单例模式

单例就是该类只能返回一个实例。
单例所具备的特点：
1.私有化的构造函数
2.私有的静态的全局变量
3.公有的静态的方法
单例分为懒汉式、饿汉式和双层锁式
饿汉式:

public class Singleton1 {      private Singleton1() {};      private static Singleton1 single = new Singleton1();      public static Singleton1 getInstance() {          return single;     } }

懒汉式：

public class Singleton2 {      private Singleton2() {}      private static Singleton2 single=null;      public tatic Singleton2 getInstance() {           if (single == null) {                single = new Singleton2();          }           return single;     } }

线程安全：

public class Singleton3 {      private Singleton3() {}      private static Singleton3 single ;      public static Singleton3 getInstance() {        if(null == single){            synchronized(Singleton3.class){                if(null == single){                    single = new Singleton3();                }            }        }        return single;      }}