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
     
       map = new HashMap
      
       ();                    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);                        }      }