Web Client - The J2EE 1.4 Tutorial

Web Client

In the Duke's Bank application, the web client is used by customers to access account information and perform operations on accounts. Table 36-2 lists the functions the client supports, the URLs used to access the functions, and the components that implement the functions. Figure 36-6 shows an account history screen.

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Note: The source code for the web client is in the <INSTALL>/j2eetutorial14/examples/bank/src/com/sun/ebank/web/ and <INSTALL>/j2eetutorial14/examples/bank/web/ directories.

Table 36-2 Web Client 

Function	URL Aliases	JSP Pages	JavaBeans Components
Home page	/main	main.jsp	CustomerBean
Log on to or off of the application	/logon /logonError /logoff	logon.jsp logonError.jsp logoff.jsp
List accounts	/accountList	accountList.jsp	CustomerBean
List the history of an account	/accountHist	accountHist.jsp	CustomerBean, AccountHistoryBean
Transfer funds between accounts	/transferFunds /transferAck	transferFunds.jsp transferAck.jsp	CustomerBean, TransferBean
Withdraw and deposit funds	/atm /atmAck	atm.jsp atmAck.jsp	CustomerBean, ATMBean
Error handling	/error	error.jsp

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Figure 36-6 Account History

Design Strategies

The main job of the JSP pages in the Duke's Bank application is presentation. To achieve this, most dynamic processing tasks are delegated to enterprise beans, custom tags, and JavaBeans components.

In the Duke's Bank application, the JSP pages use enterprise beans to handle interactions with the database and rely on JavaBeans components for interactions with the enterprise beans. In the Duke's Bookstore application, discussed in Chapters 11 to 22, the BookDB JavaBeans component acts as a front end to a database. In the Duke's Bank application, TransferBean acts as a facade to the TxControllerBean enterprise bean. However, the other JavaBeans components have much richer functionality. ATMBean invokes enterprise bean methods and sets acknowledgment strings according to customer input, and AccountHistoryBean massages the data returned from the enterprise beans in order to present the view of the data required by the customer.

The web client uses a template mechanism implemented by custom tags (discussed in A Template Tag Library) to maintain a common look across all the JSP pages. The template mechanism consists of three components:

template.jsp determines the structure of each screen. It uses the insert tag to compose a screen from subcomponents.
screendefinitions.jspf defines the subcomponents used by each screen. All screens have the same banner, but different title and body content (specified in the JSP Pages column in Table 36-2).
Dispatcher, a servlet, processes requests and forwards them to template.jsp.

Finally, the web client uses logic tags from the JSTL core tag library to perform flow control and tags from the JSTL fmt tag library to localize messages and format currency.

Client Components

All the JavaBeans components used in the web client are instantiated by Dispatcher. The BeanManager and CustomerBean components are instantiated for the session and request, respectively. The other beans--AccountHistoryBean, TransferBean, and ATMBean--are instantiated depending on which request URL is being handled.

Responsibility for managing the enterprise beans used by the web client rests with the BeanManager. It creates customer, account, and transaction controller enterprise beans and provides methods for retrieving the beans.

When instantiated by Dispatcher, the BeanManager component retrieves the home interface for each bean from the helper class EJBGetter and creates an instance by calling the create method of the home interface. Because these enterprise beans apply to a particular customer or session, Dispatcher stores a BeanManager as a session attribute.

public class BeanManager {
  private CustomerController custctl;
  private AccountController acctctl;
  private TxController txctl; 
  public BeanManager() {
    if (custctl == null) {
      try {
        CustomerControllerHome home =
          EJBGetter.getCustomerControllerHome();
        custctl = home.create();
      } catch (RemoteException ex) {
        Debug.print("Couldn't create customer bean."  + 
          ex.getMessage());
      } catch (CreateException ex) {
        Debug.print("Couldn't create customer bean."  + 
          ex.getMessage());
      } catch (NamingException ex) {
        Debug.print("Unable to look up home: " +
          CodedNames.CUSTOMER_CONTROLLER_EJBHOME  +
          ex.getMessage());
    }
  }
  public CustomerController getCustomerController() {
    return custctl;
  }
  ...
} 

CustomerBean maintains the customer and account information for the current request. Although the customer is the same for each request, the account may change, so Dispatcher stores a CustomerBean as a request attribute.

public class CustomerBean {
  private BeanManager beanManager;
  private String customer;
  private String account;

  public AccountDetails getAccountDetails() {
  AccountDetails ad = null;
    try {
      ad = beanManager.getAccountController().
          getDetails(this.account);
    } catch (InvalidParameterException e) {
      ...
    return ad;
  }

  public ArrayList getAccounts() {
    ArrayList accounts = null;
    try {
      accounts = beanManager.getAccountController().
        getAccountsOfCustomer(this.customer);
    } catch (InvalidParameterException e) {
    ...
    }      
    return accounts;
} 

The page fragment template/links.jsp generates the list of bank function links at the top of every page. Notice that the customer is retrieved from the userPrincipal object, which is set when the customer logs in (see Protecting the Web Client Resources). After the customer is set, the page can retrieve from CustomerBean the collection of accounts for the customer. The collection is assigned to the accounts variable, and the first item in the collection is used as the default account ID for the ATM operation.

