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JavaServer Pages

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JavaServer Pages (JSP) is a Java technology that allows software developers to dynamically generate HTML, XML or other types of documents in response to a Web client request. The technology allows Java code and certain pre-defined actions to be embedded into static content.

The JSP syntax adds additional XML tags, called JSP actions, to be used to invoke built-in functionality. Additionally, the technology allows for the creation of JSP tag libraries that act as extensions to the standard HTML or XML tags. Tag libraries provide a platform independent way of extending the capabilities of a Web server.

JSPs are compiled into Java Servlets by a JSP compiler. A JSP compiler may generate a servlet in Java code that is then compiled by the Java compiler, or it may generate byte code for the servlet directly.

Contents

JSP and Servlets

Architecturally speaking, JSP can be viewed as a high-level abstraction of servlets that is implemented as an extension of the Servlet 2.1 API. Both servlets and JSPs were originally developed by Sun Microsystems. Starting with version 1.2 of the JSP specification, JavaServer Pages have been developed under the Java Community Process. JSR 53 defines both the JSP 1.2 and Servlet 2.4 specifications and JSR 152 defines the JSP 2.0 specification. As of May 2006 the JSP 2.1 specification has been released under JSR 245 as part of Java EE 5.

JSP Syntax

A JavaServer Page may be broken down into the following pieces:

Static data

Static data is written out to the HTTP response exactly as it appears in the input file. Thus a valid JSP input would be a normal HTML page with no embedded java or actions. In that case, the same data would be sent in the response each and every time by the web server.

JSP directives

JSP directives control how the JSP compiler generates the servlet. The following directives are available:

<%@ include file="somefile.jsp" %>
  • page – There are several options to the page directive.
    • importresults in a Java import statement being inserted into the resulting file
    contentTypespecifies the content that is generated. This should be used if HTML is not used or if the character set is not the default character set.
    errorPageindicates the page that will be shown if an exception occurs while processing the HTTP request.
    isErrorPageif set to true, it indicates that this is the error page.
    isThreadSafeindicates if the resulting servlet is thread safe. 

    <%@ page import="java.util.*" %> //example import
<%@ page contentType="text/html" %> //example contentType
<%@ page isErrorPage=false %> //example for non error page
<%@ page isThreadSafe=true %> //example for a thread safe JSP
    Note: Only the "import" page directive can be used multiple times in the same JSP.

    <%@ taglib prefix="myprefix" uri="taglib/mytag.tld" %>

    JSP scripting elements and variables

    Standard scripting variables

    The following scripting variables are always available:

    Scripting elements

    There are three basic kinds of scripting elements that allow java code to be inserted directly into the servlet. 

    <%! int serverInstanceVariable = 1; %>
  • A scriptlet tag places the contained statements inside the _jspService() method of the java servlet class.
    • <% int localStackBasedVariable = 1; out.println(localStackBasedVariable); %>
  • An expression tag places an expression to be evaluated inside the java servlet class. Expressions should not be terminated with a semi-colon .
    • <%= "expanded inline data " + 1 %>

    JSP actions

    JSP actions are XML tags that invoke built-in web server functionality. The following actions are provided:

    jsp:includeSimilar to a subroutine, the Java servlet temporarily hands the request and response off to the specified JavaServer Page. Control will then return to the current JSP, once the other JSP has finished. Using this, JSP code will be shared between multiple other JSPs, rather than duplicated.
    jsp:paramCan be used inside a jsp:include, jsp:forward or jsp:params block. Specifies a parameter that will be added to the request's current parameters.
    jsp:forwardUsed to hand off the request and response to another JSP or servlet. Control will never return to the current JSP.
    jsp:pluginOlder versions of Netscape Navigator and Internet Explorer used different tags to embed an applet. This action generates the browser specific tag needed to include an applet.
    jsp:fallbackThe content to show if the browser does not support applets.
    jsp:getPropertyGets a property from the specified JavaBean.
    jsp:setPropertySets a property in the specified JavaBean.
    jsp:useBeanCreates or re-uses a JavaBean available to the JSP page.

