CSC/ECE 517 Fall 2011/ch4 4h lp

Introduction

This wiki article discusses about some of the commonly used & easy to understand Desgin patterns[1] in the software industry. Specifically, we will be studying about the Singleton, Adapter, Command & Strategy patterns.

Case Study

Singleton

Singleton is a design pattern which imposes a restriction on the class to instantiate exactly one object of that class.

Implementation

• In Java, We can create a thread-safe & lazy version of Singleton as follows. The comments in the code help you understand the Lazy & thread-safe aspect of it.

public class Singleton {
        private static Singleton instance;
 
        private Singleton() {
                //Do nothing. Initialize the object only when the first time getInstance() is called.
        }
 
        public static synchronized Singleton getInstance() { 

        //The keyword "synchronized" makes it thread safe so that two threads invoking getInstance()  
        //at the same time cannot create two instances when a context switch happens so as to 
        //facilitate this scenario. In general this can happen even with 'n' threads invoking getInstance().
        //"synchronized" keyword is used to eliminate such scenarios.

                if (null == instance) {
                        instance = new Singleton();
                }
                return instance;
        }
}

When should we use Singleton ? <ref>When should we use Singleton ?</ref>

There is a lot of criticism to the use of singleton pattern. So, how do we decide whether an object should be singleton or not ? For an object to be considered as a singleton object, they must satisfy three requirements:

1. controls concurrent access to a shared resource.
2. access to the resource will be requested from multiple, disparate parts of the system.
3. there can be only one object.

Advantages

• Singletons are often preferred to global variables as they don't pollute the global namespace with unnecessary variables & also can be instantiated only when needed unlike global variables which always consume resources.

Drawbacks

• Introduces a global state into the application.
• Can make unit testing classes in isolation difficult.

Checkout the this video from Misko Hevery which explains in detail when the usage of singletons is not exercised.<ref>Why Singletons are bad</ref>

References

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