設計模式-單例模式

第一種方式：通過synchronized解決，性能下降

 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 }

第二種方式：JVM載入這個類時馬上創建唯一的單件，可能創建時負擔太重而該單件未使用，造成浪費.

 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 }

第三種方式：雙重檢查加鎖

 1 package singleton;
 2 
 3 public class Singleton {
 4     private Singleton() {
 5     }
 6 
 7     /**
 8      * volatile確保當instance被初始化成Singleton實例時，多個線程正確的處理instance變數
 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 }