Question

What is the best/correct way to create a singleton class in java?

One of the implementation I found is using a private constructor and a getInstance() method.

package singleton;

public class Singleton {

    private static Singleton me;

    private Singleton() {
    }

    public static Singleton getInstance() {
        if (me == null) {
            me = new Singleton();
        }

        return me;
    }
}

But is implementation fails in the following test case

package singleton;

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

public class Test {

    /**
     * @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 {
        Singleton singleton1 = Singleton.getInstance();
        System.out.println(singleton1);

        Singleton singleton2 = Singleton.getInstance();
        System.out.println(singleton2);

        Constructor<Singleton> c = Singleton.class
                .getDeclaredConstructor((Class<?>[]) null);
        c.setAccessible(true);
        System.out.println(c);

        Singleton singleton3 = c.newInstance((Object[]) null);
        System.out.println(singleton3);

        if(singleton1 == singleton2){
            System.out.println("Variable 1 and 2 referes same instance");
        }else{
            System.out.println("Variable 1 and 2 referes different instances");
        }
        if(singleton1 == singleton3){
            System.out.println("Variable 1 and 3 referes same instance");
        }else{
            System.out.println("Variable 1 and 3 referes different instances");
        }
    }

}

How to resolve this?

Thank you

Answer 1

As per the comment on your question:

I’ve a properties file containing some keys value pairs, which is need across the application, that is why I was thinking about a singleton class. This class will load the properties from a file and keep it and you can use it from anywhere in the application

Don’t use a singleton. You apparently don’t need one-time lazy initialization (that’s where a singleton is all about). You want one-time direct initialization. Just make it static and load it in a static initializer.

E.g.

public class Config {

    private static final Properties PROPERTIES = new Properties();

    static {
        try {
            PROPERTIES.load(Thread.currentThread().getContextClassLoader().getResourceAsStream("config.properties"));
        } catch (IOException e) {
            throw new ExceptionInInitializerError("Loading config file failed.", e);
        }
    }

    public static String getProperty(String key) {
        return PROPERTIES.getProperty(key);
    }

    // ...
}