EJB Architecture and Design What is EJB z

EJB Architecture and Design

What is EJB? z An EJB is just a collection of Java classes and XML file, bundled into a single unit. The Java classes must follow certain rules and provide certain callback methods. z EJB is just a specification. It is not a product. z EJBs are reusable components.

What is EJB? z EJB is a widely-adopted server-side component architecture for J 2 EE. z EJB components are designed to encapsulate business logic, and to protect the application developer from having to worry about system level issues.

Contents Services provided by EJB container Circumstances of EJB component usage How an EJB component looks like? View of an EJB component by client programmer and EJB developer z Mechanisms by which EJB container provides its services z z z Rules an EJB developer must follow and how to use EJBs in a web architecture?

Key features of EJB technology z EJB components are server-side components written entirely in the Java programming language z EJB components contain business logic only - no System-level programming z System-level services (i. e. "plumbing") such as transactions, security, Life-cycle, threading, persistence, etc. are automatically managed for the EJB component by the EJB server

Key features of EJB technology z EJB architecture is inherently transactional, distributed, portable, multi-tier, scalable and secure z EJB components are fully portable across any EJB server and any OS, work with any client. z Components are declaratively customized z There are four major parts to every bean: the home interface, the remote interface, the implementation class and the XML deployment descriptor

EJB vs Java. Beans z The Java. Beans architecture is meant to provide a format for general-purpose components whereas the EJB architecture provides a format for encapsulation and management of business logic. z Java. Beans has tier of execution at Client and EJB has at Server (specifically business logic tier)

EJB vs Java. Beans z In Java. Beans the runtime execution environment provides services like Java libraries, Java application etc. The EJB runtime environment provides services of Persistence, declarative transactions and security, connection pooling and lifecycle services.

Varieties of Beans z Session Beans Ù Stateful session bean Ù Stateless session bean z Entity Beans Ù With container-managed persistence Ù With bean-managed persistence z Message-Driven Beans

Why use EJBs in your design? z EJB specification provides enterprise-level services, that is, it provides software services that are fundamental to an organization’s purpose. z EJB’s API was designed to keep the application programmer from having to provide systems-level services, so that they are free to concentrate on business logic.

Why use EJBs in your design? z A requirement of any of the services provided by an EJB container like transactions, scalability, persistence, security, future growth possibilities is an appropriate reason to use EJB in the design of the application.

EJB Architecture Client Application Logic Data J 2 EE Application Server RDBMS EJB Container Corba Client Application JMS JTA Java Mail JDBC Entity Bean RMI Session Bean Mail

Roles in EJB Development z EJB provider - a person who develops EJB Components z EJB Deployer - a person responsible for deploying EJB’s in EJB server z Application Server/ EJB Container Vendor one who provides application server on which the application is deployed

Roles in EJB Development z Application assembler - one who combine the EJB components with other software to make a complete application z System administrator - one who manages the application after it has been deployed into a target environment.

Roles in EJB Development EJB Provider Application Assembler Deployer App Server/ EJB Container Provider System Administrator

EJB Container and its Services z A container is an execution environment for a component. The component lives in the container and the container provides the services for the component. z Similarly, a container lives in an application server, which provides an execution environment for it and other containers.

Services provided by an EJB container z Persistence Ù Ex: simple connection pooling, automatic persistence, etc. EJBs created with application development tools will encapsulate data access in components.

Services provided by an EJB container z z z z Declarative transactions Data caching Declarative Security Error Handling Component Framework for Business Logic Scalability and Fall-Over Portability Manageability

How the Container Provides Services There are three basic ideas: z First, there are clearly defined responsibilities between the various parts of an application using EJB component namely the client, the EJB container and the EJB component. The definition of these responsibilities is formally known as a contract. z Second, the services that the container provides are defined in such a way that they are orthogonal to the component. In other words, security, persistence, transactions are separate from the Java files that implement the business logic of the component.

How the Container Provides Services z Third, the container interposes on each and every call to an EJB component so that it can provide its services. In other words, the container puts itself between the client and the component on every single business method call.

Contracts EJB Container/Application Server Enterprise Java. Bean Client

Rules for the bean programmer z The developer of the EJB component must implement the business methods in the implementation class z The bean provider must implement the ejb. Create(), ejb. Post. Create(), ejb. Remove() methods and the ejb. Find<METHOD>() methods if the bean is an entity with bean managed persistence z The bean provider must define the enterprise bean’s home and remote interfaces z For session beans, the bean provider must implement the container callbacks defined in the javax. ejb. Session. Bean interface

Rules for the bean programmer z For entity beans, the provider must implement the container callbacks defined in the javax. ejb. Entity. Bean interface z The bean provider must not use programming practices that would interfere with the container’s runtime management of the enterprise bean instances

Interposition : method call to an EJB Container from a remote client z First, the client makes a call on the RMI stub z This RMI stub interposes on the method call in order to marshal parameters and send the information across the network z A skeleton on the server side unmarshals the parameters and delivers them to the EJB Container

