jsp

The JSP syntax adds additional XML-like 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.

"JavaServer Pages" is a technology released by Sun.

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 at Sun Microsystems, initially created by Anselm Baird-Smith and later elaborated on as a specification by Satish Dharmaraj. 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 2.3 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 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 normal HTML page with no embedded java or actions would be valid JSP input. In that case, the same data would be sent in the response each and every time by the web server to the browser.

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. Included files generally have the extension "jspf" (for JSP Fragment):

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

• 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 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 objects

JSP Implicit Objects

The following JSP implicit objects are exposed by the JSP container and can be referenced by the programmer:

• 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.
• config – Provides servlet configuration data.
• application – Data shared by all JSPs and servlets in the application.
• exception – Exceptions not caught by application code.

Scripting elements

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

• A declaration tag places a variable definition inside the body of the java servlet class. Static data members may be defined as well.

<%! 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:

Examples of tags

jsp:include

<html>
<head></head>
<body>
<jsp:include page="mycommon.jsp" >
    <jsp:param name="extraparam" value="myvalue" />
</jsp:include>
name:<%=request.getParameter("extraparam")%>
</body></html>

jsp:forward

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

In this forwarding example, the request is forwarded to "subpage.jsp". The request handling does not return to this page.

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 plugin example illustrates a <html> 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 global variable.

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" %>
 ...
<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   
}

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.

Hello, ${param.visitor}  <%-- 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 Apache 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" %>

 <html>
 <head>
 <%! int serverInstanceVariable = 1;%>
 ...
 <% int localStackBasedVariable = 1; %>
 <table>
 <tr><td><%= "expanded inline data " + 1 %></td></tr>
 ...

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 _myservlet implements javax.servlet.Servlet, javax.servlet.jsp.HttpJspPage {
     //inserted as a
     //result of <%! int serverInstanceVariable = 1;%>
     int serverInstanceVariable = 1; 
     ...

     public void _jspService( javax.servlet.http.HttpServletRequest request,
                              javax.servlet.http.HttpServletResponse response )
       throws javax.servlet.ServletException,
              java.io.IOException
     {
         javax.servlet.ServletConfig config = ...;//get the servlet config
         Object page = this;
         PageContext pageContext = ...;//get the page context for this request 
         javax.servlet.jsp.JspWriter out = pageContext.getOut();
         HttpSession session = request.getSession( true );
         try {
             out.print( "<html>\r\n" );
             out.print( "<head>\r\n" );
             ...
             //from <% int localStackBasedVariable = 1; %>
             int localStackBasedVariable = 1; 
             ...
             out.print( "<table>\r\n" );
             out.print( "   <tr><td>" );
             //note, toStringOrBlank() converts the expression into a string or if
             // the expression is null, it uses the empty string.
             //from <%= "expanded inline data " + 1 %>
             out.print( toStringOrBlank( "expanded inline data " + 1 ) );
             out.print( "   </td></tr>\r\n" );
             ...
         } catch ( Exception _exception ) {
             //clean up and redirect to error page in <%@ page errorPage="myerror.jsp" %>
         }
    }
 }

Publications

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