设计模式之单例
java中单例模式是一种常见的设计模式，单例模式分三种：懒汉式单例、饿汉式单例、登记式单例三种。
例如：log4j日志就是一个典型的单例、spring bean创建的默认单例模式
一、懒汉式单例
package com.design.singleton;/** * 懒汉式单例--在第一次调用的时候实例化 * @author * */public class LazySingleton {/** * 构造私有，以免外部构造 */private LazySingleton(){}//创建实例private static LazySingleton INSTANCE = null;/** * 外部访问实例 * @return */public synchronized static LazySingleton getInstance(){if(INSTANCE == null)INSTANCE = new LazySingleton();return INSTANCE;}}
二、饿汉式单例
package com.design.singleton;/** * 饿汉式--在类加载的时候创建实例 * @author * */public class HungerSingleton {/** * 不能被外部实例化 */private HungerSingleton(){}//创建一个实例private static HungerSingleton INSTANCE = new HungerSingleton();/** * 调用这个实例化 * @return */public static HungerSingleton getInstance(){return INSTANCE;}}
三、登记式单例
package com.design.singleton;import java.util.concurrent.ConcurrentHashMap;import java.util.concurrent.ConcurrentMap;/** * 登记式单例 * @author * */public class SingletonFactory {//存放单例实例private static ConcurrentMap<String, Object> managedSingletons = new ConcurrentHashMap<String, Object>();/** * 根据类名(需加上包路劲)获取对应实例,若不存在则创建 * @param className * @return */public static Object getInstance(String className){if(!managedSingletons.containsKey(className)){createInstance(className);}return managedSingletons.get(className);}/** * 获取实例，若没有这个实例则创建 * @param c * @return */public static Object getInstance(Class<?> c){return getInstance(c.getName());}/** * 创建一个实例 * @param className */public static void createInstance(String className){synchronized(managedSingletons){try {managedSingletons.put(className, Class.forName(className).newInstance());} catch (InstantiationException e) {// TODO Auto-generated catch blocke.printStackTrace();} catch (IllegalAccessException e) {// TODO Auto-generated catch blocke.printStackTrace();} catch (ClassNotFoundException e) {// TODO Auto-generated catch blocke.printStackTrace();}}}/** * 创建一个实例 * @param c */public static void createInstance(Class<?> c){createInstance(c.getName());}/** * 移除实例 * @param className 类名称加路劲 */public static void removeInstance(String className){if(!managedSingletons.containsKey(className)){synchronized(managedSingletons){managedSingletons.remove(className);}}}/** * 移除实例 * @param c */public static void removeInstance(Class<?> c){removeInstance(c.getName());}}