Web Applications through Components

1
Web Applications through Components

• Alessio Bechini
• June 2002

2
The Evolution of Frameworks

• The evolution of web servers, application servers
  and frameworks as seen in 1999

3
Products Sold by Publicly Trade Companies

• WebSphere and Domino Application Servers (IBM
  Corporation)
• iPlanet/Netscape Application Server
  (AOL/Netscape/Sun)
• NetDynamics 5.0 (Sun MicroSystems)
• Oracle Application Server (Oracle Corporation)
• BEA WebLogic Server 5.1 (BEA Systems)
• WebObjects (Apple Computer, Inc.)
• Inprise Application Server (Inprise Corporation)
• Sybase Enterprise Application Server 3.0 (Sybase
  Corporation)
• Persistence PowerTier for EJB (Persistence
  Software)
• Progress Apptivity 3.0 (Progress Software)
• Dynamo Application Server (Art Technology Group)
• SilverStream (SilverStream Software)
• Iona IPortal (Iona Technologies)
• Unify eWave Engine (Unify Corporation)

4
Products Sold by Other Companies

• Sapphire/Web (Bluestone Software)
• Versata Logic Server (Versata, Inc.)
• HahtSite (Haht Corporation)
• Novera jBusiness (Novera Software)
• Gemstone/J 3.0 (Gemstone Systems)
• Parlay Application Integration Server
  (Information Builders)
• Secant Extreme Enterprise EJB Server (Secant
  Technologies)
• Proton EJB Server (Pramati Technologies)
• Ejipt EJB Application Server (Valto Systems)
• Prosyst Enterprise Beans Server (Prosyst Software
  GmBH)
• Orion Application Server (Orion)
• Voyager Application Server (ObjectSpace)

5
Open Source Products

• Enhydra Java Application Server- (Lutris
  Technologies)
• Locomotive Application Server- (Leverage
  Information Systems)
• Voyager Application Server (ObjectSpace)
• JOnAS JavaTM Open Application Server- (BullSoft)
  
• The EJBoss server- (EJBoss)
• Apache JServ Servlet engine-
• Jakarta Tomcat - a reference implementation for
  servlets and JSP

6
EJB Goals (I)

• The EJB Spec tries to meet several goals at once
• EJB is designed to help developers to create
  applications, freeing them from low-level system
  details of managing transactions, threads, load
  balancing, and so on. Developers can concentrate
  on business logic and leave the details of
  managing the data processing to the framework.
  For specialized applications, though, it's
  always possible to customize these lower-level
  services.

• The EJB Spec defines the major structures of
  the EJB framework, and then specifically defines
  the contracts between them. The responsibilities
  of the client, the server, and the individual
  components are all clearly spelled out. A
  developer creating an EJB component has a very
  different role from someone creating an
  EJB-compliant server, and the spec describes the
  responsibilities of each.

7
EJB Goals (II)

• EJB aims to be the standard way for client/server
  applications to be built in the Java language.
  Just as JavaBeans (or whatever components)from
  different vendors can be combined to produce a
  custom client, EJB server components from
  different vendors can be combined to produce a
  custom server. EJB components will of course run
  in any EJB-compliant server without
  recompilation.

• Finally, the EJB is compatible with and uses
  other Java APIs, can interoperate with non-Java
  applications, and is compatible with CORBA.

8
How an EJB System Works

• To understand how an EJB client/server system
  operates, we need to understand the basic parts
  of an EJB system
• the EJB component
• the EJB container
• the EJB object.

9
The EJB Component

• An Enterprise JavaBean is a component.
• EJBs execute within an EJB container, which in
  turn executes within an EJB server. Any server
  that can host an EJB container and provide it
  with the necessary services can be an EJB server.
  
• An EJB component is a Java class, written by an
  EJB developer, that implements business logic.
  All the other classes in the EJB system either
  support client access to or provide services
  (like persistence, etc.) to EJB component classes.

