Enterprise Java Beans

Enterprise Java Beans

What Are EJB’s? • Enterprise Java Beans • EJB: special kind of JavaBean for performing server-side business logic • More powerful version of the regular beans that we’ve used in class

Where they fit in a system

EJB’s in J2EE

Three kinds of EJB’s • Session • associate client information with a specific client • both stateless and stateful versions • Entity • groups associated information in an abstraction that provides transaction support • Message Bean - rarely used, hardly supported

What is a Session Bean? • Represents a single Client inside the J2EE server • one client at a time/ not persistent • when the client terminates, the session bean is disassociated from the client • There are two types: Stateful and Stateless...

Stateful • These represent a set of interactions between client and server. • Example: shopping cart • Saves information over several method invocations. • There is a lot of overhead associated with using stateful beans

Stateless Beans • A stateless bean does not save information between method calls. • Limited application • Little overhead • multiple clients can use the same bean instance without alteration • Example: fetch from a read-only database or send a confirmation email for an order

Entity Beans • Associates pieces of information in a group • Accessed by multiple clients at a time • Persistent and Serializable • The container loads and stores the entity beans in the database • These are more similar to regular beans

More on Entity Beans • Transactions: this is what makes an Entity Bean special. • Entity beans rely on the container to enforce robust transactions • example: Airline booking: if the flight booking action fails, then the credit card charge action fails, or vice versa.

Persistence in Entity Beans • Container managed persistence • the container controls when the bean is read from or written to the database • Bean managed persistence • the bean’s implementation performs all of the sql operations that loads, stores, and updates the bean’s data to or from the database. • Bean is responsible for connection allocation to the database

Connection Pooling • Setting up connections to the database is resource intensive • Connection pooling maintains a pool of database connections for the entity beans so that the connection is maintained when a bean finishes, and is available for other entity beans. • Specific to database and EJB container implementation

Message Beans • A message bean is an enterprise bean that allows J2EE applications to process messages asynchronously. It acts as a JMS message listener, which is similar to an event listener except that it receives messages instead of events. • Many systems do not yet support JMS, message bean use is currently not widespread

Using an Entity bean from a Session bean • An entity bean can be shared by multiple sessions. • This allows for data encapsulation; clients can interact with data via session beans within transaction boundaries. • Can do all database interaction from session bean as an alternative • encapsulation is weakened

Using EJB’s

An EJB Example... • Online Banking Application • Demonstrates how all the component technologies--enterprise beans, J2EE application clients, and Web components fit together

Online Banking Application • Two clients: • a J2EE application client used by administrators to manage customers and accounts • Web client used by customers to access account histories and perform transactions. The clients access the customer, account, and transaction information maintained in a database through enterprise beans.

Online Bank Application Overview

Session beans used • The online bank application has three session beans: AccountControllerEJB, CustomerControllerEJB, and TxControllerEJB. These session beans provide a client's view of the application's business logic. Hidden from the clients are the server-side routines that implement the business logic, access databases, manage relationships, and perform error checking.

Entity Beans used • For each business entity represented in our simple bank, the bank application has a matching entity bean: • AccountEJB, CustomerEJB, TxEJB • The purpose of these beans is to provide an object view of these database tables: account, customer, and tx. For each column in a table, the corresponding entity bean has an instance variable. Because they use bean-managed persistence, the entity beans contain the SQL statements that access the tables. For example, the create method of the CustomerEJB entity bean calls the SQL INSERT command.

Database

Security in this example • you can protect an enterprise bean by specifying the security roles that can access its methods In the bank application, two roles are defined--BankCustomer and BankAdmin • BankAdmin role is allowed to perform administrative functions: creating or removing an account, adding a customer to or removing a customer from an account, setting a credit line, and setting an initial balance. • BankCustomer role is allowed to deposit, withdraw, transfer funds, make charges and payments, and list the account's transactions. Notice that there is no overlap in functions that users in either role can perform

…security continued... • Access to these functions was restricted to the appropriate role by setting method permissions on selected methods of the CustomerControllerEJB, AccountControllerEJB, and TxControllerEJB enterprise beans. For example, by allowing only users in the BankAdmin role to access the createAccount method in the AccountControllerEJB enterprise bean, you have denied users in the BankCustomer role or any other role permission to create bank accounts. • Set method permissions in the container with the deploy tool for each role

BankCustomer Role Mapped to Customer Group

Underlying TechnologiesRMI • RMI - Remote Method Invocation • instead of invoking a method on another Java object running in the same JVM, you invoke a method in a Java object in another JVM on the same computer or another one. • Using an interface, RMI hides the fact that you are invoking a method remotely • The Remote and Home interfaces for an EJB must be RMI interfaces

Underlying Technologies:JNDI • JNDI - Java Naming and Directory Interface • JNDI provides a uniform way to access naming and directory services • You use JNDI to locate EJB’s and JDBC connection pools from within your EJB container • When a client needs to access a bean’s Home interface, it uses JNDI to locate the Home interface. After you locate an object, you communicate directly with it instead of going through JNDI • You don’t need to know much about JNDI for EJB’s except for a few setup calls.

