From: <glassfish_at_javadesktop.org>
Date: Sat, 18 Sep 2010 21:48:20 PDT

MySQL has a timeout of 8 hours (after 8 hours of connection idleness they will be removed).

You can set your connection-pool validation type to be "table" and provide a valid "table-name" so that table based validation is used. This might be of help.
http://blogs.sun.com/JagadishPrasath/entry/connection_validation_in_glassfish_jdbc
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