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Java Server Pages

posted Aug 13, 2009, 2:41 AM by Thiyagaraaj M   [ updated Aug 16, 2009, 9:31 PM ]

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-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. JSPs can also be interpreted on-the-fly, reducing the time taken to reload changes.


JSP and Servlets

Architecturally, JSP may 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. 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 with correct library

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. 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. Default value is false.isThreadSafe Indicates if the resulting servlet is thread safe.autoFlush To autoflush the contents.A value of true, the default, indicates that the buffer should be flushed when it is full. A value of false, rarely used, indicates that an exception should be thrown when the buffer overflows. A value of false is illegal when also using buffer="none".sessionTo maintain session. A value of true (the default) indicates that the predefined variable session (of type HttpSession) should be bound to the existing session if one exists, otherwise a new session should be created and bound to it. A value of false indicates that no sessions will be used, and attempts to access the variable session will result in errors at the time the JSP page is translated into a servlet.bufferTo set Buffer Size. The default is 8k and it is advisable that you increase it.isELIgnoredDefines whether EL expressions are ignored when the JSP is translated.languageDefines the scripting language used in scriptlets, expressions and declarations. Right now, the only possible value is "java".extendsDefines the superclass of the class this JSP will become. You won't use this unless you REALLY know what you're doing - it overrides the class hierarchy provided by the Container.infoDefines a String that gets put into the translated page, just so that you can get it using the generated servlet's inherited getServletInfo() method.pageEncodingDefines the character encoding for the JSP. The default is "ISO-8859-1"(unless the contentType attribute already defines a character encoding, or the page uses XML document syntax).
<%@ 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 --%>
<%@ page session="true" %> <%-- example for using session binding --%>
<%@ page autoFlush="true" %> <%-- example for setting autoFlush --%>
<%@ page buffer="20kb" %> <%-- example for setting Buffer Size --%>
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" %>


SP 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(output page).page The servlet itself.pageContext PageContext instance that contains data associated with the whole page. A given HTML page may be passed among multiple JSPs.request The HttpServletRequest object that provides HTTP request information.response The HttpServletResponse object that can be used to send data back to the client.session The HttpSession 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.

  • declaration tag places a variable definition inside the body of the java servlet class. Static data members may be defined as well. Also inner classes should be defined here.
<%! int serverInstanceVariable = 1; %>

Declaration tags also allow methods to be defined.

<%!
/**
* Converts the Object into a string or if
* the Object is null, it returns the empty string.
*/
public String toStringOrBlank( Object obj ){
if(obj != null){
return obj.toString();
}
return "";
}
%>
  • 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 %>
  • Also we can use the following tag to give comments in jsp:
<%-- give your comments here --%>



JSP actions

JSP actions are XML tags that invoke built-in web server functionality. They are executed at runtime. Some are standard and some are custom (which are developed by Java developers). The following list contains the standard ones:

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 multiple 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 JavaBean.jsp:setProperty Sets a property in the specified JavaBean.jsp:useBean Creates or re-uses a JavaBean available to the JSP page.



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. Currently, the jsp:plugin tag does not allow for dynamically called applets. For example, jsp:params cannot be used with a charting applet that requires the data points to be passed in as parameters unless the number of data points is constant. You cannot, for example, loop through a ResultSet to create the jsp:param tags. Each jsp:param tag must be hand-coded. However, each of those jsp:param tags can have a dynamic name and a dynamic value.


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(){ ... }
}

Add Body Tag description.

If you want to iterate the body a few times, then the java class (tag handler) implements IterationTag interface. It returns EVAL_BODY_AGAIN - which means to invoke the body again.


JSP Standard Tag Library (JSTL)

The JavaServer Pages Standard Tag Library (JSTL) is a component of the Java EE Web application development platform. It extends the JSP specification by adding a tag library of JSP tags for common tasks, such as XML data processing, conditional execution, loops and internationalization.


JSP Technology in the Java EE 5 Platform

The focus of Java EE 5 has been ease of development by making use of Java language annotations that were introduced by J2SE 5.0. JSP 2.1 supports this goal by defining annotations for dependency injection on JSP tag handlers and context listeners.

Another key concern of the Java EE 5 specification has been the alignment of its webtier technologies, namely JavaServer Pages (JSP),JavaServer Faces (JSF), and JavaServer Pages Standard Tag Library (JSTL).

The outcome of this alignment effort has been the Unified Expression Language (EL), which integrates the expression languages defined by JSP 2.0 and JSF 1.1.

The main key additions to the Unified EL that came out of the alignment work have been:

A pluggable API for resolving variable references into Java objects and for resolving the properties applied to these Java objects, Support for deferred expressions, which may be evaluated by a tag handler when needed, unlike their regular expression counterparts, which get evaluated immediately when a page is executed and rendered, and Support for lvalue expression, which appear on the left hand side of an assignment operation. When used as an lvalue, an EL expression represents a reference to a data structure, for example: a JavaBeans property, that is assigned some user input. The new Unified EL is defined in its own specification document, which is delivered along with the JSP 2.1 specification.

Thanks to the Unified EL, JSTL tags, such as the JSTL iteration tags, can now be used with JSF components in an intuitive way.

JSP 2.1 leverages the Servlet 2.5 specification for its web semantics

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 display parameter values.
Hello, ${param.visitor} <%-- same as: Hello, <%=request.getParameter("visitor")%> --%>
  • A clearer way to navigate nested beans
// consider some beans.
class Person {
String name;
// person nests an organization bean.
Organization organization;

public String getName() { return this.name; }
public Organization getOrganization() { return this.organization; }
}

class Organization {
String name;
public String getName() { return this.name; }
}


// then if an instance of Person was to be placed onto a request attribute under the name "person"
<!-- the JSP would have -->
Hello, ${person.name}, of company ${person.organization.name}
<%-- second expression same as
<% Person p = (Person) request.getAttribute("person");
if (p != null) {
Organization o = p.getOrganization();
if (o != null) {
out.print(o.getName());
}
}
%>
--%>

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 andcomputer 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 BarracudaApache Struts or Spring MVC 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 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><%= toStringOrBlank( "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>" );
//from <%= toStringOrBlank( "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" %>
}
}
}

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