JavaServer Pages

JSP or JavaServer Pages, known to some as the Java Scripting Preprocessor, is a Java technology that allows developers to dynamically generate HTML, XML or some other type of web page. 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 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 transforms the page into a Java servlet. For an example, see the following input, and its resulting generated Java servlet.

JSP and Servlets

Architecturally speaking, you can view JSP as a high-level abstraction of servlets that is implemented as an extension of the Servlet 2.1 API.

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 response 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 JSP 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 functionality is similar to the one provided by the C preprocessor.

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

page – There are several options to the page directive.

import	results in a java import statement being inserted into the resulting file
contentType	specifies 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.
errorPage	indicates the page that will be shown if an exception occurs while processing the HTTP request.
isErrorPage	if set to true, it indicates that this is the error page.
isThreadSafe	indicates 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 – 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 description.

    <%@ 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

There 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 java servlet as a complete separate statement.

    <% int localStackBasedVariable = 1; %>

There is a tag that can allow code to be expanded inline and written directly out to the HTTP response. The semi-colon is not to be used here as the code will be evaluated to an expression.

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

JSP Actions

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

jsp:include: Similar 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 two other JSPs, rather than duplicated.
jsp:param: Can 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:forward: Used to hand off the request and response to another JSP or servlet. Control will never return to the current JSP.
jsp:plugin: Older 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:fallback: The content to show if the browser does not support applets.
jsp:getProperty: Gets a property from the specified Java bean.
jsp:setProperty: Sets a property in the specified Java bean.
jsp:useBean: Creates or re-uses a Java bean available to the JSP page.

Examples of tags

jsp:include =

<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

<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

<jsp:plugin type=applet height="100%" width="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 no common way of embedding applets. This tag is poorly designed and hopefully future specs will allow for dynamic attributes (height="${param.height}", code="${chart}", 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

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

The scope attribute 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.

The example above will use a Bean Manager to create 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 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:

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.

When it encounters the end tag, it invokes the doEndTag() method. The method may return one of two values:

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.

JSP 2.0

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

An Expression Language (EL) which allows developers to create Velocity style templates (among other things).
A faster/easier way to create new tags.

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.

Publications

JavaServer Pages, Third Edition, Hans Bergen, O'Reilly & Associates, Inc., Sebastopol, California, 2003. ISBN 0-596-00563-6

External links

Sun's JSP product description (http://java.sun.com/products/jsp/)de:JavaServer Pages

es:Java Server Pages fr:JavaServer Pages hr:JSP ja:JavaServer Pages sv:Java Server Pages

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Categories: Java platform