<?xml version="1.0" encoding="utf-8"?>

<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
          http://java.sun.com/xml/ns/j2ee/web-app_2_4.xsd">

  <display-name>Servlet Testing</display-name>
  <description>
    This web application demonstrates the usage of Servlet/JSP (mainly Servlet).
    The contents are composed while reading Core Servlet and JSP Volume 2.
  </description>

  <!-- the following definition seems no effect... -->
  <icon>
    <small-icon>/small.png</small-icon>
    <large-icon>/large.gif</large-icon>
  </icon>

  <!-- the following definition seems no effect... -->
  <mime-mapping>
    <extension>png</extension>
    <mime-type>text/plain</mime-type>
  </mime-mapping>

  <context-param>
    <param-name>host</param-name>
    <param-value>nj-kelvin-hu</param-value>
  </context-param>

  <error-page>
    <error-code>404</error-code>
    <location>/404.jsp</location>
  </error-page>

  <error-page>
    <exception-type>java.lang.Exception</exception-type>
    <location>/exception.jsp</location>
  </error-page>

  <listener>
    <listener-class>ini.kelvin.servlet.ContextListener</listener-class>
  </listener>

  <listener>
    <listener-class>ini.kelvin.servlet.SessionListener</listener-class>
  </listener>

  <filter>
    <filter-name>print</filter-name>
    <filter-class>ini.kelvin.servlet.PrintFilter</filter-class>
    <init-param>
      <param-name>admin</param-name>
      <param-value>Kelvin Hu</param-value>
    </init-param>
  </filter>
  <filter-mapping>
    <filter-name>print</filter-name>
    <servlet-name>Hello</servlet-name>
  </filter-mapping>
  <filter-mapping>
    <filter-name>print</filter-name>
    <url-pattern>/*</url-pattern>
  </filter-mapping>

  <servlet>
    <servlet-name>Hello</servlet-name>
    <servlet-class>ini.kelvin.servlet.HelloWorldServlet</servlet-class>
    <init-param>
      <param-name>name</param-name>
      <param-value>Kelvin Hu</param-value>
    </init-param>
  </servlet>
  <servlet-mapping>
    <servlet-name>Hello</servlet-name>
    <url-pattern>/hello</url-pattern>
  </servlet-mapping>

  <welcome-file-list>
    <welcome-file>index.jsp</welcome-file>
    <welcome-file>index.html</welcome-file>
    <welcome-file>default.jsp</welcome-file>
  </welcome-file-list>

  <session-config>
    <session-timeout>1</session-timeout>
  </session-config>

</web-app>

The value of url-pattern must start with / or *..

Matching Overlapping Patterns:

1. Exact matches handled first: http://.../foo/bar will be handled by /foo/bar, not /foo/*, also, /foo/bar.html will win over *.html, if the URL is http://.../foo/bar.html.
2. Directory mapping are preferred over extension mappings: the URL http://.../foo/bar.html will be handled by /foo/*, not the pattern *.html.
3. For overlapping directory mappings, the longest path is preferred: http://.../foo/bar/baz.html will be handled by /foo/bar/*, not /foo/*.

Parameters defined in <init-param> could be read by the method getInitParameter() of ServletConfig.

<load-on-startup> tells server the sequence that the servlet should be loaded into memory, lower number goes first, if two servlets have the same number, the load sequence is decided by server. Negative number cannot guarantee the servlet loading at startup.

When defining the <location> of <error-page>, be careful of the leading slash, so <location>/404.jsp</location> will work, but if the slash / removed, 404.jsp will be not found.

1. Accept: specify the MIME types that the browser can handle
2. Accept-Charset: the character charset
3. Accept-Encoding: the encoding type, the most commonly used is "gzip, deflate"
4. Accept-Language: the language the browser excepted
5. Authorization: used to identify the browser when visiting some pages with password protection
6. Connection: to indicate if the browser can handle continuous HTTP connection, continuous connection can transfer several files(html, images, etc) in one socket, saved the cost of establishing multiple connections
7. Content-Length: only used for POST, specify the length of POST request
8. Cookie: return cookies back to http server, these cookies are formerly sent to browser by http server
9. Host: the host name and port number in the requested URL
10. If-Modified-Since: indicate the requested page is wanted only if the page has been modified after the specified date, otherwise the http server will send 304(Not Modified) instead, servlet should not handle this header directly, but implement getLastModified() method, let the server to handle the date comparison
11. If-Unmodified-Since: the opposite one of If-Modified-Since
12. Referer: the URL which referenced current page
13. User-Agent: to identify the client browser which sent the request

1. Allow: specify request methods the server supported
2. Cache-Control: tell client how to cache the document, can be set with the following values:
   • public: the document can be cached
   • private: the document can only be cached in private cache for single user
   • no-cache: do not cache the document (old browsers use Pragma, so this header should also be set to no-cache for old browsers)
   • no-store: do not cache, even do not store in temp folder on local disk
   • must-revalidate: client must communicate with server to validate the document everytime when using it
   • proxy-revalidate: similar to previous one, only adapted to shared cache
   • max-age=XXX: the document will be invalid after XXX seconds, it is the replacement of header Expires, and it has higher priority when both of them exist
   • s-max-age=XXX: the shared cache should make the document invalid after XXX seconds
3. Connection: used for continuouse connection, the value "close" tells the browser not to use continuous connection, default is continuous connection

4. Content-Disposition: make the browser to ask user to store the response with the specified name on disk, as below:

   Content-Disposition: attachment; filename=some-file-name
   
   

5. Content-Encoding: the encoding used during response transmission
6. Content-Language: the language response used
7. Content-Length: the byte count of response, only used for continuous connection

8. Content-Type: the MIME of response, example:

   Content-Type: text/html; charset=utf-8
   
   

9. Expires: see Cache-Control and Pragma
10. Last-Modified: the last modified time of the document
11. Location: required when status code is between 300 and 399, used to notify the browser where the document is stored, the browser will automatically connect to the new address and get the new document
12. Pragma: see Cache-Control and Expires

13. Refresh: tell browser to send request for the newest page after the interval, example use:

    Refresh: 5; URL=http://host/path
    
    

14. Retry-After: used with status code 503, tell the client to retry after the interval
15. Set-Cookie: specify a cookie, every cookie needs a stand alone header
16. WWW-Authenticate: used with 401, tell browser the authentication type and domain needed in request header Authorization