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The Page Directive
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generated servlet as usual When javascript is specified, there is still a generated Java servlet, but it does not contain the javascript code Instead, the servlet initializes a scripting engine that reads and interprets the original JSP page For example, if the JSP page looks like this,
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<%@ page language="java" %> <% int k = 10; %> k = <%= k %>
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ELEMENTS OF JSP
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then the generated servlet includes the statements,
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outprint("\r\n"); int k = 10; outprint("\r\nk = "); outprint(k); outprint("\r\n\r\n");
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which treats k as a Java variable, assigns a value to it, and prints it using the out JspWriter variable By contrast, if the same JSP page uses javascript as a value of the language attribute,
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<%@ page language="javascript" %> <% var k = 10; %> k = <%= k %>
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then the generated servlet initializes a JRun-specific scripting engine and invokes its evaluate method, as shown in the following:
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if (scriptEngine == null) { try { scriptEngine = ScriptEngineFactorygetScriptEngine("javascript"); scriptEngineinit(pageContext); } catch (Exception e) { throw new ServletException ("Error initializing scripting engine", e); }
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JSP: The Complete Reference
} if (requestgetAttribute(SCRIPT_KEY) != null) { scriptEngineinit( pageContext, (String) requestgetAttribute(SCRIPT_KEY), (String) requestgetAttribute(DECLARATION_KEY)); } scriptEngineevaluate(pageContext);
Obviously, any JSP pages written in a language not explicitly required by the JSP specification most likely won t be portable between JSP containers of different vendors
extends
Ordinarily, the JSP container supplies the parent class for any servlet it generates from a JSP page However, the specification enables you to subclass another parent class of your liking by specifying its fully qualified name in the extends attribute of the page directive Doing so would let you provide additional behavior to a family of JSP pages without explicitly coding the behavior in the page The JSP specification urges caution when using this capability because it may prevent the JSP container from providing vendor-specific performance and reliability enhancements For example, the standard JSP parent class used by JRun provides methods for determining dependencies and their last modification times Similarly, Tomcat implements a parent class that stores a reference to a specialized class loader If you use a different parent class, it ought to provide important functionality that outweighs these features
Required Interfaces for a JSP Superclass
For a class to be used as the superclass for JSP pages, it must implement one of the following interfaces: I javaxservletjspJspPage A generic interface, not necessarily for use with HTTP Few servlets implement this interface directly I javaxservletjspHttpJspPage Intended for JSP pages that operate under the HTTP protocol, this interface is an extension of JspPage These interfaces define three methods you must implement, which are described in Table 10-1
10:
The Page Directive
Method
public void jspInit()
Description
Method called automatically from the servlet init() method when the JSP page is loaded Although you must implement this method, your implementation needn t do anything The method is designed as a placeholder to be overridden by JSP page subclasses for any initialization work they need to do The counterpart of jspInit(), this method is automatically called from the servlet s destroy() method when a JSP page class is unloaded This method is the heart of the JSP request processing logic It must not be explicitly defined in the JSP page because it is the work of the JSP container to generate the method from the JSP s scriptlets, expressions, and directives This method is typically declared to be abstract in the JSP parent class
ELEMENTS OF JSP
public void jspDestroy()
public void _jspService( request, response) throws ServletException, IOException
Table 10-1
Methods Required to Be Declared in JSP Superclasses
The exact types of the request and response parameters in the _jspService method are dictated by the protocol they support For the HTTP environment, these types are javaxservlethttpHttpServletRequest and javaxservlethttpHttpServletResponse If you are implementing a different protocol, you need to define request and response classes to be used in the method signature HttpJspPage extends JspPage to provide HTTP-specific behavior JspPage, in turn, extends javaxservletServlet, which defines the methods listed in Table 10-2
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