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From "Stefan Larsson (JIRA)" <j...@apache.org>
Subject [jira] [Created] (HTTPCLIENT-1523) DateUtils ThreadLocal leak if referencing non-JDK classes
Date Thu, 26 Jun 2014 14:16:25 GMT
Stefan Larsson created HTTPCLIENT-1523:
------------------------------------------

             Summary: DateUtils ThreadLocal leak if referencing non-JDK classes
                 Key: HTTPCLIENT-1523
                 URL: https://issues.apache.org/jira/browse/HTTPCLIENT-1523
             Project: HttpComponents HttpClient
          Issue Type: Bug
          Components: HttpClient
    Affects Versions: 4.3.3
         Environment: Java 7
            Reporter: Stefan Larsson


org.apache.http.client.utils.DateUtils use of a SoftReference is not enough to avoid a memory
leak. 

Tomcat displays these kinds of warnings when undeploying a webapp:
{noformat}
Jun 26, 2014 3:04:27 PM org.apache.catalina.loader.WebappClassLoader checkThreadLocalMapForLeaks
SEVERE: The web application [/myapp] created a ThreadLocal with 
key of type [org.apache.http.client.utils.DateUtils$DateFormatHolder$1] (value [org.apache.http.client.utils.DateUtils$DateFormatHolder$1@22b76cd])
and a 
value of type [java.lang.ref.SoftReference] (value [java.lang.ref.SoftReference@5e63cdf6])
but failed to remove it when the web application was stopped. Threads are going to be renewed
over time to try and avoid a probable memory leak. 
{noformat}

ThreadLocal is implemented through a reference from each Thread instance to the _value_ (a
SoftReference, so OK after a GC takes place) but also to the ThreadLocal instance itself.
In the case of DateUtils the JDK ThreadLocal is overridden in order to implement the initialValue()
method, meaning that object's class has a reference to the classloader which loaded DateUtils.DateFormatHolder.
When used in combination with a Tomcat webapp this eventually causes the permgen to fill up
after a few redeployments.

(Thread.threadLocals is declared as a ThreadLocal.ThreadLocalMap, which is a collection of
pairs of ThreadLocal instance plus its value)

Since there's already extensive code in DateUtils.DateFormatHolder.formatFor() to re-initialize
the Map of date formats, one might as well perform the "new SoftReference(new HashMap()) there
to avoid overriding the ThreadLocal.

References:
Second comment of https://plumbr.eu/blog/when-and-how-to-use-a-threadlocal

There will still be a tiny leak of ThreadLocal and SoftReference instances I believe with
this change (max 1 per thread), but that should have much less impact than leaking a reference
to a webapp's class loader.

ThreadLocals are supposed to be cleaned up either by letting the thread terminate (not an
option if Tomcat is managing the thread) or by using ThreadLocal.remove() from the thread
which assigned the ThreadLocal's value. I generally try to avoid using ThreadLocal whenever
possible.



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