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Thread Dump and Concurrency Locks

Posted by mandychung on November 23, 2005 at 4:12 PM PST

Thread dumps are very useful for diagnosing synchronization related problems such as deadlocks on object monitors. Ctrl-\ on Solaris/Linux or Ctrl-Break on Windows has been a common way to get a thread dump of a running application. On Solaris or Linux, you can send a QUIT signal to the target application. The target application in both cases prints a thread dump to the standard output and also detects if there is any deadlock involving object monitors.

jstack, a new troubleshooting utility introduced in Tiger (J2SE 5.0), provides another way to obtain a thread dump of an application. Alan Bateman has a nice blog about jstack and its several improvements in Mustang (Java SE 6). Mustang jstack works like a remote Ctrl-\ or Ctrl-Break if you are on Windows.

jconsole is JMX-complaint GUI tool which allows you to get a thread dump on the fly. The "Using JConsole to Monitor Applications" article gives you an overview of the Tiger monitoring and management functionality.

Mustang extends the thread dump, jstack, and jconsole to support java.util.concurrent.locks to improve its diagnosability. For example, the Threads tab in the Mustang jconsole now shows which synchronizer a thread is waiting to acquire when the thread is blocked to lock a ReentrantLock and also which thread is owning that lock.


In addition, it has a new "detect deadlock" button (in the bottom). When you click on the "detect deadlock" button, it will send a request to the target application to perform the deadlock detection operation. If the target application is running on Mustang, it finds deadlocks involving both object monitors as well as the java.util.concurrent.locks. If the target application is running on Tiger, it finds deadlocks involving object monitors only. Each deadlock cycle will be displayed in a separate Deadlock tab.


Click here to see a wider form of this screenshot.

JDK 6 has a nice demo FullThreadDump under $JDK_HOME/demo/management/FullThreadDump where JDK_HOME is the location of your JDK 6. This demo has been included in JDK 5.0 and is updated to use the new Mustang API. It demonstrates the use of the API to get the thread dump and detect deadlock programmatically.

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