java -javaagent:myagent.jar=mode=test Test

jvmtiEventCallbacks callbacks;
jvmtiEnv *          jvmtienv = jvmti(agent);
jvmtiError          jvmtierror;
memset(&callbacks, 0, sizeof(callbacks));
callbacks.ClassFileLoadHook = &eventHandlerClassFileLoadHook;
jvmtierror = (*jvmtienv)->SetEventCallbacks( jvmtienv,
                                             &callbacks,
                                             sizeof(callbacks));

JNIEXPORT jint JNICALL
Agent_OnLoad(JavaVM *vm, char *options, void *reserved);

JNIEXPORT jint JNICALL
Agent_OnAttach(JavaVM* vm, char* options, void* reserved);

JNIEXPORT void JNICALL
Agent_OnUnload(JavaVM *vm); 

struct _JPLISAgent {
    JavaVM *                mJVM;                   /* handle to the JVM */
    JPLISEnvironment        mNormalEnvironment;     /* for every thing but retransform stuff */
    JPLISEnvironment        mRetransformEnvironment;/* for retransform stuff only */
    jobject                 mInstrumentationImpl;   /* handle to the Instrumentation instance */
    jmethodID               mPremainCaller;         /* method on the InstrumentationImpl that does the premain stuff (cached to save lots of lookups) */
    jmethodID               mAgentmainCaller;       /* method on the InstrumentationImpl for agents loaded via attach mechanism */
    jmethodID               mTransform;             /* method on the InstrumentationImpl that does the class file transform */
    jboolean                mRedefineAvailable;     /* cached answer to "does this agent support redefine" */
    jboolean                mRedefineAdded;         /* indicates if can_redefine_classes capability has been added */
    jboolean                mNativeMethodPrefixAvailable; /* cached answer to "does this agent support prefixing" */
    jboolean                mNativeMethodPrefixAdded;     /* indicates if can_set_native_method_prefix capability has been added */
    char const *            mAgentClassName;        /* agent class name */
    char const *            mOptionsString;         /* -javaagent options string */
};

struct _JPLISEnvironment {
    jvmtiEnv *              mJVMTIEnv;              /* the JVM TI environment */
    JPLISAgent *            mAgent;                 /* corresponding agent */
    jboolean                mIsRetransformer;       /* indicates if special environment */
};

VirtualMachine vm = VirtualMachine.attach(pid);
vm.loadAgent(agentPath, agentArgs);

void JNICALL
eventHandlerClassFileLoadHook(  jvmtiEnv *              jvmtienv,
                                JNIEnv *                jnienv,
                                jclass                  class_being_redefined,
                                jobject                 loader,
                                const char*             name,
                                jobject                 protectionDomain,
                                jint                    class_data_len,
                                const unsigned char*    class_data,
                                jint*                   new_class_data_len,
                                unsigned char**         new_class_data) {
    JPLISEnvironment * environment  = NULL;

    environment = getJPLISEnvironment(jvmtienv);

    /* if something is internally inconsistent (no agent), just silently return without touching the buffer */
    if ( environment != NULL ) {
        jthrowable outstandingException = preserveThrowable(jnienv);
        transformClassFile( environment->mAgent,
                            jnienv,
                            loader,
                            name,
                            class_being_redefined,
                            protectionDomain,
                            class_data_len,
                            class_data,
                            new_class_data_len,
                            new_class_data,
                            environment->mIsRetransformer);
        restoreThrowable(jnienv, outstandingException);
    }
}

 private byte[]
    transform(  ClassLoader         loader,
                String              classname,
                Class               classBeingRedefined,
                ProtectionDomain    protectionDomain,
                byte[]              classfileBuffer,
                boolean             isRetransformer) {
        TransformerManager mgr = isRetransformer?
                                        mRetransfomableTransformerManager :
                                        mTransformerManager;
        if (mgr == null) {
            return null; // no manager, no transform
        } else {
            return mgr.transform(   loader,
                                    classname,
                                    classBeingRedefined,
                                    protectionDomain,
                                    classfileBuffer);
        }
    }
    
  public byte[]
    transform(  ClassLoader         loader,
                String              classname,
                Class               classBeingRedefined,
                ProtectionDomain    protectionDomain,
                byte[]              classfileBuffer) {
        boolean someoneTouchedTheBytecode = false;

