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6.2

How to Callback Java Using JNI From A Global Mouse/Keyboard Hook

Zones: Java Native Interface (JNI), C++ Programming Language, Java Programming Language

Tags: Java, C++

I'm using this code to catch keyboard events (and later mouse events when I finally get this working) and send the details of each event back to Java. My problem is that when HookKeyboardProc is triggered by any process other than Java (i.e. when the java app does not have focus) then env->CallVoidMethod(hookObj, processKeyID, (jint)(wParam), true); is not called.

MessageBox() is always called so I know I have the global hook setup correctly.

Also, if remove the "if (jvm != NULL) {" statement, then test outside of the Java window, it no longer works when the focus is once again on the Java application. I read somewhere that windows simply "turns off" the callback method if it detects an error such as a null pointer or malformed api call. So I presume that jvm is evaluating to NULL whenever the HookKeyboardProc function is called "outside of java" (i.e. when another application is active)

Can anyone give me some guidance as to how I solve this problem and correctly return global keyboard events back to my java app?

         
           
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             // syshook.h
 
#include "jni.h"
 
#ifdef __cplusplus
extern "C" {
#endif
 
JNIEXPORT void JNICALL Java_inputhooks_PollThread_AttachKeyboardHook
  (JNIEnv *, jobject);
JNIEXPORT void JNICALL Java_inputhooks_PollThread_UnHookKeyboard
  (JNIEnv *, jobject);
 
#ifdef __cplusplus
}
#endif
 
 
 
 
 
// syshook.cpp
 
#include <windows.h>
 
#include "jni.h"
#include "syshook.h"
 
#pragma data_seg(".HOOKDATA") //Shared data among all instances.
static HHOOK hkb = NULL;
static HINSTANCE hinstDLL = NULL;
 
static JavaVM *jvm = NULL;
static jobject hookObj = NULL;
static jmethodID processKeyID = NULL;
static DWORD hookThreadId = 0; 
#pragma data_seg()
#pragma comment(linker, "/SECTION:.HOOKDATA,RWS")
 
 
JNIEXPORT LRESULT CALLBACK HookKeyboardProc(INT nCode, WPARAM wParam, LPARAM lParam) {
    if (nCode < 0)  // do not process message 
		return CallNextHookEx(hkb, nCode, wParam, lParam); 
 
	MessageBox(NULL,"key event","Hook", MB_OK | MB_ICONEXCLAMATION);
	printf("C++: test\n");
 
	JNIEnv *env;
 
	if (jvm != NULL) {
		if (jvm->AttachCurrentThread((void **)&env, NULL) >= 0) {
			env->CallVoidMethod(hookObj, processKeyID, (jint)(wParam), true);
		}
    }
	return CallNextHookEx(hkb, nCode, wParam, lParam);
}
 
/*
  Called from java side when the DLL is loaded.
*/
JNIEXPORT void JNICALL Java_inputhooks_PollThread_AttachKeyboardHook(JNIEnv *env, jobject obj) {
    hinstDLL = LoadLibrary((LPCTSTR) "InputHooks.dll"); 
    hkb = SetWindowsHookEx(WH_KEYBOARD, (HOOKPROC)HookKeyboardProc, hinstDLL, 0);
    
    hookObj = env->NewGlobalRef(obj);
    jclass cls = env->GetObjectClass(hookObj);
    processKeyID = env->GetMethodID(cls, "test", "(IZ)V");
    env->GetJavaVM(&jvm);
    hookThreadId = GetCurrentThreadId(); 
 
    if (hkb == NULL) {
          DWORD dw = GetLastError(); 
          LPSTR lpstrMyprompt;
          sprintf(lpstrMyprompt,"OpenProcess failed with last error %d.",GetLastError());
          
          MessageBox(NULL,lpstrMyprompt,"HookError", MB_OK | MB_ICONEXCLAMATION);
          
          LocalFree(lpstrMyprompt);
          ExitProcess(dw);
    }
}
 
JNIEXPORT void JNICALL Java_inputhooks_PollThread_UnHookKeyboard(JNIEnv *env, jobject obj) {
	if(hkb != NULL)
		UnhookWindowsHookEx(hkb);
 
	hkb = NULL;
}

           
         

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Related Solutions: How to Callback Java Using JNI From A Global Mouse Hook

Question Stats

Zone: Programming

Question Asked By: frube

Solution Provided By: jkr

Participating Experts: 1

Solution Grade: A

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