[xxs]

There are many things that must go right. Threads being the primary one - don't start threads, pretty much anything that can start threads (outside the webapp) have to guard for the CCL and the thread group at the very least.

The entire app must have a clear start/stop lifecycle correctly implemented. Static registrations in non-webapp services must be removed, and/or those services must use weak references.

The list is quite long. (Source - I have done my fair share of middleware and know tomcat source quite well)

[bzzzt]

While this is possible, it takes ages for an application to gracefully shutdown so any time you save not having to start up a new server is lost. And then you still have classloader, memory and thread leaks since you can't really prove they are absent unless you have 100% test coverage + load testing on every possible code path.

Modern Java webservers (even 'enterprise' containers like JBoss etc) start up so quickly now there's not much to gain by not restarting them.

[xxs]

>And then you still have classloader, memory and thread leaks since you can't really prove they are absent unless you have 100% test coverage + load testing on every possible code path.

Those are trivially proven and diagnosed with memory dumps or even "jmap -histo"(incl. where the leak started). Zero test coverage for that needed. Some bugs were even in JDK where it'd start a new thread in a threadpool and copy all the CCL/Threadgroup/ACL and any inheritable thread local.