Java serialization: readFields() beyond of readObject()?

Question

ObjectInputStream.readFields() is eligible only within private void readObject(ObjectInputStream) method.

public ObjectInputStream.GetField readFields() throws IOException, ClassNotFoundException {
  SerialCallbackContext ctx = curContext;
  if (ctx == null) {
    throw new NotActiveException("not in call to readObject");
  }
...

I'm in situation when I can't use default serialisation for reading object (i.e. ObjectInputStream.defaultReadObject()) and don't wish to implement readObject() method in all my classes. In ideal case I would like to have ownDefaultReadObject() method that will construct new object from serialized fields (e.g. by reflection).

Any ideas?

If someone would like to know more. Field names in some of my classes were renamed (e.g. by obfuscator) to a, b, c etc. Such classes were serialized with renamed fields using default Java serialization. I need to deserialise them to original classes (I know pairs of field names for each class; a=> fieldName, b=> age, c=>gender etc.).

Answer 1

To rename fields from an object stream, the method you need to override is ObjectInputStream.readClassDescriptor which returns an ObjectStreamClass.

Instances ObjectStreamClass fulfil one of two different roles through large different subsets of the interface. For the avoidance of doubt, this design choice should not be copied.

• Describes the fields of a serialisable class running in the current JVM instance. Find these instance through ObjectStreamClass.lookup.
• Describes the fields of a serialisable class as represented in a particular serialised stream. These instances are returned by the implementations of ObjectInputStream.readClassDescriptor.

In your override call super.readClassDescriptor. This will read in the data from the stream. Substitute the value from the stream with one having the new fields names, if it's a class you're interested in.

How to create you own ObjectStreamClass? Write dummy instance of the classes you are interested in to an ObjectOutputStream. You can do this as part of the built, just keeping the binary data. Read with another ObjectInputStream with readClassDescriptor overridden to stash the descriptors.

ObjectInputStream.defaultReadObject/readFields wouldn't make any sense outside of readObject (or similar) because they rely on the current deserialising object rather than an argument. There are other limitations to prevent other code calling defaultReadObject to rewrite fields that must remain constant, copied validated, security checked or similar.