[IBATIS-721] PermGen leak on undeploy in Tomcat 6.x (ThreadLocal in ErrorContext)

• Type: Bug
• Status: Closed
• Priority: Critical
• Resolution: Fixed
• Affects Version/s: 3.0 Beta 7
• Fix Version/s: 3.0 Beta 8
• Component/s: Core
• Environment: Java SE 1.6.0_17 (64 bit); Mac OS X 10.6.2; Tomcat 6.0.20
• Assignee: Clinton Begin
• Reporter: Tom Kunicki
• Created: 23/Dec/09 04:13
• Updated: 23/Dec/09 15:45
• Resolved: 23/Dec/09 15:45

Description

The current use of ThreadLocal in org.apache.ibatis.executor.ErrorContext can lead to permgen memory leaks on webapp undeploy on Tomcat 6.0.20

The potential of a request thread being used as the ThreadLocal key without removal will pin the webapp class loader (and all loaded classes) in permgen memory after an undeploy.

To reproduce:

1) Deploy a webapp using ibatis 3.0 to tomcat
2) Hit the webapp once with an execution path invoking ibatis
3) Undeploy the webapp
4) GC the tomcat process 2+ times to invoke a full GC (i.e. w/ jconsole or jvisualvm)
5) dump the heap
6) examine with jhat from later JDK 6 version

(http://blogs.sun.com/fkieviet/entry/how_to_fix_the_dreaded)

You should be able to see the webapp classes still loaded, by looking at the associated class loader and the reference chain from rootset you'll see a System class reference with the ErrorContext ThreadLocal instance chaining the class loader.

I am using "-XX:+UseConcMarkSweepGC -XX:+CMSClassUnloadingEnabled -XX:+CMSPermGenSweepingEnabled" when launching tomcat.

As a quick fix I was able to clean this up by adding LOCAL.remove() to ErrorContext.reset() on my local build. I don't know if this is a good long term solution...