How to inform users that a plugin's data structure is not backward compatible
Scenario: Adding a new field
Persistence is XML-based, and when the data is read back from disk and XML doesn't contain data for a particular field, the existing field value is left as-is. "As-is" normally means the JVM default value (0, null, etc.), because the persistence doesn't invoke your constructor to create your object, like Java serialization.
If you want to fill in your field with a non-trivial default value, you can write the
readResolve method, which gets invoked right after your object is resurrected from persistence.
readResolve is called by XStream, but is not part of a Jenkins class hierachy, so there is no override. Just put it right in your class alongside your fields, for example in the build pipeline plugin:
If you need to force the Manage Old Data screen to list jobs, builds, etc. using your data in the old format so that it can be saved in the new format in bulk, you cannot use
readResolve since it will not notify this system of the problem. Instead you must create a static nested class called
XStream2.PassthruConverter, which should clean up the storage of your instance and finally call
OldDataMonitor.report to record the conversion.
Scenario: Remove a field
Removing a field requires that you leave the field with the transient keyword. When Jenkins reads the old XML, XStream will set the value for this field, but it will no longer be written back to XML when data is saved.
Scenario: Rename a field
Renaming a field is a combination of the above: mark your old field as transient, declare your new field, and then migrate the data from the old format to the new in your
Scenario: Make a class abstract and introduce concrete subtypes
- You decide to extend a class and create new choosable classes, e.g. more browsers for a SCM-plugin.
The old data structure looked like this when you had only one class
Now you decide to add a new
NewSCMBrowser, all your
SCMBrowserBaseand your XML suddenly looks like this:
- With new jobs, no problem. Old jobs however will probably break.
SCMBrowserBaseclass add a method
readResolve(see XStream FAQ):
Scenario: rename a class
Sometimes, you need to rename packages or class names. If your serialization data includes a fully qualified class name (which happens for example if you have a collection of them), then a measure must be taken to maintain backward compatibility.
To do this, use
XSTREAM2.addCompatibilityAlias(String,Class) to register aliases. You need to do this against the right XStream instance, as a few different instances are used to persist different parts of data.
Items.XSTREAM2 is used for serializing project configuration, and
Run.XSTREAM2 is used for serializing build and its associated Actions.
For example, to alias
Foo in the "old" package to the "updated" one, you can use this method call:
To ensure your alias is registered early in the Jenkins boot sequence, you can use the Initializer annotation on a static method, e.g. in your
Scenarion: rename or move the descriptor class of a plugin
Since 1.507 Descriptor#getConfigFile() is overridable and XmlFile can be instantiaded with any XStream instance.
TODO Examplecode (Weltramschaf)