Java单例模式实现的几种方式

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();
 }
}

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

来源：http://www.cnblogs.com/jycboy/p/5892185.html