1 package org.study2.javabase.ThreadsDemo.sync;
2
3 /**
4 * @Auther:GongXingRui
5 * @Date:2018/9/20
6 * @Description: 单例模式 - 二次确认，提高效率
7 **/
8 public class DanDemo {
9 public static void main(String args[]) {
10 Dan dan1 = new Dan();
11 Dan dan2 = new Dan();
12 dan1.start();
13 dan2.start();
14 }
15 }
16
17 class Dan extends Thread {
18 @Override
19 public void run() {
20 System.out.println(JVM.getInstance());
21 }
22
23 }
24
25 /**
26 * 单例模式 - 懒汉式
27 */
28 class JVM {
29 private static JVM instance = null;
30
31 private JVM() {
32
33 }
34
35 // 二次确认提高效率
36 public static JVM getInstance() {
37 if (null == instance) { // 提高效率
38 synchronized (JVM.class) {
39 if (null == instance) { // 安全
40 instance = new JVM();
41 }
42 }
43 }
44 return instance;
45 }
46
47 public static JVM getInstance2() {
48 synchronized (JVM.class) {
49 if (null == instance) {
50 instance = new JVM();
51 }
52 }
53 return instance;
54 }
55
56 public static synchronized JVM getInstance1() {
57 if (null == instance) {
58 instance = new JVM();
59 }
60 return instance;
61 }
62 }
63
64 /**
65 * 单例模式 - 饿汉式
66 * 线程安全
67 */
68 class JVM2 {
69 private static JVM2 instance = new JVM2();
70
71 private JVM2() {
72
73 }
74
75 public static JVM2 getInstance() {
76 return instance;
77 }
78 }
