I have a legacy app written in Java 8, built for Google App Engine, which had been using Jackson 2.3. Now I need to update the app to use Jackson 2.10. However, once I do so, the app's webservices (which had previously been working) won't return anything except a 503 page saying "503" and "Powered by Jetty://9.4.36v20210114".
The logs display no errors. All that they say is
com.google.appengine.tools.development.jetty9.DevAppEngineWebAppContext disableTransportGuarantee
INFO: Ignoring <transport-guarantee> for /my/endpoint/* as the SDK does not support HTTPS. It will still be used when you upload your application.
which is a completely normal and expected message.
My dependency tree shows no reference to Jetty. I'm not sure what that means; is Jetty built into the JDK now or something?
Does anyone have any idea what might be causing this? or how to debug it?