Recommend to use the retransform command.

mc-redefine online tutorial

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

Reference: Instrumentation#redefineClasses

Frequently asked questions

Recommend to use the retransform command.

  • The class of redefine cannot modify, add or delete the field and method of the class, including method parameters, method names and return values.

  • If mc fails, you can compile the class file in the local development environment, upload it to the target system, and use redefine to hot load the class.

  • At present, redefine conflicts with watch / trace / jad / tt commands. Reimplementing redefine function in the future will solve this problem.

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.

The reset command is not valid for classes that have been processed by redefine. If you want to reset, you need redefine the original bytecode.

The redefine command will conflict with the jad/watch/trace/monitor/tt commands. After executing redefine, if you execute the above mentioned command, the bytecode of the class will be reset. The reason is that in the JDK redefine and retransform are different mechanisms. When two mechanisms are both used to update the bytecode, only the last modified will take effect.


Name Specification
[c:] hashcode of the class loader
[classLoaderClass:] The class name of the ClassLoader that executes the expression.


redefine /tmp/Test.class
redefine -c 327a647b /tmp/Test.class /tmp/Test\$Inner.class
redefine --classLoaderClass sun.misc.Launcher$AppClassLoader /tmp/Test.class /tmp/Test\$Inner.class

Use with the jad/mc command

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

mc /tmp/ -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.

  1. Convert the .class file to base64 first, then save it as result.txt

    Base64 < Test.class > result.txt
  2. Login the server, create and edit result.txt, copy the local content, paste and save

  3. Restore result.txt on the server to .class

    Base64 -d < result.txt > Test.class
  4. 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) {
                // 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());