The concurrent library is developed by Doug Lea on http://gee.cs.oswego.edu/dl/classes/EDU/oswego/cs/dl/util/concurrent/intro.html

Here are some Release Notes:
All classes are released to the public domain and may be used for any purpose whatsoever without permission 
or acknowledgment. Portions of the CopyOnWriteArrayList and ConcurrentReaderHashMap classes are adapted from 
Sun JDK source code. These are copyright of Sun Microsystems, Inc, and are used with their kind permission,  
as described in this license.

You don't need a license to use the concurrent library, here is an answer for such question from Doug Lea:
Do I need a license to use it? Can I get one?
    No! 