How to create static JNI Environment Pointer?

Question

Here I create a class in JAVA in which I have function (callback) which I must call from C file.

class DSMInitializeClassParameter {

    /**
     * Callback         


        

Answer 1

Another way to ensure that you get a reference to the JavaVM as the first order of business is to add the JNI_OnLoad method and cache the reference. This will be called when the shared library gets loaded.

Ex.

static JavaVM* cachedJVM;

JNIEXPORT jint JNICALL JNI_OnLoad(JavaVM *jvm, void *reserved)
{
    cachedJVM = jvm;
    // ... Any other initialization code.
}

Once you have the ref to the JavaVM pointer you can then use the method Michael Mrozek described in the above post.

Answer 2

It's generally unsafe to cache a JNIEnv* instance and keep using it, as it varies depending on the currently active thread. You can save a JavaVM* instance, which will never change. In a native initializer function, call GetJavaVM and pass it the address of a JavaVM pointer:

static JavaVM *jvm;
JNIEXPORT void JNICALL Java_SomeClass_init(JNIEnv *env, jclass) {
    int status = (*env)->GetJavaVM(env, &jvm);
    if(status != 0) {
        // Fail!
    }
}

Now you can use that JavaVM* to get the current JNIEnv* with AttachCurrentThread:

dsmResult_t dsmInitializeCall( dsmResult_t status, void * pUserData, dsmEvent_t * hEvent ) {
    JNIEnv *env;
    (*jvm)->AttachCurrentThread(jvm, (void **)&env, NULL);
    (*env)->CallVoidMethod(env, classObject, mid);
}