The generated externalization code reads and writes a data stream according to the following order, with field values for each class ordered alphabetically:

To maintain externalization compatibility of a class from version to version, it is most important to understand the resulting stream layout for each version of a class. When the persistent signature of a class is being changed significantly, it may be helpful to map out the stream format for each version, in order to understand clearly what is necessary to read a previous version of the stream.

Beyond the fundamental understanding of the stream format, points in the following sections provide guidance concerning when and what kind of manual intervention is necessary to maintain compatibility.