserController > /tmp/UserController.java

mc /tmp/UserController.java -d /tmp

redefine /tmp/com/example/demo/arthas/user/UserController.class

Use jad command to decompile bytecode, and then you can use other editors, such as vim to modify the source code.
mc command to compile the modified code
Load new bytecode with redefine command

Tips for uploading .class files to the server¶

The mc command may fail. You can modify the code locally, compile it, and upload it to the server. Some servers do not allow direct uploading files, you can use the base64 command to bypass.

Convert the .class file to base64 first, then save it as result.txt
```
Base64 < Test.class > result.txt
```
Login the server, create and edit result.txt, copy the local content, paste and save
Restore result.txt on the server to .class
```
Base64 -d < result.txt > Test.class
```
Use the md5 command to verify that the .class files are consistent.

Restrictions of the redefine command¶

New field/method is not allowed

The function that is running, no exit can not take effect, such as the new System.out.println added below, only the run() function will take effect.

public class MathGame {
    public static void main(String[] args) throws InterruptedException {
        MathGame game = new MathGame();
        while (true) {
            game.run();
            TimeUnit.SECONDS.sleep(1);
            // This doesn't work because the code keeps running in while
            System.out.println("in loop");
        }
    }

    public void run() throws InterruptedException {
        // This works because the run() function ends completely every time
        System.out.println("call run()");
        try {
            int number = random.nextInt();
            List<Integer> primeFactors = primeFactors(number);
            print(number, primeFactors);

        } catch (Exception e) {
            System.out.println(String.format("illegalArgumentCount:%3d, ", illegalArgumentCount) + e.getMessage());
        }
    }