weak references : Java Glossary

2008-06-24 by Roedy Green ©1996-2009 Canadian Mind Products
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weak reference

A weak reference is one that does not prevent the referenced object from being garbage collected. You might use them to manage a HashMap to look up a cache of objects. A weak reference is a reference that does not keep the object it refers to alive. A weak reference is not counted as a reference in garbage collection. If the object is not referred to elsewhere as well, it will be garbage collected.

The GC will send some sort of "finalize" message to the object and then set any weakly-referencing variables to null whenever it disposes of the referenced object. This allows "finalization" logic to be run before the object is disposed of (e.g., close a file if still open, commit any open transaction(s), etc.). Java 1.1 does not support weak references other than via an undocumented Ref class that is not supported under Netscape. Weak references arrived officially with JDK 1.2. Java has three kinds of weak references, called soft references, weak references, and phantom references, in order of increasing weakness.

Java has four orders of strength in holding onto Objects. In descending order from strongest to weakest they are:

The JVM holds onto regular Objects until they are no longer reachable by either clients or any container. In other words Objects are garbage collected when there are no more live references to them. Dead references don’t count.
garbage collection
Soft references can be deleted from a container if the clients are no longer referencing them and memory is tight.
Weak references are automatically deleted from a container as soon clients stop referencing them.
Phantom references point to objects that are already dead and have been finalised.

Soft vs Weak vs Phantom References
Type	Purpose	Use	When GCed	Implementing Class
Strong Reference	An ordinary reference. Keeps objects alive as long as they are referenced.	normal reference.	Any object not pointed to can be reclaimed.	default
Soft Reference	Keeps objects alive provided there’s enough memory.	to keep objects alive even after clients have removed their references (memory-sensitive caches), in case clients start asking for them again by key.	After a first gc pass, the JVM decides it still needs to reclaim more space.	java.lang.ref.SoftReference
Weak Reference	Keeps objects alive only while they’re in use (reachable) by clients.	Containers that automatically delete objects no longer in use.	After gc determines the object is only weakly reachable	java.lang.ref.WeakReference java.util.WeakHashMap
Phantom Reference	Lets you clean up after finalization but before the space is reclaimed (replaces or augments the use of finalize())	Special clean up processing	After finalization.	java.lang.ref.PhantomReference

Learning More

Sun’s Javadoc on the WeakHashMap class : available:

on the web at java.sun.com
in the current JDK 1.6.0_14 or in the old JDK 1.5.0_19 on your local Windows J: drive.

phantom references
soft references
Using Weak References

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