Weak References in Java

Jussi Pohjolainen

TAMK University of Applied Sciences » ICT

Intro

• In Java, object becomes eligible to garbage collector, when there is no reference to the object.
• Weak reference is a reference that isn't strong enough to force an object to remain in memory
• There are multiple levels of weak reference strength: soft, weak and phantom (package java.lang.ref)

Different levels of weak references

• There are three levels of weak reference length:
  1. java.lang.ref.SoftReference keeps objects alive until memory is tight.
  2. java.lang.ref.WeakReference keeps objects alive until there are no references to them.
  3. java.lang.ref.PhantomReference points to objects that are already dead and have been finalised.

SoftReferences

• When object has a SoftReference, object is removed from memory before OutOfMemory - error.
• Usage:

          
  SoftReference a = new SoftReference( new Foo() );
  ( (Foo) a.get() ).doSomething();
  

WeakReferences

• When object has a WeakReference, it is not protected from GC when there are no strong references to the object
• For tracking objects in memory
• Usage

          
  WeakReference a = new WeakReference( new Foo() );
  ( (Foo) a.get() ).doSomething();
  

PhantomReferences

• Keeping track when objects are cleared from memory
• Example usage: load next image to memory when the previous one has been deleted
• Example usage: clean up processing
• Differs from Weak- and SoftReferences, PhantomReference's get() - method always returns null.
• When creating, you must pass object and java.lang.ref.ReferenceQueue
• When GC starts, it adds PhantomReference to ReferenceQueue