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JSP Syntax and Semantics
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} catch (Exception ex) { if (outgetBufferSize() != 0) outclear(); pageContexthandlePageException(ex); } finally { outflush(); _jspxFactoryreleasePageContext(pageContext); } } }
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ELEMENTS OF JSP
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Let s consider the JSP page and the generated code section by section
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A Page Directive
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The JSP page begins with a page directive indicating the page uses the javautil package:
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<%@ page import="javautil" %>
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This directive shows up in the servlet source code at the end of its list of imported classes:
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orgapachejasperruntime*; javabeans*; orgapachejasperJasperException; javautil*;
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A <jsp:include> Action
The page uses a style sheet to set the look and feel of the output The style sheet is incorporated using a <jsp:include> action:
<STYLE> <jsp:include page="stylecss" flush="true"/> </STYLE>
JSP: The Complete Reference
The <jsp:include> action causes the following style sheet to be read at request time:
body { color: #000000; background-color: #FEFEF2; font: Verdana 9pt; };
Scriptlet
Two scriptlets are on the page, with HTML template data located before, between, and after them The HTML data
<HTML> <HEAD> <TITLE>Echo</TITLE>
is passed through unchanged by means of write statements:
outwrite("\r\n"); outwrite("<HTML>\r\n\r\n"); outwrite("<HEAD>\r\n "); outwrite("<TITLE>Echo</TITLE>\r\n ");
Then the first scriptlet is simply copied to the servlet:
Enumeration eNames = requestgetHeaderNames(); while (eNamesHasMoreElements()) { String name = (String) eNamesnextElement(); String value = normalize(requestgetHeaderName());
Notice the code fragment has an unclosed curly brace on the second line The matching brace is supplied by the second scriptlet
6:
JSP Syntax and Semantics
JSP Expressions
During each iteration of the loop, the scriptlet extracts a header name and header value from the request object Rather than printing these values using outwrite(), the page author switches back into HTML mode and uses JSP expression tags,
%> <TR> <TD><%= name %></TD> <TD><%= value %></TD> </TR> <% ELEMENTS OF JSP
which generates the following servlet code:
// HTML // begin [file="Echojsp";from=(19,2);to=(20,12)] outwrite("\r\n <TR> <TD>"); // end // begin [file="Echojsp";from=(20,15);to=(20,21)] outprint( name ); // end // HTML // begin [file="Echojsp";from=(20,23);to=(20,33)] outwrite("</TD> <TD>"); // end // begin [file="Echojsp";from=(20,36);to=(20,43)] outprint( value ); // end // HTML // begin [file="Echojsp";from=(20,45);to=(21,0)] outwrite("</TD> </TR>\r\n"); // end
JSP: The Complete Reference
A Declaration
Header values that are lists can be very long and cause the table width to be distorted You can get around this problem by scanning the header value for semicolons and inserting <BR> tags wherever they are found This function is performed by a method called normalize(), which is found at the end of the JSP file:
<%! private String normalize(String value) { StringBuffer sb = new StringBuffer(); for (int i = 0; i < valuelength(); i++) { char c = valuecharAt(i); sbappend(c); if (c == ';') sbappend("<BR>"); } return sbtoString(); } %>
As was the case with the two scriptlets, the declaration code is copied verbatim to the generated servlet, except it is not placed inside the _jspService() method Instead, it is written inside the class block, but outside any other method, near the beginning of the servlet:
// begin [file="Echojsp";from=(27,3);to=(39,0)] private String normalize(String value) { StringBuffer sb = new StringBuffer(); for (int i = 0; i < valuelength(); i++) { char c = valuecharAt(i); sbappend(c); if (c == ';') sbappend("<BR>"); } return sbtoString(); } // end
The resulting output is shown in Figure 6-1
6:
JSP Syntax and Semantics
ELEMENTS OF JSP
Figure 6-1
Output of Echojsp
Summary
The JSP development environment provides a means for generating HTML pages dynamically with server-side Java programming The syntax allows most of the HTML to be coded directly, with sections marked off for Java code that controls the page generation There is support for including other resources, both static and dynamic JavaBeans are fully integrated into the framework, and custom tags allow functionality to be encapsulated and made available to nonexpert page authors The key point this chapter makes is this: a mental model of the JSP development cycle is crucial to understanding how to create and debug Web applications Knowledge of what happens at translation time (static resources are included via the <%@ include %> directive, for example) versus request time (dynamic request dispatching with <jsp:include>) provides insight into which features to use and when to use them The remaining chapters of Part II discuss each of these features of the application model in more detail
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