Remote Method Invocation (rmi)

Remote Method Invocation (RMI)

Object to Object Communication between different Java Virtual Machines (JVM) One JVM can invoke methods of an object in another JVM. “In the Java distributed object model, a remote object is the one whose method can be invoked from another java virtual machine, potentially a different host. An Object of type is described by one or more remote Interfaces, which are Java interfaces that declare the methods of the remote object. Remote method Invocation is the action of invoking a method of a remote interface on a remote object..” -- RMI Specification

Need For RMI

An Application comprised of smaller applications spread across different locations in a network. For Using the processing power of multiple computers. Sharing resources and processing load across systems.

RMI's Purpose To make objects in separate JVMs look and act like local objects One of the most important aspect of RMI design is its transparency, where Applications do not know whether an object is remote or local. A Remote object is always accessed via its remote Interface. In other words the client invokes methods on the object only after casting the reference to the remote interface.

Stubs Stub is the client side object that represents or acts as a proxy for the remote object. The stub has the list of methods, as the remote object. Sequence of tasks performed by a stub: 1. Initiates a connection with the remote VM containing the actual remote object. 2. Marshals the parameters to the remove VM 3. Waits for the result of method invocation 4. Unmarshals the return value or exception returned 5. Returns the value to the caller.

Skeletons On the server side, the skeleton object takes care of all of the details of “Remoteness” so that the remote object doesn't need ot worry about them. In other words, You can pretty much code a remote object the same way as if it were local. Sequence of tasks performed by a skeleton 1.Unmarshals the parameters for the remote method. 2.Invokes the method on the actual remote object implementation 3.Marshals the result or exception to the caller.

Protocols Involved 1.JRMP – Java Remote Method Protocol.(Sun Microsystems) 2.IIOP – Internet Inter-ORB Protocol.(Object Management Group) 3.NinjaRMI – (BEA Weblogic) These Protocols uses Stream based network connections over TCP/IP between the JVMs.

Remote Interface java.rmi.Remote – A marker – used to mark object as being remote. 1.Remote Interface must be declared public. 2.It must extend the java.rmi.Remote Interface. 3.Each method in the Interface has to throw java.rmi.RemoteException

Parameters Passed to Remote Methods

Primitives are passed by Value Objects Think.!!!! Little More.!!!

In RMI , Objects are passed by value

In RMI Objects are passed by value, not by reference . A reference to an object doesn't make sense across multiple JVMs , since a reference, points to a value in the heap. Different JVMs do not share heap memory. So RMI sends the object itself, not its reference, between JVMs.

Enterprise Java Beans (EJB)

EJBs are Server Side Components that encapsulates a bigger chunk of functionality (Business Logic). The primary reason that you would write an Enterprise Java Bean Component is to take the advantage of services provided by the container. EJB Specification clearly defines the contracts for Component development. (Client, Bean provider, Container, Server, Deployer etc.)

Contracts & Container Contracts A Statement of responsibilites between different layers of the software. Container A Container is an execution environment for a component. The Component lives in the container. The Container provides services for the component. A component interacts only with its container and the resources the container provides.

An Application Server

A Container lives in a server which provies an execution environment for it. A Server may have one or more containers.

Container Services 1.Transaction 2.Security 3.Concurrency 4.Networking 5.Resource Management 6.presistence 7.Messaging 8.Deploy-time customization Doing programmatically – writing the code by yourself. Doing declaratively – Telling the container.

Being a Bean Provider

An EJB is a single unit bundled with collections of java classes and an XML file.

1. Home Interface 2.Remote Interface 3.Bean Class 4.Deployment Descriptor

Writing Classes for a Component 1.Write Home (extends javax.ejb.EJBHome) Interface The Home interface is for life cycle operations : Creating, Finding and removing EJBs. The home interface isn't associated with a particular Bean. 2.Write Remote (extends javax.ejb.EJBObject) Interface. The Remote Interface is for business methods, Logically, It represents a particular bean on the server.

Writing Classes for a Component

3.Implement Interface’s (Either SessionBean or Entity Bean) container callback methods in your bean class. 4.Write business methods in the Bean Class. 5.Write deployment descriptor file (ejb-jar.xml) This file tells the container about how to deploy your bean.

What is the need to Implement these Interfaces.?

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The Client will never access the bean directly, the container intercepts the call to provide its services. How will it Intercepts??? You are making a call to the EJBObject stub. Your Remote Interface which extend EJBObject is implemented by the Container. The Container implementation of Remote Interface has your business methods in which the Bean Business method is called.

