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Overview. Topics: Java Remote Method Invocation (RMI) Enterprise JavaBeans (EJB). Java RMI. distributed object computing de velopment of distributed Java programs same syntax and semantics as in non-distributed programs. Computer A class AClass: myAClassMethod(){}. Computer B

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  • Topics:

    • Java Remote Method Invocation (RMI)

    • Enterprise JavaBeans (EJB)

Java rmi
Java RMI

  • distributed object computing

  • development of distributed Java programs

  • same syntax and semantics as in non-distributed programs

Computer A

class AClass:


Computer B

class BClass:

AClass a;

/* initialize a */



Rmi architecture
RMI architecture

  • The RMI achitecture defines

  • how objects behave

  • how and when exceptions can occur

  • how memory is managed

  • how parameters are passed to and returned from remote methods

Principle of rmi 1
Principle of RMI (1)

  • RMI separates:

    • definition of behaviour

    • implementation of that behaviour

  • each of them is allowed to run on different JVMs

  • interfaces: define definition

  • classes: define implementation

Principle of rmi 2
Principle of RMI (2)

  • 2 classes that implement the same interface

    • service implementation on server

    • service proxy on client

  • client program makes method calls to proxy

  • RMI sends request to remote JVM

  • return values are sent back to proxy / client program

Rmi architecture layers
RMI architecture layers

  • 3 abstract layers:

    • Stubs & SkeletonsLayer

    • Remote Reference Layer

    • Transport Layer

  • advantages of layer architecture:

    • implementation of layers independent from each other

    • each layer can be enhanced / replaced without affecting rest of the system

Stubs skeletons layer 1
Stubs & Skeletons Layer (1)

  • lies just beneath developer

  • intercepts method calls made by client

  • redirects them to remote RMI service (= remote object)

  • Proxy Pattern (Book: Design Patterns by Gamma, Helm, Johnson):

    • one object in one context

    • represented by another object (the proxy) in another context

  • proxy forwards method calls between participating objects

  • in RMI, stub class is the proxy

Stubs skeletons layer 2
Stubs & Skeletons Layer (2)

proxy represents the RealSubject

in RMI: stub class

Stubs skeletons layer 3
Stubs & Skeletons Layer (3)

  • skeleton class:

    • helper class on server

    • generated for RMI to use

    • communicates with stub accross the link

    • reads parameters for the method call from the link

    • makes the call to the service object

    • accepts the return value, writes it back to the stub


<< parameters


service object

client program


network link


return values >>

Remote reference layer
Remote Reference Layer

  • defines & supports the invocation semantics of the RMI connection

  • client gets remote reference to remote objects via naming service (discussed later)

  • Remote Reference Layer manages these remote reference

  • provides RemoteRef object (represents link to remote object)

  • stub object: forwards method callsbyRemoteRef.invoke()

  • in JDK1.1:

    • unicast point-to-point connection

    • service must be instantiated and exported to the RMI system, before client can use it

  • in Java 2 SDK:

    • multicast and other types of connection semantics

    • activatable objects

Transport layer
Transport Layer

  • Transport Layer:

    • stream-based network connections that use TCP/IP

    • basic connectivity and firewall penetration strategies

  • on top of TCP/IP, RMI uses Java Remote Method Protocol (JRMP)

  • RMI-IIOP: instead of JRMP, OMG IIOP is used to communicate between clients and servers

Rmi iiop

  • RMI-IIOP: co-production of Sun and IBM

  • uses Object Management Group Internet Inter-ORB Protocol (OMG IIOP)

  • OMG: defines a vendor-neutral, distributed object architecture called Common Object Request Broker Architecture (CORBA)

  • CORBA Object Request Broker (ORB) servers and clients use IIOP for communication

  • RMI-IIOP: RMI programs can inter-operate with CORBA ORBs

  • modifications on the source code: RMI programs can be compiled with rmic –iiop to use IIOP

  • EJB:

    • based on RMI / RMI-IIOP

    • communication with CORBA ORBs easier with same protocol

Naming remote objects 1
Naming Remote Objects (1)

  • clients: find remote services by using a naming or directory service, running on a well known host and port number

  • RMI

    • can use different directory services, e.g. the Java Naming and Directory Service (JNDI)

    • includes simple service called RMI Registry (rmiregistry, default on port 1099)

  • host: server program creates a remote service by

    • creating a local object that implements the service

    • exporting that object to RMI

    • after exporting, server registers object in the RMI Registry under a public name

    • RMI creates a listening service that waits for client connections and requests for the service

Naming remote objects 2
Naming Remote Objects (2)

  • host (continued):

    • provides needed class files for client (HTTP, FTP, ...)

