详解Java序列化如何破坏单例模式

public class Singleton {
    private Singleton(){}

    private static class SingletonInstance{
        private static final Singleton instance = new Singleton();
    }

    public static Singleton getInstance(){
        return SingletonInstance.instance;
    }
}

public class SerializableSingleton implements Serializable {
    // 省略部分代码
}

public class Test {
    public static void main(String[] args) throws Exception {
        ObjectOutputStream oos = new ObjectOutputStream(new FileOutputStream("Singleton.txt"));
        SerializableSingleton singleton = SerializableSingleton.getInstance();
        oos.writeObject(singleton);

        ObjectInputStream ois = new ObjectInputStream(new FileInputStream("Singleton.txt"));
        SerializableSingleton singleton1 = (SerializableSingleton) ois.readObject();

        System.out.println("singleton = singleton1：" + (singleton == singleton1));
    }
}

    public final Object readObject() throws IOException, ClassNotFoundException {
        //...
        int outerHandle = passHandle;
        try {
            Object obj = readObject0(false);
            //...
            return obj;
        } finally {
            //...
        }
    }

    private Object readObject0(boolean unshared) throws IOException {
        // ...
        try {
            switch (tc) {
                // ...

                case TC_OBJECT:
                    // readObject0()
                    return checkResolve(readOrdinaryObject(unshared));
                // ...
            }
        } finally {
            depth--;
            bin.setBlockDataMode(oldMode);
        }
    }

    private Object readOrdinaryObject(boolean unshared) throws IOException {
        // ...

        Object obj;
        try {
            // 核心代码
            // 反射创建一个新对象
            obj = desc.isInstantiable() ? desc.newInstance() : null;
        } catch (Exception ex) {
            throw (IOException) new InvalidClassException(
                desc.forClass().getName(),
                "unable to create instance").initCause(ex);
        }
        
        // ...

        if (obj != null &&
            handles.lookupException(passHandle) == null &&
            desc.hasReadResolveMethod()) {
            Object rep = desc.invokeReadResolve(obj);
            if (unshared && rep.getClass().isArray()) {
                rep = cloneArray(rep);
            }
            if (rep != obj) {
                // Filter the replacement object
                if (rep != null) {
                    if (rep.getClass().isArray()) {
                        filterCheck(rep.getClass(), Array.getLength(rep));
                    } else {
                        filterCheck(rep.getClass(), -1);
                    }
                }
                handles.setObject(passHandle, obj = rep);
            }
        }

        return obj;
    }

obj = desc.isInstantiable() ? desc.newInstance() : null;

if (obj != null && handles.lookupException(passHandle) == null &&
      desc.hasReadResolveMethod()) {
     //....
 }        

public class SerializableSingleton implements Serializable {
    // ...
    private Object readResolve() {
        return SingletonInstance.instance;
    }
}