Interposition diagram Client RMI Stub Network RMI Stub Container generated class Interposition class EJB

Interposition : from EJB Container to EJBs z The container will examine the security credentials of the caller of the method z It will start or join with any required transactions z It will make any necessary calls to persistence functions z It will trigger various callbacks to allow the EJB Component to acquire resources z Only after all this is done will the actual business method be called z Once it is called, the container will do some more work with transactions, persistence, callbacks and returns data or exception to the remote client

Working with EJBs The Enterprise Java. Beans specification is written for three audiences: z The Client developer z The EJB container developer

EJB Clients z EJB Clients are applications that access EJB components in EJB containers. There are two possible types. The first category is application clients which are stand-alone applications accessing the EJB components using the RMIIIOP protocol. The second category of application clients are components in the web container. They are java servlets and JSPs which also access the EJB components via the RMI-IIOP protocol.

The Client Developer’s View z The client has a smaller set of concerns then a bean developer with regard to using EJBs. Basically, he need to know : Ù how to find or create a bean, Ù how to use its methods and Ù how to release its resources z The client need not worry about the implementation of the EJB, callbacks that the EJB container will make on the EJB or nature of the services provided to the EJB.

EJB’s interface z Home Interface : It is primarily for the life cycle operations of the bean: creating, finding, and removing EJBs. The home interface is not associated with a particular bean, just with a type of bean. z Remote Interface : It is for business methods. Logically, it represents a particular bean on the server. The remote interface also provides some infrastructure methods associated with a bean instance, rather than a bean type.

Sample client application pseudo code z A client programmer will acquire an EJB’s home interface through JNDI, and they use this home interface to : Create or find instance of bean Execute methods Reference (Handle) Remove bean

Client. java Package order. Mgmt; import java. util. properties; import java. naming. Context; // for name-to-object findings import java. naming. Initial. Context; // context for naming operations public class Client { try { Properties prop = new Properties(); // server dependent properties for Initial. Context prop. put(Context. INITIAL_CONTEXT_FACTORY, “org. jnp. interfaces. Naming. Context. Factory”); prop. put(Context. PROVIDER_URL, “localhost: 1099”); Context ctx = new Initial. Context(prop); Object objref = ctx. lookup(“Order. Management”);

Client contd. . // casting home interface reference to the Order. Management. Home home = (Order. Management. Home) javax. rmi. Portable. Remote. Object. narrow(objref, Order. Management. Home. class); // home interface to create an instance of the Order. Management order. Management = home. create(); // calling place. Order() order. Management. place. Order("Dan OConnor", "Wrox books on programming", 1000); order. Management. remove(); System. out. println("Order successfully placed. "); } catch (Exception e) { e. print. Stack. Trace(); } }}

The Bean Programmer’s view z Main responsibility is write business logic and structure the code in a particular structure. The structure has 4 files, the home interface, remote interface, business logic class file and the XML file. The XML file called the deployment descriptor, contains the structural information about the bean, declares the bean’s external dependencies and specifies certain information about how services such as transaction and security work.

Interface EJBObject package javax. ejb; public interface javax. ejb. EJBObject extends java. rmi. Remote { EJBHome get. EJBHome() throws java. rmi. Remote. Exception; Handle get. Handle() throws java. rmi. Remote. Exception; Object get. Primary. Key() throws java. rmi. Remote. Exception; boolean is. Identical(EJBObject obj) throws java. rmi. Remote. Exception; void remove() throws java. rmi. Remote. Exception; }

Order. Management code. . z package order. Mgmt; z import javax. ejb. *; z public interface Order. Management extends javax. ejb. EJBObject z { z public void place. Order(String cust. Name, String prod. Name, int quantity) throws java. rmi. Remote. Exception; z public void cancel. Order(String cust. Name, String prod. Name) z throws java. rmi. Remote. Exception; z public boolean is. Shipped(String cust. Name, String prod. Name) throws java. rmi. Remote. Exception; z }

Order. Management. Bean code. . z package order. Mgmt; z import javax. ejb. *; z public class Order. Management. EJB implements javax. ejb. Session. Bean z { z public void place. Order(String cust. Name, String prod. Name, int z z z quantity) { //. . . Business logic. . . } public void cancel. Order(String cust. Name, String prod. Name) { //. . . Business logic. . . } public boolean is. Shipped(String cust. Name, String prod. Name) { //. . . Business logic … return true; }

Order. Management. Bean code. . z public void ejb. Create() z { // Can be empty } z public void ejb. Remove() z { // Can be empty } z public void ejb. Activate() z { // Can be empty} z public void ejb. Passivate() z { // Can be empty} z public void set. Session. Context( Session. Context ctx ) z { // Can be empty} z }

Interface EJBHome Package javax. ejb; public interface EJBHome extends java. rmi. Remote { EJBMeta. Data get. EJBMeta. Data () throws java. rmi. Remote. Exception; Home. Handle get. Home. Handle() throws java. rmi. Remote. Exception; void remove(Handle handle) throws java. rmi. Remote. Exception, java. ejb. Remove. Exception; void remove(Object primary key) throws java. rmi. Remote. Exception, java. ejb. Remove. Exception; }

