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Enterprise Java Beans [EJB]

Enterprise JavaBean

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Simple EJB2 Architecture

Enterprise JavaBeans (EJB) is a managed, server-side component architecture for modular construction of enterprise applications.

The EJB specification is one of several Java APIs in the Java Platform, Enterprise Edition. EJB is a server-side model that encapsulates the business logic of an application. The EJB specification was originally developed in 1997 by IBM and later adopted by Sun Microsystems (EJB 1.0 and 1.1) in 1999[1] and enhanced under the Java Community Process as JSR 19 (EJB 2.0), JSR 153 (EJB 2.1) and JSR 220 (EJB 3.0).

The EJB specification intends to provide a standard way to implement the back-end 'business' code typically found in enterprise applications (as opposed to 'front-end' interface code). Such code was frequently found to address the same types of problems, and it was found that solutions to these problems are often repeatedly re-implemented by programmers. Enterprise JavaBeans were intended to handle such common concerns as persistence, transactional integrity, and security in a standard way, leaving programmers free to concentrate on the particular problem at hand.

Accordingly, the EJB specification details how an application server provides:

Additionally, the Enterprise JavaBean specification defines the roles played by the EJB container and the EJBs as well as how to deploy the EJBs in a container.

Contents

[hide]

[edit] History

[edit] Rapid adoption followed by criticism

 The neutrality of this section is disputed. Please see the discussion on the talk page. Please do not remove this message until the dispute is resolved. (December 2007)

This vision was persuasively presented by EJB advocates such as IBM and Sun Microsystems, and Enterprise JavaBeans were quickly adopted by large companies. Problems were quick to appear, however, and the reputation of EJBs began to suffer as a result. Some developers felt that the APIs of the EJB standard were far more complex than those developers were used to. An abundance of checked exceptions, required interfaces, and the implementation of the bean class as an abstract class were all unusual and counter-intuitive for many programmers. Granted, the problems that the EJB standard was attempting to address, such as object-relational mapping and transactional integrity, are complex. However many programmers found the APIs to be just as difficult if not more so, leading to a widespread perception that EJBs introduced complexity without delivering real benefits.

In addition, businesses found that using EJBs to encapsulate business logic brought a performance penalty. This is because the original specification only allowed for remote method invocation through CORBA (and optionally other protocols), even though the large majority of business applications actually do not require this distributed computing functionality. The EJB 2.0 specification addressed this concern by adding the concept of local interfaces which could be called directly without performance penalties by applications that were not distributed over multiple servers. It was introduced largely to address the performance problems that existed with EJB 1.0.[2]

The complexity issue, however, continued to hinder EJB's acceptance. Although high-quality developer tools made it easy to create and use EJBs by automating most of the repetitive tasks, these tools did not make it any easier to learn how to use the technology. Moreover, a counter-movement had grown up on the grass-roots level among programmers. The main products of this movement were the so-called 'lightweight' (i. e. in comparison to EJB) technologies of Hibernate (for persistence and object-relational mapping) and Spring Framework (which provided an alternate and far less verbose way to encode business logic). Despite their lacking the support of big businesses that EJBs had, these technologies grew in popularity and were adopted more and more by businesses who had become disillusioned with EJBs.

EJBs were promoted by Sun's Java Pet Store demo Java BluePrints. The use of EJBs was controversial and influential J2EE programmers such as Rod Johnson took positions in response to Java Pet Store that sought to deemphasize EJB use. Sun itself produced an alternative called Java Data Objects. Later, EJBs, Java Data Forms, and many of the ideas underlying Hibernate were combined to form EJB 3.0 which included the Java Persistence API and Plain Old Java Objects (POJOs). EJB 3.0 was less heavy weight than EJB 2.0 and provided more choices to developers.

[edit] Reinventing EJBs

Gradually an industry consensus emerged that the original EJB specification's primary virtue — enabling transactional integrity over distributed applications — was of limited use to most enterprise applications, and the functionality delivered by simpler frameworks like Spring and Hibernate was more useful. Accordingly, the EJB 3.0 specification (JSR 220) was a radical departure from its predecessors, following this new paradigm. It shows a clear influence from Spring in its use of POJOs, and its support for dependency injection to simplify configuration and integration of heterogeneous systems. Gavin King, the creator of Hibernate, participated in the EJB 3.0 process and is an outspoken advocate of the technology. Many features originally in Hibernate were incorporated in the Java Persistence API, the replacement for entity beans in EJB 3.0. The EJB 3.0 specification relies heavily on the use of annotations, a feature added to the Java language with its 5.0 release, to enable a much less verbose coding style.

Accordingly, in practical terms EJB 3.0 is very nearly a completely new API, bearing little resemblance to the previous EJB specifications.

