Some of the basic services provided by the ServletContext interface are in Java

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Some of the basic services provided by the ServletContext interface are
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setAttribute: Stores information in the context getAttribute: Retrieves information stored in the ServletContext getAttributeNames: Obtains the names of attributes in the context
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removeAttribute: Removes an attribute in the context
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An approach similar to the one discussed for servlet forwarding and shown inFigure 10-8 can be employed to model servlet interactions with the ServletContext
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Servlet Session Management
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Given the stateless nature of the HTTP protocol, managing repeat interaction and dialog with the same client (such as that required for an ongoing shopping session) poses some serious challenges There are various means of overcoming these challenges:
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Hidden fields: Hidden fields are embedded within the page displayed to the client These fields are sent back to the client each time a new request is made, thereby permitting client identification each time a client makes a request
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Dynamic URL rewriting: Extra information is added to each URL the client clicks on This extra information is used to uniquely identify each client for the duration of the client session, for example, adding a " sessionid=97859" to the end of each URL the client clicks to identify that the request is associated with session id 97859
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Cookies: Stored information can later be passed back to the client repeatedly The Web server provides the cookie to the browser Cookies are one of the more popular means of setting up a servlet session
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Server-side session object: Cookies and URL encoding suffer from limitations on how much information can be sent back with each request In server-side session management, the session information is maintained on the server in a session object and can be accessed as required Server-side session objects are expensive to use, so it is best to use them sparingly
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The Java Servlet Application Programming Interface (API) provides abstractions that directly support some of the session management techniques discussed in the preceding list
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The core abstraction provided by the servlet API is the HTTP session, which facilitates handling of multiple requests from the same user
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Figure 10- 1 0
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gives an example of servlet session management
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Figure 10-10 Servlet session usage
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importjavaxservlethttp*; // locate a session object HttpSession theSession = requestgetSession (true); // add data to the session object theSessionputValue("Sessionid", "98579"); // get the data for the session object sessionid = theSessiongetValue("SessionID");
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Activity diagrams can be used to model the servlet and session interaction This is similar to the approach discussed for servlet forwarding and shown in Figure
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A descriptor based on XML is used in the deployment of servlets on a Web server The compiled servlet class, additional supporting Java classes, and the deployment descriptor are packaged together into a Web archive file, also known as a "war" file
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The deployment descriptor is an XML-based file that contains specific configuration and deployment information for use by the servlet container
Figure 10- 1 1
shows an example of a vanilla XML deployment descriptor for an HttpServlet
Additional required fields in the descriptor are filled in during configuration and deployment on the Web server
Figure 10-11 A simple vanilla XML deployment descriptor for a sample HttpServlet
< xml version="10" encoding="UTF-8" > <!DOCTYPE web-app PUBLIC "- / / Sun Microsystems, Inc / / DTD Web Application 22 / / EN" "http: / / javasuncom/j2ee/dtds/web-app_2_2dtd"> <web-app> <servlet> <servlet-name>LoginServlet</servlet-name> <servlet-class>LoginServlet</servlet-class> </servlet> </web-app>
We discuss servlet deployment descriptors and Web archive files and their role in the context of modeling in
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Now that you have become intimately familiar with servlets, it is time to return to building the HomeDirect online banking example
At the beginning of this chapter, we identified the need to evolve the control object in the Transfer funds use case by splitting it into two, one focused on the external interaction and the other focused on the internal interaction
Of course, the question remains: How do you actually arrive at this division of responsibilities The answer is partly based on understanding what a servlet is capable of doin g and the rest on judgment and experience In general, the role of the servlet is that of a coordinator between the boundary objects and the rest of the system All the interaction between the boundary object and the composite control class belongs in the new servlet How you split the interaction that is shown between the control object and the entity objects is somewhat less clear The key factor to remember is that the servlet is primarily a coordinator; and hence, it should only take on lightweight responsibilities, which could include initiating some business logic However, actual business logic, computations, interaction with entity objects, and so on would all fall outside of these responsibilities
With this in mind, let's take another look at the i nteractions involving the control object as shown in
Figure 10- 1 2