Embedded Jetty is one of the more popular servers for ring applications. JMX can be useful for poking around the guts of Jetty, as well as making runtime config changes. Unfortunately, enabling JMX for an embedded Jetty isn’t a straightforward config change, and the process for doing so in Clojure is largely undocumented. So this is the guide that I wish existed when I found the need to profile Jetty. If you’d rather skip the commentary, I’ve put up a minimal clojure jmx-enabled server for perusal.

Most essentially, the version of Jetty that comes bundled in ring-jetty-adapter is too old (currently 7.6.13) to expose meaningful JMX hooks. Thankfully there’s a modern ring adapter that you can add to your dependency list:

[info.sunng/ring-jetty9-adapter "0.8.1"]

which serves as a drop-in replacement for the official ring-jetty-adapter. Another relevant dependency is Jetty’s JMX artifact:

[org.eclipse.jetty/jetty-jmx "9.2.7.v20150116"]

The jetty-jmx version should match with the version of jetty-server provided by ring-jetty9-adapter.

While editing project.clj, it’s important enable JMX on the JVM level, and select a port:

:jvm-opts ["-Dcom.sun.management.jmxremote"
           "-Dcom.sun.management.jmxremote.ssl=false"
           "-Dcom.sun.management.jmxremote.authenticate=false"
           "-Dcom.sun.management.jmxremote.port=8001"]

Finally, the running Jetty server must opt-in to JMX by pointing to the appropriate “MBean,” which can be imported with:

(ns jetty-jmx.core
  (:require [ring.adapter.jetty9 :refer [run-jetty]])
  (:import (java.lang.management ManagementFactory)
           (org.eclipse.jetty.jmx MBeanContainer)))

The server can then be started with:

(let [mb-container (MBeanContainer. (ManagementFactory/getPlatformMBeanServer))]
    (doto (run-jetty app {:port 8000
                          :join? false})
      (.addEventListener mb-container)
      (.addBean mb-container)))

which attaches the MBean to the running Jetty server. Since the run-server command calls .start on the Server object before returning it, it’s important to configure :join? false to allow thread execution to continue, preventing the following .addEventListener and .addBean from being blocked.

With all of this, it should now be possible to start the server and connect to the JMX port using jconsole:

jconsole localhost:8001

Relevant info will be under the MBeans tab. Useful fields include

org.eclipse.jetty.util.thread.queuedthreadpool.threads

for how many threads are allocated, and

org.eclipse.jetty.util.thread.queuedthreadpool.queueSize

to find out how many requests are waiting on threads.