URL

The class java.net.URL is designed to make it easier to handle fetching url data. You don't have to open sockets yourself

URLConnection

If you want to get extra information about the url, the class URLConnection can be used

Forms

HTTP connections write information from client to HTTP server and read the reply. This can be used to fetch static URLs (as above). It can also be used to post form data and read the reply. The URLConnection can be used for this

Authorization

• To access a password-protected site, you need to send the user name and password
• The article http://www.javaworld.com/javaworld/javatips/jw-javatip46.html describes how to do this

Proxies

• To fetch a url using a proxy, you send the HTTP request to the proxy
• The full url of the destination must be given
• e.g. using telnet:

  
        telnet proxy.monash.edu.au 8080
        GET http://jan.newmarch.name/index.html HTTP/1.0
        

• Java has builtin support for proxies; you only need to define some properties

  
        java -DproxySet=true \
             -DproxyHost=proxy.monash.edu.au \
             -DproxyPort=8080 \
             SimpleURL http://jan.newmarch.name/index.html
        

Proxy Authorization

• Some proxies (e.g. the Monash proxy) require authentication before passing on requests
• The following is supposed to handle this

  
        String password = "newmarch:xxxxxxx";
        String encPasswd = new sun.misc.BASE64Encoder().encode(password.getBytes
  ());
        connect.setRequestProperty("Proxy-Authorization", "Basic " + encPasswd);
        

Displaying URLs

Some classes that are useful on the client side for displaying documents

• JLabel can be used to display images such as GIF images
• JEditorPane can be used to display HTML documents using HTML 3.2

HTTP server

• Java has no explicit support for writing an HTTP server
• Here is an "echo server"

Server-side processing - 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

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)

HelloWorld servlet (HTML)

Form parameters

• Available in request.getParameter(String name)

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