[HTTPCLIENT-1523] DateUtils ThreadLocal leak if referencing non-JDK classes

• Type: Bug
• Status: Closed
• Priority: Major
• Resolution: Not A Problem
• Affects Version/s: 4.3.3
• Component/s: HttpClient (classic)
• Environment: Java 7
• Assignee: Unassigned
• Reporter: Stefan Larsson
• Created: 26/Jun/14 14:15
• Updated: 04/May/17 09:49
• Resolved: 26/Jun/14 14:27

Description

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:

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. 

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.

Issue Links

• is related to: HTTPCLIENT-1216 org.apache.http.impl.cookie.DateUtils creates but doesn't cleanup a ThreadLocal instance
• is related to: HTTPCLIENT-659 DateUtils should cache SimpleDateFormat