设计模式-单例模式

 1 package singleton;
 2 
 3 public class Singleton {
 4     private Singleton() {
 5     }
 6 
 7     private static Singleton instance ;
 8 
 9     public static synchronized Singleton getInstance() {
10         if (instance == null) { 
11             synchronized (instance) {
12                 instance = new Singleton(); 
13             }
14         }
15         return instance;
16     }
17 }

 1 package singleton;
 2 
 3 public class Singleton {
 4     private Singleton() {
 5     }
 6 
 7     private static Singleton instance = new Singleton();
 8 
 9     public static Singleton getInstance() {
10         return instance;
11     }
12 }

 1 package singleton;
 2 
 3 public class Singleton {
 4     private Singleton() {
 5     }
 6 
 7     /**
 8      * volatile确保当instance被初始化成Singleton实例时，多个线程正确的处理instance变量
 9      */
10     private volatile static Singleton instance;
11 
12     public static Singleton getInstance() {
13         if (instance != null) {
14             synchronized (Singleton.class) {
15                 if (instance != null) {
16                     instance = new Singleton();
17                 }
18             }
19         }
20         return instance;
21     }
22 }