单实例模式Singleton

• 有且只能有一个实例，必须给其他角色提供该实例
• 推荐使用 避免了多线程的同步开销，实现也较为简单

饿汉单实例模式

public class className{
        public static className instance = new className();
        private className(){}
        public static className getInstance(){
            return instance;
        } 

懒汉单实例模式

public class className{
        public static className instance = null; 


        private className(){}


        public static  synchronized className getInstance(){ 

                 if(instance == null){ 

             instance = new className(); 

    }
            return instance; 

}

注册式单例模式

import java.util.HashMap; 

public class RegSinglePattern {
    private static HashMap regMap = new HashMap();
    static {
        RegSinglePattern x = new RegSinglePattern();
        regMap.put(x.getClass().getName(), new RegSinglePattern());
    }
    private RegSinglePattern() {
    }
    public synchronized RegSinglePattern getInstance(String name) {
        if(name == null ) {
            name = "com.javapatterns.singleton.demos.RegSigleton";
        }
        if(regMap.get(name) == null) {
            regMap.put(name, new RegSinglePattern());
        }
        return (RegSinglePattern) regMap.get(name);
    }
}