        TransformerInfo[]  transformerList = getSnapshotTransformerList();

        byte[]  bufferToUse = classfileBuffer;

        // order matters, gotta run 'em in the order they were added
        for ( int x = 0; x < transformerList.length; x++ ) {
            TransformerInfo         transformerInfo = transformerList[x];
            ClassFileTransformer    transformer = transformerInfo.transformer();
            byte[]                  transformedBytes = null;

            try {
                transformedBytes = transformer.transform(   loader,
                                                            classname,
                                                            classBeingRedefined,
                                                            protectionDomain,
                                                            bufferToUse);
            }
            catch (Throwable t) {
                // don't let any one transformer mess it up for the others.
                // This is where we need to put some logging. What should go here? FIXME
            }

            if ( transformedBytes != null ) {
                someoneTouchedTheBytecode = true;
                bufferToUse = transformedBytes;
            }
        }

        // if someone modified it, return the modified buffer.
        // otherwise return null to mean "no transforms occurred"
        byte [] result;
        if ( someoneTouchedTheBytecode ) {
            result = bufferToUse;
        }
        else {
            result = null;
        }

        return result;
    }   

public void
    redefineClasses(ClassDefinition[]   definitions)
            throws  ClassNotFoundException {
        if (!isRedefineClassesSupported()) {
            throw new UnsupportedOperationException("redefineClasses is not supported in this environment");
        }
        if (definitions == null) {
            throw new NullPointerException("null passed as 'definitions' in redefineClasses");
        }
        for (int i = 0; i < definitions.length; ++i) {
            if (definitions[i] == null) {
                throw new NullPointerException("element of 'definitions' is null in redefineClasses");
            }
        }
        if (definitions.length == 0) {
            return; // short-circuit if there are no changes requested
        }

        redefineClasses0(mNativeAgent, definitions);
    }

jvmtiError
JvmtiEnv::RedefineClasses(jint class_count, const jvmtiClassDefinition* class_definitions) {
//TODO: add locking
  VM_RedefineClasses op(class_count, class_definitions, jvmti_class_load_kind_redefine);
  VMThread::execute(&op);
  return (op.check_error());
} /* end RedefineClasses */

public void
retransformClasses(Class<?>[] classes) {
    if (!isRetransformClassesSupported()) {
        throw new UnsupportedOperationException(
          "retransformClasses is not supported in this environment");
    }
    retransformClasses0(mNativeAgent, classes);
}

private:
  void post_all_envs() {
    if (_load_kind != jvmti_class_load_kind_retransform) {
      // for class load and redefine,
      // call the non-retransformable agents
      JvmtiEnvIterator it;
      for (JvmtiEnv* env = it.first(); env != NULL; env = it.next(env)) {
        if (!env->is_retransformable() && env->is_enabled(JVMTI_EVENT_CLASS_FILE_LOAD_HOOK)) {
          // non-retransformable agents cannot retransform back,
          // so no need to cache the original class file bytes
          post_to_env(env, false);
        }
      }
    }
    JvmtiEnvIterator it;
    for (JvmtiEnv* env = it.first(); env != NULL; env = it.next(env)) {
      // retransformable agents get all events
      if (env->is_retransformable() && env->is_enabled(JVMTI_EVENT_CLASS_FILE_LOAD_HOOK)) {
        // retransformable agents need to cache the original class file
        // bytes if changes are made via the ClassFileLoadHook
        post_to_env(env, true);
      }
    }
  }

     Class[] getAllLoadedClasses();

  Class[] getInitiatedClasses(ClassLoader loader);

  long getObjectSize(Object objectToSize);

void appendToBootstrapClassLoaderSearch(JarFile jarfile);

void appendToSystemClassLoaderSearch(JarFile jarfile);

void setNativeMethodPrefix(ClassFileTransformer transformer, String prefix);