Java Singleton and Synchronization

Question

Please clarify my queries regarding Singleton and Multithreading:

• What is the best way to implement Singleton in Java, in a multithreaded environment?
• What happens when multiple threads try to access getInstance() method at the same time?
• Can we make singleton's getInstance() synchronized?
• Is synchronization really needed, when using Singleton classes?

Answer 1

You can also use static code block to instantiate the instance at class load and prevent the thread synchronization issues.

public class MySingleton {

  private static final MySingleton instance;

  static {
     instance = new MySingleton();
  }

  private MySingleton() {
  }

  public static MySingleton getInstance() {
    return instance;
  }

}

Answer 2

public class Elvis { 
   public static final Elvis INSTANCE = new Elvis();
   private Elvis () {...}
 }

Source : Effective Java -> Item 2

It suggests to use it, if you are sure that class will always remain singleton.