According to ‘ , 2nd Edition’, “ … The best way to ensure that callbacks are garbage collected promptly is to store only to them, for instance, by storing them only as keys in a WeakHashMap.” Effective Java weak refer- ences There is an example in source code which uses WeakHashMap to save callbacks to prevent memory leak. JDK Let’s look at the class BaseResourceFactory There is a field: listenerMap , which is a WeakHashMap that saves listeners that are being added. // Use a WeakHashMap as it automatically removes dead objects when they're// collectedprivate final WeakHashMap<ResourceFactoryListener,Boolean> listenerMap =new WeakHashMap<ResourceFactoryListener,Boolean>(); To add a listener, the client just need to save the listener as the key, and Boolean.TRUE as the value to the map. @Override public void addFactoryListener(ResourceFactoryListener l) {listenerMap.put(l, Boolean. );} TRUE Why using Boolean.TRUE? Because it is a constant object: public static final Boolean = new Boolean(true); TRUE So we are using it as a placeholder without creating any additional object or using any additional memory. And to get all listeners, the getFactoryListeners method returns a defensive copy of the keySet of the WeahHashMap. protected ResourceFactoryListener[] getFactoryListeners() {return listenerMap.keySet().toArray(new ResourceFactoryListener[0]);} Lastly, there is remove method. @Override public void removeFactoryListener(ResourceFactoryListener l) {// remove will return null if there is no mapping, so it's safe to call// with unregistered listenerslistenerMap.remove(l);}
