Java

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• Form with 2 mandatory fields. This example uses 2 forms: one submits a form the other gets and displays the form's results.
  If the user fails to supply a value for any of the fields in the form, it re-displays the form but with an error message saying that the value is missing. Doesn't make the user reenter values that they’ve entered already.
  Download source code

• Servlet with a counter:
  When the counter hits number 10 the servlet redirects to "nytimes" site, otherwise the counter redirects to "washingtonpost" site.
  NOTE: dont forget to synchronize access to shared resources (the counter).
  Download source code
   
• Idea: generate list of large (e.g., 150-digit long) prime numbers
  – Show partial results until completed (if two users submit the same input, the last one will start with the numbers already returned to the first user);
  – Let new clients make use of results from others (the program keeps a ArrayList with the last 10 results of prime numbers);
  Demonstrates:
   - use of the Refresh header.
   - how easy it is for servlets to maintain state between requests.
   - that servlets can handle multiple simultaneous connections - each request is in a separate thread.
  Download source code [4]
   
• Visits count using cookies. Things to test with this app:
        - Open 2 distinct browsers (ex. firefox and chrome) and check that each one gets their own count
        - The cookie has a maxAge time set to 10 seconds, so it will persist in the client and if you close and re-open a browser before the 10s end it will keep counting. If you wait more than 10 seconds from each request the cookie will be destroyed and, so, the counter will start from zero again
  (remember maxAge has this special values:
               -"-1" means the cookie will exist till the session expires (the client close the browser;
               - "0" means destroy the cookie;
               - any value >0 means the cookie will be destroyed when that time expires;
  Download source code [5]
• MVC (aka "Model 2") example with servlets+jsp [7]

Solutions to common problems [4]

Useful Annotations and XML descriptors

Useful Classes/Interfaces and their methods

Managing HTTP headers with Servlets

GET /search-servlet?keywords=servlets+jsp HTTP/1.1
Accept: image/gif, image/jpg, */*
Accept-Encoding: gzip
Connection: Keep-Alive
Cookie: userID=id456578
Host: www.somebookstore.com
Referer: http://www.somebookstore.com/findbooks.html
User-Agent: Mozilla/4.0 (compatible; MSIE 6.0; Windows NT 5.0)

•  NOTE: Always check that request.getHeader is non-null before trying to use it:

  String val = request.getHeader("Some-Name");
  if (val != null) {
  …
  }

• The HTTP headers are optional, so you cannot expect that they are always sent by the client browser. The code bellow can be used to show all the headers and their values sent by the client:

  String html = "";
  Enumeration<String> headerNames = request.getHeaderNames();
  String headerName;
  while (headerNames.hasMoreElements()) {
      headerName = headerNames.nextElement();
      html += "<tr>"
              + "<td>" + headerName + "</td>"
              + "<td>" + request.getHeader(headerName) + "</td>"
              + "</tr>"; 
  }
  

• Checking which browser the client is using (checks for Microsoft IE):
  

  String userAgentHeaderValue = request.getHeader("User-Agent");
         if(userAgentHeaderValue.contains("MSIE")){...}

• Checking if the client came to this page from the expected page and, if not, redirect him back:

  String refererHeaderValue = request.getHeader("referer");            
      if(refererHeaderValue == null ||
         !refererHeaderValue.contains("TutorialServletJSP/tutorial4ex3.xhtml")) {
              response.sendRedirect("tutorial4ex3.xhtml");
      }
  

@WebServlet(name = "ControllerServlet", loadOnStartup = 1, urlPatterns = {
    "/category",
    "/addToCart",
    "/viewCart",
    "/updateCart",
    "/checkout",
    "/purchase",
    "/chooseLanguage"})
public class ControllerServlet extends HttpServlet {

public void init() throws ServletException {

        // store category list in servlet context
        getServletContext().setAttribute("categories", categoryFacade.findAll());
}

synchronized(this) {
SomeBean value = SomeLookup.findResult(...);
getServletContext().setAttribute("key", value);
RequestDispatcher dispatcher =
request.getRequestDispatcher
("/WEB-INF/SomePage.jsp");
dispatcher.forward(request, response);
}

${key.someProperty}

<jsp:useBean id="key" 
    type="somePackage.SomeBean"
    scope="application" />

<jsp:getProperty 
    name="key" 
    property="someProperty" />

SomeBean value = LookupService.findResult(...);
HttpSession session = request.getSession();
session.setAttribute("key", value);
RequestDispatcher dispatcher =
request.getRequestDispatcher
    ("/WEB-INF/SomePage.jsp");
dispatcher.forward(request, response);