[edit] Types

An EJB container holds three major types of beans:

  • Session Beans that can be either "Stateful" or "Stateless"
  • Entity Beans that can be either "CMP" (Container managed persistence) or "BMP" (Bean managed persistence)
  • Message Driven Beans (also known as MDBs or Message Beans)

Stateful Session Beans are distributed objects having state: that is, they keep track of which calling program they are dealing with throughout a session. For example, checking out in a web store might be handled by a stateful session bean that would use its state to keep track of where the customer is in the checkout process. Stateful session beans' state may be persisted, but access to the bean instance is limited to only one client.

Stateless Session Beans are distributed objects that do not have state associated with them thus allowing concurrent access to the bean. The contents of instance variables are not guaranteed to be preserved across method calls. The lack of overhead to maintain a conversation with the calling program makes them less resource-intensive than stateful beans. Sending an e-mail to customer support might be handled by a stateless bean, since this is a one-off operation and not part of a multi-step process.

Message Driven Beans were introduced in the EJB 2.0 specification that is supported by J2EE 1.3 or higher. The message bean represents the integration of JMS (Java Message Service) with EJB to create an entirely new type of bean designed to handle asynchronous JMS messages. Message Driven Beans are distributed objects that behave asynchronously. That is, they handle operations that do not require an immediate response. For example, a user of a website clicking on a "keep me informed of future updates" box may trigger a call to a Message Driven Bean to add the user to a list in the company's database. (This call is asynchronous because the user does not need to wait to be informed of its success or failure.) These beans subscribe to JMS (Java Message Service) message queues or message topics. They were added in the EJB 2.0 specification to allow event-driven processing inside EJB Container. Unlike other types of beans, MDB does not have a client view (Remote/Home interfaces), i. e. clients can not look-up an MDB instance. It just listens for any incoming message on a JMS queue (or topic) and processes them automatically.

Previous versions of EJB also used a type of bean known as an Entity Bean. These were distributed objects having persistent state. Beans in which their container managed the persistent state were said to be using Container-Managed Persistence (CMP), whereas beans that managed their own state were said to be using Bean-Managed Persistence (BMP). Entity Beans were replaced by the Java Persistence API in EJB 3.0, though as of 2007, CMP 2.x style Entity beans are still available for backward compatibility.

Other types of Enterprise Beans have been proposed. For instance, Enterprise Media Beans (JSR 86) address the integration of multimedia objects in Java EE applications.

[edit] Execution

EJBs are deployed in an EJB container within the application server. The specification describes how an EJB interacts with its container and how client code interacts with the container/EJB combination. The EJB classes used by applications are included in the javax.ejb package. (The javax.ejb.spi package is a service provider interface used only by EJB container implementations.)

With EJB 2.1 and earlier, each EJB had to provide a Java implementation class and two Java interfaces. The EJB container created instances of the Java implementation class to provide the EJB implementation. The Java interfaces were used by client code of the EJB.

The two interfaces, referred to as the Home and the Component interface, specified the signatures of the EJB's remote methods. The methods were split into two groups:

Class methods 
Not tied to a specific instance, such as those used to create an EJB instance (factory method) or to find an existing entity EJB (see EJB Types, above). These were declared by the Home interface.
Instance methods 
I. e. methods tied to a specific instance. These are placed in the Component interface.

Because these are merely Java interfaces and not concrete classes, the EJB container must generate classes for these interfaces that will act as a proxy in the client. Client code invokes a method on the generated proxies that in turn places the method arguments into a message and sends the message to the EJB server.

[edit] Remote communication

The EJB specification requires that EJB containers support accessing the EJBs using RMI-IIOP. EJBs may be accessed from any CORBA application or provide Web Services.

[edit] Transactions

EJB containers must support both container managed ACID transactions and bean managed transactions. Container-managed transactions use a declarative syntax for specifying transactions in the deployment descriptor.

[edit] Events

JMS is used to send messages from the beans to client objects, to let clients receive asynchronous messages from these beans. MDB can be used to receive messages from client applications asynchronously using either a JMS Queue or a Topic.

[edit] Naming and directory services

Clients of the EJB locate the Home Interface implementation object using JNDI. The Home interface may also be found using the CORBA name service. From the home interface, client code can find entity beans, as well as create and delete existing EJBs.

[edit] Security

The EJB Container is responsible for ensuring the client code has sufficient access rights to an EJB.

[edit] Deployment

The EJB specification defines a mechanism that allows EJBs to be deployed in a consistent manner regardless of the specific EJB platform that is chosen. Information about how the bean should be deployed (such as the name of the home or remote interfaces, whether and how to store the bean in a database, etc.) are specified in the deployment descriptor.

