Definition

In computer programming, a weak reference is a reference that does not protect the referenced object from collection by a garbage collector, unlike a strong reference.

Uses

• in the observer pattern (particularly in event handling), if a strong reference is kept, objects must be explicitly unregistered, otherwise a memory leak occurs (the lapsed listener problem), while a weak reference removes the need to unregister.

• the most known use of these references is the WeakHashMap class. It’s the implementation of the Map interface where every key is stored as a weak reference to the given key. When the Garbage Collector removes a key, the entity associated with this key is deleted as well.

Examples

since Java 1.2 there are two types:-

• soft rerefernce
• weak reference

for example

import java.lang.ref.WeakReference;

public class ReferenceTest {
	public static void main(String[] args) throws InterruptedException {

            WeakReference wr = new WeakReference("I'm here");
            StrongReference sr = new StrongReference("I'm here");
            System.out.println("before gc: r=" + wr.get() + ", static=" + sr.get());
            System.gc();
            Thread.sleep(100);

            // only r.get() becomes null
            System.out.println("after gc: r=" + wr.get() + ", static=" + sr.get());

	}
}

eXTRA

• These special behaviour of SoftReference and WeakReference makes them useful in certain cases e.g. SoftReference looks perfect for implementing caches, so when JVM needs memory it removes object which have only SoftReference pointing towards them.

• On the other hand WeakReference is great for storing meta data e.g. storing ClassLoader reference. If no class is loaded then no point in keeping reference of ClassLoader, a WeakReference makes ClassLoader eligible for Garbage collection as soon as last strong reference removed.

• Some cases

  • Strong referece prevent object from GC.
  • Weak referece does not prevent object from GC.
  • Soft referece does not prevent object from GC but delay it until need a memory.
  • Phantom referece does not prevent object from GC (GC collects it whenever it like).