How to write a thread-safe Singleton?

[Method 1]




public

 class

 MySingleton 
{

private

 static

 MySingleton instance = new

 MySingleton();
private

 MySingleton() 
{

// construct the singleton instance

}


public

 static

 MySingleton getInstance() 
{

return

 instance;
}

}



[Method 2]



public

 class

 MySingleton {

private

 static

 MySingleton instance = null

;
private

 MySingleton() {

// construct the singleton instance

}

public

 synchronized static

 MySingleton getInstance() {

if

 (instance == null

) {


            instance = new

 MySingleton();
}

return

 instance;
}

}



[Method 3]



public

 class

 MySingleton {

private

 static

 boolean

 initialized = false

;
private

 static

 Object lock = new

 Object();
private

 static

 MySingleton instance = null

;
private

 MySingleton() {

// construct the singleton instance

}

public

 static

 MySingleton getInstance() {

if

 (!initialized) {

synchronized

(lock) {

if

 (instance == null

) {


                    instance = new

 MySingleton();

                    initialized = true

;
}

}

}

return

 instance;
}

}