mlogagent_OnLoad.c

JNIEXPORT jint JNICALL 
Agent_OnLoad(JavaVM *vm, char *options, void *reserved) {
  char *option = strtok(options, ",");
  // ...
  // Load the config file using options from `option`. Also parse out an option for
  // a log file to write to.
  // .. 
  
  fprintf(stderr, "mlogagent: Loaded agent\n");

  // Get a JVMTI env object, which is used to make function calls into JVMTI
  jvmtiEnv *env;
  assert(JVMTI_ERROR_NONE == (*vm)->GetEnv(vm, (void **)&env, JVMTI_VERSION));

  // Let JVMTI know we are going to be registering breakpoints and accessing local
  // variables. If these capabilities are not supported by the JVM, then the AddCapabilities
  // function will return an error code, and our agent will terminate the VM.
  jvmtiCapabilities capabilities = { 0 };
  capabilities.can_generate_breakpoint_events = 1;
  capabilities.can_access_local_variables = 1;
  assert(JVMTI_ERROR_NONE == (*env)->AddCapabilities(env, &capabilities));

  // Register callbacks for ClassPrepare and Breakpoint events. We'll dig into these callback
  // functions soon.
  jvmtiEventCallbacks callbacks = { 0 };
  callbacks.ClassPrepare = &ClassPrepareCallback;
  callbacks.Breakpoint = &BreakpointCallback;
  assert(JVMTI_ERROR_NONE == (*env)->SetEventCallbacks(env, &callbacks, sizeof(callbacks)));

  // Tell JVMTI to enable breakpoint and class prepare events so our callbacks will be invoked.
  assert(JVMTI_ERROR_NONE == (*env)->SetEventNotificationMode(env, 
    JVMTI_ENABLE, JVMTI_EVENT_BREAKPOINT, NULL));
  assert(JVMTI_ERROR_NONE == (*env)->SetEventNotificationMode(env, 
    JVMTI_ENABLE, JVMTI_EVENT_CLASS_PREPARE, NULL));

  return JNI_OK;
}