10
The EJB Container

• The EJB container is where the EJB component
  "lives."
• The EJB container provides services such as
  transaction and resource management,
  versioning, scalability, mobility,
  persistence, and security to the EJB
  components it contains. Since the EJB container
  handles all of these functions, the EJB component
  developer can concentrate on business rules, and
  leave database manipulation and other such fine
  details to the container.
• Multiple EJB component instances typically exist
  inside a single EJB container.

11
The EJB Object and the Component Interface

• Client programs execute methods on remote EJBs by
  way of an EJB object. The EJB object implements
  the component interface" of the EJB component on
  the server. The C.I. represents the "business"
  methods of the EJB component.
• An EJB component runs on the server in an EJB
  container and implements the business logic. The
  EJB object runs on the client and remotely
  executes the EJB component's methods.

12
EJB Clients

• An EJB client is an object that interacts with
  an enterprise bean in other words, a bean must
  undertake a given activity upon a request from a
  client.
• Such an interaction does not involve any instance
  of the EJB class, but it takes place by means
  of the component interface of the EJB.
• In EJB 2.0 there are two types of component
  interfaces
• Remote
• Local (introduced in EJB 2.0)

13
Local vs. Remote Clients

• Local clients are always placed on the same Java
  Virtual Machine that handles the enterprise bean.
• The direct coupling of an enterprise bean with a
  local client yields a number of disadvantages,
  because of the lack of location transparency.
• The usage of local clients is mainly motivated
  by performance issues

• Remote clients are placed on a different Java
  Virtual Machine, respect to the enterprise bean.
• There is no direct coupling of an enterprise
  bean with a remote client the location
  transparency is achieved.
• The remote client does not know, and does not
  care about the actual physical position of the
  enterprise bean.

14
Accessing EJBs by Component Interfaces

• If the EJB is accessed by local clients, its
  component interface must extend
  javax.ejb.EJBLoc-alObject

• If the EJB is accessed by remote clients, its
  component interface must extend
  javax.ejb.EJBObject

The bean class doesnt actually implement its
own component interface, but it must contain the
same operating methods defined in the component
interface. To get the object that implements the
C.I., we can use either javax.ejb.SessionContext
or javax.ejb.EntityConext, depending on the
bean type.
15
Use of the Home Interface

• A client can invoke a method of the C.I. once it
  has obtained a reference to an object that
  implements such an interface.
• The component in charge of creating C.I.
  instances is the bean Home Interface.
• An Home Interface must be provided for each
  enterprise bean in the application.
• The home interface defines methods to allow
  clients to create, find, and remove EJB objects.
• The H.I. must extend either javax.ejb.EJBLocalHome
  or javax.ejb.EJBHome, depending on the bean type

16
The Container the Home Interface

• As for the C.I, the container creates an object
  that implements the bean home interface.
• The home object is aimed at behaving like a
  factory for creating objects that implements the
  component interface.
• An home interface manages the lifecycle of all
  the instances (it has created) of a certain EJB.
• For local and remote clients two different home
  factories are used.

17
Types of Enterprise JavaBeans

• There are three different types of beans
• Session beans
• Stateful session beans
• Stateless session beans
• Entity beans
• Message-driven beans

18
Goals for a Server Component Technology

• The fundamental goal of a server component
  technology is this
• How do you go about creating a server whose
  methods are extensible at runtime?
• How can someone, for example, buy an accounting
  package, drop it into a running server, and use
  the client software that came with the package to
  access the server functionality, all without even
  shutting the server down, much less recompiling
  it?

19
Conclusion Extensible application servers
realized

• The answer that Enterprise JavaBeans provides is
  to create the client in terms of component
  interfaces. These component interfaces are
  implemented on the client as remote method
  invocations, and the very same interface is
  implemented on the server side as domain
  functionality. The EJB server uses JNDI to
  publish the availability of these services. The
  client uses JNDI to locate a new class's home
  interface by name, and then uses the home
  interface to create objects with the remote
  interface that provides the service.
• This late-binding between available interfaces
  on a server and the interface names is what
  makes an EJB server runtime-extensible.