When placing a JDBC URL into the /WEB-INF/web.xml configuration file, make sure to URL-encode the connection string (especially the ampersand symbol):
<context-param>
<param-name>ConnectionString</param-name>
<param-value>jdbc:mysql://127.0.0.1/?user=java&password=java</param-value>
</context-param>
To access this variable simply use the ServletContext class:
request.getServletContext().getInitParameter("ConnectionString")