Cesar Chew at 17:50, 24 November 2014

2014-11-24T17:50:34Z

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{{Cat|Java}}

<slideshow style="nobleprog" headingmark="⌘" incmark="…" scaled="true" font="Trebuchet MS" >
;title: Java Debugging
;author: Sam Bashton
</slideshow>

=== Debugging Java applications ⌘===

* Debugging tools are more limited on RHEL5
* Still many options

=== Showing JVM properties ⌘===

jinfo <java process id>

=== Showing heap usage ⌘===

* You can see the heap usage of an application with jmap
jmap <java process id>

=== Showing object heap usage ⌘===

jmap -histo <java process id>
* It may be necessary to 'force' on newer Java versions:
jmap -F -histo <java process id>

=== Taking a thread dump ⌘===

* Shows more details on what an application is doing
* Dumps thread output to stdout
- If you started your Java process redirecting stdout to /dev/null,
you're out of luck..
* Take two thread dumps to show stuck threads

pkill -3 java; sleep 5; pkill -3 java

=== Take a stack trace ⌘===

* `jstack <java process id>`
* If the app uses JNI, take a mixed-mode trace:
jstack -m <java process id>

=== Showing garbage collection details ⌘===

* Start the JVM with `-verbose:gc` to get details about garbage collection printed to stdout on each collection
For even more verbosity, use `-XX:+PrintGCDetails

=== Disabling explicit garbage collection ⌘===

* Applications can manually invoke full garbage collection by calling `System.gc()`
* This can be disabled by adding command line option
-XX:+DisableExplicitGC` at JVM startup

=== Exercise ⌘===

* Try all of the mentioned techniques on your tomcat instance

== Visualization⌘ ==
* jvisualvm

Java Debugging - Revision history

Cesar Chew at 17:50, 24 November 2014