what is wrong with this call to the java method?

问题

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 telling which key was pressed. Following are the snippets that play a role in this.

C Snippet:

mid = (*env)->GetMethodID(env,cls,"callBack","(Ljava/lang/String;)V");
Env = env;
if(called)
    switch(param) {
        case VK_CONTROL:
            printf("Control pressed !\n");
            (*Env)->CallVoidMethodA(Env,Obj,mid,"11"); // calling the java method
            break;
        case VK_SHIFT:
            printf("Shift pressed !\n");
            (*Env)->CallVoidMethodA(Env,Obj,mid,"10"); // calling the java method
            break;
        case VK_ESCAPE:
            printf("Escape pressed !\n");
            (*Env)->CallVoidMethodA(Env,Obj,mid,"1B"); // calling the java method
            break;
        default:
            printf("The default case\n");
            break;
    }

Java Snippet:

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

When I run the program and press the Escape key I get this on the console:

Escape pressed !
#
# A fatal error has been detected by the Java Runtime Environment:
#
#  EXCEPTION_ACCESS_VIOLATION (0xc0000005) at pc=0x5c8b809a, pid=7588, tid=8088
#
# JRE version: 7.0
# Java VM: Java HotSpot(TM) Client VM (20.0-b01 mixed mode, sharing windows-x86 )
# Problematic frame:
# V  [jvm.dll+0x19809a]
#
# An error report file with more information is saved as:
# W:\UnderTest\NetbeansCurrent\KeyLoggerTester\build\classes\hs_err_pid7588.log
#
# If you would like to submit a bug report, please visit:
#   http://java.sun.com/webapps/bugreport/crash.jsp
#

I know I am calling the Java function the wrong way, but I don't know where I am wrong. As from the output, it satisfies the case when I press the Escape key and then an unexpected error occurs.

Link to the LOG FILE

EDIT:

After the answer by mavroprovato I still get the same errors.

I edited this way:

(*Env)->CallVoidMethodA(Env,Obj,mid,(*Env)->NewStringUTF(Env,"1B"));

EDIT:

COMPLETE CODE version 1

COMPLETE CODE version 2

回答1:

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.

回答2:

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

回答3:

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.

来源：https://stackoverflow.com/questions/10895826/what-is-wrong-with-this-call-to-the-java-method

标签

java

java-native-interface

jnienv