Tuesday, July 10, 2007

New series - reference types in Java

Starting this week, I will post a series of blogs on "Reference Types In Java". So here goes...

Since version 1.2, Java supports references other than the commonly used strong reference. An object is said to be strongly referenced, when at least one path to the object from the root set of references contains a strong reference. A strong reference is the traditional type of reference which is created by the = operator. The garbage collector considers a strongly referenced object to be in use and will not reclaim it.

This statement creates a strong reference.

String s = new String("abc"); 

The String object holding the value “abc” is strongly referenced because the variable 's' directly refers to it. Java 1.2 supports three more types of references; SoftReference, WeakReference, and PhantomReference. Whenever we want an object to be softly, weakly, or phantomly referenced, we do not directly reference the object using an '=' operator. We instead reference the object through another object. This object is a reference object and encapsulates a reference.

SoftReference sf = new SoftReference(new String(“abc”));

he String object “abc” is not strongly referenced because the only path to it is through a Reference object. The String object “abc” will remain live as long as there is sufficient memory in the JVM. But if the garbage collector has finished reclaiming all unused objects, and the program demands more memory, then “abc” will be reclaimed. What happens if we change the code to add a strong reference to the object as well?

String s = new String(“abc”);

SoftReference sf = new SoftReference(s);

Now, is the String object “abc” softly referenced, or is it strongly referenced? It is strongly referenced because there is a path from the root set of references to the String object, which has a strong reference. The variable s has a strong reference to the String object. The garbage collector will not reclaim the object because there is a strong reference to it.

The Reference object shown in the above diagram, is really an abstract superclass for different types of reference objects in Java; soft references, weak references, and phantom references. Each type denotes the strength of reachability to an object. Before we proceed, remember, the reference type of an object is the weakest type in the strongest link.

The next few posts will discuss different reference types in more detail.

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Note: This text was originally posted on my earlier blog at http://www.adaptivelearningonline.net
Here are the comments from the original post

AUTHOR: Sanket Daru
URL: http://sankdadevildaru.blogspot.com
DATE: 07/16/2007 05:02:18 AM
Dear Sir,
In this series, you are taking us through the references, weak, strong, etc... But I have a question...

Why will I explicitly want my references to be weak or soft??? Can you please guide us through one real life scenario where these references come into use???

I remember one which you discussed in class about loading registry entries in memory which might end up taking a lot of space and hence it is wiser to make it weak referenced... But still, in day-to-day usage, can you provide some good examples???

Also, I will like to ask, will it not be beautiful if Java (JVM) will do this work of assigning weak-soft-strong references for us??? The garbage collection series pointed out when and how JVM reclaims the memory, but wouldn't it be nice if JVM also assigned specific reference types on its own during compile or run time as needed??? Just a random thought... Please guide...

Thank You.

Sanket Daru.
DATE: 07/16/2007 09:09:10 AM
Hi Sanket,

I have answered your question on the newsgroup.

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