The deployment descriptor is an XML document having an entry for each EJB to be deployed. This XML document specifies the following information for each EJB:

  • Name of the Home interface
  • Java class for the Bean (business object)
  • Java interface for the Home interface
  • Java interface for the business object
  • Persistent store (only for Entity Beans)
  • Security roles and permissions
  • Stateful or Stateless (for Session Beans)

EJB containers from many vendors require more deployment information than that in the EJB specification. They will require the additional information as separate XML files, or some other configuration file format. An EJB platform vendor generally provides their own tools that will read this deployment descriptor, and possibly generate a set of classes that will implement the Home and Remote interfaces.

Since EJB3.0 (JSR 220), the XML descriptor is replaced by Java annotations set in the Enterprise Bean implementation (at source level), although it is still possible to use an XML descriptor instead of (or in addition to) the annotations. If an XML descriptor and annotations are both applied to the same attribute within an Enterprise Bean, the XML definition overrides the corresponding source-level annotation.

[edit] Version history

[edit] EJB 3.1, in development

JSR 318. The purpose of the Enterprise JavaBeans 3.1 specification is to further simplify the EJB architecture by reducing its complexity from the developer's point of view, while also adding new functionality in response to the needs of the community:

Topics under consideration:

  • Local view without interface
  • .war packaging of EJB components
  • EJB Lite: definition of a subset of EJB
  • Portable EJB Global JNDI Names
  • Singletons
  • Application Initialization and Shutdown Events
  • EJB Timer Service Enhancements
  • Simple Asynchrony

[edit] EJB 3.0, final release (2006)

JSR 220 - Major changes: This release made it much easier to write EJBs, using 'annotations' rather than the complex 'deployment descriptors' used in version 2.x. The use of home and remote interfaces and the ejb-jar.xml file were also no longer required in this release, having been replaced with a business interface and a bean that implements the interface.

[edit] EJB 2.1, final release (2003-11-24)

JSR 153 - Major changes:

  • Web service support (new): stateless session beans can be invoked over SOAP/HTTP. Also, an EJB can easily access a Web service using the new service reference.
  • EJB timer service (new): Event-based mechanism for invoking EJBs at specific times.
  • Message-driven beans accepts messages from sources other than JMS.
  • Message destinations (the same idea as EJB references, resource references, etc.) has been added.
  • EJB query language (EJB-QL) additions: ORDER BY, AVG, MIN, MAX, SUM, COUNT, and MOD.
  • XML schema is used to specify deployment descriptors, replaces DTDs

[edit] EJB 2.0, final release (2001-08-22)

JSR 19 - Major changes: Overall goals:

  • The standard component architecture for building distributed object-oriented business applications in Java.
  • Make it possible to build distributed applications by combining components developed using tools from different vendors.
  • Make it easy to write (enterprise) applications: Application developers will not have to understand low-level transaction and state management details, multi-threading, connection pooling, and other complex low-level APIs.
  • Will follow the Write Once, Run Anywhere - philosophy of Java. An enterprise Bean can be developed once, and then deployed on multiple platforms without recompilation or source code modification.
  • Address the development, deployment, and runtime aspects of an enterprise application’s life cycle.
  • Define the contracts that enable tools from multiple vendors to develop and deploy components that can interoperate at runtime.
  • Be compatible with existing server platforms. Vendors will be able to extend their existing products to support EJBs.
  • Be compatible with other Java APIs.
  • Provide interoperability between enterprise Beans and Java 2 Platform Enterprise Edition (J2EE) components as well as non-Java programming language applications.
  • Be compatible with the CORBA protocols (RMI-IIOP).

[edit] EJB 1.1, final release (1999-12-17)

Major changes:

  • XML deployment descriptors
  • Default JNDI contexts
  • RMI over IIOP
  • Security - role driven, not method driven
  • Entity Bean support - mandatory, not optional

Goals for Release 1.1:

  • Provide better support for application assembly and deployment.
  • Specify in greater detail the responsibilities of the individual EJB roles.

[edit] EJB 1.0 (1998-03-24)

Announced at JavaOne 1998, Sun's third Java developers conference (March 24 through 27) Goals for Release 1.0:

  • Defined the distinct “EJB Roles” that are assumed by the component architecture.
  • Defined the client view of enterprise Beans.
  • Defined the enterprise Bean developer’s view.
  • Defined the responsibilities of an EJB Container provider and server provider; together these make up a system that supports the deployment and execution of enterprise Beans.

[edit] External links

[edit] References

  1. ^ J2EE Design and Development, © 2002 Wrox Press Ltd., p. 5.
  2. ^ J2EE Design and Development, © 2002 Wrox Press Ltd., p. 19.
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Java Server Pages... A web technology by Sun Microsystem.