  • client:

    • RMI Registry is accessed through the static class Naming

    • client uses its method lookup() to query registry

    • methodlookup() accepts a URL that specifies the server host name and the name of the desired service

    • if needed: class files downloaded from server

    • method returns a remote reference to the service object

URLs accepted by Naming.lookup():


Using rmi
Using RMI

  • working RMI system:

    • interface definitions for the remote services

    • implementation of the remote services

    • stub and skeleton files

    • a server to host the remote services

    • an RMI Naming service for clients to find the remote services

    • a class file provider (HTTP or FTP server)

    • a client program that needs the remote services

Enterprise java beans ejb
Enterprise Java Beans (EJB)

  • architecture for development of

    • transactional

    • 'distributed object applications'-based

    • server-side software

  • based on RMI / RMI-IIOP

  • enterprise beans:

    • server-side components

    • distributed objects

    • short name for such a component: "bean"

    • hosted in container (Enterprise JavaBeans)

    • provide remote services for clients distributed thoughout the network

Ejb containers 1
EJB Containers (1)

  • enterprise beans: software components that run in a special environment called EJB container

  • container:

    • hosts and manages an enterprise bean

    • hosting comparable to

      • webserver that hosts a servlet

      • browser that hosts an applet

  • enterprise bean can't function outside an EJB container

  • EJB container manages at runtime:

    • remote access, security

    • persistence, transactions

    • concurrency

    • access to and pooling of resources

Ejb containers 2
EJB Containers (2)

  • EJB container: isolates enterprise bean from direct access by client application

  • client invokes a method call on an enterprise bean:

    • container intercepts invocation

    • automatically manages persistence, transactions, security

  • developer doesn't have to care about writing that code

  • bean doesn't contain any of this logic

Ejb containers 3
EJB Containers (3)

  • containers:

    • pool resources, manage life cycles of all beans

    • reduces memory consumption and processing (scalability)

  • client application is unaware of this resource management

  • enterprise bean depends on container for everything:

    • JDBC connections

    • connections to other beans

    • identity of caller

    • reference to itself

Interaction bean container 1
Interaction bean-container (1)

  • 3 types of interaction enterprise bean EJB container

  • callback methods

    • every bean implements a subtype of the EnterpriseBean interface

    • this i/f defines several "callback methods"

    • container invokes these methods to

      • activate a bean

      • persist its state to the database

      • end a transaction

      • remove the bean from memory

      • ...

Interaction bean container 2
Interaction bean-container (2)

  • EJBContext

    • object EJBContext: reference to container

    • provides methods to interact with the container

    • e.g. request information about the bean's environment:

      • identity of its client

      • status of transaction

      • remote reference to itself

      • ...

Interaction bean container 3
Interaction bean-container (3)

  • Java Naming and Directory Service (JNDI)

    • standard extension to the Java platform for accessing naming systens like

      • LDAP

      • NetWare

      • file systens

      • ...

