对于单例模式的理解

 //饿汉式
 //简单的单例模式，线程安全，可是性能低， 当singletontest被建立的时候 会被实例化 占用内存
public class SingletonTest(){
   //构造函数私有化
   private SingletonTest(){}

   private static final SingletonTest instance = new SingletonTest();
   private static SingletonTest getInstance(){
      return instance;
}
}
 //懒汉式 
 //线程不安全 调用getInstance的时候可能屡次实例化
public class SingletonTest{
   //构造函数私有化
   private SingletonTest(){}
   private static SingletonTest instance;
   private static SingletonTest getInstance()[
      if(instance==null){
         instance = new SingletonTest();
      }
      return instance;
   }
}

 //简单优化
 //线程安全，可是性能低，被加了 synchronized
public class SingletonTest{
   //构造函数私有化
   private SingletonTest(){}
   private static SingletonTest instance;
   private static synchronized SingletonTest getInstance()[
      if(instance==null){
         instance = new SingletonTest();
      }
      return instance;
   }
}
 //最优方案
public class SingletonTest{
   //构造函数私有化
   private SingletonTest(){}
   private static SingletonTest instance;
   private static SingletonTest getInstance()[
      if(instance==null){
    //同步代码块
         synchronized(SingletonTest.class){
            if(instance==null){
               instance = new SingletonTest();
            }
         }
      }
      return instance;
   }
}