c - Keeping a global reference to the JNIEnv environment

Question

I am storing off JNIEnv in a global so I can call static java methods later. But is it nessasary to store off a global pointer to the JNIEnv, they way one would with any other java object, or is it a special case that does not require this.

JNIEnv* globalEnvPointer;

[JNICALL etc] void init(JNIENv* env, [etc])
{
   //required?
   globalEnvPointer = (JNIENv*) (env*)->GetGlobalRef(env, env);
   //or is this OK?
   globalEnvPointer = env;
}

Edit

I'm bing a bit dumb here, all the methods that will use globalEnvPointer, are invoked within my init because my init is actually my c program's main method, which won't return until the end of the program. I am also using no other threads in the c program. I think this simplifies the answer.

JNIEnv* globalEnvPointer;

[JNICALL etc] void main(JNIENv* env, [etc])
{
   //required?
   globalEnvPointer = (JNIENv*) (env*)->GetGlobalRef(env, env);
   //or is this OK?
   globalEnvPointer = env;
   someMethod();
}

void someMethod()
{
   //use globalEnvPointer here
}

question from:https://stackoverflow.com/questions/12420463/keeping-a-global-reference-to-the-jnienv-environment

Answer 1

You cannot cache the JNIEnv pointer. Read about it here:

The JNI interface pointer (JNIEnv) is valid only in the current thread. Should another thread need to access the Java VM, it must first call AttachCurrentThread() to attach itself to the VM and obtain a JNI interface pointer. Once attached to the VM, a native thread works just like an ordinary Java thread running inside a native method. The native thread remains attached to the VM until it calls DetachCurrentThread() to detach itself.

What you can do is to cache the JavaVM pointer instead.

static JavaVM *jvm;

[JNICALL etc] void init(JNIENv* env, [etc])
{
   jint rs = (*env)->GetJavaVM(env, &jvm);
   assert (rs == JNI_OK);
}

And then whenever you need then JNIEnv pointer from a context where it is not given you do this:

void someCallback() {
    JNIEnv *env;
    jint rs = (*jvm)->AttachCurrentThread(jvm, &env, NULL);
    assert (rs == JNI_OK);
    // Use the env pointer...
}

But whenever you call a native method from Java the env pointer to use is given:

JNIEXPORT jint JNICALL Java_package_Class_method(JNIEnv *env, jobject obj) {
    // just use the env pointer as is.
}