Cookies were invented by Netscape as a debugging technique, but have
since become ubiquitous: all modern browsers, including MSIE, and
text browsers such as Lynx accept and store them. A cookie is, at
heart, a small piece of text -- a name and value pair -- that
the server side generates and sends to the client. The browser
remembers them (nontransient cookies are stored to your hard disk;
Netscape creates a file called cookies or
cookies.txt, for example). The browser then
sends them back to the server on any subsequent visit to a page from
the same site. The Cookie class is part of the
javax.servlet.http package, so any
servlet implementation will include it. The
constructor is passed a name and value, but there are other
parameters you can set. Most
important is the expiry time, which is
in seconds from the time you first send it. The default is -1; if the
value is negative, the cookie is not saved to disk; it becomes a
"transient cookie” that exists only
until the browser exits and is then forgotten. For cookies that are
stored to disk, the expiry time is converted to a base of January 1,
1970, the beginning of Unix time and of the modern computing era.
When the browser visits a site that has sent it a cookie or cookies,
it returns all of them as part of the HTTP headers. You retrieve them
all (as an array) using the getCookies( )
method, and iterate through them looking
for the one you want.
for (int i=0; i<mySiteCookies.length; i++) {
Cookie c = mySiteCookies[i];
if (c.getName( ).equals(name-you're-looking-for)) {
someString = c.getValue( );
break;
}
}Suppose you want the user to pick a favorite color for the servlet to
use as the background
color for pages from that
point on. Let’s use a name of prefs.bgcolor
for the color-coding cookie. The main
servlet is
CookieServlet, which checks for the cookie. If it
was not set previously, it jumps off to an HTML page, which will
eventually return here via another servlet. On the other hand, if the
color cookie was previously set, CookieServlet
(shown in Example 18-5) displays the welcome page
with the user’s color set.
Example 18-5. CookieServlet.java
import java.io.*;
import java.util.*;
import javax.servlet.*;
import javax.servlet.http.*;
/** Simple Cookie-based Page Color Display servlet demo.
*/
public class CookieServlet extends HttpServlet {
/** The preferences cookie name */
protected final static String PREFS_BGCOLOR = "prefs.bgcolor";
/** Where to go if we have not yet been customized. */
protected final static String CUSTOMIZER = "/ColorCustomize.html";
/** The user's chosen color, if any */
protected String faveColor = null;
public void doGet(HttpServletRequest request, HttpServletResponse response)
throws ServletException, IOException {
// Go through all the cookies we have, looking for a faveColor.
Cookie[] mySiteCookies = request.getCookies( );
for (int i=0; i<mySiteCookies.length; i++) {
Cookie c = mySiteCookies[i];
if (c.getName( ).equals(PREFS_BGCOLOR)) {
faveColor = c.getValue( );
break;
}
}
// if we did not find a faveColor in a cookie,
// punt to customization servlet to bake one up for us.
if (faveColor == null) {
ServletContext sc = getServletContext( );
// Requires Servlet API 2.1 or later!
// RequestDispatcher rd =
// sc.getRequestDispatcher(CUSTOMIZER");
//rd.forward(request, response);
// Do it the old way
response.sendRedirect(CUSTOMIZER);
}
// OK, we have a color, so we can do the page.
PrintWriter out = response.getWriter( );
response.setContentType("text/html");
out.println("<html><title>A Custom-Colored Page</title>");
out.print("<body bgcolor="");
out.print(faveColor);
out.println("">");
out.println("<P>Welcome! We hope you like your colored page!</P>");
out.println("</body></html>");
out.flush( );
}
}If the user has not yet set a color customization cookie, the
CookieServlet passes control (by sending an HTTP
redirect in the old API, or by use of a
ServletDispatcher
under the Servlet API 1.2 or
later) to this HTML page.
<BODY BGCOLOR="pink">
<H1>Please choose a color</H1>
<FORM ACTION="/servlet/ColorCustServlet" METHOD=GET>
<SELECT NAME="color_name">
<OPTION VALUE="green">Green</>
<OPTION VALUE="white" SELECTED>White</>
<OPTION VALUE="gray">Grey</>
</SELECT>
<INPUT TYPE="submit" VALUE="OK">
</FORM>Finally, the HTML page will jump to the customization servlet (Example 18-6), which contains the code shown here to save
the user’s preference as a cookie, and then return to the
CookieServlet by sending an HTTP
“redirect,” causing the browser to load the specified
replacement page.
Example 18-6. ColorCustServlet.java
import java.io.*;
import javax.servlet.*;
import javax.servlet.http.*;
/** Color customization servlet */
public class ColorCustServlet extends HttpServlet {
protected final static String DEFAULT_COLOR = "white";
protected String faveColor = DEFAULT_COLOR;
public void doGet(HttpServletRequest request, HttpServletResponse response)
throws IOException {
response.setContentType("text/html");
PrintWriter out = response.getWriter( );
String cand=request.getParameter("color_name");
if (cand != null) {
faveColor = cand;
Cookie c = new Cookie(CookieServlet.PREFS_BGCOLOR, faveColor);
c.setMaxAge(60*60*24*365);
response.addCookie(c);
}
response.sendRedirect("/servlet/CookieServlet");
}
}Of course, there are issues to consider when using cookies. Some users disable cookies out of justifiable fear that web sites will use them for gathering more information than a person might want to have known. In this case, our servlet would keep coming back to the customization page. It should probably have a warning to the effect that “cookies must be enabled to view this site.” Or you could use other techniques, such as session tracking (see Section 18.5).
And realistically, you probably want to keep more than one preference item for a user. If you let them set the screen background, you also need to set the text color, for example. It’s probably better to keep the preferences in a database on the server side, and just set a token that identifies the user (possibly the database primary key). Even then, remember that cookies can be altered! See Section 18.12 for a program to allow modification of the cookies stored on your hard drive.