Finally, I got the reason:
I have to put <%@ page isELIgnored="false" %> in jsp to make EL work when My web.xml is this:
<!DOCTYPE web-app
PUBLIC "-//Sun Microsystems, Inc.//DTD Web Application 2.3//EN" "http://java.sun.com/dtd/web-app_2_3.dtd">
When I update my web.xml to 1.4, EL is enable by default, i.e. I don't have to put <%@ page isELIgnored="false" %> in jsp.
<web-app
xmlns="http://java.sun.com/xml/ns/j2ee"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xsi:schemaLocation="http://java.sun.com/xml/ns/j2ee/web-app_2_4.xsd" version="2.4">