Why the "response.addCookie(..)" can't work in the filter?

[Migrated From Jforum.net]

Hello,everybody!

The problem is that:

I want to add/refresh a cookie to response after processed ervery page, so
I put a servlet filter in the web.xml, the web.xml looks like that:

<?xml version="1.0" encoding="UTF-8"?>
<web-app version="2.4" 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 
	http://java.sun.com/xml/ns/j2ee/web-app_2_4.xsd">
	
	<display-name>JForum - Powering Communities</display-name>
	<description>Open Source Java Forum Software</description>

	<filter>
		<filter-name>SSOFilter</filter-name>
		<filter-class>cn.com.ecoll.web.SSOFilter</filter-class>
	</filter>
	<filter-mapping>
		<filter-name>SSOFilter</filter-name>
		<url-pattern>*.page</url-pattern>
	</filter-mapping>
	<!-- Clickstream -->
	<filter>
		<filter-name>clickstream-jforum</filter-name>
		<filter-class>
			net.jforum.util.legacy.clickstream.ClickstreamFilter
		</filter-class>
	</filter>

	<filter-mapping>
		<filter-name>clickstream-jforum</filter-name>
		<url-pattern>*.page</url-pattern>
	</filter-mapping>

	<listener>
		<listener-class>net.jforum.ForumSessionListener</listener-class>
	</listener>

	<!-- JForum Controller -->
	<servlet>
		<servlet-name>jforum</servlet-name>
		<servlet-class>net.jforum.JForum</servlet-class>

		<init-param>
			<param-name>development</param-name>
			<param-value>true</param-value>
		</init-param>
	</servlet>

	<!-- Installer -->
	<servlet>
		<servlet-name>install</servlet-name>
		<servlet-class>net.jforum.InstallServlet</servlet-class>

		<init-param>
			<param-name>development</param-name>
			<param-value>true</param-value>
		</init-param>
	</servlet>

	<!-- AJAX -->
	<servlet>
		<servlet-name>dwr-invoker</servlet-name>
		<servlet-class>uk.ltd.getahead.dwr.DWRServlet</servlet-class>

		<init-param>
			<param-name>configJForum</param-name>
			<param-value>WEB-INF/dwr-jforum.xml</param-value>
		</init-param>

		<init-param>
			<param-name>debug</param-name>
			<param-value>false</param-value>
		</init-param>
		<init-param>
			<param-name>logLevel</param-name>
			<param-value>ERROR</param-value>
		</init-param>
	</servlet>

	<!-- Mapping -->
	<servlet-mapping>
		<servlet-name>install</servlet-name>
		<url-pattern>/install/install.page</url-pattern>
	</servlet-mapping>

	<servlet-mapping>
		<servlet-name>dwr-invoker</servlet-name>
		<url-pattern>/dwr/*</url-pattern>
	</servlet-mapping>

	<servlet-mapping>
		<servlet-name>jforum</servlet-name>
		<url-pattern>*.page</url-pattern>
	</servlet-mapping>

	<!-- SESSION -->
	<session-config>
		<session-timeout>10</session-timeout>
	</session-config>

	<welcome-file-list>
		<welcome-file>index.htm</welcome-file>
	</welcome-file-list>

</web-app>

and the SSOFilter.java is below:

import java.io.*;
import javax.servlet.*;
import javax.servlet.http.*;
import javax.servlet.Filter;
import javax.servlet.FilterChain;
import javax.servlet.FilterConfig;
import javax.servlet.ServletContext;
import javax.servlet.ServletException;
import javax.servlet.ServletRequest;
import javax.servlet.ServletResponse;
import org.apache.commons.logging.*;

import cn.com.ecoll.auth.*;

public class SSOFilter implements Filter {
	Log logger = LogFactory.getLog(SSOFilter.class);
	
	private static String cookieName="cookieName";
	public void init(FilterConfig filterConfig) {
//		cookieName=filterConfig.getInitParameter("cookieName");
		cookieName=AuthConfiguration.getInstance().getCookieName();
		logger.info("cookieName:"+cookieName);
	}

	public void doFilter(ServletRequest req, ServletResponse res,
			FilterChain chain) throws IOException,ServletException {
		
		HttpServletRequest request = (HttpServletRequest) req;
		HttpServletResponse response = (HttpServletResponse) res;
		
// do someting before the *.page processing
// .....
// .....
		
		chain.doFilter(req, res);
		
                 //do someting after the *.page processing	
                 // .....
                 // .....
		

                 // there I want to add/refresh a cookie after the page processing 
                  response.addCookie(new Cookie("filtercookie","filterCookieadd"));
		
				
	}

	public void destroy() {

	}
}

But there is no cookie in the Browser after refresh the page. Howerver, these codes work well in other web applications.
why is it? and what should I do to add a cookie after the *.page processing.

Thanks.
[originally posted on jforum.net by aaron]

[Migrated From Jforum.net]

Adding a cookie requires modifying the HTTP headers of the response. Once the response has been "committed", e.g. anything written to the output stream, you can not modify the HTTP headers. The response has been committed by the time you try to add your cookie.
[originally posted on jforum.net by monroe]

[Migrated From Jforum.net]

thans for the reply.
yes, I put the "addCookie" before "chain.filter(...)"
and it works.
and the only problem is:
why it can works in some web application?
[originally posted on jforum.net by aaron]

[Migrated From Jforum.net]

Most JSP implimentations and some servlets will actually generate the response in a buffer. This means the response isn't committed the output is closed or manually flushed. So, this would work in some applications and not in others.

I imagine that the either jForum or the Template engine does a specific flush of the output stream at some point.

FWIW, there is also a JSP page directive to set the buffer size and turn on autoflush. These are useful for long running pages that you want users to see some status info coming back. Your cookie setting method would fail for pages like these too.

[originally posted on jforum.net by monroe]