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Java单例模式实现的几种方式
单例模式好多书上都是这么写的：
public class SingleTon1 { private SingleTon1(){ } private static SingleTon1 instance = null; public static SingleTon1 getInstance(){ if(instance == null){ instance = new SingleTon1(); } return instance; }}但是实际开发中是不会这么写的，因为有一个严重的问题，多线程并发访问的时候，可能会产生多个实例！！
下面列举几个常用的方法：
1.使用synchronized 关键字
package singleton;public class SingleTon1 { private SingleTon1(){ } private static SingleTon1 instance = null; //多线程问题解法一，但是效率不高！因为每次调用都会加锁！ 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的时候加锁，之后就不加啦 public static SingleTon1 getInstance2(){ if(instance == null){ synchronized (object){ instance = new SingleTon1(); } } return instance; }}2.加锁
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){ //注意这里还要判断下！！ instance = new SingleTon2(); } lock.unlock(); } return instance; }}3.利用静态变量：
package singleton;public class SingleTon3 { public static void print(){ System.out.println("thread_id:"+Thread.currentThread().getId()); } public static Nested getNested(){ return Nested.instance; } //这个是单例创建的类 static class Nested{ private Nested(){ } static Nested instance = new Nested(); }}以上就是常用的创建单例的模式：
Test测试代码：
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){ //是同一个 System.out.println("是同一个"); }else{ System.out.println("不是同一个"); } }} 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(); } }输出：
是同一个
thread_id:11
thread_id:10
Java单例模式实现的几种方式
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