Android JNI Global Reference Subtleties

Question

Ok so given the following steps taken from Cpp

• Use jni to make a dictionary
• Make the jobject of the dictionary returned into a globalref
• Use jni to call a Java method that returns an object (we will call this object *A*)
• Add *A* to the dictionary WITHOUT making the ref of *A* global

What is the lifespan of the *A* ?

My expectations are as follows. The dictionary itself is global and so is protected from garbage collection, when I call the 'Add' method from jni *A* is passed 'back into java' and then the dictionary will hold a new reference to it, protecting it too from garbage collection. So I expect *A* to last as long as the dictionary (ignoring outside meddling).

Am I on the right track here? Thanks.

Answer 1

when I call the 'Add' method from jni A is passed 'back into java' and then the dictionary will hold a new reference to it, protecting it too from garbage collection.

No. You are conflating two properties of local and global references. A local reference is only valid for the duration of the JNI call it is created in. After that, it's invalid. If you want to use it again in a subsequent JNI call, make it a GlobalRef.

So I expect A to last as long as the dictionary (ignoring outside meddling).

Yes, but the reference itself has become invalid so you can't use it anyway.