Java servlets

Java servlets run as modules within an HTTP server
Servlets run as threads within a continuous Java process
The startup costs for a thread are lower than the startup costs for a process
Servlets are faster than the corresponding CGI script
Servlets have a standard API for accessing cookies, etc, just like Perl's CGI module and mod_perl module
Servlets also have session management
Servlets are Java programs, and can make use of any Java features

Servlet template


import java.io.*;
import javax.servlet.*;
import javax.servlet.http.*;

public class SomeServlet extends HttpServlet {
  public void doGet(HttpServletRequest request,
                    HttpServletResponse response)
      throws ServletException, IOException {
      
    // Use "request" to read incoming HTTP headers (e.g. cookies)
    // and HTML form data (e.g. data the user entered and submitted)
    
    // Use "response" to specify the HTTP response line and headers
    // (e.g. specifying the content type, setting cookies).
    
    PrintWriter out = response.getWriter();
    // Use "out" to send content to browser
  }
}

Servlet engines

The major Open Source servlet engine is "Jakarta Tomcat" available from www.apache.org
When unpacked the bin directory contains Unix shell and DOS batch files to start a server on port 8088
Compile the source, including the Apache the servlet.jar in the classpath
Java classes can be copied to a directory such as webapps/jan/WEB-INF/classes

HelloWorld servlet (plain text)

Servlet return

If a request is made to the server, it returns


telnet localhost 8088
Trying 127.0.0.1...
Connected to localhost.
Escape character is '^]'.
GET /jan/servlet/HelloWorld HTTP/1.1

HTTP/1.0 200 OK
Servlet-Engine: Tomcat Web Server/3.2.1 (JSP 1.1; Servlet 2.2; Java 1.4.2; Linux 2.6.10-1.12_FC2custom i386; java.vendor=Sun Microsystems Inc.)

Hello World
Connection closed by foreign host.

HelloWorld servlet (HTML)

Servlet return


telnet localhost 8088
Trying 127.0.0.1...
Connected to localhost.
Escape character is '^]'.
GET /jan/servlet/HelloWWW HTTP/1.1

HTTP/1.0 200 OK
Content-Type: text/html
Servlet-Engine: Tomcat Web Server/3.2.1 (JSP 1.1; Servlet 2.2; Java 1.4.2; Linux 2.6.10-1.12_FC2custom i386; java.vendor=Sun Microsystems Inc.)

<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN">
<HTML>
<HEAD><TITLE>Hello WWW</TITLE></HEAD>
<BODY>
<H1>Hello WWW</H1>
</BODY></HTML>
Connection closed by foreign host.

Deploying servlets

Deployment of servlets and JSP pages seems to me to be far more complex than it needs to be
Under the Tomcat home there is a directory webapps
Create your own subdirectories
- jan
- jan/jsp
- jan/WEB-INF
- jan/WEB-INF/classes
Compiled servlet files go into jan/WEB-INF/classes
JSP pages go into jan/jsp

A servlet such as HelloWWW is referenced by


      http://jan.newmarch.name:8080/jan/servlet/helloWWW

A JSP page such as hello.jsp is referenced by


      http://jan.newmarch.name:8080/jan/jsp/hello.jsp

A deployment descriptor must be created for servlets in the file jan/WEB-INF/web.xml to link servlet URLs and classes
That was the simple version - it can get much messier
When you modify a JSP page or a servlet class, you may need to restart Tomcat to register the changes

HTTP headers

HTTP headers can be found by

Servlet return

The output from a request by Firefox with language set to French, English US and then English is


Cache-Control max-age=0
Host localhost:8088
Accept text/xml,application/xml,application/xhtml+xml,text/html;q=0.9,text/plain;q=0.8,image/png,*/*;q=0.5
User-Agent Mozilla/5.0 (X11; U; Linux i686; en-US; rv:1.7.6) Gecko/20050225 Firefox/1.0.1
Keep-Alive 300
Accept-Language fr,en-us;q=0.7,en;q=0.3
Accept-Encoding gzip,deflate
Accept-Charset ISO-8859-1,utf-8;q=0.7,*;q=0.7
Connection keep-alive

Getting user locales

The following program uses the Accept-Language field to find possible locales:

Getting properties

The following program checks if there is a properties file matching any of the desired locales

Form parameters

Available in request.getParameter(String name)


package hall;

import java.io.*;
import javax.servlet.*;
import javax.servlet.http.*;
import java.util.*;

public class ThreeParams extends HttpServlet {
  public void doGet(HttpServletRequest request,
                    HttpServletResponse response)
      throws ServletException, IOException {
    response.setContentType("text/html");
    PrintWriter out = response.getWriter();
    String title = "Reading Three Request Parameters";
    out.println(ServletUtilities.headWithTitle(title) +
                "<BODY>\n" +
                "<H1 ALIGN=CENTER>" + title + "</H1>\n" +
                "<UL>\n" +
                "  <LI>param1: "
                + request.getParameter("param1") + "\n" +
                "  <LI>param2: "
                + request.getParameter("param2") + "\n" +
                "  <LI>param3: "
                + request.getParameter("param3") + "\n" +
                "</UL>\n" +
                "</BODY></HTML>");
  }

  public void doPost(HttpServletRequest request,
                     HttpServletResponse response)
      throws ServletException, IOException {
    doGet(request, response);
  }
}

Cookies

To set a cookie, use


      Cookie cookie = new Cookie("sessionID", "1234");
      response.addCookie(cookie);

To access a cookie, use


      Cookie[] cookies = request.getCookies();

Session tracking

There is a session tracking class so that you don't need to bother with the dtails of cookies, URL rewriting, etc
To get a session
```
      HttpSession session = request.getSession(true);
      
```
This creates a session object if one did not exist before, or returns one if it did exist before
Different sessions from different browsers will each get their own sesion object - they are not shared

To add to a session


      session.putValue(key, value);

To retrieve,
```
      session.getValue(key);
      
```
The retrieved value will need to be cast to the right type

Setting the charset

Set the charset of the response document before setting the output writer


      response.setContentType("text/html; charset=Big5");
      Writer out = response.getWriter();

This can be based on looking at the Accept-Charset HTTP parameter from the browser

Form parameter encoding

When you type characters into a Form, you use the input method for the browser in its locale
These characters get translated into the charset of the form's page
e.g. from http://tagunov.tripod.com/i18n/i18n.html
Let us assume you have the following form in your web page
a.jsp:

<FORM METHOD="GET" ACTION="b.jsp">
<INPUT TYPE="TEXT" NAME="n">
<INPUT TYPE="SUBMIT">
</FORM>
Let us assume that you enter two characters in the text field: a latin letter 'a' and a cyrillic letter 'a'.
This is what you will get depending on the character encoding that page a.jsp has been marked with (for details on the character encoding for a page see the beginning of this document):

character encoding of a.jsp result of submitting the query

KOI8-R b.jsp?n=a%C1

windows-1251 b.jsp?n=a%E0

UTF-8 b.jsp?n=a%D0%B0

character encoding of a.jsp	result of submitting the query
KOI8-R	b.jsp?n=a%C1
windows-1251	b.jsp?n=a%E0
UTF-8	b.jsp?n=a%D0%B0

Decode from e.g. windows-1251 by


    String original = request.getParameter(...);
    String converted = new String(original.getBytes("ISO-8859-1"),
                                  "windows-1251:);

Work out what the original encoding is by sending it as a "hidden form parameter" when you first send the form

Jan Newmarch (http://jan.newmarch.name)