Question

I was going through singleton pattern, below is my singleton class..

    package CrackingSingleton;

public class SingletonObject {
    private static SingletonObject ref; 
    private SingletonObject () //private constructor
    {   
    }

    public  static synchronized   SingletonObject getSingletonObject()
    {if (ref == null)
    ref = new SingletonObject();
    return ref;
        }

    public Object clone() throws CloneNotSupportedException
    {throw new CloneNotSupportedException ();
    }
    }

and I was going through the way to break it, below is my piece of code…

 package CrackingSingleton;
import java.lang.reflect.Constructor;
import java.lang.reflect.InvocationTargetException;

public class CrackingSingleton {    
     public static void main(String[] args) throws ClassNotFoundException,
       IllegalArgumentException, SecurityException,
       InstantiationException, IllegalAccessException,
       InvocationTargetException {      
        //First statement retrieves the Constructor object for private constructor of SimpleSingleton class.
        Constructor pvtConstructor = Class.forName("CrackingSingleton.SingletonObject.java").getDeclaredConstructors()[0];
        //Since the constructor retrieved is a private one, we need to set its accessibility to true.
        pvtConstructor.setAccessible(true);
        //Last statement invokes the private constructor and create a new instance of SimpleSingleton class.
         SingletonObject  notSingleton1 = ( SingletonObject) pvtConstructor.newInstance(null);
         SingletonObject  notSingleton2 = ( SingletonObject) pvtConstructor.newInstance(null);
    }
}

But upon executing the above class CrackingSingleton it throws the error…

Exception in thread "main" java.lang.ClassNotFoundException: CrackingSingleton.SingletonObject.java
at java.net.URLClassLoader$1.run(Unknown Source)
at java.security.AccessController.doPrivileged(Native Method)
at java.net.URLClassLoader.findClass(Unknown Source)
at java.lang.ClassLoader.loadClass(Unknown Source)
at sun.misc.Launcher$AppClassLoader.loadClass(Unknown Source)
at java.lang.ClassLoader.loadClass(Unknown Source)
at java.lang.Class.forName0(Native Method)
at java.lang.Class.forName(Unknown Source)
at CrackingSingleton.CrackingSingleton.main(CrackingSingleton.java:14)

Answer 1

This happens because you are asking for a wrong class. Which is by the way clearly stated in the stacktrace also.

// This should work
// There is no need to add .java or .class declaration. 
Class.forName("CrackingSingleton.SingletonObject")

That being said, then you might want to consider using ENUM singleton pattern.