30 Sep 2008

Tracking down threading issues

One of the projects we’ve been working on involves kicking off a number of threads to check on availability of certain services. Despite having read the fantastic Java Concurrency in Practice book (which you should buy, possibly from Amazon UK) we’d run into a situation where Tomcat wasn’t shutting down. Inevitably it’d be because we’d starting a thread that wasn’t stopping. As a reminder to myself here are a bunch of useful tools for figuring out what might be going on….

Step 1: find out what’s running with jps -lm:

$ jps -lm
1001 org.apache.catalina.startup.Bootstrap start
1302 sun.tools.jps.Jps -lm
1066 com.j_spaces.core.client.SpaceFinder /./taykt

This is a list of the Java process IDs which are “typically, but not necessarily, the operating system’s process identifier for the JVM process”, according to the JPS manual page. In my case, they were the OS process IDs, although I should point out that I’m running this on Mac OS 10.5 using JDK 5, and the tools do vary by operating system.

What the list is showing me is that the Tomcat process is still running (process 1001). All the other processes I expect to see on my machine, including the mysterious 176 which happens to be IntelliJ.

Step 2: take a look at what threads are running in the process, as described in the J2SE 5.0 Trouble-Shooting and Diagnostic Guide PDF:

$ kill -QUIT 1001 

Hmm. No output from that command… of course, because I’ve signalled to the Tomcat process to give me a thread dump the output will be in the Tomcat logs:

==> catalina.out Full thread dump Java HotSpot(TM) Client VM
mixed mode, sharing):

"DestroyJavaVM" prio=5 tid=0x010017f0 nid=0xb0801000 waiting 
 on condition [0x00000000..0xb0800060]

"SelfCleaningTable\$Cleaner" daemon prio=5 tid=0x01049030 
 nid=0x8e7a00 in Object.wait() [0xb171f000..0xb171fd90]
 at java.lang.Object.wait(Native Method)
 - waiting on (a java.lang.ref.ReferenceQueue\$Lock)
 at java.lang.ref.ReferenceQueue.remove(ReferenceQueue.java:120)
 - locked (a java.lang.ref.ReferenceQueue\$Lock)
 at java.lang.ref.ReferenceQueue.remove(ReferenceQueue.java:136)
 at com.j\_spaces.obf.eb.run(SourceFile:231)

"pool-1-thread-1" prio=5 tid=0x01021470 nid=0x8f0c00 waiting
 on condition [0xb0c8a000..0xb0c8ad90]
 at sun.misc.Unsafe.park(Native Method)
 at java.util.concurrent.locks.LockSupport
 at java.util.concurrent.locks.AbstractQueuedSynchronizer$
 at java.util.concurrent.LinkedBlockingQueue
 at java.util.concurrent.ThreadPoolExecutor
 at java.util.concurrent.ThreadPoolExecutor$Worker
 at java.lang.Thread.run(Thread.java:613)

…it goes on for a bit. The parts I was interested in were non-daemon processes (because daemon ones are shutdown when the JVM shutsdown). In my case, the “pool-1-thread-1” process reminded me that we’d put a few things into an Executor and possibly not cleaned up properly. I know it was that thread name, because I’d seen it in the application log files. A quick dip into Java Concurrency in Practice to remind myself about The Rules, and I had a few modifications that resolved the issue. But…

Step 3: try out JConsole. The trick here is that you need to set a flag at Tomcat start up time:

$ export CATALINA_OPTS="-Dcom.sun.management.jmxremote"
$ sh startup.sh

Then a quick jps -lm to find the process number, then fire up JConsole with jconsole pid-goes-here and you’re set. Not only do you get easier access to the thread list, you also have buttons in the MBean tab with tempting names.