Underlying Technologies:JDBC • JDBC - Java Database Connectivity • gives you a standard API in which to communicate with different types of databases • If you use CMP (Container Managed Persistence) there’s a chance that you won’t use JDBC at all. However there are still a few cases in which CMP doesn’t handle all the different ways that you can access data.

Bean DesignThe Home interface • The 2 main functions of the home interface are creating beans and locating beans • Only entity beans need methods to locate beans import java.rmi.*; import javax.ejb.*; public interface HelloWorldHome extends EJBHome{ public MyBean create() throws RemoteException,CreateException; MyBean findByPrimaryKey(String myEntityKey) throws RemoteException,CreateException; }

Bean design:The Remote Interface • Consists of the remote methods you can call on a bean after the bean has been created and located. import java.rmi.*; public interface HelloWorld{ public String getGreeting() throws RemoteException; }

Bean DesignBean Implementation import java.rmi.*; import java.util.*; import javax.ejb.*; public class HelloWorldImpl implements SessionBean{ protected String greeting; private SessionContext context; /** An EJB must have a public, parameterless constructor */ public HelloWorldSessionImpl(){} /** Called by the EJB container when a client calls the create() method in the home interface */ public void ejbCreate() throws CreateException{ greeting = "Hello World!"; } /** Returns the session's greeting */ public String getGreeting(){ return greeting; }

/** Called by the EJB container to wake this session bean up after it has been put to sleep with the ejbPassivate method. */ public void ejbActivate(){} public void ejbPassivate(){} public void ejbRemove(){} /** Called by the EJB container to set this session's context */ public void setSessionContext(SessionContext aContext) { context = aContext; } }

Bean DesignClient to access bean import java.util.*; import javax.naming.*; import javax.rmi.*; public class TestHello{ public static void main(String[] args) { try{ /** Creates a JNDI naming context for location objects */ Context context = new InitialContext(); /** Asks the context to locate an object named "HelloWorld" and expects the object to implement the HelloWorldSessionHome interface */ HelloWorldSessionHome home = (HelloWorldSessionHome)portableRemoteObject.narrow( context.lookup("HelloWorld"),HelloWorldSessionHome.class); /** Asks the Home interface to create a new session bean */ HelloWorldSession session = (HelloWorldSession) home.create(); System.out.println(session.getGreeting()); /** Destroy this session */ session.remove(); } catch (Exception exc){exc.printStackTrace()} } }

Some Restrictions • EJB’s cannot • create or manage threads • access files using java.io • create a ServerSocket or change the socket and stream handler factories • load a native library • use the AWT to interact with the user These restrictions keep EJB’s from making too many assumptions or interfering with its environment.

OpenEJB • OpenEJB is an open source EJB container and EJB server • Allows use of EJB’s in your apps with little configuration • Why use it? • Can be combined to work with Tomcat • A web app in Tomcat will become an OpenEJB client in a fully J2EE compliant way

Server-Container Contract • Defines responsibilities between app server and OpenEJB container • Container manages EJBs at runtime according. Container provides transaction, authorization, and connector support for beans servicing requests from the server.

The Major Steps • Install OpenEJB • Deploy an EJB from OpenEJB • Configure Tomcat • Create and Deploy the Web app

Configuring Tomcat • Tomcat is a servlet container. OpenEJB is an EJB container. • Open EJB creates a runtime environment to run EJBs and let other access them by JNDI. • Tomcat allows EJB’s to be bound to names so JSPs and servlets can access them, but there must be a bridge...

JNDI Object Factory • Bridge between Tomcat and OpenEJB runtime environment • The OpenEJB class, TomcatEJBFactory interprets bean requests and passes them to OpenEJB • OpenEJB processes this and responds back • Installation of the factory also requires some classes be available for Tomcat, a simple web app provided with OpenEJB loads the jars into Tomcat’s class path

Configure server.xml • EJB name bindings appear between <Context> of the web app using the bean <Ejb name="ejb/hello" type="Session" home="org.acme.HelloHome" remote="org.acme.Hello"/> <ResourceParams name="ejb/hello"> . . . . . </ResourceParams>

That all folks • Questions?

Enterprise Java Beans