• Purpose of Cookies in HTTP:
  • Allow for the preservation of state information
• Internals:
  • Name-Value pairs
• Transmission:
  • In a Request: Use a Cookie header
  • In a Response: Use a Set-Cookie header

• The Class:
  • HttpCookie
• Obvious Methods:
  • getName()
  • getValue()
• Methods for Retrieving Attributes:
  • getDiscard()
  • getDomain()
  • getMaxAge()
  • getPath()

• The CookieHandler Interface:
  • get() gets all applicable cookies from the cache
  • put() sets all applicable cookies
  • set() is a static method that sets the universal cookie handler for all subsequent HTTP connections
• The CookieManager Class:
  • A concrete implementation provides security and uses a CookieStore to save/retrieve the cookies
• The CookieStore Interface:
  • The requirements of an object that can save/retrieve cookies

Display the Cookies Set by an HTTP Response

javaexamples/internet/DisplayCookies.java

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

/**
 * An application that reports on the cookies that are written in response
 * to an HTTP GET request.
 * 
 * @author  Prof. David Bernstein, James Madison University
 * @version 1.0
 */
public class DisplayCookies
{
    /**
     * The entry point of the application.
     * 
     * @param args Command-line arguments (args[0] is the URL)
     */
    public static void main(String[] args)
    {
        CookieManager   manager;
        CookieStore     store;
        String  arg;
        URL    url;

        // Process the command line argument
        if ((args == null) || (args.length == 0)) arg = "https://www.google.com/";
        else                                      arg = args[0];

        try
        {
            // Construct the URL
            url = new URL(arg);

            // Set the (universal) CookieHandler
            manager = new CookieManager();
            CookieHandler.setDefault(manager);
            
            // Get the CookieStore (which does not provide 
            // persistence by default)
            store = manager.getCookieStore();
            
            try
            {
                // Send the HTTP GET request. The response will
                // cause a call-back to the CookieHandler.
                InputStream in = url.openStream();
                in.close();
                
                // Print the cookies
                List<HttpCookie> cookies = store.getCookies();
                System.out.println("Cookies from " + arg);
                for (HttpCookie c: cookies)
                {
                    System.out.println(c);
                }
            }
            catch (IOException ioe)
            {
                System.out.println("Unable to make a connection.");
            }
        }
        catch (MalformedURLException mue)
        {
            System.out.println("Invalid URL: " + arg);
        }
    }
}
        

• What's Needed:
  • A class that implements CookieStore (and probably Runnable for reasons discussed below)
• When to Load/Save?
  • Loading should happen at construction time
  • Saving depends on the application
• Ensuring a Save at Shutdown:
  • Call Runtime.getRuntime().addShutdownHook() and pass it a Thread (with an associated Runnable)