What happens to JBoss if I restart the database?

You can configure the following in your -ds.xml

<valid-connection-checker-class-name>[see [jboss]/docs/examples/jca/[mydb]-ds.xml]</valid-connection-checker-class-name>

JBoss will use the jdbc drivers ping operation to check the connection.  If the ping fails the connection will be destroyed/closed and new ones will be created. The valid-connection-checker-class-name must implement the following ValidConnectionChecker interface:

/*
* JBoss, Home of Professional Open Source
* Copyright 2005, JBoss Inc., and individual contributors as indicated
* by the @authors tag. See the copyright.txt in the distribution for a
* full listing of individual contributors.
*
* This is free software; you can redistribute it and/or modify it
* under the terms of the GNU Lesser General Public License as
* published by the Free Software Foundation; either version 2.1 of
* the License, or (at your option) any later version.
*
* This software is distributed in the hope that it will be useful,
* but WITHOUT ANY WARRANTY; without even the implied warranty of
* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
* Lesser General Public License for more details.
*
* You should have received a copy of the GNU Lesser General Public
* License along with this software; if not, write to the Free
* Software Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA
* 02110-1301 USA, or see the FSF site: http://www.fsf.org.
*/
package org.jboss.resource.adapter.jdbc;

import java.sql.Connection;
import java.sql.SQLException;

/**
 * Checks that a connection is valid
 *
 * @author <a href="mailto:adrian@jboss.org">Adrian Brock</a>
 * @version $Revision: 1.3.4.1 $
 */
public interface ValidConnectionChecker
{
   /**
    * Checks the connection is valid
    *
    * @param c the connection
    * @return Exception when not valid, null when valid
    */
   SQLException isValidConnection(Connection c);
}

OR you can use:

<check-valid-connection-sql>some cheap sql statement</check-valid-connection-sql>
e.g. for Oracle "select 1 from dual"

JBoss will run the SQL statement before handing out the connection from the pool. If the SQL fails, the connection will be destroyed/closed and new ones will be created.

<check-valid-connection> SQL for Sybase:

If you are using

select @@VERSION

with Sybase, then it may not always work. See this for details http://community.jboss.org/thread/156667?tstart=0