import javax.servlet.*;
import javax.servlet.http.*;
import java.io.*;
/** A quiz-show "buzzer" servlet: the first respondant wins the chance
* to answer the skill-testing question.
* <p>
* Previous versions of this code used shared static variables, but this
* is not reliable, since most web engines now use custom class loaders
* that may load a servlet class more than once. The "right" way is to
* synchronize on an object stored in the Servlet Application Context.
*/
public class BuzzInServlet extends HttpServlet {
/** The attribute name used throughout. */
protected final static String WINNER = "buzzin.winner";
/** doGet is called from the contestants web page.
* Uses a synchronized code block to ensure that
* only one contestant can change the state of "buzzed".
*/
public void doGet(HttpServletRequest request, HttpServletResponse response)
throws ServletException, IOException
{
ServletContext application = getServletContext();
boolean iWon = false;
String user = request.getRemoteHost() + '@' + request.getRemoteAddr();
// Do the synchronized stuff first, and all in one place.
synchronized(application) {
if (application.getAttribute(WINNER) == null) {
application.setAttribute(WINNER, user);
application.log("BuzzInServlet: WINNER " + user);
iWon = true;
}
}
response.setContentType("text/html");
PrintWriter out = response.getWriter();
out.println("<html><head><title>Thanks for playing</title></head>");
out.println("<body bgcolor=\"white\">");
if (iWon) {
out.println("<b>YOU GOT IT</b>");
// TODO - output HTML to play a sound file :-)
} else {
out.println("Thanks for playing, " + request.getRemoteAddr());
out.println(", but " + application.getAttribute(WINNER) +
" buzzed in first");
}
out.println("</body></html>");
}
/** The Post method is used from an Administrator page (which should
* only be installed in the instructor/host's localweb directory).
* Post is used for administrative functions:
* 1) to display the winner;
* 2) to reset the buzzer for the next question.
* <p>
* In real life the password would come from a Servlet Parameter
* or a configuration file, instead of being hardcoded in an "if".
*/
public void doPost(HttpServletRequest request, HttpServletResponse response)
throws ServletException, IOException
{
ServletContext application = getServletContext();
response.setContentType("text/html");
PrintWriter out = response.getWriter();
if (request.getParameter("password").equals("syzzy")) {
out.println("<html><head><title>Welcome back, host</title><head>");
out.println("<body bgcolor=\"white\">");
String command = request.getParameter("command");
if (command.equals("reset")) {
// Synchronize what you need, no more, no less.
synchronized(application) {
application.setAttribute(WINNER, null);
}
out.println("RESET");
} else if (command.equals("show")) {
String winner = null;
synchronized(application) {
winner = (String)application.getAttribute(WINNER);
}
if (winner == null) {
out.println("<b>No winner yet!</b>");
} else {
out.println("<b>Winner is: </b>" + winner);
}
}
else {
out.println("<html><head><title>ERROR</title><head>");
out.println("<body bgcolor=\"white\">");
out.println("ERROR: Command " + command + " invalid.");
}
} else {
out.println("<html><head><title>Nice try, but... </title><head>");
out.println("<body bgcolor=\"white\">");
out.println(
"Your paltry attempts to breach security are rebuffed!");
}
out.println("</body></html>");
}
}
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