Java设计模式之单例模式

SINGLETON （Object Creational） 

Purpose 
    Ensures that only one instance of a class is allowed within a system. 
Use When 
   1 Exactly one instance of a class is required. 
   2 Controlled access to a single object is necessary. 
Example 
    Most languages provide some sort of system or environment 
object that allows the language to interact with the native operating 
system. Since the application is physically running on only one 
operating system there is only ever a need for a single instance of 
this system object. The singleton pattern would be implemented 
by the language runtime to ensure that only a single copy of the 
system object is created and to ensure only appropriate processes 
are allowed access to it. 

package javaPattern;

public class Singleton {

    private static Singleton singleton;
    private Singleton(){}//构造器必须私有
    public static Singleton getSingleton(){
        if(singleton == null){
            singleton =  new Singleton();
        }
        return singleton;
    }
    public static void main(String[] args) {
        Singleton s1 = Singleton.getSingleton();
        Singleton s2 = Singleton.getSingleton();
        System.out.println(s1 == s2);
    }
}