How does Tomcat redirect verbose:gc console output to catalina.out

Question

I need to understand how Tomcat is able to catch JVM garbage collection console output logs and put it in catalina.out.

I tried to redirect System.out and System.err , but no can do. I was not able to get the output of GC logs.
However, when activating -verbose:gc in Tomcat (in catalina.sh, *JAVA_OPT*) and later cheking the output in catalina.out I was able to see the GC logs. How is this possible? How was Tomcat able to redirect that output to a file?

Maybe do you know any way to redirect the GC log output inside some java application using java code?

Thank you in advance.

Answer 1

It has nothing to do with code; the catalina.out file is not a "real log", it's just a file into which the standard tomcat startup command pushes all it's output.

In other words, this is a shell question, not a java one :)

Answer 2

Tomcat must indeed be using System.setErr() and System.setOut(). There's a great discussion about how that works elsewhere on this site: java: "final" System.out, System.in and System.err?

Tomcat redirects all console output to catalina.out. You might be able to configure this via the JULI log settings.