[java] Java Singleton and Synchronization

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?

Yes, you need to make getInstance() synchronized. If it's not there might arise a situation where multiple instances of the class can be made.

Consider the case where you have two threads that call getInstance() at the same time. Now imagine T1 executes just past the instance == null check, and then T2 runs. At this point in time the instance is not created or set, so T2 will pass the check and create the instance. Now imagine that execution switches back to T1. Now the singleton is created, but T1 has already done the check! It will proceed to make the object again! Making getInstance() synchronized prevents this problem.

There a few ways to make singletons thread-safe, but making getInstance() synchronized is probably the simplest.

This pattern does a thread-safe lazy-initialization of the instance without explicit synchronization!

public class MySingleton {

     private static class Loader {
         static final MySingleton INSTANCE = new MySingleton();
     }

     private MySingleton () {}

     public static MySingleton getInstance() {
         return Loader.INSTANCE;
     }
}

It works because it uses the class loader to do all the synchronization for you for free: The class MySingleton.Loader is first accessed inside the getInstance() method, so the Loader class loads when getInstance() is called for the first time. Further, the class loader guarantees that all static initialization is complete before you get access to the class - that's what gives you thread-safety.

It's like magic.

It's actually very similar to the enum pattern of Jhurtado, but I find the enum pattern an abuse of the enum concept (although it does work)

What is the best way to implement Singleton in Java, in a multithreaded environment?

Refer to this post for best way to implement Singleton.

What is an efficient way to implement a singleton pattern in Java?

What happens when multiple threads try to access getInstance() method at the same time?

It depends on the way you have implemented the method.If you use double locking without volatile variable, you may get partially constructed Singleton object.

Refer to this question for more details:

Why is volatile used in this example of double checked locking

Can we make singleton's getInstance() synchronized?

Is synchronization really needed, when using Singleton classes?

Not required if you implement the Singleton in below ways

1. static intitalization
2. enum
3. LazyInitalaization with Initialization-on-demand_holder_idiom

Refer to this question fore more details

Java Singleton Design Pattern : Questions

Enum singleton

The simplest way to implement a Singleton that is thread-safe is using an Enum

public enum SingletonEnum {
  INSTANCE;
  public void doSomething(){
    System.out.println("This is a singleton");
  }
}

This code works since the introduction of Enum in Java 1.5

Double checked locking

If you want to code a “classic” singleton that works in a multithreaded environment (starting from Java 1.5) you should use this one.

public class Singleton {

  private static volatile Singleton instance = null;

  private Singleton() {
  }

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

This is not thread-safe before 1.5 because the implementation of the volatile keyword was different.

Early loading Singleton (works even before Java 1.5)

This implementation instantiates the singleton when the class is loaded and provides thread safety.

public class Singleton {

  private static final Singleton instance = new Singleton();

  private Singleton() {
  }

  public static Singleton getInstance() {
    return instance;
  }

  public void doSomething(){
    System.out.println("This is a singleton");
  }

}

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;
  }

}

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.

If you are working on a multithreaded environment in Java and need to gurantee all those threads are accessing a single instance of a class you can use an Enum. This will have the added advantage of helping you handle serialization.

public enum Singleton {
    SINGLE;
    public void myMethod(){  
    }
}

and then just have your threads use your instance like:

Singleton.SINGLE.myMethod();