Session Beans Based on: Patel, Brose, Silverman, Mastering Enterprise JavaBeans 3.0

Enterprise Java Beans (EJB) An EJB is a component that provides reusable business logic functionality and/or a representation of a persistent business entity An EJB Container executes an EJB on behalf of clients.  Container provides the plumbing necessary to execute the EJB including non-business logic related functionality such as transactions, security, concurrency, remote access, etc. life cycle functions, e.g., creating, destroying, etc. Client uses an interface to access the Bean indirectly A deployment descriptor describes the structure of the Bean and how to execute the Bean as part of an application

Session Beans Session Beans are one type of EJBs  other types are Entity Beans and Message Driven Beans A session bean executes on behalf of a single client  Focus on functionality, application logic and application state May be transaction aware May access shared data in an underlying DB but does not directly represent this shared data Is relatively short-lived Is removed when the EJB Container crashes

Stateless Session Beans A Stateless Session Bean executes a request and returns a result without saving any client specific state information  Can use instance variables only if they are not client related (do not expect instance variables value to remain the same) All Stateless Session Beans are equivalent  A container can choose To serve the same instance of a Bean to multiple clients To serve difference Bean instances to the same client at different times  A container may maintain a pool of Stateless Session Beans No necessary relation between when a client creates the Bean and when the Container creates the Bean Provide very high scalability

Stateful Session Beans Maintain per client state across multiple client requests May be “passivated” – allows a degree of pooling  The container serializes the state of a Bean non currently being used and writes it to a secondary storage  Frees JVM resources held for Bean  When a new request arrives for the Bean, it must be “activated” State read from secondary storage and deserialized  Can only passivate a Bean if it is not in a transaction

Stateful Session Beans PassiveReadyDoes Not Exist create In Transaction commit or rollback start transaction timeout remove passivate activate

EJB Development Steps 1.Write Business Interface  Plain Java Interface exposing the EJB business methods can be local or remote 2.Write Bean Class  Implementation of Business Interface  Use annotations (e.g. to specify type of EJB)  Use resource injection (e.g. for persistence access) 3.Provide deployment descriptor (if needed) 4.Package and deploy on container  Usually done by a container provided tool

EJB Development Steps Local versus Remote Interface There are two types of Interface to EJBs  local interface – for clients running in the same Java Virtual Machine (JVM) as the EJB  remote interface – for clients running in different (JVM) than the EJB A local interface should be used whenever possible local interface provides a better performance (an actual reference to an instance of EJB is used) remote interface involves RMI (creation of stub and skeleton objects, marshalling/unmarshalling messages,...)

EJB Development Steps package helloejb; import javax.ejb.Remote; /** * This is the business interface for HelloEJB enterprise bean. // specify as a remote interface public interface Hello { public String hello(); } Interface The annotation for the type of interface could be omited from the interface and specified in the EJB implementation.

EJB Development Steps Stateless Session EJB Implementation package helloejb; import // for a stateless session EJB public class HelloEJBBean implements helloejb.Hello { /** Creates a new instance of HelloEJBBean */ public HelloEJBBean() { } /** Implementation of business method */ public String hello() { return "Hello from ejb !!!"; }

EJB Client An EJB client can be managed by the same container as the EJB (e.g. a Servlet or another EJB) or external.  Container managed clients can use ressource injection for access (using  Non-container managed clients must use JNDI to locate the EJB.

EJB Client Web application client, managed by the same container package servlet; import helloejb.Hello;... public class HelloClientServlet extends HttpServlet // EJB ressource injection private Hello hello; protected void processRequest(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException { response.setContentType("text/html;charset=UTF-8"); PrintWriter out = response.getWriter(); out.println(" "); out.println(" EJB returns " + hello.hello()); out.println(" "); out.close(); }... }

EJB Client Stand-alone client, managed outside of container. Need to use JNDI to lookup. import helloejb.Hello; import javax.naming.Context; import javax.naming.InitialContext; import javax.naming.NamingException; public class HelloClient { public static void main(String[] args) { Context ctx; try { /** Use JNDI to lookup */ ctx = new InitialContext(); Hello helloEjb = (Hello) ctx.lookup("helloejb.Hello"); System.out.println(helloEjb.hello()); } catch (NamingException ex) { ex.printStackTrace(); } }

Session EJBs declares a stateless session public class TaxRateBean implements TaxRate { }  By default, name is the same as the class declares a stateful session can be use to annotate public class TaxRateBean implements TaxRate { }  In that case, the interface do not have annotation

Session EJBs Annotations Annotations are also used for dependency injection  mechanism allowing to obtain object references on resources from the set up a dependency to a resource in the bean's SessionContext context; TaxRate tr = (TaxRate)context.lookup(TaxRate.class.getName());  Any resource available to the application server can be referred to (javax.sql.DataSource, javax.transaction.UserTransaction,...)

Session EJBs can be used to inject a reference to an EJB or bind a name in the EJB environment  Injection of a EJB private Pricer pricer;  Binding a name in the public class PricerBean implements Pricer {... public double getFinalPrice() { InitialContext ctx = new InitialContext(); TaxRate tr = (TaxRate)context.lookup(“java:comp/env/ejb/TaxRate”); }

Session EJBs to inject an entity manager object or bind a name at the class level  Injection of an entity manager EntityManager em;  Binding public class HelloWorldBean implements HelloWorld SessionContext context; public void hello() { EntityManager em = (EntityManager)context.lookup(“HelloWorldEntity”); }

StateFul Session EJBs Activation and Passivation annotate a Stateful Session EJB method as a callback that is called just before the EJB is public class AstatefulBean public void passivate() { // close socket connections,... }

StateFul Session EJBs Activation and Passivation annotate a Stateful Session EJB method as a callback that is called just before the EJB is public class AstatefulBean public void passivate() { // open socket connections,... }

Session EJBs Construction and Destruction annotate a method as a callback called right after a new instance of a bean class is created by the container  can be used for initializations needed by the annotate a method as a callback called right before a bean class is destroyed  can be used to free allocated resources