JavaServer Pages

The JSP syntax adds additional XML tags, called JSP actions, to be used to invoke built-in functionally. 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 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. In either case, it is helpful to understand how the JSP compiler transform the page into a Java servlet. For an example, see the following input, and its resulting generated java servlet.

JSP Syntax

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

• static data such as HTML
• JSP directives such as the include directive
• JSP scripting elements and variables
• JSP actions
• custom tags

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 with no embedded java or actions. In that case, the same data would be sent in the resonse each and every time by the web server. Of course, the point of JSP is to allow dynamic data to be inserted into the static content.

JSP Directives

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

• include . The include directive informs the JSP compiler to include a complete file into the current file. It is as if the contents of the included file were pasted directly into the original file. This functionally is similar to the one provided by the C preprocessor.

 <%@ include file="somefile.ext" %>

• page . There are several options to the page directive.

 <%@ page import="java.util.*" %> //example import
 <%@ page contentType="text/html" %> //example contentType
 <%@ page isErrorPage=false %> //example for non error page
 <%@ page isThreadSave=true %> //example for a thread safe JSP

• taglib . The taglib directive indicates that a JSP tag library is to be used. The directive requires that a prefix be specified (much like a namespace in C++) and the URI for the tag library

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

JSP Scripting Elements and Variables

Standard Scripting Variables
The following scripting variables are always available:

• out . The JSPWriter used to write the data to the response stream.
• page . The servlet itself.
• pageContext . A PageContext instance that contains data associated with the whole page. A given HTML page may be passed among multiple JSPs.
• request . The HTTP request object.
• response . The HTTP response object.
• session . The HTTP session object that can be used to track information about a user from one request to another.

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

• There is a tag that allows the code to be placed just inside the class. This can be used to define data members of the java class (static data members may be defined as well).

 <%! int serverInstanceVariable = 1;%>

• There is a tag that allow code to be placed within the _jspService() method of the resulting.

 <% int localStackBasedVariable= 1; %>

• There is a tag that can allow code to be expanded inline and written directly out to the HTTP response.

 <%= "expanded inline data " + 1 %>

JSP Actions

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

Examples of tags:

--

 <jsp:include page="mycommon.jsp" >
   <jsp:param name="extraparam" value="myvalue" />
 </jsp:include>

In the example above, generation of the output is handed off to a common file, mycommon.jsp. An extra request parameter with the name "extraparam" and value "myvalue" is set in the request before passing control to mycommon.jsp. Once mycommon.jsp has written all of its output to the response, control returns to the current JSP.

--

 <jsp:forward page="subpage.jsp" >
    <jsp:param name="forwardedFrom" value="this.jsp" />
 </jsp:forward>

In this forwarding example, control is handed over completely to the JSP called subpage.jsp. As in the first example, an extra parameter this time with the name "forwardedFrom" is set in the request before the hand-off. Control will never return to this JSP.

--

 <jsp:plugin type=applet height="100%" weight="100%"
       archive="myjarfile.jar,myotherjar.jar"
       codebase="/applets"
       code="com.foo.MyApplet" >
   <jsp:params>
      <jsp:param name="enableDebug" value="true" />
   </jsp:params>
   <jsp:fallback>
     Your browser does not support applets.
   </jsp:fallback>
 </jsp:plugin>

The plug-in example illustrates a uniform way of embedding applets in a web page. Before the advent of the <OBJECT> tag, there was not a common way of embedding applets.

--

The scope attribute below can be request, page, session or application. It has the following meanings:

• request - the attribute is available for the lifetime of the request. Once the request has been processed by all of the JSPs, the attribute will be de-referenced.
• page - the attribute is available for the current page only.
• session - the attribute is available for the lifetime of the user's session.
• application - the attribute is available to every instance and is never de-referenced. Same as a static (i.e. global) variable.

 <jsp:useBean id="myBean" "com.foo.MyBean" scope="request" />
 <jsp:getProperty name="myBean" property="lastChanged" />
 <jsp:setProperty name="myBean" property="lastChanged" value="<%= new Date()%>" />

The example above will use a Bean Manager to instantiate an instance of the class com.foo.MyBean and store the class 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" %>
  ...
 <myprefix:myaction> <%-- the start tag %>
  ...
 </myprefix:myaction> <%-- 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 instantiate 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:

• SKIP_BODY - the body between the tag is not processed
• EVAL_BODY_INCLUDE - evaluate the body of the tag
• EVAL_BODY_TAG - evaluate the body of the tag and push the result onto stream (stored in the body content property of the tag).

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.

• EVAL_PAGE - this indicates that the rest of the JSP file should be processed.
• SKIP_PAGE - this indicates that no further processing should be done. Control leaves the JSP page. This is what is used for the forwarding action.

The myaction tag above would have an implementation class that looked like something below:

 public class MyActionTag extends  TagSupport {
    //Releases all instance variables.
    public void release() {...}

    public MyActionTag() { ... }
    //called for the start tag
    public int doStartTag() { ... }
    //called at the end tag   
    public int doEndTag() throws JspTagException { ... }

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[?].

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. 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. This JSP should contain only HTML, XML plus either the built-in JSP actions or custom JSP tag extensions. The JSP file may make use of Java beans to get the data.

Additional Information

For more information on JSPs, see:

• J2EE
• Sun's JSP product description at http://java.sun.com/products/jsp/index