Naïve Singleton (multithreaded)

The same class behaves incorrectly in a multithreaded environment. Multiple threads can call the creation method simultaneously and get several instances of Singleton class.

Singleton.java: Singleton

package refactoring_guru.singleton.example.non_thread_safe;

public final class Singleton {
    private static Singleton instance;
    public String value;

    private Singleton(String value) {
        // The following code emulates slow initialization.
        try {
            Thread.sleep(1000);
        } catch (InterruptedException ex) {
            ex.printStackTrace();
        }
        this.value = value;
    }

    public static Singleton getInstance(String value) {
        if (instance == null) {
            instance = new Singleton(value);
        }
        return instance;
    }
}

DemoMultiThread.java: Client code

package refactoring_guru.singleton.example.non_thread_safe;

public class DemoMultiThread {
    public static void main(String[] args) {
        System.out.println("If you see the same value, then singleton was reused (yay!)" + "\n" +
                "If you see different values, then 2 singletons were created (booo!!)" + "\n\n" +
                "RESULT:" + "\n");
        Thread threadFoo = new Thread(new ThreadFoo());
        Thread threadBar = new Thread(new ThreadBar());
        threadFoo.start();
        threadBar.start();
    }

    static class ThreadFoo implements Runnable {
        @Override
        public void run() {
            Singleton singleton = Singleton.getInstance("FOO");
            System.out.println(singleton.value);
        }
    }

    static class ThreadBar implements Runnable {
        @Override
        public void run() {
            Singleton singleton = Singleton.getInstance("BAR");
            System.out.println(singleton.value);
        }
    }
}

OutputDemoMultiThread.txt: Execution result

If you see the same value, then singleton was reused (yay!)
If you see different values, then 2 singletons were created (booo!!)

RESULT:

FOO
BAR