Question

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what is wrong with this call to the java method?

The following C snippet calls a java function whenever the escape key is pressed. The first block of C snippet executes on a different thread and second block executes on a different thread.When i press the escape key,the message escape key pressed is displayed and then the jvm crashes.Why doesn’t the java function get called ?

C Code :

    if(called) {// starts on a different thread
    switch(param) {
        case VK_CONTROL:
            printf("Control pressed !\n");
            //(*Env)->CallVoidMethodA(Env,Obj,mid,"11");
            break;
        case VK_SHIFT:
            printf("Shift pressed !\n");
            //(*Env)->CallVoidMethodA(Env,Obj,mid,"10");
            break;
        case VK_ESCAPE:
            printf("Escape pressed !\n");
            JavaVM *jvm;
            JNIEnv *env;
            jmethodID mid;
            env = (*jvm)->AttachCurrentThread(jvm,&env,NULL);
            jclass cls = (*env)->GetObjectClass(env,Obj);
            mid = (*env)->GetMethodID(env,cls,"callBack","(Ljava/lang/String;)V");
            (*env)->CallVoidMethodA(env,Obj,mid,(*env)->NewStringUTF(env,"1B")); // Calling the java function
            break;
        default:
            printf("The default case\n");
            break;
    }}

    void Java_keylogger_TestKeys_initializeJNIVars
     (JNIEnv *env, jobject obj) { // starts on a different thread

      Obj = (*env)->NewGlobalRef(env,obj);
      if(Obj == NULL) {
        printf("Obj is Null\n");
      }
      called = TRUE;
   }

Java Snippet :

    public void callBack(String key) {
      String x = KeyEvent.getKeyText(Integer.parseInt(key, 16));
      System.out.println(x);
    }

I have uploaded the complete code with the output here.

Answer 1

You need to initialize your jvm variable. You can do this using JNI_CreateJavaVM()method. See here for more information: http://docs.oracle.com/javase/1.4.2/docs/guide/jni/spec/invocation.html