Java Debugging

title

Java Debugging

author

Sam Bashton

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