Singleton Class

[Vinney Shanmugam]

Please explain about singleton class. I know that, only on instance can be created for a singleton class. Is that correct?
Throw some light on that please......

Any sample code will be appreciated.

[Sidd Kulk]

You are right in saying that a singleton class allows only one instance of that class. But it is important to know that ONE INSTANCE PER JVM. That is to say that as long as the JVM is up and running, only one instance can be used. This is used when a Class is Behavior based rather than State based.

Example.

   
public class Singleton
{
    private static Singleton singleton;
    private Singleton()
    {
        // To prevent from instance creation
    }
    
    // Static method to get the same instance everytime
    public static Singleton getSingleton()
    {
       if(singleton==null){
           singleton=new Singleton();
       }
       return this.singleton;
    }
}
 

Cheers!
Sid

[Harshit Rastogi]

You dont need to use the this operator with a static variable.
Example.

   
public class Singleton
{
    private static Singleton singleton;
    private Singleton()
    {
        // To prevent from instance creation
    }
    
    // Static method to get the same instance everytime
    public static Singleton getSingleton()
    {
       if(singleton==null){
           singleton=new Singleton();
       }
       return singleton;
    }
}
 

but if you are using in threaded environment the object need to be early loaded.

   
public class Singleton
{
    private static Singleton singleton = new Singleton();
    private Singleton()
    {
        // To prevent from instance creation
    }
    
    // Static method to get the same instance everytime
    public static Singleton getSingleton()
    {
       return singleton;
    }
}
 

[Rob Spoor]

Harshit Rastogi wrote:You dont need to use the this operator with a static variable.

Even more important, you can't. There is no "this" in a static method / block.

[Maneesh Godbole]

If you are accessing the static variable and initializing it inside the getSingleton() method, you should be declaring it as synchronized.

[Bharat Mehta]

Thanks folks for your insights on Singleton Class. But I have a query. Shouldn't we override the clone method which every class inherits from the Object class in Java, to prevent any cloning of the object of our Singleton class.

[Maneesh Godbole]

From the API of Cloneable

Invoking Object's clone method on an instance that does not implement the Cloneable interface results in the exception CloneNotSupportedException being thrown.

[Olivier Scalbert]

Hi,

Do not forget the thread safety aspect !
If you are using the previous code, in a multi-threading context, then you can create several singletons.
Which is not good !

You can found interesting infos there:
webpage

Olivier

[amit punekar]

Hi,
In Singletons also make sure that the class can not be serialized.
Once serialized another instance can very well be created.
Probably make variable " singleton " as transient to prevent it from Serializing.
OR
Do not implement java.io.Serializable.

Regards,
Amit

[Rob Spoor]

That's where the readResolve method comes in:

private Object readResolve()
{
    return getInstance();
}

This method will actually replace the object that was just deserialized with the return of getInstance(). The result is, ObjectInputStream's readObject() will return the singleton instance.

[Sidd Kulk]

The best way could be:

 
    public class SingletonClass
    {
         private static SingletonClass singleton=new SingletonClass();
         private SingletonClass()
        {
             
        }
        
        public static SingletonClass getInstance()
       {
             return singleton;
        }
    }