设计模式-单例

发表于 阅读次数： 
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
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
public class P66{
    private static void main(String []args) {
        for (int i = 0; i < 10000; i++) {
            new Thread(() -> {
                Singleton.getInstance().doSth(); // ok, only create one instance
//                Singleton.getInstance2().doSth(); // error, will create more than one instance
//                Singleton.getInstance3().doSth(); // error, will create more than one instance, too
//                Singleton.getInstance4().doSth(); // error, maybe cause NullPointerException
            }).start();
        }
    }

    private static class Singleton {
        private static Singleton instance;
        private static int count;

        private Singleton() {
            count = count + 1;
            if (count > 0) {
                System.out.println(count);
            }
        }

        private void doSth() {
            // do something
        }

        // double check
        public static Singleton getInstance() {
            if (instance == null) {
                try {
                    Thread.sleep(100);
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
                synchronized (Singleton.class) {
                    if (instance == null) {
                        instance = new Singleton();
                    }
                }
            }
            return instance;
        }

        public static Singleton getInstance2() {
            if (instance == null) {
                try {
                    Thread.sleep(100);
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
                instance = new Singleton();
            }
            return instance;
        }

        public static Singleton getInstance3() {
            if (instance == null) {
                try {
                    Thread.sleep(100);
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
                synchronized (Singleton.class) {
                    instance = new Singleton();
                }
            }
            return instance;
        }

        public static Singleton getInstance4() {
            if (instance == null) {
                try {
                    Thread.sleep(100);
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }

                if (count > 0) {
                    return instance;
                } else {
                    instance = new Singleton();
                }
            }
            return instance;
        }
    }
}