    Examples of tags

    jsp:include
    





name:<%=request.getParameter("extraparam")%>
    jsp:forward
    In this forwarding example, the request is forwarded to "subpage.jsp". The request handling does not return to this page.

    jsp:plugin

    




Your browser does not support applets.
    The plugin example illustrates a uniform way of embedding applets in a web page. Before the advent of the <OBJECT> tag, there was no common way of embedding applets. This tag is poorly designed and hopefully future specs will allow for dynamic attributes (height="$", code="$", etc) and dynamic parameters. Currently, the jsp:plugin tag does not allow for dynamically called applets. For example, if you have a charting applet that requires the data points to be passed in as parameters, you can't use jsp:params unless the number of data points are constant. You can't, for example, loop through a ResultSet to create the jsp:param tags. You have to hand code each jsp:param tag. Each of those jsp:param tags however can have a dynamic name and a dynamic value.

    jsp:useBean
    The scope attribute can be request, page, session or application. It has the following meanings: The example above will use a Bean Manager to create an instance of the class com.foo.MyBean and store the instance in the attribute named "myBean". The attribute will be available for the life-time of the request. It can be shared among all of the JSPs that were included or forwarded-to from the main JSP that first received the request.

    JSP tag libraries

    In addition to the pre-defined JSP actions, developers may add their own custom actions using the JSP Tag Extension API. Developers write a Java class that implements one of the Tag interfaces and provide a tag library XML description file that specifies the tags and the java classes that implement the tags.

    Consider the following JSP.

    <%@ taglib uri="mytaglib.tld" prefix="myprefix" %>
...
 <%-- the start tag %>
...
 <%-- the end tag %>
...
    The JSP compiler will load the mytaglib.tld XML file and see that the tag 'myaction' is implemented by the java class 'MyActionTag'. The first time the tag is used in the file, it will create an instance of 'MyActionTag'. Then (and each additional time that the tag is used), it will invoke the method doStartTag() when it encounters the starting tag. It looks at the result of the start tag, and determines how to process the body of the tag. The body is the text between the start tag and the end tag. The doStartTag() method may return one of the following:
    NOTE: If tag extends the BodyTagSupport class, the method doAfterBody() will be called when the body has been processed just prior to calling the doEndTag(). This method is used to implement looping constructs.
    When it encounters the end tag, it invokes the doEndTag() method. The method may return one of two values:
    The myaction tag above would have an implementation class that looked like something below:

    public class MyActionTag extends  TagSupport 
    

public MyActionTag() 
    

//called for the start tag
public int doStartTag() 
    

//called at the end tag
}
    Add Body Tag description.

    Internationalization

    Internationalization in JSP is accomplished the same way as in a normal Java application, that is by using resource bundles.

    JSP 2.0

    The new version of the JSP specification includes new features meant to improve programmer productivity. Namely:

    Hello, $  <%-- same as: Hello, <%=request.getParameter("visitor")%> --%>

    Model-view-controller paradigm

    Sun recommends that the Model-view-controller pattern be used with the JSP files in order to split the presentation from request processing and data storage. Either regular servlets or separate JSP files are used to process the request. After the request processing has finished, control is passed to a JSP used only for creating the output. There are several platforms based on Model-view-controller pattern for web tiers (such as Struts and Spring framework).

    Example

    Regardless of whether the JSP compiler generates Java source code for a servlet or emits the byte code directly, it is helpful to understand how the JSP compiler transforms the page into a Java servlet. For an example, consider the following input JSP and its resulting generated Java Servlet.

    Input JSP 

    

    <%@ page errorPage="myerror.jsp" %>
<%@ page import="com.foo.bar" %>

    

    

<%! int serverInstanceVariable = 1;%>
...
<% int localStackBasedVariable = 1; %>

...

    

Resulting servlet

    

    package jsp_servlet;
import java.util.*;
import java.io.*;
import javax.servlet.*;
import javax.servlet.http.*;
import javax.servlet.jsp.*;
import javax.servlet.jsp.tagext.*;

    

    import com.foo.bar; //imported as a result of <%@ page import="com.foo.bar" %>
import ...

    

    class _myserlvet implements javax.servlet.Servlet, javax.servlet.jsp.HttpJspPage  catch ( Exception _exception ) 
}
}

    


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