JavaServer Pages, 3rd Edition

Buffering

There’s one important thing about how a JSP page is processed that has not been covered in any example so far: buffering of the response body. As you may recall from Chapter 2, an HTTP response message contains both headers and a body. The headers tell the browser such things as what type of data the body contains (HTML text, an image), the size of the body, if the body can be cached, and so forth. Headers are also used to set cookies and to tell the browser to automatically get another page (a redirect). All response headers must be sent to the browser before the body is sent.

As soon as a JSP page writes something to the body of the message, the JSP container may start sending the response to the browser. It’s then too late to set headers, since they have to be sent first. In a servlet, you have full control over when something is written to the response body, so you can make sure that you set all headers you need before you generate the body. In a JSP page, however, it’s not that easy. Everything you put in a JSP page that is not a JSP element is written to the response body automatically by the JSP container. Here’s the top part of the autheticate.jsp page from Chapter 13:

<%@ taglib prefix="c" uri="http://java.sun.com/jsp/jstl/core" %>
<%@ taglib prefix="sql" uri="http://java.sun.com/jsp/jstl/sql" %>
<%@ taglib prefix="ora" uri="orataglib" %>
  
<%-- Remove the validUser session bean, if any --%>
<c:remove var="validUser" />
...

It doesn’t contain any HTML, so you may think ...