如何在jni中创建CBTHook，然后在回调函数中调用Java函数？

Question

Update (23/02/13): Tested the Hook separatedly, it works. The problem was that you can't use printf in a .dll, unless you associate a console with it.

I instead replaced every printf with fprintf and wrote the output in a logfile.

Update (22/02/13): Initialized the variable clazz in a correct way.

I want to call a method in my java application, if any desktop window is created/activated/destroyed .

I wrote a simple Java class with two native functions:

public class Hook {

public Hook(){}

public void setstatus(){
    System.out.println("status set");
}

public native void starthook();
public native void stophook();
}

And in CI wrote the following:

#include "Hook.h"
#include <windows.h>
#include <jni.h>
#include <stdio.h>

#pragma data_seg("Shared")
HHOOK   g_hHook  = NULL;
FILE *pFile=NULL;
//do mid, clazz and jvm have to be shared ?
//jmethodID mid=NULL;
//jclass clazz=NULL;
//static JavaVM *jvm = NULL;

#pragma data_seg()

HINSTANCE   g_hInstance = NULL;

BOOL APIENTRY DllMain( HANDLE hModule, DWORD  ul_reason_for_call, LPVOID lpReserved ){
    switch (ul_reason_for_call){
    case DLL_PROCESS_ATTACH:
        g_hInstance  = (HINSTANCE) hModule;
        break;

    case DLL_THREAD_ATTACH:  break;
    case DLL_THREAD_DETACH:  break;
    case DLL_PROCESS_DETACH: break;
    }

    return TRUE;
}

LRESULT CALLBACK CBTProc(int nCode, WPARAM wParam, LPARAM lParam){
    if (nCode < 0)
        return CallNextHookEx(g_hHook, nCode, wParam, lParam);       
    /*
    JNIEnv *env;
    int res =(*jvm)->AttachCurrentThread(jvm,(void **)&env, NULL);
        if(res<0){
            fprintf(pFile,"AttachCurrentThread failed\n");
            return 0;
        }
    */

    HWND hWnd = (HWND)wParam;
    if (!hWnd){
        fprintf(pFile,"hWnd==NULL\n");
        //(*jvm)->DetachCurrentThread(jvm);
        return 0;
    }
    if (nCode == HCBT_ACTIVATE){            
        // is printed
        fprintf(pFile,"activated\n");
        // isn't called
        //(*env)->CallVoidMethod(env, clazz, mid);
        // Nothing is logged
        //if((*env)->ExceptionOccurred(env))
        //      (*env)->ExceptionDescribe(env);
    }
    else if (nCode == HCBT_DESTROYWND){
        fprintf(pFile,"destroyed\n");
    }
    else if (nCode == HCBT_CREATEWND){
        fprintf(pFile,"created\n");
    }
    //(*jvm)->DetachCurrentThread(jvm);
    return 0;
}

JNIEXPORT void JNICALL Java_Hook_starthook(JNIEnv *e, jobject o){
/*
if(jvm==NULL)
    if(((*e)->GetJavaVM(e,&jvm))<0){
        printf("GetJavaVM failed\n");
        return;
        }
    jclass localRef = (*e)->GetObjectClass(e, o);
        clazz = (*e)->NewGlobalRef(e,localRef);
    if(clazz==NULL){
        printf("GetObjectClass failed\n");
        return;
    }
    mid = (*e)->GetMethodID(e, clazz, "setstatus", "()V");
    if(mid==0){
        printf("GetMethodID failed\n");
        return;
    }
*/
    pFile = fopen("C:/workspace/CBTHook/log.txt","a");
    g_hHook      = SetWindowsHookEx(WH_CBT, (HOOKPROC) CBTProc, g_hInstance, 0);
    printf("Hook started\n");
}

JNIEXPORT void JNICALL Java_Hook_stophook(JNIEnv *e, jobject o){
    if (g_hHook){
        UnhookWindowsHookEx(g_hHook);
        g_hHook = NULL;
        //(*e)->DeleteGlobalRef(e,clazz);
    }
    if(pFile)
        fclose(pFile);
    }
    printf("Hook stopped\n");
}

The CBTHook works, but I can't call the setstatus function from within the callback function.

(*env)->CallVoidMethod(env, clazz, mid);

doesn't do anything and no Exception is thrown.

What do I have to do ?

Furthermore do I have to share the global variables: mid , clazz and jvm ? Every variable in the

#pragma data_seg("SHARED")

segment is "shared", which means that they are not unique to a specific process.

And what would be the correct way to attach/detach the other threads, if this is necessary?

Answer 1

Given the lack of clear explanation about what " don't receive messages " exactly means, i will speculate that your CBTProc is being called , and you just don't get the JVM callback. If CBTProc is not called, then it's not a JNI problem at all .

You did it almost right. jmethodID is reusable between threads and between JNI calls, so it can be cached as you did. The way you get JavaVM* in starthook() and cache it is also correct. You are only wrong about jclass clazz . You try to cache it in starthook() and then try to use it not only in different call, but most probably even in different thread. What you get from GetObjectClass is a local reference which means that it's validity is not ensured after the current JNI call exits back to JVM. It might work, but you can't rely on it. And it definitely won't work when used from different thread. You must make a global reference :

jclass localRef = (*e)->GetObjectClass(e,o);
clazz = (*e)->NewGlobalRef(e,localRef);

Then you can use clazz in the way you already have in CBTProc commented out. The only work left is in stophook :

(*e)->DeleteGlobalRef(e,clazz);

So that it doesn't leak.

Bottom note : i don't understand what you mean by " sharing the global variables ".