Java Naming and Directory Interface

Usage Of JNDI • Simple usage: binding and looking up objects by name • Putting Objects into JNDI (binding objects into the name space) • Get an initial context - the first handle into the name space • Bind object to context • Getting Objects out of JNDI (resolving objects out of the name space)

Bean Class • Implements the bean’s business methods • Does NOT implement home or remote interface • However, it must have methods that match the signatures of the remote interface.

Deploying EJBs

After the Home interface, Remote interface, Bean Class implementation and Deployment Descriptor are Written The class files and descriptor are bundled together as a jar, using some tools given by App Server vendors an ejb jar is created The EJBs must be added to the EJB container. This process is called deployment.

Deployment Descriptors file tells the EJB server which are all the classes

….

<ejb-name>StatelessSessionBeanExample examples.ejb.basic.statelessSession.TraderHome examples.ejb.basic.statelessSession.Trader <ejb-class>examples.ejb.basic.statelessSession.TraderBean <session-type>Stateless Container ......... <jndi-name>statelessSession.TraderHome

Types of EJBs  Session Beans Stateful session Beans Stateless session Beans  Entity Beans Container Managed Persistence (CMP) Bean Managed Persistence (BMP)

What Is a Session Bean?

A session bean represents a single client inside the J2EE server acts as a logical interface of the client Non-persistence object that implements business logic The business logic is implemented in methods defined in remote interface.

Stateless Session Beans

Stateless Session Beans

 Does not maintain any conversational state.  Stateless session beans are pooled by their container to handle multiple requests from multiple clients.

Life Cycle of Stateless Session Beans

Home Interface

import java.rmi.RemoteException; import javax.ejb.CreateException; import javax.ejb.EJBHome; public interface TraderHome extends EJBHome { Trader create() throws CreateException, RemoteException; }

Remote Interface

import java.rmi.RemoteException; import javax.ejb.EJBObject; public interface Trader extends EJBObject { public TradeResult buy (String stockSymbol, int shares) throws RemoteException; public TradeResult sell (String stockSymbol, int shares) throws RemoteException; }

Bean Class import javax.ejb.*; import javax.naming.InitialContext; import javax.naming.NamingException; public class TraderBean implements SessionBean { private SessionContext ctx; private int tradeLimit; public void ejbActivate() { } public void ejbRemove() { } public void ejbPassivate() { } public void setSessionContext(SessionContext ctx) { this.ctx = ctx; } public void ejbCreate () throws CreateException {} public TradeResult buy(String stockSymbol, int shares) { if (shares > tradeLimit) { shares = tradeLimit; } return new TradeResult(shares, stockSymbol); } public TradeResult sell(String stockSymbol, int shares) { if (shares > tradeLimit) { shares = tradeLimit; } return new TradeResult(shares, stockSymbol); }}

Client Class // Lookup the beans home using JNDI Object home = ctx.lookup(JNDI_NAME); return (TraderHome) PortableRemoteObject.narrow(home, TraderHome.class); // create a Trader Trader trader = (Trader) narrow(home.create(), Trader.class); String [] stocks = {"BEAS", "MSFT", "AMZN", "HWP" }; // execute some buys for (int i=0; i<stocks.length; i++) { int shares = (i+1) * 100; trader.buy(stocks[i], shares); } // execute some sells for (int i=0; i<stocks.length; i++) { int shares = (i+1) * 100; trader.sell(stocks[i], shares); } // remove the Trader log("Removing the trader"); trader.remove();

ejb-jar.xml <ejb-jar> <enterprise-beans> <session> <ejb-name>statelessSession examples.ejb.basic.statelessSession.TraderHome examples.ejb.basic.statelessSession.Trader <ejb-class>examples.ejb.basic.statelessSession.TraderBean <session-type>Stateless Container <env-entry> <env-entry-name>tradeLimit <env-entry-type>java.lang.Integer <env-entry-value>500 <method> <ejb-name>statelessSession <method-intf>Remote <method-name>* Required

weblogic-ejb-jar.xml Stateless Session Bean – weblogic-ejb-jar.xml <weblogic-ejb-jar> <weblogic-enterprise-bean> <ejb-name>statelessSession <max-beans-in-free-pool>100 <jndi-name>statelessSession.TraderHome Stateful Session Bean – weblogic-ejb-jar.xml <weblogic-ejb-jar> <weblogic-enterprise-bean> <ejb-name>statefulSession <max-beans-in-cache>100 <jndi-name>statefulSession.TraderHome

Stateful Session Beans

Stateful Session Beans

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Maintains client-specific session information (called

conversational state) across multiple method calls. 

Each stateful session bean has a timeout value.

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The bean instance is destroyed and theremote reference

is invalidated after the timeout period is elapsed.

