为什么我收到java.io.EOFException

Question

If I have a class with nested classes and I serialize it, how can I read data from one of the nested classes?

I know how to read from the main class like this:

FileInputStream fis = new FileInputStream(eventsFile); 
ObjectInputStream ois = new ObjectInputStream(fis);  

GreenhouseControls gc = (GreenhouseControls) ois.readObject();

But then I try to read from one of the nested classes like this:

WindowMalfunction ce = (WindowMalfunction)ois.readObject();

I get this exception:

java.io.EOFException
    at java.io.ObjectInputStream$BlockDataInputStream.peekByte(Unknown Source)
    at java.io.ObjectInputStream.readObject0(Unknown Source)
    at java.io.ObjectInputStream.readObject(Unknown Source)
    at greenhouse.GreenhouseControls$Restore.action(GreenhouseControls.java:348)
    at other.Controller.run(Controller.java:45)
    at greenhouse.GreenhouseControls.main(GreenhouseControls.java:555)

When I serialized the GreenhouseControls class I did it like this:

public void saveState() {
      try{
          // Serialize data object to a file
          ObjectOutputStream out = new ObjectOutputStream(new FileOutputStream("dump.out"));
          out.writeObject(GreenhouseControls.this);
          out.close();
          } catch (IOException e) {
          }
  }

Answer 1

Classes and other methods do not get serialized. Serialization is the process of converting an object into a stream.

If your question is "How do I serialize/deserialize a nested class," just make the nested class serializable and then use it as you would a top level class.

If your question is "How do I access a nested class from a serialized class, you can do this:

((GreenhouseControls) ois.readObject()).new WindowMalfunction(<constructor arguments>);

Answer 2

Firstly, as @user1549476 says, writeObject serializes objects. Not classes. This means that:

• the classes / methods of a class are not serialized, and
• the state of a class (ie values of statics) are not serialized.

Second, a class with nested / inner / anonymous inner classes is not a container.

• For inner and anonymous inner classes, there is a one-way relationship from an inner class instance to an outer class instance. But there is no reverse relationship. That means that there is no way to find all inner instances from the outer instance.

• For a nested class, there is no relationship at all.

So what you are trying to do simply does not make sense. And the EOF exception occurs because you are trying to read back objects that weren't written in the first place.

---

If you want the outer class instance to act as a container, you need to add an instance attribute to the outer class that holds references to the inner class instances. And then you need to implement "container management" operations. If you do that, and ensure that the outer and inner classes are serializable, then serializing an outer instance will serialize the inner instances in its container data structure.

---

For a nested class (but not an inner class!) you could make the class serializable and store the instances independently.

For an inner class, the serialization needs to include the outer class, but I think this will happen automatically. So for example if had something like this:

  Outer outer = ...
  Inner inner1 = outer.new Inner(...);
  Inner inner2 = outer.new Inner(...);
  ...
  ObjectOutputStream oos = ...
  oos.writeObject(inner1);
  oos.writeObject(inner2);
  oos.close();
  ...
  ObjectInputStream ois = ...
  Inner deserialized_inner1 = (Inner) ois.readObject();
  Inner deserialized_inner2 = (Inner) ois.readObject();

... you should get a pair of Inner instances whose "outer" is an Outer instance that has just been deserialized!