We used have  Singleton Design Pattern in our applications whenever it is needed. As we know that in singleton design pattern we can create only one instance and can access in the whole application. But in some cases, it will break the singleton behavior.

There are mainly 3 concepts which can break singleton property of a singleton class in java. In this post, we will discuss how it can break and how to prevent those.

Here is sample Singleton class and SingletonTest class.

Singleton.Java

package demo1;

public final 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;
    }
}

SingletonTest.java

package demo1;

public class SingletonTest {
    public static void main(String[] args) {
        Singleton object1 = Singleton.getInstance();
        Singleton object2 = Singleton.getInstance();
        System.out.println("Hashcode of Object 1 - " + object1.hashCode());
        System.out.println("Hashcode of Object 2 - " + object2.hashCode());
    }
}

Here is output, you can see it the same hashcode for objectOne and objectTwo

Hashcode of Object 1 - 1836019240
Hashcode of Object 2 - 1836019240

Now we will break this pattern. First, we will use java reflection.

Reflection

ReflectionSingleton.java

package demo1;

import java.lang.reflect.Constructor;

public class ReflectionSingleton {
    public static void main(String[] args)  {

        Singleton objOne = Singleton.getInstance();
        Singleton objTwo = null;
        try {
            Constructor constructor = Singleton.class.getDeclaredConstructor();
            constructor.setAccessible(true);
            objTwo = (Singleton) constructor.newInstance();
        } catch (Exception ex) {
            System.out.println(ex);
        }

        System.out.println("Hashcode of Object 1 - "+objOne.hashCode());
        System.out.println("Hashcode of Object 2 - "+objTwo.hashCode());

    }
}

Example to show how reflection can break the singleton pattern with Java reflect. You will get two hash code as below. It has a break on the singleton pattern.

Hashcode of Object 1 - 1836019240
Hashcode of Object 2 - 325040804

Prevent Singleton pattern from Reflection

There are many ways to prevent Singleton pattern from Reflection API, but one of the best solutions is to throw run time exception in the constructor if the instance already exists. In this, we can not able to create a second instance.

    private Singleton() {
        if( instance != null ) {
           throw new InstantiationError( "Creating of this object is not allowed." );
        }
    }

Deserialization

In serialization, we can save the object of a byte stream into a file or send over a network. Suppose if you serialize the Singleton class and then again de-serialize that object will create a new instance, hence deserialization will break the Singleton pattern.

Below code is to illustrate how the Singleton pattern breaks with deserialization.

Implements Serializable interface for Singleton Class.

DeserializationSingleton.Java

The output is below and you can see two hashcodes.

hashCode of instance 1 is - 2125039532
hashCode of instance 2 is - 381259350

Prevent Singleton Pattern from Deserialization

To overcome this issue, we need to override readResolve() method in Singleton class and return same Singleton instance. Update Singleton.java, with below method.

   protected Object readResolve() { 
           return instance; 
     }  

Now run above DeserializationDemo class and see the output.

hashCode of instance 1 is - 2125039532
hashCode of instance 2 is - 2125039532

Cloning

Using the "clone" method we can create a copy of original object, samething if we applied clone in singleton pattern, it will create two instances one original and another one cloned object. In this case will break Singleton principle as shown in below code.

Implement the "Cloneable" interface and override the clone method in the above Singleton class.

Singleton.java

    @Override
    protected Object clone() throws CloneNotSupportedException  {
        return super.clone();
    }

Then Test with cloning for breaking the singleton
CloningSingleton.java

public class CloningSingleton {
    public static void main(String[] args) throws CloneNotSupportedException, Exception {
        Singleton instanceOne = Singleton.getInstance();
        Singleton instanceTwo = (Singleton) instanceOne.clone();
        System.out.println("hashCode of instance 1 - " + instanceOne.hashCode());
        System.out.println("hashCode of instance 2 - " + instanceTwo.hashCode());
    }

}

Here is the output

hashCode of instance 1 - 1836019240
hashCode of instance 2 - 325040804

If we see the above output, two instances have different hashcodes means these instances are not the same.


Prevent Singleton Pattern from Cloning

In the above code, breaks the Singleton principle i. e created two instances. To overcome the above issue we need to implement/override clone() method and throw an exception CloneNotSupportedException from clone method. If anyone try to create clone object of Singleton, it will throw an exception as see below code.

    @Override
    protected Object clone() throws CloneNotSupportedException  {
        throw new CloneNotSupportedException();
    }

Now we can run the CloningSingleton class, it will throw CloneNotSupportedException while creating a clone object of Singleton object.