How to explore which classes are loaded from which JARs?

Question

Is there a way to determine which classes are loaded from which JARs at runtime?

I'm sure we've all been in JAR hell before. I've run across this problem a lot troubleshooting ClassNotFoundExceptions and NoClassDefFoundErrors on projects. I'd like to avoid finding all instances of a class in JARs and using process of elimination on the code causing a CNFE to find the culprit.

Will any profiling or management tools give you this kind of information?

This problem is super annoying purely because we should have this information at the time the class gets loaded. There has to be a way to get to it, or record it and find it, yet I know of nothing that will do this, do you?

I know OSGi and versioned bundles/modules aim to make this a non issue... but it doesn't seem to be going away any time soon.

Note: I found this question is a subset of my question related to classes loaded from versioned jars.

Somewhat related, this post explains a strategy to search for a class within JARs (either under the current directory) or in your M2_REPO: JarScan, scan all JAR files in all subfolders for specific class

Also somewhat related, JBoss Tattletale

Answer 1

Passing the -verbose:class switch to the java command will print each class loaded and where it was loaded from.

Joops is also a nice tool for finding missing classes ahead of time.

Answer 2

From code you can call:

myObject.getClass().getProtectionDomain().getCodeSource()

(Note, getProtectionDomain may unfortunately return null (bad design), so "proper code" would check for that.)

Answer 3

There is an MBean for the JVM flag mentioned by Jason Day above.

If you are using JBoss, you can twiddle this on demand using JMX, if you add the native JMX MBean server to your config. Add the following -D's:

-Dcom.sun.management.jmxremote.port=3333
-Dcom.sun.management.jmxremote.authenticate=false
-Dcom.sun.management.jmxremote.ssl=false
-Djboss.platform.mbeanserver 
-Djavax.management.builder.initial=org.jboss.system.server.jmx.MBeanServerBuilderImpl
-DJBOSS_CLASSPATH="../lib/jboss-system-jmx.jar"

And then you can see this setting under the java.lang:Classloading MBean and can cut it on/off on the fly. This is helpful if you only want it on while executing a certain piece of code.

There is also an MBean which will allow you to enter a fully qualified classname and see where it was loaded from in the class hierarchy. The MBean is called LoaderRepository and you'll want to invoke the displayClassInfo() operation, passing in the FQCN.