模式設計1--單例設計模式

單例設計模式的要求:

    1在某些情況下爲了避免死其他程序過多的該類的對象,則需禁止其他程序建立該類的對象.

    2但是又需要讓其他程序能夠訪問到該類的對象,只好在本類中自定義一個對象

    3這個自定義對象要被其他程序訪問,就必須在該類中對外提供一些訪問方式

由以上要求,單例模式的設計思路:

    1將構造器函數私有化,這樣就可以避免其他程序創建該類的對象.

    2在該類中創建一個本來對象

    3提供一個方法以便獲取該對象---因爲外界程序無法創建該類對象,所以調用該方法只能通過類名調用的形式來調用方法,所以該方法必須爲static修飾的,要被外界訪問,所以爲public修飾權限

 

package cn.imust.day;

//單例設計模式
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 
	}
}

具體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 "餓漢式單例設計模式" ,the other is called "懶漢單例設計模式",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;
	}

 

 

 

 

 

多線程中的懶漢式單例:

class Single
{
	private static Single s=null;
	private Single(){}
	public static Single getInstance(){//there you may not use the 同步函數,becuase if so,you need to check the 鎖棋標誌 many times,witch will decline the efficiency,so use the 同步代碼塊 like below:
		if (s==null)
		{
			synchronized(Single.class){
				if (s==null)
				{
					s=new Single();
				}
			}
		}
		return s;
	}
}