设计模式之单例模式

单例模式：
             保证只有一个实例存在

             避免产生多个对象消耗资源，或者某种对象只应该有且仅有一个

           注意： 1.构造函数不对外开发，一般private
                        2.通过静态方法或枚举返回单例类对象
                        3.确保单例对象有且仅有一个，尤其在多线程环境下

                        4.确保单例对象在反序列化时不会重新构建对象

 

    //饿汉式  
      
    public class SingleTon {  
        private SingleTon(){}  
        private static final SingleTon single = new SingleTon();  
        public static SingleTon getSingle(){  
            return single;  
        }  
    }  

 

    //懒汉模式  
      
    public class SingleTon{  
        private SingleTon(){}  
        private static SingleTon single = null;  
        public static synchronized SingleTon getSingle(){  
            if(single ==null){  
                single = new SingleTon();  
            }  
            return single;  
        }  
    }  

 

    //Double check Lock(DCL)  
      
    public class SingleTon{  
        private SingleTon(){}  
        private static SingleTon single = null;  
        public static  SingleTon getSingle(){  
            if(single == null)  
                synchronized(SingleTon.class){  
                    if(single ==null){  
                        single = new SingleTon();  
                    }  
                }  
            return single;  
        }  
    }  

 

    //静态内部类  
      
      
    public class SingleTon{  
        private SingleTon(){}  
        public static  SingleTon getSingle(){  
            return SingletoHolder.single;  
        }  
          
        private static class SingletoHolder{  
            private static final SingleTon single = new SingleTon();  
        }  
          
        private Object readResolve() {  
            return SingletoHolder.single;  
              
        }  
          
    }  

 

    //枚举单例  
    public enum Single {  
        INSTACE;  
    }  

 

一般情况下DCL 使用最多但是反序列化还是可以重新生成对象

    private Object readResolve() throws ObjectStreamException{  
            return single;  
    }  

 还有一种用容器实现：

    public class SingletoManager {  
        private SingletoManager(){  }  
          
        private static Map<String,Object> map = new HashMap<String,Object>();  
          
        public  static void registerService(String key,Object instance){  
            if(!map.containsKey(key)){  
                map.put(key, instance);  
            }  
        }  
          
        public static Object getService(String key){  
            return map.get(key);  
              
        }  
    }