<%@ taglib uri="http://java.sun.com/jsp/jstl/core" prefix="c" 
%>
<%@ taglib uri="http://java.sun.com/jsp/jstl/fmt" prefix="fmt" 
%>
<jsp:useBean id="customerBean"
  class="com.sun.ebank.web.CustomerBean" scope="request"/>
<jsp:setProperty name="customerBean" property="customer"
  value="${pageContext.request.userPrincipal.name}"/>

<c:set var="accounts" value="${customerBean.accounts}" />
<c:forEach items="${accounts}" begin="0" end="0" var="ad">
  <c:set var="accountId" value="${ad.accountId}" />
</c:forEach>
<center> 
<table border=0 cellpadding=10 cellspacing=25 
  width=600 summary="layout">
  <tr>      
    <c:url var="url" value="/accountList" />
    <td bgcolor="#CE9A00"><a href="${url}">
      <fmt:message key="AccountList"/></a></td>
    <c:url var="url" value="/transferFunds" />
    <td bgcolor="#CE9A00"><a href="${url}">
      <fmt:message key="TransferFunds"/></a></td>
    <c:url var="url" 
      value="/atm?accountId=${accountId}&operation=0" />
    <td bgcolor="#CE9A00"><a href="${url}">
      <fmt:message key="ATM"/></a></td>
    <c:url var="url" value="/logoff" />
    <td bgcolor="#CE9A00"><a href="${url}">
      <fmt:message key="Logoff"/></a></td>
  </tr>
</table>
</center> 

Request Processing

All requests for the URLs listed in Table 36-2 are mapped to the dispatcher web component, which is implemented by the Dispatcher servlet:

public class Dispatcher extends HttpServlet {
  public void doPost(HttpServletRequest request,
    HttpServletResponse response) {
    ...
    String selectedScreen = request.getServletPath();
    ...
    if (selectedScreen.equals("/accountHist")) {
      ...
    } else if (selectedScreen.equals("/transferAck")) {
      String fromAccountId = 
        request.getParameter("fromAccountId");
      String toAccountId =
        request.getParameter("toAccountId");
      if ( (fromAccountId == null) || (toAccountId == null)) {
        request.setAttribute("errorMessage",
          messages.getString("AccountError"));
        try {
          request.getRequestDispatcher(
            "/error.jsp").forward(request, response);
        } catch(Exception ex) {
        }
    } else { 
      TransferBean transferBean = new TransferBean();
      request.setAttribute("transferBean",
        transferBean);
      try {
        transferBean.setMessages(messages);
        transferBean.setFromAccountId(fromAccountId); 
        transferBean.setToAccountId(toAccountId);
        transferBean.setBeanManager(beanManager);
        transferBean.setTransferAmount(new
          BigDecimal(request.
            getParameter("transferAmount")));
        String errorMessage = transferBean.doTx();
        if (errorMessage != null) {
          request.setAttribute("errorMessage",
            errorMessage);
          try {
            request.getRequestDispatcher(
              "/error.jsp").forward(request, response);
          } catch(Exception ex) {
          }
        } 
      } catch (NumberFormatException e) {
        request.setAttribute("errorMessage",
          messages.getString("AmountError"));
        try {
          request.getRequestDispatcher(
            "/error.jsp").forward(request, response);
        } catch(Exception ex) {
        }
      }
    }
    ...
    try {
      request.getRequestDispatcher(
      "/template/template.jsp").forward(request, response);
    } catch(Exception e) {
    }
  }
} 

When a request is delivered, Dispatcher does the following:

Retrieves the incoming request URL and extracts the requested screen. Dispatcher performs business logic and updates model objects based on the requested screen.
Creates a JavaBeans component and stores the bean as a request attribute.
Parses and validates the request parameters. If a parameter is invalid, Dispatcher may reset the request alias to an error page. Otherwise, it initializes the JavaBeans component.
Calls the doTx method of the JavaBeans component. This method retrieves data from the enterprise beans and processes the data according to options specified by the customer.
Forwards the request to template.jsp.

As mentioned earlier, template.jsp generates the response by including the responses from subcomponents. The body subcomponent in turn usually retrieves data from the JavaBeans components initialized by Dispatcher.

Figure 36-7 depicts the interaction among these components.

Figure 36-7 Web Component Interaction

Protecting the Web Client Resources

In the J2EE platform, you protect a web resource from anonymous access by specifying which security roles can access the resource. The web container guarantees that only certain users acting in those roles can access the resource. For the web container to enforce the security constraint, the application must specify a means for users to identify themselves, and the web container must support mapping a role to a user.

In the Duke's Bank web client, you restrict all the URLs listed in Table 36-2 to the security role bankCustomer. The application requires users to identify themselves via the form-based login mechanism. When a customer tries to access a web client URL and has not been authenticated, the web container displays the JSP page logon.jsp. This page contains a form that requires a customer to enter an identifier and password.

<form action="j_security_check" method=post>
<table>
<tr>
  <td align="center" >
  <table border="0">
  <tr>
  <td><b><fmt:message key="CustomerId"/></b></td>
  <td>
    <input type="text" size="15" name="j_username"> 
  </td>
  </tr>
  <tr>
  <td><b><fmt:message key="Password"/></b></td>
  <td> 
    <input type="password" size="15" name="j_password">
  </td>
  ...
</form> 

Note that the action invoked by the form, j_security_check, is specified by the Java Servlet specification, as are the request parameters j_username and j_password. The web container retrieves this information, maps it to a security role, and verifies that the role matches that specified in the security constraint. Note that in order for the web container to check the validity of the authentication information and perform the mapping, you must perform these two steps when you deploy the application:

Add the customer's group, ID, and password to the default realm of the container using the Admin Console.
Map the bankCustomer role to the customer or the customer's group in deploytool.

After the customer has been authenticated, the identifier provided by the customer is used as a key to identify the customer's accounts. The identifier is retrieved from the request using the following expression:

${pageContext.request.userPrincipal.name} 

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