为什么我没有得到NotSerializableException？

Question

class NotSerializable {}

class MyClass implements Serializable {
   private NotSerializable field; // class NotSerializable does not implement Serializable!!
}

public class Runner {
   public static void main(String[] args) {
      MyClass ob = new MyClass();

      try {
         FileOutputStream fs = new FileOutputStream("testSer.ser");
         ObjectOutputStream os = new ObjectOutputStream(fs);
         os.writeObject(ob);
         os.close();
      } catch (IOException e) { 
          e.printStackTrace(); 
      }

      try {
         FileInputStream fis = new FileInputStream("testSer.ser");
         ObjectInputStream ois = new ObjectInputStream(fis);
         MyClass copyOb = (MyClass) ois.readObject();
         ois.close();
      } catch (Exception e) { 
          e.printStackTrace(); 
      }
   }
}

This program executes correctly and successfully serializes the object ob . But I expected to get java.io.NotSerializableException at runtime. Because MyClass has reference of class which does not implement the Serializable interface! What is really going on?

Answer 1

Because the field is null. And null can be serialized fine.

The serialization mechanism checks the actual, concrete type of each field, and not its declared type. You could have an instance of a subclass of NotSerializable, that is Serializable, and it would then serialize fine as well. If that was not the case, you would not be able to serialize any object having a member of type List for example, because List doesn't implement Serializable.