How to call Java function from c

Question

I'm stuck with this, I need to call a Java Function from c/c++.

In the examples and tutorials i only see a java app calling a c method, and in this same method calling another java method, but what I want to do is to call a java method from any part of the code. This is what I have:

static JNIEnv mEnv;
static jclass mClassAndroidActivity;
static mMethodSayHello;
JNIEXPORT void JNICALL JNI_FUNCTION(AndroidActivity_nativeInit)(JNIEnv* env, jobject obj, int width, int height)
{
    mEnv = env;
    jclass cls = (*env)->GetObjectClass(env, obj);
    mClassAndroidActivity = (*env)->NewGlobalRef(env, cls);
    mMethodSayHello = (*env)->GetMethodID (env, mClassAndroidActivity, "SayHello", "(Ljava/lang/String;)V");
}

//this method is called from a cpp
void nativeSayHello(char* msg)
{
    jstring string = (*mEnv)->NewStringUTF(mEnv, msg);
    (*mEnv)->CallVoidMethod(mEnv, mClassAndroidActivity, mMethodSayHello, string);
}

but is always crashing, I've tried without the NewGlobalRef, using mEnv instead of env in the JNI_Function, I've tried getting the method id from the JNI_OnLoad, but always crashes.

This is the log i get:

02-15 18:09:48.520: W/dalvikvm(27904): JNI WARNING: threadid=1 using env from threadid=0

'The "best" log that i get said that env from thread 1 was different from thread 0' This is correct, I believe. You should have a different env pointer per Java thread. — Yourpalal
– Yourpalal, Commented Feb 16, 2012 at 1:25
yeah but how can i save the first env pointer so i can used in different threads — cbedoya
– cbedoya, Commented Feb 16, 2012 at 1:31

praetorian droid · Accepted Answer · 2012-02-16 02:51:42Z

You can't reuse JNIEnv because it is specific to the calling thread. To call (non-static) Java method from the native code, you need something like this:

static JavaVM *gJavaVM;
static jobject gCallbackObject = NULL;

JNIEXPORT jint JNI_OnLoad(JavaVM *vm, void *reserved) {
    gJavaVM = vm;
    return JNI_VERSION_1_6;
}

JNIEXPORT void JNICALL JNI_FUNCTION(AndroidActivity_nativeInit)(JNIEnv* env, jobject obj, int width, int height) {
    // ...
    gCallbackObject = (*env)->NewGlobalRef(env, obj);
}

JNIEXPORT void JNICALL JNI_FUNCTION(AndroidActivity_nativeRelease)(JNIEnv* env, jobject obj) {
    (*env)->DeleteGlobalRef(env, gCallbackObject);
    gCallbackObject = NULL;
}

//this method is called from native code
void nativeSayHello(char* msg) {
    int status;
    JNIEnv *env;
    int isAttached = 0;

    if (!gCallbackObject) return;

    if ((status = (*gJavaVM)->GetEnv(gJavaVM, (void**)&env, JNI_VERSION_1_6)) < 0) {
        if ((status = (*gJavaVM)->AttachCurrentThread(gJavaVM, &env, NULL)) < 0) {
            return;
        }
        isAttached = 1;
    }

    jclass cls = (*env)->GetObjectClass(env, gCallbackObject);
    if (!cls) {
        if (isAttached) (*gJavaVM)->DetachCurrentThread(gJavaVM);
        return;
    }

    jmethodID method = (*env)->GetMethodID(env, cls, "SayHello", "(Ljava/lang/String;)V");
    if (!method) {
        if (isAttached) (*gJavaVM)->DetachCurrentThread(gJavaVM);
        return;
    }

    jstring string = (*mEnv)->NewStringUTF(mEnv, msg);
    (*env)->CallVoidMethod(env, gCallbackObject, method, string);

    if (isAttached) (*gJavaVM)->DetachCurrentThread(gJavaVM);
}

This code snippet is not tested. To prevent memory leak, don't forget to call nativeRelease() method in your Java code when the reference to the object will not needed any more.

See The Java Native Interface documentation for more details.

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