We have collected various implementation methods of singleton.

Singleton implementation type

Typical

The most typical way to write it is Non Thread Safe.

public class Singleton1 {
    private static Singleton1 singleton = null;

    private Singleton1() {
        try {
            Thread.sleep(2 * 1000);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    }

    public static Singleton1 getInstance() {
        if (singleton == null) {
            singleton = new Singleton1();
        }
        return singleton;
    }
}

Non-Thread-Safe

ExecutorService executor = Executors.newFixedThreadPool(2);

Future<Singleton1> f1 = executor.submit(Singleton1::getInstance);
Future<Singleton1> f2 = executor.submit(Singleton1::getInstance);

Singleton1 s1 = f1.get();
Singleton1 s2 = f2.get();

System.out.println(s1);
System.out.println(s2);
System.out.println(s1 == s2);

Different ID

design.singleton.Singleton1@7ba4f24f
design.singleton.Singleton1@3b9a45b3
false

synchronized Add synchronized to the typical writing style to make it Thread Safe. However, there is a performance concern because the synchronized lock cannot be released even after the instance is created.

synchronized

public class Singleton2 {
    private static Singleton2 singleton = null;

    private Singleton2() {
        try {
            Thread.sleep(2 * 1000);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    }

    public static synchronized Singleton2 getInstance() {
        if (singleton == null) {
            singleton = new Singleton2();
        }
        return singleton;
    }
}

Double Check Lock By adding Check again on top of synchronized, It avoids the synchronized lock.

But it's not actually Thread Safe due to atomicity issues! I don't know the details, but it is because new Singleton3 () is broken down into several parts and executed.

DCL

public class Singleton3 {
    private static Singleton3 singleton = null;

    private Singleton3() {
        try {
            Thread.sleep(2 * 1000);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    }

    public static Singleton3 getInstance() {
        //Not thread-safe due to atomicity
        if (singleton == null) {
            synchronized (Singleton3.class) {
                if (singleton == null) {
                    singleton = new Singleton3();
                }
            }
        }
        return singleton;
    }
}

volatile + DCL It uses volatile, guarantees primitiveness, and implements Double Check Lock.

volatile

public class Singleton4 {
    private static volatile Singleton4 singleton = null;

    private Singleton4() {
        try {
            Thread.sleep(2 * 1000);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    }

    public static Singleton4 getInstance() {
        //Volatile guarantees atomicity
        if (singleton == null) {
            synchronized (Singleton4.class) {
                if (singleton == null) {
                    singleton = new Singleton4();
                }
            }
        }

        return singleton;
    }
}

static fianl Thanks to the class loader, it is Thread Safe and simple to write. However, resources can be wasted.

static_final

public class Singleton5 {

    private static final Singleton5 singleton = new Singleton5();

    private Singleton5() {}

    public static Singleton5 getInstance() {
        return singleton;
    }
}

Holder It is an implementation that makes Thread Safe with static final, but does not waste resources by generating delays.

Holder

public class Singleton6 {

    private static class SingletonHolder {
        private static final Singleton6 singleton = new Singleton6();
        private SingletonHolder() { }
    }

    private Singleton6() {
    }

    public static Singleton6 getInstance() {
        return SingletonHolder.singleton;
    }
}

enum Use enum. However, resources can be wasted.

enum

public enum Singleton7 {
    SINGLETON;
}

The essence is not much different from static final, but it is public. public static final Singleton7 SINGLETON = new Singleton7();

Summary

Overall, volatile + DCL or Holder is the best!