${key.someProperty}

<jsp:useBean 
    id="key" 
    type="somePackage.SomeBean"
    scope="session" />

<jsp:getProperty 
    name="key" 
    property="someProperty" />

SomeBean value = LookupService.findResult(...);
request.setAttribute("key", value);
RequestDispatcher dispatcher =
request.getRequestDispatcher("/WEB-INF/SomePage.jsp");
dispatcher.forward(request, response);

<%= request.getAttribute("key");

${key.someProperty}

<jsp:useBean
    id="key" 
    type="somePackage.SomeBean"
    scope="request" />

<jsp:getProperty 
    name="key" 
    property="someProperty" />

• set/get a application-scoped variable in a Servlet:
  

  // store category list in servlet context
  getServletContext().setAttribute("categories", categoryFacade.findAll());
  //get category list from servlet context
  List <Category> list = (List<Category>) getServletContext().getAttribute("categories"); 

  access the variable from JSP (used in a forEach statement in this example):
  

  <c:forEach var="category" items="${applicationScope.categories}">

• set/get a session-scoped variable in a Servlet:
  

  HttpSession session = request.getSession();
  // place selected category in session scope
  session.setAttribute("selectedCategory", selectedCategory);
  

  access this variable  in JSP:
  

  ${sessionScope.selectedCategory.name}

• set/get a page-scoped variable in JSP (this variables are only accessed by JSP):
  

  <!-- set the variable -->
  <c:set var="myVar" value="Hello World" />
  <!-- get the value-->
  ${pageScope.myVar}

${categoryProducts}

${sessionScope.categoryProducts}

@Override
public void init() throws ServletException {
    // store category list in servlet context
    getServletContext().setAttribute("categories", categoryFacade.findAll());
}

<div id="indexRightColumn">
    <c:forEach var="category" items="${categories}">
        <div class="categoryBox">
            <a href="category?${category.id}">
                <span class="categoryLabelText">${category.name}</span>               
                <img src="${initParam.categoryImagePath}${category.name}.jpg" alt="${category.name}"/>
            </a>
        </div>
    </c:forEach>   
</div>

 @Override
    protected void doGet(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {

        String userPath = request.getServletPath();
        Category selectedCategory;
        Collection<Product> categoryProducts;

        // if category page is requested
        if (userPath.equals("/category")) {
            // get categoryId from request
            String categoryId = request.getQueryString();

            if (categoryId != null) {
                // get selected category
                selectedCategory = categoryFacade.find(Short.parseShort(categoryId));

                // place selected category in request scope
                request.setAttribute("selectedCategory", selectedCategory);

                // get all products for selected category
                categoryProducts = selectedCategory.getProductCollection();

                // place category products in request scope
                request.setAttribute("categoryProducts", categoryProducts);
            }

<c:forEach var="product" items="${categoryProducts}" varStatus="iter">
            <tr class="${((iter.index%2)==0) ? 'lightBlue': 'white'}">           
                <td>
                    <img src="${initParam.productImagePath}${product.name}.png" alt="product image">
                </td>
                <td>
                    ${product.name}
                    <br>
                    <span class="smallText">${product.description}</span>
                </td>
                <td>${product.price}</td>
                <td>
                    <form action="addToCart" method="post">
                        <input type="hidden"
                               name="productId"
                               value="${product.id}">
                        <input type="submit"
                               value="add to cart">
                    </form>
                </td>
            </tr>
        </c:forEach>

Session Tracking Options

1. By far the most common is with cookies.
2. URL rewriting can be applied in the event that cookies are not supported or disabled.
3. Hidden form fields can also be used as a means of "maintaining state" over multiple requests, but these are limited to usage within forms.

HttpSession session = request.getSession();

ShoppingCart cart = new ShoppingCart();
session.setAttribute("cart", cart);

cart = (ShoppingCart) session.getAttribute("cart");

${cart}

HttpSession session = request.getSession();
synchronized (session) {
    SomeClass value =
            (SomeClass) session.getAttribute("someID");
    if (value == null) {
        value = new SomeClass();
    }
    doSomethingWith(value);
    session.setAttribute("someID", value);
}

Redirecting or Forwarding to a new page [7]

• using RequestDispatcher.forward
• or using response.sendRedirect

RequestDispatcher dispatcher =
    request.getRequestDispatcher
    ("/WEB-INF/SomePage.jsp");
dispatcher.forward(request, response);
return;//stop any further processing on the servlet

response.sendRedirect("somePage.jsp");
return;//stop any further processing on the servlet

References