If you specify a string value with the syntax in the Summary, 
DataSourceUtil.getDataSource() will throw an exception if it's not a valid 
JNDI path. The call to DataSourceUtil.setUsingParams() is never made.

Here's the pertinent code from DataSourceUtil:

if (rawDataSource instanceof String) {
            try {
                Context ctx = new InitialContext();
                // relative to standard JNDI root for J2EE app
                Context envCtx = (Context) ctx.lookup("java:comp/env");
                dataSource = (DataSource) envCtx.lookup((String)rawDataSource);
            } catch (NamingException ex) {
                throw new JspException(ex.toString(), ex);
            }
            if (dataSource == null) {
                setUsingParams((String)rawDataSource);
            }
        }