How to create thread safe Singleton in Java - Java Singleton Example

Thread safe Singleton means a Singleton class which returns exactly same
instance even if exposed to multiple threads. Singleton in Java has been a
classical design pattern like Factory
method pattern or Decorator
design pattern and has been used a lot even inside JDK like java.lang.Runtime
is an example of Singleton class. Singleton pattern ensures that exactly
one instance of class will remain in Java program at any time. In our last post
10
Interview questions on Singleton in Java we have discussed many different
questions asked on Singleton pattern, One of them was writing Thread safe
singleton in Java. Prior to Java 5 double
checked locking mechanism is used to create thread-safe singleton in Java which breaks if one Thread doesn't
see instance created by other thread at same time and eventually you will end
up with more than one instance of Singleton class. From Java 5 onwards volatile
variable guarantee can be used to write thread safe singleton by using
double checked locking pattern. I personally  don't prefer that way as there are many other
simpler alternatives to write thread-safe singleton is available available like
using static
field to initialize Singleton instance or using Enum
as Singleton in Java. Let’s see example of both ways to create thread-safe
Singleton in Java.

Java Singleton Example – Thread safe
Singleton in Java using Enum

This is one of the example of Enum which I missed while writing 10
Examples of Enum in Java. Using Enum to create Singleton is by far most
simple and effective way to create thread-safe Singleton in Java, as
thread-safety guarantee is provided by Java programming language itself. You
don't need to bother about thread-safety issue. Since Enum instances are by
default final
in Java, it also provides safety against multiple instance due to serialization.
One point worth remembering is that, when we talk about thread-safe Singleton,
we are talking about thread-safety during instance creation of Singleton class
and not when we call any method of Singleton class. If your Singleton class
maintain any state and contains method to modify that state, you need to write
code to avoid and thread-safety
and synchronization
issues. Any way here is code example
of creating thread safe Singleton in Java using Enum.

 public
enum Singleton{

    INSTANCE;

 

    public void show(){

        System.out.println("Singleton using Enum in Java");

    }

}

//You can access this Singleton
as Singleton.INSTANCE and call any method like below

Singleton.INSTANCE.show();

If this suits your need than this is the most easy way of writing
thread-safe Singleton in Java. Using Enum as Singleton also provide couple of
more benefits which you can find out on Why
Enum Singleton are better in Java.

Java Singleton Example - Thread Safe
Singleton using Static field Initialization

You can also create thread safe Singleton in Java by creating Singleton
instance during class
loading. static fields are initialized during class loading and Classloader
will guarantee that instance will not be visible until its fully created. Here
is example of creating thread safe singleton in Java using static factory
method. Only disadvantage of this implementing Singleton patter using static
field is that this is not a lazy
initialization and Singleton is initialized even before any clients call
there getInstance() method.

public class
Singleton{

    private static final
Singleton INSTANCE = new Singleton();

 

    private Singleton(){ }



    public static Singleton getInstance(){

        return
INSTANCE;

    }

    public void show(){

        System.out.println("Singleon using static initialization in Java");

    }

}

//Here is how to access this
Singleton class

Singleton.getInstance().show();

here we are not creating Singleton instance inside getInstance() method
instead it will be created by ClassLoader. Also private
constructor makes impossible to create another instance , except one case.
You can still access private constructor by reflection and calling setAccessible(true). By the way
You can still prevent creating another instance of Singleton by this way by throwing
Exception from constructor.

That's all on how to create thread safe Singleton in Java. Both
the approach are safe with thread-safety issue but my personal favorite is
using Enum because of its simplicity, prevention of multiple instance against
Serialization attack and concise code.

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Source:http://javarevisited.blogspot.com/2012/12/how-to-create-thread-safe-singleton-in-java-example.html