Tomcat fails to scan JAR file that I never asked for

With a recent Tomcat, 9.0.13, I was really puzzled with loads of warnings about Tomcat being unable to scan a JAR file which I never had reference nor did exist on the file system:

org.apache.tomcat.util.
scan.StandardJarScanner.scan Failed to scan
[file:/opt/tomcat-engine1/escenic/lib/serializer.jar] from classloader
hierarchy

It turned out, with Tomcat 8.0.41 or thereabouts, it has started to scan all manifests for other JARs and add these to the classpath. If any of these cannot be found, a big fat warning and stack trace is added to the Tomcat standard out log.

The remedy is to turn off scanning the MANIFEST.MF inside all JAR files:

$ vim conf/context.xml

<JarScanner scanManifest="false"/>

Now, I get about 400 lines less of spam in my logs and can concentrate on the messages I actually care about 😃.

For further details on the JAR scanner component, see the Tomcat documentation.