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Java单例模式实现的几种方式

  • 时间:2016-09-22
  • 发布:昆明Java培训
  • 来源:达内新闻

昆明Java培训机构的老师发现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!!

下面昆明Java培训机构的老师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

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