Life Cycle of Stateful Session Beans

Activation and Passivation 

During passivation, the state of the EJB instance is stored automatically by the container.

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During activation, the state of the EJB instance is loaded automatically by the container.

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An EJB instance may be passivated if none of its methods

are invoked during a specified amount of time (customisable) 

An EJB instance is activated by any method call (if not

active).

Entity Beans

Entity Beans

 An entity bean represents a business object in a persistent storage  mechanism like customers, orders, and products.  Each instance of the bean corresponds to a row in that table.  Entity lifetime is not dependent on the client connection.  Shared data between clients.  May be accessed by multiple clients concurrently.  Concurrent access is managed by the container or by the  underlying database

Object / Relational Mapping •

Associate a database table with a class each instance of the class (each object) is stored in a row

•

class

Associate a database column with an attribute of the

What Makes Entity Beans Different from Session Beans?

• • • •

Persistence Shared Access Primary Key Relationships When to Use Entity Beans ?

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The bean represents a business entity, not a procedure.

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The bean's state must be persistent.

Life Cycle of Entity Beans

Database Calls Method ejbCreate ejbFindByPrimaryKey ejbFindByLastName ejbLoad ejbRemove ejbStore

SQL Statement INSERT SELECT SELECT SELECT DELETE UPDATE

Container-Managed Persistence Bean developer doesn't need to write any data access logic. EJB server will take care of all the persistence needs. Home Interface To create a new instance of a CMP entity bean, and therefore insert data into the database, thecreate() method on the bean's home interface must be invoked •

•A

bean's home interface may declare zero or more create() methods, each of which must have corresponding ejbCreate() and ejbPostCreate() methods in the bean class. •Finder

Methods are used to query the EJB server for specific entity beans, based on the name of the method and arguments passed. •The

deployer will use vendor specific tools to tell the container how a particular find method should behave.

Home Interface import javax.ejb.CreateException; import javax.ejb.EJBHome; import javax.ejb.FinderException; import java.rmi.RemoteException; import java.util.Enumeration; public interface AccountHome extends EJBHome { public Account create(String accountId, double initialBalance, String type) throws CreateException, RemoteException; public Account findByPrimaryKey(String primaryKey) throws FinderException, RemoteException; public Account findAccount(double balanceEqual) throws FinderException, RemoteException; public Enumeration findBigAccounts(double balanceGreaterThan) throws FinderException, RemoteException; public Enumeration findNullAccounts() throws FinderException, RemoteException;}

Remote Interface The remote interface defines the business methods of the entity bean. The business methods in the remote interface are delegated to the matching business methods in the bean instance. import java.rmi.RemoteException; import javax.ejb.*; public interface Account extends EJBObject { public double deposit(double amount) throws RemoteException; public double withdraw(double amount) throws ProcessingException, RemoteException; public double balance() throws RemoteException; public String accountType() throws RemoteException; }

Bean Class

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No database access logic in the bean.

EJB vendor provides tools for mapping the fields in the Bean Class to the database. 

Could be mapped to any database providing it contains data that is similar to the fields in the bean 

Bean Class //Nessary imports public class AccountBean implements EntityBean { private EntityContext ctx; public String accountId; public double balance; public String accountType; // Implement the callback methods }

Callback Methods

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setEntityContext() unsetEntityContext() ejbLoad() ejbStore() ejbPassivate() ejbActivate()

ejb-jar.xml <ejb-jar> <enterprise-beans> <entity> <ejb-name>containerManaged examples.ejb.basic.containerManaged.AccountHome examples.ejb.basic.containerManaged.Account <ejb-class>examples.ejb.basic.containerManaged.AccountBean Container <prim-key-class>java.lang.String False accountId balance accountType <primkey-field>accountId

weblogic-ejb-jar.xml <weblogic-ejb-jar> <weblogic-enterprise-bean> <ejb-name>containerManaged <max-beans-in-cache>1000 isModified WebLogic_CMP_RDBMS 5.1.0 META-INF/weblogic-cmp-rdbms-jar.xml WebLogic_CMP_RDBMS 5.1.0 <jndi-name>containerManaged.AccountHome

Weblogic-cmp-rdbms-jar.xml <weblogic-rdbms-bean> <pool-name>demoPool ejbAccounts accountId id balance bal <method-name>findAccount <method-params> <method-param>double

EJB 2.0

New Features of EJB 2.0 Local Interfaces Reengineered Container Managed Persistence (CMP) EJB - Query Language (EJB - QL) Message Driven Beans Container Managed Relationship (CMR) Home Business Methods

Thank You