redefine

Load the external *.class files to re-define the loaded peer class 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 -p /tmp/Test.class
redefine -c 327a647b -p /tmp/Test.class /tmp/Test$Inner.class

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());
          }
      }