Java Servlets

Intro

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

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
  }
}

HelloWorld servlet (plain text)


package hall;

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

public class HelloWorld extends HttpServlet {
  public void doGet(HttpServletRequest request,
                    HttpServletResponse response)
      throws ServletException, IOException {
    PrintWriter out = response.getWriter();
    out.println("Hello World");
  }
}

HelloWorld servlet (HTML)


package hall;

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

public class HelloWWW extends HttpServlet {
  public void doGet(HttpServletRequest request,
                    HttpServletResponse response)
      throws ServletException, IOException {
    response.setContentType("text/html");
    PrintWriter out = response.getWriter();
    out.println("<!DOCTYPE HTML PUBLIC \"-//W3C//DTD HTML 4.0 " +
                                        "Transitional//EN\">\n" +
                "<HTML>\n" +
                "<HEAD><TITLE>Hello WWW</TITLE></HEAD>\n" +
                "<BODY>\n" +
                "<H1>Hello WWW</H1>\n" +
                "</BODY></HTML>");
  }
}

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

Jan Newmarch (http://pandonia.canberra.edu.au)