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;
      }
}