Classloader Hierarchy Agent

An agent to print the classloader hierarchy of a running JVM (16+).

To lear more on this topic, please consider reading the accompanying blog post Class Loader Hierarchies.

Now, we no further ado, let's look at how to use the agent for an example:

# build it
mvn package

# download a benchmark
test -e renaissance.jar || wget https://github.com/renaissance-benchmarks/renaissance/releases/download/v0.14.2/renaissance-gpl-0.14.2.jar -O renaissance.jar

# run it
java -javaagent:target/classloader-hierarchy-agent.jar -jar renaissance.jar finagle-http -r 1

This results in the following output:

...
[root]
  platform
    Thread: finagle/netty4-2-6
       java.sql
       sun.util.resources.provider
       sun.text.resources.cldr.ext
       sun.util.resources.cldr.provider
    app
         me.bechberger
         org.renaissance
         org.renaissance.core
      null
           scala
           scala.collection
           scala.jdk
           scala.io
           scala.runtime
[cumulative]   # accumulates the classloaders over time
  platform
    Thread: main
       java.sql
       sun.util.resources.provider
       sun.text.resources.cldr.ext
       sun.util.resources.cldr.provider
    ...

Be aware that the bootstrap classloader is always implicitly the root of the hierarchy, but it not printed as it represented as null. To quote from the documentation of Class.getClassLoader():

Returns the class loader for the class. Some implementations may use null to represent the bootstrap class loader. This method will return null in such implementations if this class was loaded by the bootstrap class loader.

The agent has two options:

Usage: java -javaagent:classloader-hierarchy-agent.jar[=maxPackages=10,everyNSeconds=0] <main class>
  maxPackages: maximum number of packages to print per classloader
  every: print the hierarchy every N seconds (0 to disable)

parttimenerd/classloader-hierarchy-agent

Classloader Hierarchy Agent

License