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Anatomy of Request Processing
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The servlet engine that handles servlet and JSP requests can be part of the Web server itself (referred to as the in-process model) or it can run in a separate process In the latter case, the Web server contains a component referred to as a connector The connector intercepts servlet requests and passes them on to the servlet engine by an implementation-dependent protocol1 Other requests are handled by the Web server as usual Figure 9-1 illustrates this out-of-process model When the servlet engine receives a request, it assembles all the details about the request into an HttpServletRequest object These details include the request headers, the URI, the query string, any parameters sent, and so on Similarly, it initializes an HttpServletResponse object that can hold response headers and the response output stream It then invokes the servlet s service() method (the _jspService() method, if the servlet is a JSP), passing it references to the two objects, as shown in Figure 9-2
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1 JRun features connectors for several widely used Web servers, and employs a proprietary protocol to communicate requests to its servlet engine Tomcat uses a protocol known as ajp12 (developed originally for Apache JServ) to send requests and responses between components
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Request Dispatching
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ELEMENTS OF JSP
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The out-of-process servlet engine model
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A simple JSP can extract what it needs from the request object, perform the necessary calculations and other logic, and then create output using the response object The remainder of this chapter examines how larger and more complex Web applications can operate on these request and response objects, passing them through more than one servlet or JSP
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Figure 9-2
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The servlet engine passing request and response objects to a servlet
JSP: The Complete Reference
Including Other Resources
HTML itself does not have a direct means for including data from other files in its output This is unfortunate, because a great deal of HTML markup is common to a number of pages in a typical Web site corporate logos, copyright notices, navigation links, and other features Besides these static sources of text and images, dynamic content may need to be included JSP provides two means incorporating such data: I The <%@ include %> directive is used to copy static text into the JSP source code before it is transformed into Java servlet source code and compiled Typically, this text is HTML code, but it can be anything that could appear in a JSP page I The <jsp:include> action causes the servlet engine to invoke another URL and merge its output with that of the original JSP page A key point to remember in building a mental model is the <%@ include %> directive is performed once, at compilation time, whereas the <jsp:include> action is performed each time a request is made The next two sections describe each of these JSP components and how they operate
The include Directive
The syntax of the include directive is as follows: <%@ include file="filename" %> The included filename must be a relative URL specification, meaning it contains only path information, not protocol or server information As a consequence, only resources in the current servlet context can be included by this means If the filename begins with / , it is considered to be absolute with respect to the top of the servlet context Otherwise, the filename is considered to be relative to the current JSP page For example, if a Web application has a products subdirectory and the products/searchjsp page contains the directive
<%@ include file="/includes/headerinc" %>
then the file that would be included is <path>/includes/headerinc, where <path> is the Web application mount point If, instead, the directive is
<%@ include file="includes/headerinc" %>
then the file would be <path>/products/includes/headerinc
9:
Request Dispatching
How It Works
When a <%@ include %> directive is encountered, the JSP container reads the specified file and merges its contents into the JSP source code currently being parsed For example, if flavorsjsp contains
<H3>Flavors</H3> Our most popular flavors are: <%@ include file="flavor_listhtml" %> Try them all!
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