I wrote a small bytecode transformation engine so that we can further
evolve code while maintaining the backward compatibility:
https://github.com/jenkinsci/bytecode-compatibility-transformer
This is a pair of an annotation and a bytecode transformer.
For example, I recently (somewhat inadvertently) changed
AbstractProject.triggers from List to DescribableList, and this is
causing various linkage errors when loading plugins. But with this
library, I can add:
@AdaptField
protected volatile DescribableList<...> triggers = ...
... and the bytecode transformer can rewrite a reference to this field
so that things will work (unless you try to set something like ArrayList
to this field, which would fail with ClassCastException.)
Since Jenkins controls classloading of all plugins, installing such a
transformer can be done in a core without changing plugins.
Another use for this is to get rid of a singleton, for example like in
the FreeStyleProject class:
@Restricted(NoExternalUse.class)
@Extension(ordinal=1000)
public static final DescriptorImpl DESCRIPTOR = new DescriptorImpl();
public static final class DescriptorImpl extends
AbstractProjectDescriptor {
....
}
I could change this to:
@Extension(ordinal=1000)
public static final class DescriptorImpl extends
AbstractProjectDescriptor {
....
}
@AdaptField("DESCRIPTOR")
@Restricted(NoExternalUse.class)
public static DescriptorImpl _getDescriptor() {
return ...;
}
For these benefits, I'd like to integrate this into the main line unless
I hear some objections.
So far we've relied on bridge method injector [1] to do some code
evolutions, and I think this will be another valuable tool to push that
even further. And yes, this is not a tool for an inexperienced. Plugins
would be able to use the same mechanism, but use this at your own risk.
I haven't thought about how to integrate this into remoting, but we
control classloading there as well, so it should be doable.
Besides, this is useful enough just on the master, and there are many
objects in the master that really can't be sent to slaves, like Jenkins,
Job, and Run.
[1] http://bridge-method-injector.infradna.com/
--
Kohsuke Kawaguchi | CloudBees, Inc. | http://cloudbees.com/
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