When does .getThreadCpuTime return nil?

[lawrence...@gmail.com]

I've been using this code for years. I copy and pasted it from someone else on this group, someone much smarter than I am. I have never seen an error from this code. Now suddenly I get a null pointer exception at the last line of this code: 

(ns denormalize_mysql_to_mongodb.monitoring

  (:import

   java.lang.management.ManagementFactory)

  (:require [clojure.string :as st]))

(defn thread-top

  "Return a seq of threads sorted by their total userland CPU usage."

  []

  (let [mgr (ManagementFactory/getThreadMXBean)

        cpu-times (map (fn [t]

                         [(.getThreadCpuTime mgr (.getId t)) t])

                    (threads))]

    (map

      (fn [[cpu t]] [cpu (.getName t) (.getId t) t])

      (reverse (sort-by first cpu-times)))))

This error is thrown at startup. I don't know much about ThreadMXBean. Can anyone give me a clue why any of this would be nil? 

[Neil Okamoto]

Maybe it's because you attempted method invocation via reflection but the object is nil. So for example if (threads) returns a list containing nils, then calling (.getId t) or (.getName t) would probably throw a null pointer exception. I'm not sure it's possible for (ManagementFactory/getThreadMXBean) to return nil, but if it does then (.getThreadCpuTime mgr ...) would similarly fail.

So best guess is, take a look at the implementation of "threads".

[lawrence...@gmail.com]

Thanks for the answer. I am surprised because I used this code for years without ever seeing that error. I'm trying to imagine what I did differently this one time. I've added error detection, we'll see if it happens again. 

[Neil Okamoto]

Ok, hope it works out. 

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