Occasionally, I get the following log statements in catalina.out while stopping Tomcat where my Hudson.war is deployed.

Aug 10, 2010 12:45:35 PM org.apache.catalina.loader.WebappClassLoader clearReferencesThreads
SEVERE: The web application [/hudson] appears to have started a thread named [com.google.common.base.internal.Finalizer] but has failed to stop it. This is very likely to create a memory leak.
Aug 10, 2010 12:45:35 PM org.apache.catalina.loader.WebappClassLoader clearThreadLocalMap
SEVERE: The web application [/hudson] created a ThreadLocal with key of type [null] (value [org.kohsuke.stapler.Stapler$1@1ae637f0]) and a value of type [java.text.SimpleDateFormat] (value [java
.text.SimpleDateFormat@c4dc9900]) but failed to remove it when the web application was stopped. This is very likely to create a memory leak.
Aug 10, 2010 12:45:35 PM org.apache.catalina.loader.WebappClassLoader clearThreadLocalMap
SEVERE: The web application [/hudson] created a ThreadLocal with key of type [java.lang.ThreadLocal] (value [java.lang.ThreadLocal@627b5c]) and a value of type [org.acegisecurity.context.Securit
yContextImpl] (value [org.acegisecurity.context.SecurityContextImpl@ffffffff: Null authentication]) but failed to remove it when the web application was stopped. This is very likely to create a
memory leak.
Aug 10, 2010 12:45:35 PM org.apache.catalina.loader.WebappClassLoader clearThreadLocalMap
SEVERE: The web application [/hudson] created a ThreadLocal with key of type [null] (value [net.sf.json.AbstractJSON$1@4190d536]) and a value of type [java.util.HashSet] (value [[]]) but failed
to remove it when the web application was stopped. This is very likely to create a memory leak.
Aug 10, 2010 12:45:35 PM org.apache.catalina.loader.WebappClassLoader clearThreadLocalMap
SEVERE: The web application [/hudson] created a ThreadLocal with key of type [null] (value [org.kohsuke.stapler.Stapler$1@1ae637f0]) and a value of type [java.text.SimpleDateFormat] (value [java
.text.SimpleDateFormat@c4dc9900]) but failed to remove it when the web application was stopped. This is very likely to create a memory leak.
Aug 10, 2010 12:45:35 PM org.apache.catalina.loader.WebappClassLoader clearThreadLocalMap
SEVERE: The web application [/hudson] created a ThreadLocal with key of type [java.lang.ThreadLocal] (value [java.lang.ThreadLocal@627b5c]) and a value of type [org.acegisecurity.context.Securit
yContextImpl] (value [org.acegisecurity.context.SecurityContextImpl@ffffffff: Null authentication]) but failed to remove it when the web application was stopped. This is very likely to create a
memory leak.
Aug 10, 2010 12:45:35 PM org.apache.catalina.loader.WebappClassLoader clearThreadLocalMap
SEVERE: The web application [/hudson] created a ThreadLocal with key of type [null] (value [org.kohsuke.stapler.Stapler$1@1ae637f0]) and a value of type [java.text.SimpleDateFormat] (value [java
.text.SimpleDateFormat@c4dc9900]) but failed to remove it when the web application was stopped. This is very likely to create a memory leak.
Aug 10, 2010 12:45:35 PM org.apache.catalina.loader.WebappClassLoader clearThreadLocalMap
SEVERE: The web application [/hudson] created a ThreadLocal with key of type [java.lang.ThreadLocal] (value [java.lang.ThreadLocal@627b5c]) and a value of type [org.acegisecurity.context.Securit
yContextImpl] (value [org.acegisecurity.context.SecurityContextImpl@ffffffff: Null authentication]) but failed to remove it when the web application was stopped. This is very likely to create a
memory leak.
Aug 10, 2010 12:45:35 PM org.apache.catalina.loader.WebappClassLoader clearThreadLocalMap
SEVERE: The web application [/hudson] created a ThreadLocal with key of type [null] (value [net.sf.json.AbstractJSON$1@4190d536]) and a value of type [java.util.HashSet] (value [[]]) but failed
to remove it when the web application was stopped. This is very likely to create a memory leak.
Aug 10, 2010 12:45:35 PM org.apache.catalina.loader.WebappClassLoader clearThreadLocalMap
SEVERE: The web application [/hudson] created a ThreadLocal with key of type [null] (value [org.kohsuke.stapler.Stapler$1@1ae637f0]) and a value of type [java.text.SimpleDateFormat] (value [java
.text.SimpleDateFormat@c4dc9900]) but failed to remove it when the web application was stopped. This is very likely to create a memory leak.
Aug 10, 2010 12:45:35 PM org.apache.catalina.loader.WebappClassLoader clearThreadLocalMap
SEVERE: The web application [/hudson] created a ThreadLocal with key of type [java.lang.ThreadLocal] (value [java.lang.ThreadLocal@627b5c]) and a value of type [org.acegisecurity.context.Securit
yContextImpl] (value [org.acegisecurity.context.SecurityContextImpl@ffffffff: Null authentication]) but failed to remove it when the web application was stopped. This is very likely to create a
memory leak.
Aug 10, 2010 12:45:35 PM org.apache.catalina.loader.WebappClassLoader clearThreadLocalMap
SEVERE: The web application [/hudson] created a ThreadLocal with key of type [null] (value [org.kohsuke.stapler.Stapler$1@1ae637f0]) and a value of type [java.text.SimpleDateFormat] (value [java
.text.SimpleDateFormat@c4dc9900]) but failed to remove it when the web application was stopped. This is very likely to create a memory leak.
Aug 10, 2010 12:45:35 PM org.apache.catalina.loader.WebappClassLoader clearThreadLocalMap
SEVERE: The web application [/hudson] created a ThreadLocal with key of type [java.lang.ThreadLocal] (value [java.lang.ThreadLocal@627b5c]) and a value of type [org.acegisecurity.context.Securit
yContextImpl] (value [org.acegisecurity.context.SecurityContextImpl@ffffffff: Null authentication]) but failed to remove it when the web application was stopped. This is very likely to create a
memory leak.
Aug 10, 2010 12:45:35 PM org.apache.catalina.loader.WebappClassLoader clearThreadLocalMap
SEVERE: The web application [/hudson] created a ThreadLocal with key of type [null] (value [org.kohsuke.stapler.Stapler$1@1ae637f0]) and a value of type [java.text.SimpleDateFormat] (value [java
.text.SimpleDateFormat@c4dc9900]) but failed to remove it when the web application was stopped. This is very likely to create a memory leak.
Aug 10, 2010 12:45:35 PM org.apache.catalina.loader.WebappClassLoader clearThreadLocalMap
SEVERE: The web application [/hudson] created a ThreadLocal with key of type [java.lang.ThreadLocal] (value [java.lang.ThreadLocal@627b5c]) and a value of type [org.acegisecurity.context.Securit
yContextImpl] (value [org.acegisecurity.context.SecurityContextImpl@ffffffff: Null authentication]) but failed to remove it when the web application was stopped. This is very likely to create a
memory leak.
Aug 10, 2010 12:45:35 PM org.apache.coyote.http11.Http11Protocol destroy

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Perhaps it is better to use WeakHashMaps in the said ThreadLocals?