Question

To implement a callback function from the native code to Java code, I have to create a global reference using NewGloabRef . From the memory profile , I found that ,once I called env->NewGlobalRef(weak_this), even it was a weak reference of the player object, the Player object will be available as Root Objects, which I think will prevent it from being garbage collected.

But my understanding is the weak reference won’t prevent the object from garbage collected.

//java code

Player{

native_init(new WeakReference(this)),

}

//JNi code

//listener 
Listener::Listener(jobject weak_this)
{

//will use mObject for callback 
mObject = env->NewGlobalRef(weak_this);

}


xxxx_Player_native_init(xxxx. Object weak_this)
{

Listener l = new Listener(weak_this);

}

EDIT:

memory profile :

 Root Object 0x2C820E10 <com/trident/tv/media/player/JniTPlayer>
  com/trident/tv/media/player/JniTPlayer.trace : 0x2C83CC54 <java/lang/String>
  com/trident/tv/media/player/JniTPlayer.listenerList : 0x2C820E64 <java/util/Vector>

log of JNI

[JNI] NewGlobalRef(0x2C820E10 [com/trident/tv/media/player/JniTPlayer]) : 0x2C820E10

Answer 1

A WeakReference is a Java object with an ordinary reference to it. It contains a reference to another object. It is the contained reference that is “weak”, not the reference to the WeakReference itself.

So when you call env->NewGlobalRef(weak_this) (assuming weak_this is a WeakReference), the effect is the same as assigning weak_this to a static. It doesn’t cause the object reference contained by the WeakReference to be strongly reachable.

I think you may be misinterpreting what the memory profiler is telling you. In particular, I would expect it to show the contained reference of a WeakReference to be reachable … up until the GC decided to break the link. Try an experiment with an WeakReference in an ordinary static variable.

---

UPDATE

I’m starting to think that this is normal behaviour for JNI NewGlobalRef. The JNI documentation is (as always) very vague about the method’s behaviour.

Note that there is also a JNI method called NewGlobalWeakRef; see http://java.sun.com/docs/books/jni/html/refs.html#27531. If nothing else, NewGlobalWeakRef provides an alternative way of doing what you are trying to do.