    • bean has access to a special naming system Environment Naming Context (ENC)

    • JNDI ENC allows access to

      • JDBC connections

      • other enterprise beans

      • properties specific to that bean

Enterprise beans
Enterprise Beans

  • 2 interfaces + 1 implementation class

  • client uses interfaces to interact with the bean

  • interfaces:

    • remote & home i/f

    • expose the capabilities of the bean

    • provide all methods needed to create, update, interact with, delete the bean

  • implementation class:

    • represents a business concept (like a addressbook / contact)

Interfaces types of beans
Interfaces & types of beans

  • interfaces:

    • home interface

    • remote (or business) interface

  • entity beans

    • container managed persistence (CMP)

    • bean managed persistence (BMP)

  • session beans

    • stateful beans

    • stateless beans

Remote home interfaces
Remote & Home interfaces

  • home interface:

    • extends javax.ejb.EJBHome

    • life cycle methods to create, find and destroy a bean

  • remote interface:

    • extends javax.ejb.EJBObject

    • represents business methods

Life cycle methods
Life cycle methods

  • home interface provides life cycle methods

    • for creating

    • for destroying

    • for locating beans

  • represent behaviours that are not specific to a single bean instance

  • separated from remote interface

/* some imports */

public interface ContactHome extends EJBHome{

public Contact create(int number)

throws RemoteException, CreateException;

public Contact findByPrimaryKey(int pk)

throws FinderException;


Business methods 1
Business methods (1)

  • can perform tasks

  • this type of bean is called session bean

import javax.ejb.EJBObject;

import java.rmi.RemoteException;

public interface AddressBook extends EJBObject{

public Contact findByContactData(String name, String firstname, String email)

throws RemoteException;

public Enumeration findByName(String firstname)

throws RemoteException;

public Contact findByEmail(String email)

throws RemoteException;


Business methods 2
Business methods (2)

  • can also represent business data stored in a database

  • this type of business object is called entity bean

  • each instance of an enterprise bean represents 1 row in a table

entity beans

database table




Entity type beans 1
Entity type beans (1)

  • example for an entity bean (remote interface):

import javax.ejb.EJBObject;

import java.rmi.RemoteException;

public interface Contact extends EJBObject{

public String getName() throws RemoteException;

public String setName(String name) throws RemoteException;

public String getFirstName() throws RemoteException;

public String setFirstName(String firstname)

throws RemoteException;

public String getEmailAddress() throws RemoteException;

public String setEmailAddress(String emailaddres)

throws RemoteException;


Entity type beans 2
Entity type beans (2)

  • two types of entity beans:

    • Container-Managed Persistence (CMP)

    • Bean-Managed Persistence (BMP)

  • CMP:

    • container (EJB server) manages persistence

    • developer doesn't have to write db access code

    • programmer has to specify in deployment desciptor which fields (instance variables/columns) are to be managed

  • BMP:

    • entity bean contains db access code (via JDBC)

    • responsible for reading and writing its own state to the database

Session type beans 1
Session type beans (1)

  • session beans:

    • manage interactions of entity and other session beans

    • access resources

    • perform tasks on behalf of the client

    • not persistent business objects, do not represent data in db

  • interface javax.ejb.SessionBean provides methods ejcActivate() and ejbPassivate()

  • two types of session beans:

    • stateless session beans

    • stateful session beans

Session type beans 2
Session type beans (2)

  • stateless beans:

    • made up of business methods that behave like procedures

    • operate only on passed through arguments on invocation

    • do not maintain business state between method invocations

    • each invocation of a business method is independent from previous invocations

    • doesn't remember anything from one invocation to the other

    • faster request processing, less resources used

    • don't use ejbActivate() and ejbPassivate() methods (overridden with empty implementation)

Session type beans 3
Session type beans (3)

  • stateful beans:

    • encapsulate business logic and state specific to a client

    • dedicated to one client

    • clients do not share stateful beans

    • maintain business state / conversational state between method invocations

    • held in memory, not persistent

Access to beans
Access to beans

  • access to beans from outside:

    • client application (as mentioned)

    • Java Server Pages (JSP) or Servlets

  • popularly used method: JSP

    • HTML pages with embedded Java code

    • client opens JSP page in browser

    • server processes page

    • beans can be used to perform tasks or access database

    • results are sent back to browser (HTML)

Using ejb
Using EJB

  • working EJB system:

    • for each bean

      • home interface

      • remote interface

      • implementation

    • EJB server (e.g. j2ee)

    • naming service (usually JNDI)

    • a class file provider (HTTP or FTP server)

    • for entity beans: relational database (e.g. cloudscape)

    • client program