设计模式 之 “单例模式[Singleton Pattern]”

单例模式[Singleton Pattern]类图


这里我们来看下这个单例模式注册器如何工作的：

package m.utils;
 import java.util.HashMap;   
 import org.apache.log4j.Logger;   
     
 /**
  * 单例模式注册器
  *
  */
 public class SingletonRegistry {   
    public static SingletonRegistry REGISTRY = new SingletonRegistry();   
     
    private static HashMap map = new HashMap();   
    private static Logger logger = Logger.getRootLogger();   
     
    protected SingletonRegistry() {   
       // Exists to defeat instantiation   
    }   
    public static synchronized Object getInstance(String classname) {   
       Object singleton = map.get(classname);   
     
       if(singleton != null) {   
          return singleton;   
       }   
       try {   
          singleton = Class.forName(classname).newInstance();   
          logger.info("created singleton: " + singleton);   
       }   
       catch(ClassNotFoundException cnf) {   
          logger.fatal("Couldn't find class " + classname);       
       }   
       catch(InstantiationException ie) {   
          logger.fatal("Couldn't instantiate an object of type " +    
                        classname);       
       }   
       catch(IllegalAccessException ia) {   
          logger.fatal("Couldn't access class " + classname);       
       }   
       map.put(classname, singleton);   
       return singleton;   
    }   
 }

光有代码是看不出如何调用的，我们需要看看如何调用这个注册器，例如：

 public class Singleton {       
		    protected Singleton() {   
		       // Exists only to thwart instantiation.   
		    }   
		    public static Singleton getInstance() {   
		       return (Singleton)SingletonRegistry.REGISTRY.getInstance("m.pattern.Singleton");   
		    }   
		 }

这里将需要实现单例模式的类的构造方法使用protected来修饰，通过getInstance方法来获得这个单例的实例。

单例模式[Singleton Pattern] example code