Expose ServletHolder getter in ServletHandler$ChainEnd for auditing libraries to use

[joakime]

Jetty version(s)
9.4.42

Java version/vendor (use: java -version)
All

OS type/version
All

Description
If an audit library wants to hook into the FilterChain of the ServletHandler, they use the new APIs such as ServletHandler.newFilterChain(FilterHolder filterHolder, FilterChain chain) introduced in Issue #5022

However accessing the ServletHolder isn't that easy, and you have to resort to reflection trickery to access it.

We should expose the ServletHolder as a formal API.

[gregw]

@joakime I'm closing this as both PRs are merged.

[joakime]

@gregw any reason why Chain and ChainEnd are still private?

[gregw]

@joakime well the question really should be put the other way. What is the reason for not having them private?
I guess they could be made protected so they are available to extensions, but they probably need better names if they are to be exposed... maybe: FilterChainLink and FilterChainTarget

[joakime]

@gregw to allow people to implement the newFilterChain() properly, and actually be able to call this .getServletHolder() that the associated PRs were all about.

[gregw]

@joakime developers that implement newFilterChain() can do so without use of our utility classes for creating links in the chain and a termination. However, I have no issue with making our utility classes available as protected classes, so long as we think about the names a bit more.