Changing data format requires a particular care so that users with the old data format can migrate to the newer data format smoothly.
Adding a field is the easiest. When you read an old XML that does not have any data, the newly added field is left to the VM-default value (if you let XStream create the object, such as {@link #read()} — which is the majority), or to the value initialized by theconstructor (if the object is created via new and then its value filled by XStream, such as {@link #unmarshal(Object)}.)
Removing a field requires that you actually leave the field with transient keyword. When you read the old XML, XStream will set the value to this field. But when the data is saved, the field will no longer will be written back to XML. (It might be possible to tweak XStream so that we can simply remove fields from the class. Any help appreciated.)
Changing the data structure is usually a combination of the two above. You'd leave the old data store with transient, and then add the new data. When you are reading the old XML, only the old field will be set. When you are reading the new XML, only the new field will be set. You'll then need to alter the code so that it will be able to correctly handle both situations, and that as soon as you see data in the old field, you'll have to convert that into the new data structure, so that the next save operation will write the new data (otherwise you'll end up losing the data, because old fields will be never written back.)
In some limited cases (specifically when the class is the root object to be read from XML, such as {@link Descriptor}), it is possible to completely and drastically change the data format. See {@link Descriptor#load()} for more about this technique.
There's a few other possibilities, such as implementing a custom {@link Converter} for XStream, or {@link XStream#alias(String,Class) registering an alias}. @author Kohsuke Kawaguchi
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