Question

I know in Java we can create an instance of a Class by new, clone(), Reflection and by serializing and de-serializing.

I have create a simple class implementing a Singleton.

And I need stop all the way one can create instance of my Class.

public class Singleton implements Serializable{
    private static final long serialVersionUID = 3119105548371608200L;
    private static final Singleton singleton = new Singleton();
    private Singleton() { }
    public static Singleton getInstance(){
        return singleton;
    }
    @Override
    protected Object clone() throws CloneNotSupportedException {
        throw new CloneNotSupportedException("Cloning of this class is not allowed"); 
    }
    protected Object readResolve() {
        return singleton;
    }
    //-----> This is my implementation to stop it but Its not working. :(
    public Object newInstance() throws InstantiationException {
        throw new InstantiationError( "Creating of this object is not allowed." );
    }
}

In this Class I have managed to stop the class instance by new, clone() and serialization, But am unable to stop it by Reflection.

My Code for creating the object is

try {
    Class<Singleton> singletonClass = (Class<Singleton>) Class.forName("test.singleton.Singleton");
    Singleton singletonReflection = singletonClass.newInstance();
} catch (ClassNotFoundException e) {
    e.printStackTrace();
} catch (InstantiationException e) {
    e.printStackTrace();
} catch (IllegalAccessException e) {
    e.printStackTrace();
}

Answer 1

By adding below check inside your private constructor

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