单例设计模式

1. 单例：一个类只能有惟一一个实例对象；

2. 如何实现单例？
   1）饿汉式：不管使用者是否须要这个对象，都间接先创立好这个对象；

   //① 枚举
   public class SingletonPattern1 {public static void main(String[] args) {
           SingleEnum singleEnum1 = SingleEnum.INSTANCE;
           SingleEnum singleEnum2 = SingleEnum.INSTANCE;
           System.out.println(singleEnum1==singleEnum2);//true
       }
   }
   enum SingleEnum{INSTANCE}

   //② 结构器私有化，用全局的动态变量来保留这个惟一的实例对象；public class SingletonPattern2 {public static void main(String[] args) {
   
           Single single1 = Single.INSTANCE;
           Single single2 = Single.INSTANCE;
       
           System.out.println(single1==single2);//true
       }
   }
   
   class Single{public static final Single INSTANCE = new Single();
   
       private Single(){}
   }

   //③ 结构器私有化，用公有的动态称量来保留这个惟一的实例对象，提供静态方法返回这个常量对象；public class SingletonPattern3 {public static void main(String[] args) {SingleClass single1 = SingleClass.getInstance();
           SingleClass single2 = SingleClass.getInstance();
           System.out.println(single1==single2);//true
       }
   }
   
   class SingleClass{private static final SingleClass INSTANCE = new SingleClass();
   
       private SingleClass(){}
       public static SingleClass getInstance(){return INSTANCE;}
   }

   2）懒汉式：提早创建对象，只有当使用者应用的时候才来创立这个对象；

   //① 一般模式
   public class SingletonPattern4 {public static void main(String[] args) {SingleClass4 single1 = SingleClass4.getINSTANCE();
           SingleClass4 single2 = SingleClass4.getINSTANCE();
           System.out.println(single1==single2);
       }
   }
   
   class SingleClass4{
       private static SingleClass4 INSTANCE;
       private SingleClass4(){}
   
       public static SingleClass4 getINSTANCE(){if (INSTANCE == null) { // 提高效率
               synchronized (SingleClass4.class) { // 保障线程平安
                   if (INSTANCE == null) { // 判断 实现单例
                       INSTANCE = new SingleClass4();}
               }
           }
           return INSTANCE;
       }
   }    

   //② 外部类模式：public class SingletonPattern5 {public static void main(String[] args) {SingleClass5 s1 = SingleClass5.getINSTANCE();
           SingleClass5 s2 = SingleClass5.getINSTANCE();
           System.out.println(s1==s2);
       }
   }
   class SingleClass5{
   
       // 私有化结构器
       private SingleClass5(){}
       // 创立外部类 创立外部类的惟一对象；static class Inner{private static final SingleClass5 INSTANCE = new SingleClass5();
       }
       // 定义方法提供类对象
       public static SingleClass5 getINSTANCE(){return Inner.INSTANCE;}
   }

申明：本文章是自学后编写，仅供参考，如有疏漏之处感激斧正；