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This tag is called a directive because it directs the page compiler to take some action In this case, when it sees the directive, the page compiler will embed the contents of the contentsjsp file directly into the servlet that it is building for indexjsp Subsequently, if the contentsjsp file is edited, indexjsp will not change Browsers will continue to get the old message until the page compiler is forced to rebuild the indexjsp servlet Note that the include directive specifies what is to be included with file=, whereas the jsp:include tag specifies page= This nicely encapsulates the differences between the two: The directive includes a file as it is building the servlet at request time, and the tag includes another page at request time In some obscure instances, the include directive and the jsp:include tag are not equivalent For the most part, though, the two are functionally identical, and as the jsp:include tag is more convenient, it will be used in preference to the directive throughout this book
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25 Creating Custom Error Pages
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Many other directives are available for issuing instructions to the page translator One of the most useful is called, not surprisingly, page This directive takes a number of forms, many of which will be encountered throughout this book as needed One immediately useful option allows a JSP to specify where the user should be directed in the event of an error Tomcat's default error page, as already shown in Figure 21, can be useful to developers but more than a little scary to end users Ideally, users will never see an error page, but a good site plans for all contingencies and so should include an error page that fits visually with the rest of the site and that allows the user to continue what he or she was doing, to whatever extent possible Using a custom error page involves two steps The first is to create the page, which can be another JSP However, error pages need to be treated differently from regular pages, and so the page translator must be notified that a JSP will be used as an error page by use of the following page directive at the very top of the file:
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<%@ page isErrorPage="true" %>
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Once such an error page has been created and properly identified, it can be used in one of two ways The first is globally, by telling Tomcat which error page to use for each type of error This is done through a configuration file and is shown in Appendix B In addition to the global approach, each JSP can specify its own error page through another variation of the page directive, as in
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Here, error_page_url is the URL of the error page, relative to the current page Both of these versions of the page directive will be demonstrated in the next section
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26 Java News Today
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Java News Today, our fictional news site, is ready to begin constructing its site JNT has decided to start with the new home page and for the moment will not worry about the dynamic elements The first version is shown in Listing 24 Listing 24 The JNT index page
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<html> <head>
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<link rel="StyleSheet" href="stylejsp" TYPE="text/css" media="screen"> <title>Java News Today: Welcome!</title> </head>
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<table width="100%" border="0" cellspacing="0" cellpadding="0">
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<tr> <td width="15%" class="borders"> <%-- Big Empty Corner --%> </td>
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<td width="20" class="borders"> <%-- Little buffer for the curvy bit --%> <img src="1x1gif"> </td>
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<td class="borders"> <%-- start header --%> <center><h2>Java News Today: Welcome!</h2></center> <%-- end header --%> </td> </tr>
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<tr> <td width="15%" class="borders" valign="top"> <%-- start navigation --%> Navigation - none yet <%-- end navigation --%> </td> <td width="20"><img src="1x1gif"></td>
<td valign="top" align="left"> <%-- All content goes here! --%> </td> </tr>
</table> </body> </html>
JNT saves the contents of Listing 24 into a file called indexjsp, points its browsers at the corresponding URL, and as expected sees the page in Figure 22 Figure 22 The JNT home page
Conceptually, this page consists of four major elements: the header, the navigation, the content, and the HTML that connects it all together Note the use of JSP comments to delineate these sections It is generally wise to mark off major functional areas of a page, but as the end user is probably not interested in these fences, they might as well be JSP comments instead of HTML comments Different pages will have different content, but it is reasonable to expect that the header and navigation will be repeated all over the site, although doing so can be a major headache The author of each new page will have to remember to put these pieces in and will have to worry about getting everything right Worst of all, sooner or later will come the hateful day when a new section is introduced and everyone has to go back and reedit all their pages The solution for this nightmare scenario is, of course, the jsp:include tag The header and footer will be split into separate files The headerjsp file will contain everything in Listing 24 from <%-- start header --%> to <%-- end header --%> Likewise, navigationjsp will hold everything from <%-- start navigation --%> to <%-- end navigation --%> This technique of pulling out common chunks of HTML and putting them in separate files is called templating, although the use of the word here is slightly different from that in 1 Here, a template is merely an HTML page with some "holes" where text should be, along with a way to indicate where this text should be found The advantage is that many pages can have the same spaces, and all these holes can be filled from the same
place This makes it possible to keep the header in exactly one file and let each page have a space that should be filled by this file So far, this might seem like a rather goofy thing to do, as the header and navigation are currently so small However, rest assured that they will grow in subsequent chapters, and the advantages of removing them from the main page will become increasingly obvious One such advantage is that it now becomes easy to create alternative versions of the home page Because all the HTML elements remain in the main file, a simplified version suitable for text-only browsers, such as Lynx, could be created with a page like that shown in Listing 25 Listing 25 A version of the home page without tables