1
2
3
4
5
6
7
8
9
10
11
12
13

public class Mgr01 {
    private static final Mgr01 INSTANCE = new Mgr01();
    private Mgr01(){}
    private static Mgr01 getInstance(){return INSTANCE;}
    public void m(){
        System.out.println("m");
    }
    public static void main(String[] args) {
        Mgr01 m1 = Mgr01.getInstance();
        Mgr01 m2 = Mgr01.getInstance();
        System.out.println(m1 == m2);
    }
}

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31

public class Mgr02 {
    private volatile static Mgr02 INSTANCE;
    private Mgr02(){};
    



    public static  Mgr02 getInstance(){
        if (INSTANCE == null) {
            
            synchronized (Mgr02.class){
                if (INSTANCE == null ){
                    INSTANCE = new Mgr02();
                }
            }
        }
        return INSTANCE;
    }
    public void m(){
        System.out.println("m");
    }

    public static void main(String[] args) {
        for (int i = 0; i < 100; i++) {
            new Thread(()->{
                
                System.out.println(Mgr02.getInstance().hashCode());
            }).start();
        }
    }
}

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17

public class Mgr03 {
    private Mgr03(){}
    private static class Mgr03Holder {
        private static final Mgr03 INSTANCE = new Mgr03();
    }
    public static Mgr03 getInstance() {
        return Mgr03Holder.INSTANCE;
    }
    public static void main(String[] args) {
        for (int i = 0; i < 100; i++) {
            new Thread(()->{
                
                System.out.println(Mgr03.getInstance().hashCode());
            }).start();
        }
    }
}

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15

public enum  Mgr04 {
    
    INSTANCE;
    public void m(){
        System.out.println("m");
    }
    public static void main(String[] args) {
        for (int i = 0; i < 100; i++) {
            new Thread(()->{
                
                System.out.println(Mgr04.INSTANCE.hashCode());
            }).start();
        }
    }
}