HTTP and Java

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



/**
 * SimpleURL.java
 */

import java.net.*;
import java.io.*;

public class SimpleURL{
    public SimpleURL (){
	
    }
    
    public static void main(String[] args){
	if (args.length != 1) {
	    System.err.println("Usage: java SimpleURL url");
	    System.exit(1);
	}

	URL url = null;
	try {
	    url = new URL(args[0]);
	} catch(MalformedURLException e) {
	    e.printStackTrace();
	    System.exit(1);
	}
	Object content = null;
	try {
	    content = url.getContent();
	} catch(IOException e) {
	    e.printStackTrace();
	    System.exit(2);
	}
	// The content is actually a subclass of InputStream
	BufferedReader reader = null;
	reader = new BufferedReader(new InputStreamReader((InputStream) content));
	String line;
	try {
	    while ((line = reader.readLine()) != null) {
		System.out.println(line);
	    }
	} catch (IOException e) {
	    e.printStackTrace();
	    System.exit(3);
	}
    }
} // SimpleURL

URLConnection

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


/**
 * URLInfo.java
 */
import java.net.*;
import java.io.*;

public class URLInfo{
    public URLInfo (){
	
    }
    
    public static void main(String[] args){
	URL url = null;
	try {
	    url = new URL("http://localhost/ecommerce/cgiapps.gif");
	} catch(MalformedURLException e) {
	    e.printStackTrace();
	    System.exit(1);
	}
	URLConnection connection = null;
	InputStream str = null;
	try {
	    connection = url.openConnection();
	    connection.connect();
	} catch(IOException e) {
	    e.printStackTrace();
	    System.exit(1);
	}
	System.out.println("Type " + connection.getContentType());
	System.out.println("Length " + connection.getContentLength());

    }
} // URLInfo

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


/**
 * CGI.java
 */
import java.net.*;
import java.io.*;

public class CGI{
    public CGI (){
	
    }
    
    public static void main(String[] args){
	URL url = null;
	try {
	    url = new URL("http://localhost/cgi-bin/test-cgi");
	} catch(MalformedURLException e) {
	    e.printStackTrace();
	    System.exit(1);
	}
	URLConnection connection = null;
	InputStream istr = null;
	OutputStream ostr = null;
	try {
	    connection = url.openConnection();
	    // doOutput must be set before connecting
	    connection.setDoOutput(true);
	    connection.connect();
	    ostr = connection.getOutputStream();
	} catch(IOException e) {
	    e.printStackTrace();
	    System.exit(1);
	}

	DataOutputStream writer = new DataOutputStream(ostr);
	try {
	    writer.writeBytes("name=jan\n");
	    writer.flush();
	    writer.close();
	} catch(IOException e) {
	    e.printStackTrace();
	    System.exit(1);
	}

	// After writing is over, we can read the reply
	try {
	    istr = connection.getInputStream();
	} catch(IOException e) {
	    e.printStackTrace();
	    System.exit(1);
	}
	BufferedReader reader = new BufferedReader(new InputStreamReader(istr));
	String line;
        try {
            while ((line = reader.readLine()) != null) {
                System.out.println(line);
            }
        } catch (IOException e) {
            e.printStackTrace();
            System.exit(3);
        }

    }
} // CGI

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




import java.awt.event.WindowAdapter;
import java.awt.event.WindowEvent;
import javax.swing.JFrame;
import javax.swing.JPanel;
import javax.swing.JScrollPane;
import javax.swing.JEditorPane;
import java.awt.event.ActionEvent;
import javax.swing.AbstractAction;
import javax.swing.text.html.HTMLEditorKit;
import javax.swing.text.html.HTMLDocument;
import java.io.StringReader;

/**
 * Browser.java
 *
 *
 * Created: Mon Aug 21 22:30:28 2006
 *
 * @author <a href="mailto:jan.newmarch@infotech.monash.edu.au">Jan Newmarch</a>
 * @version 1.0
 */

public class Browser extends JFrame{

    public static void main(String[] args) throws java.io.IOException {
	Browser f = new Browser();
	f.show();
    }
    
    public Browser() throws java.io.IOException {
	super("Browser");
	setSize(300, 300);
	addWindowListener(new WindowAdapter() {
		public void windowClosing(WindowEvent e) {System.exit(0);}
		public void windowOpened(WindowEvent e) {}
	    });

	JEditorPane jep = new JEditorPane();
	jep.setEditable(false);
	HTMLEditorKit htmlKit = (HTMLEditorKit) (jep.getEditorKitForContentType("text/html"));
	HTMLDocument doc = (HTMLDocument) htmlKit.createDefaultDocument();
	jep.setEditorKit(htmlKit);
	StringReader sr = new StringReader("<h1>HTML document</h1> <p> text </p>");
	jep.read(sr, doc);
	
	
	getContentPane().add(new JScrollPane(jep));
    }

} // Browser

HTTP server

Java has no explicit support for writing an HTTP server

Here is an "echo server"




import java.net.*;
import java.io.*;

public class EchoServer {

	final static protected int PORT = 8001;

 	public static void main(String[] argv) {
		try {
			new EchoServer();
		} catch(IOException e) {
			e.printStackTrace();
		}
	}

	public EchoServer() throws IOException {
		ServerSocket server = new ServerSocket(PORT);
		Socket socket = server.accept();

		InputStream is = socket.getInputStream();
		OutputStream os = socket.getOutputStream();
		DataOutputStream out = new DataOutputStream(os);
		BufferedReader in = new BufferedReader(new InputStreamReader(is));

		out.writeBytes("HTTP/1.1 404 OK\n");
		out.writeBytes("Content-Type: text/plain\n\n");

		String line;

		line = in.readline();
		// check that line is GET url HTTP/1.0
		// \extract url
		// find the file for that url
		// open the file
 		// write it to the out stream

		while ((line = in.readLine()) != null &&
			(line.length() != 0)) {
			out.writeBytes(line + "\n");
		}
		out.close();
		socket.close();
	}
}

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)


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)


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();
	String docType = 
		"<!DOCTYPE HTML PUBLIC \"-//W3C/DTD HTML 4.0 " +
		"Transitional//EN\"\n";
	out.println(docType +
		    "<HTML>\n" +
		    "<head><title>Hello WWW</title></head>\n" +
		    "<body>\n" +
		    "<h1> Hello WWW</h1>" +
		    "</body></html>");
    }
}

Form parameters

Available in request.getParameter(String name)


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) +
                "&lt;BODY&gt;\n" +
                "&lt;H1 ALIGN=CENTER&gt;" + title + "&lt;/H1&gt;\n" +
                "&lt;UL&gt;\n" +
                "  &lt;LI&gt;param1: "
                + request.getParameter("param1") + "\n" +
                "  &lt;LI&gt;param2: "
                + request.getParameter("param2") + "\n" +
                "  &lt;LI&gt;param3: "
                + request.getParameter("param3") + "\n" +
                "&lt;/UL&gt;\n" +
                "&lt;/BODY&gt;&lt;/HTML&gt;");
  }

  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://jan.newmarch.name)

This work is licensed under a Creative Commons License
The moral right of Jan Newmarch to be identified as the author of this page has been asserted.