Find out which Java classes are actually loaded and reduce jar

Question

Is there a way to automatically find out which Java classes are actually loaded (either during compile time, as far as that's possible, or during the runtime of an application), and to throw out all other classes from a JAR to create a smaller JAR? Does that actually make sense in practice?

I am talking about the application classes for an application JAR. Usually there are lots of libraries in an application, and an application rarely needs all features of those libraries. So I suspect that would make a considerably smaller application. In theory that might be done for example via an Java agent that logs which classes and resources are read by one or several runs of an application (or even just by java -verbose:class), and a maven plugin that throws out all other classes from a jar-with-dependencies. Is there already something like that?

Clarification: I am not talking about unused dependencies (JARs that are not used at all), but about removing unused parts of each included JAR.

Answer 1

Well, the Maven Shade Plugin has an option minimizeJar when creating an Uber-JAR for your application:

https://maven.apache.org/plugins/maven-shade-plugin/

But, as others already pointed out, this is quite dangerous, as it regularly fails to detect class accesses which are done via Reflection or other dynamic references.