Tutorials : All About the Singleton Design Pattern :

Another Partial Solution

public synchronized static SingletonClass getInstance() {
	if(singleton == null) {
    	//This method shall include a sleep for the first thread only.
		someSleepActivityForFirstThread();
		//Creating a new instance of the singleton class
       singletonClass = new SingletonClass();
	}
	return singletonClass;
}

There's a different technique to avoid performance bottlenecks in applications, and this is to modify the code to synchronize only the assignment in the getInstance() method:

public static SingletonClass getInstance() {
    if(singletonClass == null) {
    	synchronized(SingletonClass.class) { 
       	singletonClass = new SingletonClass();
       }
    }
    return singletonClass;
}

But upon further analyzation, you'll see that the first thread that enters the synchronized block has been delayed for some reason and the second thread also enters the "if" block. After the first thread acquires the instance, the second thread will also obtain a new instance of the singleton class, which again violates the requirements.

Perhaps a null check for the singleton object within the synchronized block can provide a solution?

public static SingletonClass getInstance() {
    if(singletonClass == null) {
    	synchronized(SingletonClass.class) { 
       	if(singletonClass == null)
			{
				singletonClass = new SingletonClass();
			}
       }
    }
    return singletonClass;
}

The Perfect Solution

public class SingletonClass{
	public final static SingletonClass singletonClass = new SingletonClass();
    private SingletonClass() {
    	//Cannot be instantiated     
    }
}

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