Order. Management. Home code. . z package order. Mgmt; z import javax. ejb. *; z public interface Order. Management. Home extends javax. ejb. EJBHome z { z public Order. Management create() z throws java. rmi. Remote. Exception, javax. ejb. Create. Exception; z }

The xml file : ejb-jar. xml <? xml version=“ 1. 0”? > <ejb-jar> <enterprise-beans> <session> <ejb-name>Order. Management</ejb-name> <home>order. Mgmt. Order. Management. Home</home> <remote>order. Mgmt. Order. Management</remote> <ejb-class>order. Mgmt. Order. Management. Bean</ejb-class> <session-type>Stateless</session-type> <transaction-type>Container</transaction-type> </session> </enterprise-beans>

The xml file : ejb-jar. xml <assembly-descriptor> <container-transaction> <method> <ejb-name>Order. Management</ejb-name> <method-name>*</method-name> </method> <trans-attribute>Required</trans-attribute> </container-transaction> </assembly-descriptor> </ejb-jar>

Structure of JAR file META -INF ejb-jar. xml order. Mgmt Order. Management. class Order. Management. Home. class Order. Management. Bean. class

What you can’t do in an EJB component? z You cannot use Reflection API to access information inaccessible to you. z You cannot create a class loader or replace a security manager. z You cannot set the socket factory used by Server. Socket or Socket z You cannot use the object substitution features of the serialization protocol

What you can’t do in an EJB component? z use Threads or the Threading API z use the AWT z Act as a Network Server z z z use Read/Write static fields use java. io package Load a native library use “this” as an Argument or Return value use Loopback Calls

EJB Components on the Web Three classes of objects in MVC architecture: z Model : This is the data and business-logic component. It can serve multiple views. z View : This is the presentation component or the user-interface component. There can be different presentations of a single model. z Controller : This is the component that responds to user input. Translates user-interface events into changes to the model and defines the way the user-interface reacts to those events.

Implementation of MVC in a web site Model 4 2 view 1. jsp view 2. jsp 3 Main. jsp Controller view 3. jsp Views 5 Browser Client 1

Design of the EJB Tier z UML use cases: UML is the Unified Modeling Language, the standard language for expressing the model of the software system that we intend to build. z Use cases are subset of UML that expresses the functionality of the software to be delivered. Use cases describe what to do, but not how to do it.

Analysis Objects z Interface Objects : The interface object is responsible for controlling access to the EJB tier from any client. An interface object should always be represented by a session bean in the implementation. Ex : controller servlet for the web application’s model-view-controller architecture.

Control Objects z Control objects provide services to the application. They model functionality that is not naturally associated with a particular entity or interface. Control objects should be represented by session beans in the implementation.

Entity Objects z Entity objects model those business objects that should maintain their state after the use case completes. This means they represent data in the database. Entity beans are often represented by entity beans in the implementation model.

An Example of EJB Design z Consider the case of a company that develops products, takes orders for those products, and then manufactures and ships them. z Actors in the company : An engineer, a web customer, a phone operator who takes orders from a catalog, floor manager who manages the manufacturing process, a crew member that actually builds the product ordered and a manager who tracks overdue orders.

Use Cases z z z z Create a Product Place an Order Cancel an Order Select an Order for Manufacture Build a Product Ship an Order List Overdue Orders

Use case diagram from analysis Engineer Customer Create Product Place Order Cancel Order Operator Select Order Manager Crew M’ment Build Product Ship an Order Overdue Orders

Stereotype icons in UML z Interface Object : Entity Object : Control Object :

Translation of analysis model into implementation z Actor User Interface Type Engineer Visual Basic Customer Web Application Operator Swing GUI Manager Web Application Crew Palm Pilot XHTML Management Web Application Interface Object Impl’ation Session Bean (RMI/IIOP) Java. Bean proxy / S Bean Session Bean Java. Bean proxy / S Bean Servelet to Session Bean Java. Bean proxy / S Bean

View of use case actors and their respective interface objects Engineer VB App Customer Web App Operator Swing app Manager Web App Crew Palm App Manage-ment Web App

View of interaction of interface and control objects VB App Create Web App Place Swing App Cancel Web App Select Palm App Build Product Web App Ship order List Overdue

View of interaction between control objects and entity objects Create Product Place Order Cancel Order Select for Manufacture Build Product Ship Order List Overdue Product Order Shipment Account Supplier Shipping Company Customer Routing

Summary z EJBs are intended for transactional systems z EJBs are portable, reusable server-side components that execute in a container z Assist developer productivity, extend application capability, and improve system stability z Are accessible from many different types of clients z There are three types of beans : stateful session, stateless session, and entity

Summary z There are four major parts to every bean: the home interface, the remote interface, the implementation class, and the XML deployment descriptor z The enterprise bean developer must follow certain rules to get the benefits of EJB technology z The roles of EJBs can be understood by analyzing a model of your enterprise in terms of interface, control and entity objects