coolx.net

Programming

No.

699

Name.

swindler

Subject.

Double-checked locking and the Singleton pattern

Main Cate.

Java

Sub Cate.

Java

Date.

2013-02-22 14:25

Hit.

2368 (211.36.27.3)

File.

import java.util.*;
class Singleton
{
private static Singleton instance;
private Vector v;
private boolean inUse;

private Singleton()
{
v = new Vector();
v.addElement(new Object());
inUse = true;
}

public static Singleton getInstance()
{
if (instance == null) //1
instance = new Singleton(); //2
return instance; //3
}
}

Thread 1 calls the getInstance() method and determines that instance is null at //1.

Thread 1 enters the if block, but is preempted by thread 2 before executing the line at //2.

Thread 2 calls the getInstance() method and determines that instance is null at //1.

Thread 2 enters the if block and creates a new Singleton object and assigns the variable instance to this new object at //2.

Thread 2 returns the Singleton object reference at //3.

Thread 2 is preempted by thread 1.

Thread 1 starts where it left off and executes line //2 which results in another Singleton object being created.

Thread 1 returns this object at //3.

public static Singleton getInstance()
{
if (instance == null)
{
synchronized(Singleton.class) { //1
if (instance == null) //2
instance = new Singleton(); //3
}
}
return instance;
}

Thread 1 enters the getInstance() method.

Thread 1 enters the synchronized block at //1 because instance is null.

Thread 1 is preempted by thread 2.

Thread 2 enters the getInstance() method.

Thread 2 attempts to acquire the lock at //1 because instance is still null. However, because thread 1 holds the lock, thread 2 blocks at //1.

Thread 2 is preempted by thread 1.

Thread 1 executes and because instance is still null at //2, creates a Singleton object and assigns its reference to instance.

Thread 1 exits the synchronized block and returns instance from the getInstance() method.

Thread 1 is preempted by thread 2.

Thread 2 acquires the lock at //1 and checks to see if instance is null.

Because instance is non-null, a second Singleton object is not created and the one created by thread 1 is returned.

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