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Java-设计模式-单例模式问题（反射、序列化）.md

发表于 分类于

参考：https://www.cnblogs.com/call-me-pengye/p/11169051.html、https://zhuanlan.zhihu.com/p/144092983

使用枚举(《Effective Java》作者的Josh Bloch提倡的方式)

破环单例模式的方式

破环单例模式的三种方式：反射，序列化，克隆

以双重检测方式为例测试反射，序列化，克隆是否能破环单例模式：

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public class Singleton  implements Serializable,Cloneable{
    private static final long serialVersionUID = 6125990676610180062L;
    private static Singleton singleton;
    
    private Singleton(){
    }
    public void doAction(){
        //TODO 实现你需要做的事
    }
    public  static Singleton getInstance(){
        if (singleton == null) {
            synchronized (Singleton.class) {
                if (singleton == null) {
                    singleton = new Singleton();
                }
            }
        }
        return singleton;
    }
    @Override
    protected Object clone() throws CloneNotSupportedException {
        return super.clone();
    }
}

测试用例：

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public class DestroySingleton {
    
    public static void main(String[] args) throws Exception {
        //通过getInstance()获取
        Singleton singleton = Singleton.getInstance();
        System.out.println("singleton的hashCode:"+singleton.hashCode());
        //通过反射获取
        Constructor<Singleton> constructor = Singleton.class.getDeclaredConstructor();
        constructor.setAccessible(true);
        Singleton reflex = constructor.newInstance();
        System.out.println("reflex的hashCode:"+reflex.hashCode());
        //通过克隆获取
        Singleton clob = (Singleton) Singleton.getInstance().clone();
        System.out.println("clob的hashCode:"+clob.hashCode());
        //通过序列化，反序列化获取
        ByteArrayOutputStream bos = new ByteArrayOutputStream();
        ObjectOutputStream oos = new ObjectOutputStream(bos);
        oos.writeObject(Singleton.getInstance());
        ByteArrayInputStream bis = new ByteArrayInputStream(bos.toByteArray());
        ObjectInputStream ois = new ObjectInputStream(bis);
        Singleton serialize = (Singleton) ois.readObject();
        if (ois != null)    ois.close();
        if (bis != null) bis.close();
        if (oos != null) oos.close();
        if (bos != null) bos.close();
        System.out.println("serialize的hashCode:"+serialize.hashCode());
    }
}

运行结果：

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singleton的hashCode:366712642
reflex的hashCode:1829164700
clob的hashCode:2018699554
serialize的hashCode:990368553

运行结果表明通过 getInstance()、反射、克隆、序列化这四种方式得到的 Singleton 对象的 hashCode 是不一样的，此时单例模式已然被破环

如何防止反射、克隆、序列化对单例模式的破环

  • 防止反射破环（虽然构造方法已私有化，但通过反射机制使用 newInstance() 方法构造方法也是可以被调用）:
    • 首先定义一个全局变量开关 isFristCreate 默认为开启状态
    • 当第一次加载时将其状态更改为关闭状态
  • 防止克隆破环
    • 重写 clone()，直接返回单例对象
  • 防止序列化破环
    • 添加 readResolve()，返回 Object 对象
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public class Singleton  implements Serializable,Cloneable{
    private static final long serialVersionUID = 6125990676610180062L;
    private static Singleton singleton;
    private static boolean isFristCreate = true;//默认是第一次创建
    
    private Singleton() {
            if (isFristCreate) {
                synchronized (Singleton.class) {
                    if (isFristCreate) {
                        sFristCreate = false;
                    }
                }
            } else {
                throw new RuntimeException("已然被实例化一次，不能在实例化");
            }
    }
    public void doAction(){
        //TODO 实现你需要做的事
    }
    public static Singleton getInstance(){
        if (singleton == null) {
            synchronized (Singleton.class) {
                if (singleton == null) {
                    singleton = new Singleton();
                }
            }
        }
        return singleton;
    }
    @Override
    protected Singleton clone() throws CloneNotSupportedException {
        return singleton;
    }
    private Object readResolve() {
        return singleton;
    }
}

测试用例：

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public class DestroySingleton {
    
    public static void main(String[] args) throws Exception {
        //通过getInstance()获取
        Singleton singleton = Singleton.getInstance();
        System.out.println("singleton的hashCode:"+singleton.hashCode());
        //通过克隆获取
        Singleton clob = (Singleton) Singleton.getInstance().clone();
        System.out.println("clob的hashCode:"+clob.hashCode());
        //通过序列化，反序列化获取
        ByteArrayOutputStream bos = new ByteArrayOutputStream();
        ObjectOutputStream oos = new ObjectOutputStream(bos);
        oos.writeObject(Singleton.getInstance());
        ByteArrayInputStream bis = new ByteArrayInputStream(bos.toByteArray());
        ObjectInputStream ois = new ObjectInputStream(bis);
        Singleton serialize = (Singleton) ois.readObject();
        if (ois != null)    ois.close();
        if (bis != null) bis.close();
        if (oos != null) oos.close();
        if (bos != null) bos.close();
        System.out.println("serialize的hashCode:"+serialize.hashCode());
        //通过反射获取
        Constructor<Singleton> constructor = Singleton.class.getDeclaredConstructor();
        constructor.setAccessible(true);
        Singleton reflex = constructor.newInstance();
        System.out.println("reflex的hashCode:"+reflex.hashCode());
    }
}

运行结果：

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singleton的hashCode:366712642
clob的hashCode:366712642
serialize的hashCode:366712642
Exception in thread "main" java.lang.reflect.InvocationTargetException
    at sun.reflect.NativeConstructorAccessorImpl.newInstance0(Native Method)
    at sun.reflect.NativeConstructorAccessorImpl.newInstance(NativeConstructorAccessorImpl.java:62)
    at sun.reflect.DelegatingConstructorAccessorImpl.newInstance(DelegatingConstructorAccessorImpl.java:45)
    at java.lang.reflect.Constructor.newInstance(Constructor.java:423)
    at designPatterns.singleton.doublecheck.DestroySingleton.main(DestroySingleton.java:33)
Caused by: java.lang.RuntimeException: 已然被实例化一次，不能在实例化
    at designPatterns.singleton.doublecheck.Singleton.<init>(Singleton.java:16)
    ... 5 more

从运行结果上看重写 clone()，添加 readResolve() 后通过克隆和序列化得到的对象的 hashCode 与从 getInstance() 得到的对象得而 hashCode 值相同，而通过反射运行得到的结果符合预想的报错；因为以上三种手段对防止单例被破坏起作用了