Application fails to start when server.tomcat.threads.max < 10 about spring-boot

[wilkinsona]

Thanks for the report. The change in behavior is due to an unanticipated side-effect of #36087.

The failure's occurring because the maximum number of threads is less than the minimum number of spare threads. You can avoid it by setting server.tomcat.threads.min-spare=9.

This is necessary with Spring Boot 3.3.0 as Tomcat's no longer creating its executor. When it does create the executor, it calls org.apache.tomcat.util.net.AbstractEndpoint.getMinSpareThreads() which does Math.min(getMinSpareThreadsInternal(), getMaxThreads()).

I'm not sure if we should add similar logic to our creation of the executor. Arguably, having the max threads set to something that's less than min-spare threads is a configuration error and I'm not 100% sure that silently adapting to it is the right thing to do. The counter-argument is that it used to work and it's what Tomcat does by default so we should probably reinstate the old behavior.

[fabian-froehlich]

@wilkinsona Thanks, your suggestion works. Our application starts again.

I would prefer a solution that either

• makes the application startable after upgrading the dependency right away
  or
• add a paragraph in the releasenotes / migration guide

[philwebb]

We're going to align with Tomcat default behavior.