Skip to main content

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.







  • Discuss this post in the learning forum.


  • Check out my learning journal. I am learning JSF at the moment. Do you want to join an experiment in forming an ad hoc virtual study group?




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

-----
COMMENT:
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.

Regards,
Sanket Daru.
-----
COMMENT:
AUTHOR: Parag
DATE: 07/16/2007 09:09:10 AM
Hi Sanket,

I have answered your question on the newsgroup.

Comments

Popular posts from this blog

My HSQLDB schema inspection story

This is a simple story of my need to inspect the schema of an HSQLDB database for a participar FOREIGN KEY, and the interesting things I had to do to actually inspect it. I am using an HSQLDB 1.8 database in one of my web applications. The application has been developed using the Play framework , which by default uses JPA and Hibernate . A few days back, I wanted to inspect the schema which Hibernate had created for one of my model objects. I started the HSQLDB database on my local machine, and then started the database manager with the following command java -cp ./hsqldb-1.8.0.7.jar org.hsqldb.util.DatabaseManagerSwing When I tried the view the schema of my table, it showed me the columns and column types on that table, but it did not show me columns were FOREIGN KEYs. Image 1: Table schema as shown by HSQLDB's database manager I decided to search on StackOverflow and find out how I could view the full schema of the table in question. I got a few hints, and they all pointed to

Fuctional Programming Principles in Scala - Getting Started

Sometime back I registered for the Functional Programming Principles in Scala , on Coursera. I have been meaning to learn Scala from a while, but have been putting it on the back burner because of other commitments. But  when I saw this course being offered by Martin Odersky, on Coursera , I just had to enroll in it. This course is a 7 week course. I will blog my learning experience and notes here for the next seven weeks (well actually six, since the course started on Sept 18th). The first step was to install the required tools: JDK - Since this is my work machine, I already have a couple of JDK's installed SBT - SBT is the Scala Build Tool. Even though I have not looked into it in detail, it seems like a replacement for Maven. I am sure we will use it for several things, however upto now I only know about two uses for it - to submit assignments (which must be a feature added by the course team), and to start the Scala console. Installed sbt from here , and added the path

Five Reasons Why Your Product Needs an Awesome User Guide

Photo Credit: Peter Merholz ( Creative Commons 2.0 SA License ) A user guide is essentially a book-length document containing instructions for installing, using or troubleshooting a hardware or software product. A user guide can be very brief - for example, only 10 or 20 pages or it can be a full-length book of 200 pages or more. -- prismnet.com As engineers, we give a lot of importance to product design, architecture, code quality, and UX. However, when it comes to the user manual, we often only manage to pay lip service. This is not good. A usable manual is as important as usable software because it is the first line of help for the user and the first line of customer service for the organization. Any organization that prides itself on great customer service must have an awesome user manual for the product. In the spirit of listicles - here are at least five reasons why you should have an awesome user manual! Enhance User Satisfaction In my fourteen years as a