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