單例模式好多書上都是這么寫的：
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public class SingleTon1 { private SingleTon1(){ } private static SingleTon1 instance = null; public static SingleTon1 getInstance(){ if(instance == null){ instance = new SingleTon1(); } return instance; }} |
但是實際開發中是不會這么寫的，因為有一個嚴重的問題，多線程并發訪問的時候，可能會產生多個實例??！
下面列舉幾個常用的方法：
1.使用synchronized 關鍵字
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package singleton; public class SingleTon1 { private SingleTon1(){ } private static SingleTon1 instance = null; //多線程問題解法一，但是效率不高！因為每次調用都會加鎖！ public static synchronized SingleTon1 getInstance(){ if(instance == null){ instance = new SingleTon1(); } return instance; } public void print(){ System.out.println("thread_id:"+Thread.currentThread().getId()); } private static Object object = new Object(); //很巧妙的方法，只有在null的時候加鎖，之后就不加啦 public static SingleTon1 getInstance2(){ if(instance == null){ synchronized (object){ instance = new SingleTon1(); } } return instance; } } |
2.加鎖
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package singleton; import java.util.concurrent.locks.ReentrantLock; public class SingleTon2 { private SingleTon2(){ } private static ReentrantLock lock = new ReentrantLock(); private static SingleTon2 instance = null; public void print(){ System.out.println("thread_id:"+Thread.currentThread().getId()); } public static SingleTon2 getInstance2(){ if(instance == null){ lock.lock(); if(instance == null){ //注意這里還要判斷下?。?/code> instance = new SingleTon2(); } lock.unlock(); } return instance; }} |
3.利用靜態變量：
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package singleton; public class SingleTon3 { public static void print(){ System.out.println("thread_id:"+Thread.currentThread().getId()); } public static Nested getNested(){ return Nested.instance; } //這個是單例創建的類 static class Nested{ private Nested(){ } static Nested instance = new Nested(); }} |
以上就是常用的創建單例的模式：
Test測試代碼：
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package singleton; import singleton.SingleTon3.Nested; public class Test2 { public static void main(String[] args) { // TODO Auto-generated method stub Nested singleton; Myrunnable mm = new Myrunnable(); Myrunnable m1 = new Myrunnable(); Myrunnable2 m2 = new Myrunnable2(); new Thread(m1).start(); new Thread(m2).start(); if(m1.singleton == m2.singleton){ //是同一個 System.out.println("是同一個"); }else{ System.out.println("不是同一個"); } }} class Myrunnable implements Runnable{ Nested singleton; @Override public void run() { // TODO Auto-generated method stub singleton = SingleTon3.getNested(); SingleTon3.print(); } } class Myrunnable2 implements Runnable{ Nested singleton; @Override public void run() { // TODO Auto-generated method stub singleton = SingleTon3.getNested(); SingleTon3.print(); } } |
輸出：
是同一個
thread_id:11
thread_id:10
以上就是對Java 單例模式的資料整理，后續繼續補充相關資料，謝謝大家對本站的支持！
原文鏈接：http://www.cnblogs.com/jycboy/p/5892185.html
