How to write a Singleton in proper manner?

Question

Today in my interview one interviewer asked me to write a Singleton class. And i gave my answer as

public class Singleton {

    private static Singleton re         


        

Answer 1

It could be because it does not use "double-checked-locking" (as others have said) or it could also be because it is apparently possible to invoke a private constructor using reflection (if the security policy allows it).

To invoke a constructor with no parameters pass an empty array.

package org.example;

public class Singleton {

    private static final Object LOCK = new Object();
    private static final Singleton SINGLETON = new Singleton();
    private static volatile boolean init = false; // 'volatile' to prevent threads from caching state locally (prevent optimizing) 

    private Singleton() {
        synchronized (LOCK) {
            if( init == true) {
                throw new RuntimeException("This is a singleton class!");
            }
            init=true;
        }
    }

    public static Singleton obtainClassInstance() {
        return SINGLETON;
    }

}


package org.example;

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

public class SimpleSingletonTester {

    /**
     * @param args
     * @throws NoSuchMethodException 
     * @throws SecurityException 
     * @throws InvocationTargetException 
     * @throws IllegalAccessException 
     * @throws InstantiationException 
     * @throws IllegalArgumentException 
     */
    public static void main(String[] args) throws SecurityException, NoSuchMethodException, 
    IllegalArgumentException, InstantiationException, IllegalAccessException, InvocationTargetException 
    {

        Class[] parameterTypes = {};
        Object[] initargs = {};
        Constructor constructor = Singleton.class.getDeclaredConstructor(parameterTypes);
        System.out.println( constructor.isAccessible() );
        constructor.setAccessible(true);
        System.out.println( constructor.isAccessible() );
        System.out.println( constructor.newInstance(initargs) );
        System.out.println( constructor.newInstance(initargs) );

    }

}