模式设计1--单例设计模式

单例设计模式的要求:

    1在某些情况下为了避免死其他程序过多的该类的对象,则需禁止其他程序建立该类的对象.

    2但是又需要让其他程序能够访问到该类的对象,只好在本类中自定义一个对象

    3这个自定义对象要被其他程序访问,就必须在该类中对外提供一些访问方式

由以上要求,单例模式的设计思路:

    1将构造器函数私有化,这样就可以避免其他程序创建该类的对象.

    2在该类中创建一个本来对象

    3提供一个方法以便获取该对象---因为外界程序无法创建该类对象,所以调用该方法只能通过类名调用的形式来调用方法,所以该方法必须为static修饰的,要被外界访问,所以为public修饰权限

 

package cn.imust.day;

//单例设计模式
class SingleInstance
{
	private int num;
	public void setNum(int num){
		this.num=num;
	}
	public int getNum(){
		return this.num;
	}
	//the above Javabean is order to test this Class can only have one instance

	//the private constructor
	private SingleInstance(){}
	
	//the user-defined instance
	private static  SingleInstance single =new SingleInstance();

	//the method offer to other programs
	public static SingleInstance getInstance(){
		return single;
	}
}

//definite a class to test the Singleton pattern
class TestSingle
{
	public static void main(String[] args){
		SingleInstance single1=SingleInstance.getInstance();
		SingleInstance single2=SingleInstance.getInstance();
		single1.setNum(23);
		System.out.println(single2.getNum());//the finally output is 23,indicate that the  instance is the same one 
	}
}

具体in using ,we create the class like usually we do ,but when we need the class only have one instance ,just add the three conditions .

There are two methods for creating of Singleton pattern ,the above is one,which called "饿汉式单例设计模式" ,the other is called "懒汉单例设计模式",the key point is also the difference ,look at :

	private static SingleInstance single=null;

	//the method offer to other programs
	public static SingleInstance getInstance(){
		if (single==null)
		{		
			single=new SingleInstacne();
		}
		return single;
	}

 

 

 

 

 

多线程中的懒汉式单例:

class Single
{
	private static Single s=null;
	private Single(){}
	public static Single getInstance(){//there you may not use the 同步函数,becuase if so,you need to check the 锁棋标志 many times,witch will decline the efficiency,so use the 同步代码块 like below:
		if (s==null)
		{
			synchronized(Single.class){
				if (s==null)
				{
					s=new Single();
				}
			}
		}
		return s;
	}
}