Name: ________________________________________________________________
UD Email: _____________________________________________@ udel.edu
About this exam
A hint about allocating your time:
How to get a good grade:
| 1 | 2 | 3 | 4 | 5 | 6 |
(25 pts) Below are two ways that one could do a query in JDBC.
Both have been tested, and correctly return the result set.
Explain as clearly as possible which way is preferred, and precisely why.
// Create a statement for SQL queries
try {
myStmt = myCon.createStatement();
}
catch (Exception e){
e.printStackTrace();
return brands;
}
// Set up the query.
String tableName = prefix_ + "Beers";
String query = "SELECT beer, color FROM " + tableName
+ " WHERE color='" + color + "'";
ResultSet rs;
try {
rs = myStmt.executeQuery(query);
}
catch (Exception e) {
e.printStackTrace();
return brands;
}
// Set up the query.
String tableName = prefix_ + "Beers";
String query = "SELECT beer, color FROM " + tableName + " WHERE color=?";
// Stick the value of "color" in where the first ? appears, and exec the query
ResultSet rs;
try {
PreparedStatement prepStmt = myCon.prepareStatement(query);
prepStmt.setString(1, color);
rs = prepStmt.executeQuery();
}
catch (Exception e) {
e.printStackTrace();
return brands;
}
(25 pts) The following lines of code appear in the H09 example program
String jdbcURL = getServletContext().getInitParameter("jdbcURL");
String mySQLuser = getServletContext().getInitParameter("mySQLuser");
String mySQLpw = getServletContext().getInitParameter("mySQLpw");
Answer the following questions about these lines of code:
jdbc:mysql://cisc474.acad.cis.udel.edu/pconrad, pconrad, and 97s8d9f7s, respectively?
(25 pts) Some lazy programmer who obviously didn't graduate from this university (and never learned the value of commenting) wrote the following lines of code.
public class LoginServlet extends HttpServlet {
public void doPost(HttpServletRequest request,
HttpServletResponse response)
throws IOException, ServletException {
HttpSession session = request.getSession(false);
if (session!=null)
session.invalidate();
session = request.getSession();
...
For 15 points, add some reasonable comments in each of the places indicated. To get full credit, your comments must shed light on what is happening—not just state the obvious. For example, for location (b) the comment "// if session is not equal to null, invalidate the session" is worth zero points.
You may write directly on this sheet, or you write comments after the letters (a), (b), and (c) on your separate answer page. Don't forget to answer part (d) and (e) on the separate answer page.
public class LoginServlet extends HttpServlet {
/**
* doPost is the event handler for the HTTP post method.
*/
public void doPost(HttpServletRequest request,
HttpServletResponse response)
throws IOException, ServletException {
// (a)(5 pts) Write a comment for the next line of code.
// Hints: What does it do? Why do we pass false as a parameter?
//
//
//
HttpSession session = request.getSession(false);
if (session!=null){
// (b) (5 pts) Write a comment for this block of code.
// Hints: What does it mean if session!=null? Why do we do session.invalidate() in that case?
//
//
//
//
//
session.invalidate();
}
// (c) (5 pts) Write a comment for the line of code below.
// Hints: Why are we doing this yet again? Why are we not passing false as a parameter like we did before?
//
//
//
//
//
session = request.getSession();
...
For ten more points, answer these questions:
The following code appeared in the doPost() method in LoginServlet.java for H09. The comments suggest that there is a better design that would improve the efficiency of the code by not requiring three connections to the database.
For 25 points, sketch what that solution would look like.
To assist you in developing this answer, a complete printout of the LoginChecker.java file has also been provided to you with this exam (that should be all you need to develop an answer.)
You do not need to write all of the code—your description can be
But you should provide enough detail that a person skilled in writing MVC Java web applications using SQL would understand your "road map" and be able to write the code herself or himself, filling the gaps.
// create a loginChecker object to do the database work for us
LoginChecker lc = new LoginChecker(jdbcURL,mySQLuser,mySQLpw);
// Check whether the login was valid using the userIsValid
// method of the LoginChecker object. Then set a request attribute
// that the JSP file can check
Boolean loginWasValid = lc.userIsValid(userid,password);
if (loginWasValid)
{
// beerSelect.jsp will get the attribute from the session
session.setAttribute("loginWasValid",loginWasValid);
// Store some useful attributes in the session so that
// we can greet the user by name.
// The following code is very inefficient--we are connecting
// to the SQL database three times and running three queries
// when only one would do the job. Refactoring this to
// improve the efficiency is left as an exercise to the
// CISC474 student!
String fname = lc.getField("fname",userid,password);
String lname = lc.getField("lname",userid,password);
session.setAttribute("fname",fname);
session.setAttribute("lname",lname);
// Now, off to select a beer, where we can greet the user
// by name!
RequestDispatcher view =
request.getRequestDispatcher("beerSelect.jsp");
view.forward(request, response);
}
else
...
Debugging is an important skill in working with web applications. Debugging techniques will vary from technology to technology, but we've learned some specific ones relating to working with Tomcat and SQL.
Suppose you find that when you access a screen in your web app that you "know" should produce a particular result—for example a list of all the "dark beers" in your database—but instead, you get an empty list.
(5 pts) What can you do to make sure that the database itself is set up properly (independent of the web app)?
(5 pts) Where (specifically) can you look to see if there is a stack trace being generated by some run time error deep inside your web app?
(5 pts) Suppose you suspect that an error is your model class is causing the problem. You make a change and recompile. But nothing changes. So you suspect that perhaps your changes are "not taking"—that there is perhaps a mismatch between the code actually running on the server, and the code that you are looking at in your editor.
What can you do to make absolutely sure that the code running on the Tomcat server is actually the same as the source code you are looking? (Don't just say "redeploy", or stop and restart the server—I want something even more specific and foolproof—I want close to 100% certainty!)
(5 pts) Suppose you are using a build.xml file similar to the build.xml file supplied with H08 and H09. Is it sufficient, after each time you make a change, to just run ant compile? How about ant create-war? Explain.
(5 pts) Describe at least one other useful Java/SQL/Ant/Tomcat debugging technique, or a problem and its solution.
By other, I mean describe one that is different from the ones you've referred to so far in your answer to this question.
JavaScript is an important client side technology which some CISC474 students have incorporated into their P06 and/or choice point projects.