Disable Tomcat's MBean Registry by default and provide a property to enable it

[philwebb]

See this thread https://tomcat.markmail.org/thread/kayfacujrpt2diht which appears to suggest we might be able to disable JMX.

[wilkinsona]

I tried this a while ago. The best I came up with was to provide a stubbed out org.apache.tomcat.util.modeler.Registry implementation. I couldn't find a way to plug it in without using reflection as the Registry instance is retrieved via a static method backed by a private field so I abandoned it. I wonder if Meecroweave figured out something smarter.

[wilkinsona]

Meecroweave uses reflection to swap out the value of the static registry field on org.apache.tomcat.util.modeler.Registry.

I'll measure the gains and then we can decide if the performance improvement is worth the reflective hack.

[wilkinsona]

The benefits in terms of startup time are negligible for most people. I believe that's because the background preinitializer causes the expensive descriptor parsing to be done in parallel on a separate thread. The benefits will be more apparent in CPU constrained environments where the preinitializer may be disabled.

The benefits in terms of memory usage are more noticeable. With a NoOpRegistry in place an app that previously required -Xmx13M will start with -Xmx11M. This was tested with Tomcat 9.0.17.

[wilkinsona]

With Tomcat 9.0.19 the same app starts with -Xmx9M by default (thanks to apache/tomcat#146) and with -Xmx7M with NoOpRegistry. That ~2MB reduction becomes increasingly attractive when considered as a percentage of the required heap.

[wilkinsona]

@markt-asf Would you consider a Tomcat enhancement that allows JMX to be switched off in a less hacky manner? Saving 2MB of heap is quite appealing, and it would be even more appealing if we didn't have to resort to reflection to do so.

[markt-asf]

It isn't exclusively my call but I have no objections to an enhancement request to do that. If you open an enhancement request in Bugzilla I'll add it to what is currently a rather long TODO list.

[wilkinsona]

Thanks. I've opened https://bz.apache.org/bugzilla/show_bug.cgi?id=63361.