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From "Nick Williams (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (LOG4J2-479) Use of InheritableThreadLocal in Map ThreadContext is dangerous and unhelpful
Date Fri, 03 Jan 2014 17:41:58 GMT

    [ https://issues.apache.org/jira/browse/LOG4J2-479?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13861703#comment-13861703

Nick Williams commented on LOG4J2-479:

In my opinion, we need to take one of two paths:

#Remove uses of {{InheritableThreadLocal}} completely.
#Make a system option to enable use of {{InheritableThreadLocal}}, but *disable it by default*
so that values are not inherited unless specifically enabled.

In applications that use thread pools to spin off long-running processes (and this is a lot
of applications), an {{InheritableThreadLocal}} is *dangerous*. Before the thread pool is
used for the first time, it is initially empty. It hasn't yet created any threads. If a thread
sets the value in an {{InheritableThreadLocal}} and then borrows a thread from that pool,
the pool will create a thread with the same {{InheritableThreadLocal}} values. That thread
will then always have that value, even after the application returns it to the pool. Even
worse, in a web application multiple applications could share a single thread pool, resulting
in information leaking from one application to another.

> Use of InheritableThreadLocal in Map ThreadContext is dangerous and unhelpful
> -----------------------------------------------------------------------------
>                 Key: LOG4J2-479
>                 URL: https://issues.apache.org/jira/browse/LOG4J2-479
>             Project: Log4j 2
>          Issue Type: Bug
>            Reporter: MK
> Described here http://logging.apache.org/log4j/2.x/manual/thread-context.html
> The use of InheritableThreadLocal creates subtle and hard to track bugs while not really
adding much useful.  It is counterintuitive -- I don't see why would anyone expect logging
context to be inherited.  But it breaks down completely when used with Thread Executors.

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