[jira] [Created] (OFBIZ-5395) Introduce Tomcat's JreMemoryLeakPreventionListener and why

[Nicolas Malin (Jira)]

Jacques Le Roux created OFBIZ-5395:
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             Summary: Introduce Tomcat's JreMemoryLeakPreventionListener and why
                 Key: OFBIZ-5395
                 URL: https://issues.apache.org/jira/browse/OFBIZ-5395
             Project: OFBiz
          Issue Type: Improvement
          Components: framework
    Affects Versions: SVN trunk
            Reporter: Jacques Le Roux
            Assignee: Jacques Le Roux


After reading few articles on possible memory leaks when using ThreadLocal variable with custom classes in application server context where a thread pool is used, I checked OFBiz code. There is only 2 custom classes  concerned: CompilerMatcher and RollbackOnlyCause (JDK classes are not concerned by ThreadLocal memory leaks).

First I must tell, that the memory leak problem is more clearly described in those articles when you use an external Application Server (like Tomcat) and you deploy/undeploy applications. It seems there are no major issues when you use OFBiz OOTB (with Tomcat embedded).  Nevertheless, it's a concern by and large.

I have not investigated RollbackOnlyCause, only CompilerMatcher. But, after some profiling, I believe both should only generate small amouts of memory leaks, almost not noticeable even after several deploy/undeploy cycles.

Nevertheless I tried to find a good way to get rid of CompilerMatcher possible leaks. I thought about 3 ways:
# *Reverts [CompilerMatcher related changes|http://svn.apache.org/viewvc?view=revision&revision=1075322]* done for OFBIZ-4107 (introduction of CompilerMatcher) by using Perl5Compiler.READ_ONLY_MASK which guarantees thread safety
** Pros: no need to introduce ThreadLocal
** Cons: performance, local Perl5 variables creation removes the patterns-compiled-cache CompilerMatcher introduced (note: [I found the origin of CompilerMatcher class here|http://mail-archives.apache.org/mod_mbox/jmeter-user/200212.mbox/%3Cse0ae21c.089@...%3E])
# *Uses ThreadLocal<CompilerMatcher> local variables* instead of private static members
** Pros: no need to worry about thread safety
** Cons: performance, local ThreadLocal local variables creation removes the patterns-compiled-cache ThreadLocal<CompilerMatcher> offers  when used as a private static member
# *Uses ThreadLocal<CompilerMatcher> local variables* instead of private static members
** Pros: no need to worry about thread safety
** Cons: performance, local ThreadLocal local variables creation removes the patterns-compiled-cache ThreadLocal<CompilerMatcher> gives when used as a private static member
# *Introduces [Tomcat's JreMemoryLeakPreventionListener|http://wiki.apache.org/tomcat/MemoryLeakProtection].* [What it does (in less than a minute)?|http://stackoverflow.com/questions/14882794/what-does-tomcats-threadlocalleakpreventionlistener-do-exactly] [Why JreMemoryLeakPreventionListener was created?|http://www.tomcatexpert.com/blog/2010/04/06/tomcats-new-memory-leak-prevention-and-detection] [History (29 pages presentation).|http://people.apache.org/~markt/presentations/2010-11-04-Memory-Leaks-60mins.pdf]  How it does it? [Read code!|http://svn.apache.org/viewvc/tomcat/trunk/java/org/apache/catalina/core/JreMemoryLeakPreventionListener.java?view=annotate]
** Pros:
*** no changes related to CompilerMatcher, performance enhancement the cache introduces kept
*** prevents memory leaks when an external Application Server is used or at least warn about them
** Cons: none, this should had any noticeable effects when OFBiz is used OOTB (Tomcat embedded)

So of course I decided to go with the JreMemoryLeakPreventionListener solution. Another reason for that is that when I profiled OFBiz trunk using the demo site I found that we were having a large bloc of memory retained by [sun.awt.AppContext|http://www.docjar.com/docs/api/sun/awt/AppContext.html].  I think we should not have such a thing, the web truk demo does not use AWT at all! Fortunately jreMemoryLeakPreventionListener.setAppContextProtection prevents this, even if I have still no ideas from where this comes.

I'm also considering to replace the current uses of java.util.regex.Pattern by CompilerMatcher in cases of a static pattern is used. Then the CompilerMatcher cache makes sense.

Some interesting references I noted while analysing this issue:
* [Oro is 6 times faster than regular Java regex|http://www.tusker.org/regex/regex_benchmark.html]. So, with its cache, CompilerMatcher is more than an interesting alternative to regular Java regex.
* Java regex Javadoc: [Compiler|https://docs.oracle.com/javase/6/docs/api/java/util/regex/Pattern.html], [Matcher|https://docs.oracle.com/javase/6/docs/api/java/util/regex/Matcher.html]
* Oro Javadoc: [Compiler|https://jakarta.apache.org/oro/api/org/apache/oro/text/regex/Perl5Compiler.html], [Matcher|https://jakarta.apache.org/oro/api/org/apache/oro/text/regex/Perl5Matcher.html]





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