Will a WeakHashMap's entry be collected if the value contains the only strong reference to the key?

Question

I need to associate some data with a key for its lifetime, so I am using a WeakHashMap. However, in addition I need to get a key by its corresponding value. The easy way to do it is to hold on to the reference when creating a value:

public class Key {}

public class Value { 
    final public Key key;
    public Value(Key k) {
        key = k;
    }
}

Of course, while I use Value in my program, its key won't go away. However, if there are no more references to either key or its value outside the map, will it be garbage collected? Or does the surviving strong reference in the value prevent it?

Elle

Answer 1

No it won't be garbage collected, see the Javadoc:

Implementation note: The value objects in a WeakHashMap are held by ordinary strong references. Thus care should be taken to ensure that value objects do not strongly refer to their own keys, either directly or indirectly, since that will prevent the keys from being discarded.

As mentioned by @biziclop one solution would be to store a weak reference to the key in your value object.

public class Value { 
  final public WeakReference<Key> key;
  public Value(Key k) {
    this.key = new WeakReference<Key>(k);
  }
}

Nynorsk

@biziclop Also from the Javadoc: One way to deal with this is to wrap values themselves within WeakReferences before inserting, as in: m.put(key, new WeakReference(value)), and then unwrapping upon each get. That should do the trick! – Nynorsk

Either that or only store a weak reference to the key in your value objects. Because if you wrap the value in a WR, you might end up with an existing key with the value alraedy collected. – Rosmunda

Answer 2

Looking at the implementation, the answer appears to be no.

This is from the WeakHashMap source:

/**
 * The table, resized as necessary. Length MUST Always be a power of two.
 */
private Entry[] table;

...

private static class Entry<K,V> extends WeakReference<K> implements Map.Entry<K,V> {
    private V value;
    ...
}

As you can see, Entry objects are referenced strongly by the map itself. So if the map is accessible, so will be the Entry, therefore your Value objects, and your key too.

Rosmunda