设计模式-单例模式2

单例模式安全问题

序列化和反射问题：

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();
    }
}

内部类处理方式相同

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");
        }
    }
}

Effective java 推荐的枚举类型单例

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;
        }
    }
}