JavaServer Pages

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JavaServer Pages (JSP) is a server side Java technology that allows software developers to create dynamically generated web pages, with HTML, XML, or other document types, in response to a Web client request to a Java Web Application container (server). To allow this an HTML page is given the file extension .jsp and an XML markup page is given the file extension .jspx for the Java server(container) to recognise the file requires JSP processing before sending it to the client. JSP pages are loaded in the server and operated from a structured special installed Java server packet called a J2EE Web Application often packaged as a .war or .ear file archive.

The technology allows Java code and certain pre-defined actions to be embedded into static page content and compiled on the server at runtime of each page request. Both the Java Server (J2EE specification) and the page scripts and/or extended customised programming added operate by(in the runtime context of being loaded programs used) a special pre-installed base program called a Virtual Machine that integrates with the host Operating System, this type being the Java Virtual Machine(JVM).

Because either, both a Compiler-JVM set(called an SDK or JDK) or the lone JVM(called a (JRE) Java Runtime Environment) is made for most computer platform OSs and the compiled programs for the JVM are compiled into special Java Byte code files for the JVM the Byte-code files(compiled Java program .class files) can be effectively transferred between platforms with no requirement to be recompiled excepting versioning compatibility, or special circumstance. The source code for these J2EE servlet or J2EE JSP programs is almost always supplied with J2EE JSP material and J2EE Web Applications because the server must call the compiler when loading them. These small extension programs (custom tags,servlets,beans,page scripting) are variable and likely to be updated or changed either shortly before runtime or intermittently but particularly when sending JSP page requests themselves, it requires the JSP server to have access to a Java compiler(SDK or JDK) and the required source code (not simply the JVM JRE and byte code class files) to successfully exploit the method of serving.

JSP syntax has two basic forms, scriptlet and markup though fundamentally the page is either HTML or XML markup. Scriptlet tagging(called Scriptlet Elements)(delimited) blocks of code with the markup are not effectively markup and allows any java server relevant API(e.g. the servers running binaries themselves or datatbase connections API or java mail API) or more specialist JSP API language code to be embedded in an HTML or XML page provided the correct declarations in the JSP file and file extension of the page are used. Scriptlet blocks do not require to be completed in the block itself only the last line of the block itself being completed syntactically correctly as a statement is required, it can be completed in a later block. This system of split inline coding sections is called step over scripting because it can wrap around the static markup by stepping over it. At runtime(during a client request) the code is compiled and evaluated, but compilation of the code generally only occurs when a change to the code of the file occurs. 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. JVM operated Tag libraries provide a platform independent way of extending the capabilities of a Web server. Note that not all company makes of Java servers are J2EE specification compliant.

Contents

[hide]

[edit] Servlets

Architecturally, JSP may be viewed as a high-level abstraction of Java servlets. 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.

JSPs are compiled into servlets by a JSP compiler. The compiler either generates a servlet in Java code that is then compiled by the Java compiler, or it may compile the servlet to byte code which is directly executable. JSPs can also be interpreted on-the-fly, reducing the time taken to reload changes.

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" %>
        }
    }
 }

[edit] 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 computer 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 Barracuda, Apache Struts, Stripes, and the Spring MVC framework).

[edit] Coding JSP

JSP pages embed tags within a HTML or XML document that are evaluated by the compiler. This can be done through the use of scripting tags similar to those used in PHP or ASP.NET, or by importing a JSP Tag Library.

[edit] JSP Directives

JSP directives are added at the top of a JSP page. These 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 
The page directive has several attributes:
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 address of the page that should 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 
A boolean indicating whether 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".
session
To 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.
buffer
To set Buffer Size. The default is 8k and it is advisable that you increase it.
isELIgnored
Defines whether EL (Expression Language) expressions are ignored when the JSP is translated.
language
Defines the scripting language used in scriptlets, expressions and declarations. Right now, the only possible value is "java".
extends
Defines 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.
info
Defines a String that gets put into the translated page, just so that you can get it using the generated servlet's inherited getServletInfo() method.
pageEncoding
Defines 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" %>

[edit] Implicit Objects

The JSP container exposes a number of implicit objects that can be used 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 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.

[edit] 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. 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 "";
   }
%>
  • A scriptlet tag places all of the statements contained within it, 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 %>

[edit] 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.

[edit] Examples of tags

[edit] 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>

[edit] 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.

[edit] 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.

[edit] 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.

[edit] 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.

[edit] 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: ok 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

[edit] Internationalization

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

[edit] JSP 2.0

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

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());
            }
        }
    %>
 --%>

[edit] See also

[edit] Further reading

[edit] External links

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