设计模式-单例模式2

1. 单例模式安全问题

2. 序列化和反射问题：

   ```java public class HungrySingleton implements Serializable,Cloneable{

    private final static HungrySingleton hungrySingleton;
   
    static{
        hungrySingleton = new HungrySingleton();
    }
    private HungrySingleton(){ //反射时处理，如果已存在则抛出异常
        if(hungrySingleton != null){
            throw new RuntimeException("单例构造器禁止反射调用");
        }
    }
    public static HungrySingleton getInstance(){
        return hungrySingleton;
    }
   
    private Object readResolve(){//反序列化时会调用此方法，从而防止创建多个实例
        return hungrySingleton;
    }
   
    @Override
    protected Object clone() throws CloneNotSupportedException {
        return getInstance();
    }
   

   } ```

3. 内部类处理方式相同

   java public class StaticInnerClassSingleton { private static class InnerClass{ private static StaticInnerClassSingleton staticInnerClassSingleton = new StaticInnerClassSingleton(); } public static StaticInnerClassSingleton getInstance(){ return InnerClass.staticInnerClassSingleton; } private StaticInnerClassSingleton(){ if(InnerClass.staticInnerClassSingleton != null){ throw new RuntimeException("单例构造器禁止反射调用"); } } }

4. Effective java 推荐的枚举类型单例

```java public class EnumSingleton { //私有构造函数 private EnumSingleton(){}

   public static EnumSingleton getInstance(){
       return Singleton.INSTANCE.getInstance();
   }

   private enum Singleton{
       INSTANCE;
       private EnumSingleton singleton;
       //jvm保证这个方法绝对只调用一次
       Singleton(){
           singleton = new EnumSingleton();
       }
       public EnumSingleton getInstance(){
           return singleton;
       }
   }

} ```