Java serialization: readObject() vs. readResolve()

readObject() is an existing method in ObjectInputStream class. At the time of deserialization readObject() method internally checks whether the object that is being deserialized has readResolve() method implemented. If readResolve() method exists then it will be invoked

A sample readResolve() implementation would look like this

protected Object readResolve() {
  return INSTANCE:
}

So, the intent of writing readResolve() method is to ensure that the same object that lives in JVM is returned instead of creating new object during deserialization.

readResolve can be used to change the data that is serialized through readObject method. For e.g. xstream API uses this feature to initialize some attributes that were not in the XML to be deserialized.

http://x-stream.github.io/faq.html#Serialization

readResolve is used for replacing the object read from the stream. The only use I've ever seen for this is enforcing singletons; when an object is read, replace it with the singleton instance. This ensures that nobody can create another instance by serializing and deserializing the singleton.

Item 90, Effective Java, 3rd Ed covers readResolve and writeReplace for serial proxies - their main use. The examples do not write out readObject and writeObject methods because they are using default serialisation to read and write fields.

readResolve is called after readObject has returned (conversely writeReplace is called before writeObject and probably on a different object). The object the method returns replaces this object returned to the user of ObjectInputStream.readObject and any further back references to the object in the stream. Both readResolve and writeReplace may return objects of the same or different types. Returning the same type is useful in some cases where fields must be final and either backward compatibility is required or values must copied and/or validated.

Use of readResolve does not enforce the singleton property.