2011-5-6漫谈设计模式（单例模式）

1.最简单的单例模式：

package com.singleton;
/**这是最简单的单例模式，实现的效果的是在一个jvm中只有一个实例
 * 类在加载的时候静态变量只会初始化一次。
 * 1.这种模式的问题在于不能够延迟加载，当SimpleSingleton加载到内存的时候
 * 实例就会自动生成,不能够延迟加载
  *
 */
public class SimpleSingleton {
	private SimpleSingleton(){};
	private static SimpleSingleton instance=new SimpleSingleton();
	public static SimpleSingleton getInstance()
	{
		return instance;
	}
	public static void main(String[] args)
	{
		SimpleSingleton s1=SimpleSingleton.getInstance();
		SimpleSingleton s2=SimpleSingleton.getInstance();
		System.out.println(s1==s1);
		System.out.println(s1.equals(s2));
		
	}

}

2.线程不安全的单例模式

package com.singleton;
/**
 * 这里实现了延迟加载的目的，但是却不是线程安全的
 * 原因主要在于两个线程同时到判断是否为null的时候
 * 可能一起创建实例
 * @author Administrator
 *
 */
public class UnThreadSingleton {
	private static UnThreadSingleton instance;
	public static UnThreadSingleton getInstance()
	{
		if(instance==null)
		{
			instance=new UnThreadSingleton();
		}
		return instance;
	}

	public static void main(String[] args)
	{
		UnThreadSingleton s1=UnThreadSingleton.getInstance();
		UnThreadSingleton s2=UnThreadSingleton.getInstance();
		System.out.println(s1==s2);
	}
}

3.线程安全的单例模式

package com.singleton;

public class ThreadSingleton {
	public static ThreadSingleton instance;
	public static ThreadSingleton  getInstance()
	{
		if(instance==null)
			synchronized(ThreadSingleton.class){
		if(instance==null)
		{
			instance=new ThreadSingleton();
		}
	}
	return instance;
	}
public static void main(String[] args)
{
	ThreadSingleton t1=ThreadSingleton.getInstance();
	ThreadSingleton t2=ThreadSingleton.getInstance();
	System.out.println(t1==t2);
	
}
	
}

4.内部类单例模式

package com.singleton;

public class LazyLoadSingleton {
	private static class LazyHolder
	{
		private static LazyLoadSingleton singletonInstance=new LazyLoadSingleton();
	}
	public static LazyLoadSingleton getInstance()
	{
		return LazyHolder.singletonInstance;
	}
	public static void main(String[] args)
	{
		LazyLoadSingleton l1=LazyLoadSingleton.getInstance();
		LazyLoadSingleton l2=LazyLoadSingleton.getInstance();
		System.out.println(l1==l2);
	}
}