The classes in resteasy-legacy module - in package org.jboss.resteasy.logging should be marked as deprecated:

• org.jboss.resteasy.logging.Logger
• org.jboss.resteasy.logging.impl.JULLogger
• org.jboss.resteasy.logging.impl.Log4jLogger
• org.jboss.resteasy.logging.impl.Slf4jLogger

They can be directly used by client/application and since Resteasy 3.1.0 the module resteasy-legacy module is not enabled by default, so usage of such classes causes NoClassDefFoundError on the server side.

It is not documented in javadocs - https://docs.jboss.org/resteasy/docs/3.1.0.Final/javadocs/ so it is not easy to find out which module to add to make this work or even know that this is actually deprecated.

Adding @Deprecated annotations together with the suggested replacement (jboss.logging) would make things clearer.