Question

I’ve been trying to implement simple low level keyhook using JNI and all went well til I figured I can’t call the methods while the DLL is on infinite loop(message loop). so I decided to create new thread but somehow, after I made it so the message loop runs on its own loop the lowlevel keyhook stops responding meaning that it doesn’t call the keyproc anymore, and I got no idea why is this? is there any other work around for this? I am required to be able to call the DLL’s methods while the keyboard hook is still functioning.

My current code is as simple as

register keyboard hook:

keyboardHook = SetWindowsHookEx(WH_KEYBOARD_LL, keyboardProc, hInstance, 0);

start thread

HANDLE threadHandle = CreateThread(NULL, 0, ThreadProc, NULL, 0, &threadId);

my keyboard proc and threadproc are the following:

DWORD WINAPI ThreadProc(LPVOID lpVoid) {
    MSG msg;
    while(GetMessage(&msg, NULL, 0, 0)) {
        TranslateMessage(&msg);
        DispatchMessage(&msg);
    }
    return 0;
}

LRESULT CALLBACK keyboardProc(int nCode, WPARAM wParam, LPARAM lParam) {
    KBDLLHOOKSTRUCT keyEvent = *((KBDLLHOOKSTRUCT*)lParam);
    jint vkCode = keyEvent.vkCode;
    jint flags = keyEvent.flags;
    jint action = wParam;
    (*globalEnv).CallVoidMethod(globalObj, keyboardMethodId, vkCode, flags, action);
    return CallNextHookEx(NULL, nCode, wParam, lParam);
}

where do I go wrong?
I am sure it not the java as even if I add simple logging for the keyproc its never called. But if I stop using thread and run the message loop on main thread it works fine but DLL wont respond for method calls after it.

Answer 1

You need to pump the message loop on the same thread that called SetWindowsHookEx(). So just move the call to your ThreadProc(). And of course, beware that your CallVoidMethod() callback runs on that same thread as well so be careful what you do in that function. Any shared state you access needs to be protected with a lock.