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