Inject Jar and replace classes in running JVM

问题

I want to be able to replace and add some classes to an already running JVM. I read that I need to use CreateRemoteThread, but I don't completely get it. I read this post on how to do it (Software RnD), but I can't figure out what it does and why. Besides that, it only introduces new classes, but doesn't change existing ones. How can I do it with C++?

回答1:

You don't even need CreateRemoteThread - there is an official way to connect to remote JVM and replace loaded classes by using Attach API.

1. You need a Java Agent that calls Instrumentation.redefineClasses.

   public static void agentmain(String args, Instrumentation instr) throws Exception {
       Class oldClass = Class.forName("org.pkg.MyClass");
       Path newFile = Paths.get("/path/to/MyClass.class");
       byte[] newData = Files.readAllBytes(newFile);
   
       instr.redefineClasses(new ClassDefinition(oldClass, newData));
   }
   

   You'll have to add MANIFEST.MF with Agent-Class attribute and pack the agent into a jar file.

2. Then use Dynamic Attach to inject the agent jar into the running VM (with process ID = pid).

   import com.sun.tools.attach.VirtualMachine;
   ...
   
       VirtualMachine vm = VirtualMachine.attach(pid);
       try {
           vm.loadAgent(agentJarPath, options);
       } finally {
           vm.detach();
       }
   

   A bit more details in the article.

If you insist on using C/C++ instead of Java API, you may look at my jattach utility.

来源：https://stackoverflow.com/questions/43157989/inject-jar-and-replace-classes-in-running-jvm