what is wrong with this call to the java method?

Question

I am trying to call a Java method from the code. C code listens to either Escape, Shift, Ctrl key press, then it calls the Java method

Answer 1

I think it is due to the UAC feature enabled on your Operating System. This was a bug for Java 6. Read this for further reference.

The reason I say this is because the event to the escape key is fired correctly and the problem only begins as soon as the call to the java method is done.

Answer 2

I believe you cannot call a java method that takes a String parameter and pass it a char*. You should call NewStringUTF first.

Answer 3

The JVM is crashing because the JNIEnv that is used is not a valid one. There are other issues with the code as well.

The Sun JNI documentation is providing very good information regarding threads.

Here comes some parts that are obvious:

Create a JNI_OnLoad function in your code. It will be called when the library is loaded. Then cache the JavaVM pointer because that is valid across threads. An alternative is to call (*env)->GetJavaVM in the initializeJNIVars function but I prefer the first one.

In your initializeJNIVars you can save the obj reference by calling Obj = (*env)->NewGlobalRef(obj).

In the LowLevelKeyboardProc you will have to get the env pointer:

AttachCurrentThread(JavaVM *jvm, JNIEnv &env, NULL);

---

Edit

OK, here are the code that you should add to get it working, I have tried it myself and it works. NB: I have not analyzed what your code is actually doing so I just did some fixes to get it working.

Add these variables among your other global variables:

static JavaVM *javaVM = NULL;
static jmethodID callbackMethod = NULL;
static jobject callbackObject = NULL;

You can remove your cls, mid, Env and Obj variables and use mine instead.

Create the JNI_OnLoad method where you cache the JavaVM pointer:

JNIEXPORT jint JNICALL JNI_OnLoad(JavaVM *jvm, void *reserved) {
    JNIEnv *env = 0;

    if ((*jvm)->GetEnv(jvm, (void**)&env, JNI_VERSION_1_4)) {
        return JNI_ERR;
    }

    javaVM = jvm;

    return JNI_VERSION_1_4;
}

Alter your initializeJNIVars to look like the following:

void Java_keylogger_TestKeys_initializeJNIVars(JNIEnv *env, jobject obj) {
    jclass cls = (*env)->GetObjectClass(env,obj);
    callbackMethod = (*env)->GetMethodID(env, cls, "callBack", "(Ljava/lang/String;)V");
    callbackObject = (*env)->NewGlobalRef(env, obj);
    if(cls == NULL || callbackMethod == NULL) {
        printf("One of them is null \n");
    }
    called = TRUE;
}

And finally in your LowLoevelKeyboardProc code you will have to add the following:

...
WPARAM param = kbhook->vkCode;

JNIEnv *env;
jint rs = (*javaVM)->AttachCurrentThread(javaVM, (void**)&env, NULL);
if (rs != JNI_OK) {
    return NULL; // Or something appropriate...
}
...

    case VK_ESCAPE:
        printf("Escape pressed !\n");
        jstring message = (*env)->NewStringUTF(env, "1B");
        (*env)->CallVoidMethod(env, callbackObject, callbackMethod, message);
        break;
...

In your unregisterWinHook you should delete the global reference so that objects can be GC'd.

...
(*env)->DeleteGlobalRef(env, callbackObject);

And that's it.