MULTITHREADED SINGLETON JAVA CLASS

First We see a Simple Singleton Class Here :

Here we are doing the Lazy Loading .

class Singleton {
 private static Singleton instance;

 private Singleton(){};

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

}

The Main Properties of a Singelton Class is below :-

1. There should always be a single instance of a class in an Application.
2. We can't be able to instantiate the class using the using the new keyword.
3. We can get the instance of the class using a getter method.

The Above Code looks fine as we have made our constructor and creating the instance for the first time and reusing it whenever asked .

Now , the problem arises when two parallel threads try to access the method , what happens then :

Both the threads will create one instance for the class hence breaking the singelton pattern .

How to tackle it :-

A simple way could be this .

private static Singleton instance;

private Singleton(){};

public synchronized static Singleton getInstance(){
    if(instance== null)
        instance = new Singleton();
    return instance;
}

We can use the synchronized keyword here in the method :

This will make our method a critical section , which does not allow multiple threads to access it at once .

A better approach here could be using a Synchronization block instead of Synchronization method : -

As Synchronization block provides more granular control unlike synchronization method which provide a generic control .

class Singleton {
    private static Singleton instance;

    private Singleton(){};

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

This is a better solution to solve the multithreading issue in singelton pattern.

This pattern can still be broke by
1. Reflection

2. Serialization

3. Cloneability

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Will cover the above three in next article.