Do Java listeners need to be removed? (In general)

Question

Imagine this sample java class:

class A {     void addListener(Listener obj);     void removeListener(Listener obj); }  class B {     private A a;     B() {         a = new A();         a.addListener(new Listener() {             void listen() {}         }  } 

Do I need to add a finalize method to B to call a.removeListener? Assume that the A instance will be shared with some other objects as well and will outlive the B instance.

I am worried that I might be creating a garbage collector problem here. What is the best practice?

Answer 1

There is a cycle in the reference graph. A references B and B references A. The garbage collector will detect cycles and see when there are no external references to A and B, and will then collect both.

Attempting to use the finaliser here is wrong. If B is being destroyed, the reference to A is also being removed.

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The statement: "Assume that the A instance will be shared with some other objects as well and will outlive the B instance." is wrong. The only way that will happen is if the listener is explicitly removed from somewhere other than a finalizer. If references to A are passed around, that will imply a reference to B, and B will not be garbage collected because there are external references to the A-B cycle.

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Further update:

If you want to break the cycle and not require B to explicitly remove the listener, you can use a WeakReference. Something like this:

class A {     void addListener(Listener obj);     void removeListener(Listener obj); }  class B {     private static class InnerListener implements Listener {         private WeakReference m_owner;         private WeakReference m_source;          InnerListener(B owner, A source) {             m_owner = new WeakReference(owner);             m_source = new WeakReference(source);         }          void listen() {             // Handling reentrancy on this function left as an excercise.             B b = (B)m_owner.get();             if (b == null) {                 if (m_source != null) {                     A a = (A) m_source.get();                     if (a != null) {                         a.removeListener(this);                         m_source = null;                     }                 }                  return;             }             ...         }     }      private A a;      B() {         a = new A();         a.addListener(new InnerListener(this, a));     } } 

Could be further generalised if needed across multiple classes.