定義

單例模式（Singleton），也叫單子模式，是一種常用的軟件設計模式。對於系統而言該實例有且僅有一個。

應用場景

線程池、數據庫池、用於對系統做初始化的實例，提供給關聯系統調用的接口（任務提交部分）

不適用成員變量多變的場景。

1、實現方式-餓漢模式

 1 package com.learn.designpattern.singleton;
 2 
 3 import java.util.concurrent.ArrayBlockingQueue;
 4 import java.util.concurrent.ExecutorService;
 5 import
 
 java.util.concurrent.ThreadPoolExecutor;
 6 import java.util.concurrent.TimeUnit;
 7 
 8 /**
 9  * 
10  * @author zhujj
11  */
12 
13 public class ThreadPoolUntil {
14 
15     private final static ExecutorService executorService= new ThreadPoolExecutor(5,20,0,TimeUnit.SECONDS,new ArrayBlockingQueue<>(128));
 
16 
17     private ThreadPoolUntil() {
18         // TODO Auto-generated constructor stub
19     }
20     
21     public static ExecutorService getIntance() {
22         return executorService;
23     }
24     public static void main(String[] arg0) {
25         ThreadPoolUntil.getIntance().execute(new
 
 Runnable() {
26             @Override
27             public void run() {
28                 boolean control = true;
29                 int i =0;
30                 while(control ) {
31                     System.out.println(">>>>>");
32                     i++;
33                     if(i>3) control = false;
34                 }
35             }
36         });
37     }
38 }

2、實現方式-懶漢模式+雙重校驗鎖

 1 package com.learn.designpattern.singleton;
 2 
 3 import java.util.concurrent.ArrayBlockingQueue;
 4 import java.util.concurrent.ExecutorService;
 5 import java.util.concurrent.ThreadPoolExecutor;
 6 import java.util.concurrent.TimeUnit;
 7 
 8 /**
 9  * 
10  * @author zhujj
11  */
12 
13 public class ThreadPoolUntil {
14 
15     private static volatile ThreadPoolUntil threadPoolUntil;
16 
17     private ThreadPoolUntil() {
18         // TODO Auto-generated constructor stub
19     }
20     
21     public static ThreadPoolUntil getIntance() {
22         if(threadPoolUntil==null) {
23             synchronized(ThreadPoolUntil.class) {
24                 if(threadPoolUntil==null) {
25                     threadPoolUntil = new ThreadPoolUntil();
26                 }
27             }
28         }
29         return threadPoolUntil;
30     }
31 }

推薦使用方式一

設計模式之單例模式>>應用場景及實現