J2EE n-Tier Application Architecture
J2EE middleware defines a set of four independent tiers, over which applications can be built:
- Client tier
- Presentation logic tier
- Business logic tier
- Enterprise information systems tier
A pictorial representation of these tiers is shown in Figure 1.
Figure 1 J2EE: multitier application architecture.
Presentation logic and business logic tiers fall under the application server zonewhich is nothing but the J2EE platform implementation.
Each of these tiers may physically be sitting over several machines. Moreover, within the application server zone the presentation logic can be hosted in one application server, and business logic can be hosted in another.
For example, it is possible to use iPlanet (a J2EE application server product from Sun) as an HTTP Web server and presentation logic container, and use Weblogic (another J2EE application server from BEA) to deploy business logic components.
It is common to associate the presentation logic tier with Servlet and JSP containers (discussed later in this article) and business logic tier to the Enterprise Javabeans container. It will be easier to understand tiers as conceptual entitiesmeant for ease of designand containers as physical software instancesmeant for providing the runtime environment for application components.
J2EE's multitier architecture is inspired by Model View Controller (MVC) architecture: A software methodology used for demarcating the boundaries and scope of individual application components in a complex system.
MVC advocates that those codes, which represent the business logic or core application functionality, should not be intermingled with those that present the proceed results to clients or other applications. They should possibly be independent of each other, and a controller should mediate the interactions between them.
For example, let us consider an Internet-based, bank account-transfer process. According to MVC, this process can be broken down to four independent tasks, as shown in Figure 2.
Task 1: The task of invoking the transfer from the browser (view)
Task 2: The task of invoking the mechanism that does the account transfer (controller)
Task 3: The task of doing the actual account transfer (business process or model)
Task 4: The task of showing the status of the transfer (success or failure) to the browser (view)
Figure 2 MVC look at an Internet account transfer process.
MVC states that the core business process (Task 3) should not assume anything about the clients. Instead of a browser, another application or a back office system may well invoke it. The process of orchestrating between the business logic and view elements is to be assigned to a dedicated entity called the controller.
Having been based on MVC methodology, J2EE's architecture naturally demarcates business logic from presentation logic tier. Controllers can be placed in either of these tiers or both. By doing so, J2EE provides room for reusability of business logic components.
J2EE Application Components
The J2EE standard defines a rich set of application component framework, with which almost all kinds of business applications can be built. These applications range from simple Web portals to complex, distributed, enterprise-scale transactional applications. Application components, which are the brick and mortar of enterprise systems, are built on the top of this framework.
J2EE component framework is only a rudimentary infrastructurein the form of libraries, base classes, and interfaces. Application components that are built over them usually embody business logic and the presentation/controller logic that makes up the application.
For example, let us consider servlets, which are standard application components provided by J2EE. Servlets for a given business application are to be developed on top of a basic servlet interface provided by J2EE. While doing so, developers can make use of servlet libraries and services that come along with the servlet package. Many of the system level services, such as reading data from an HTTP input stream or writing data to an HTTP output stream, are available within these libraries, so that applications can readily make use of them.
Hence, we term the component infrastructure provided by J2EE as an application framework, and those codes written on top of it as J2EE application components. But I'll be using these two terms interchangeably, throughout this series for the sake of simplicity.
J2EE application components are available for the client tier, presentation tier, and business logic tiers, as shown in Figure 3.
These components include the following:
Client tier: applets, Javabeans
Presentation logic tier: servlets, Java server pages, Javabeans
Business logic tier: Enterprise Javabeans
Figure 3 J2EE application components.
It is very important to note that normal Java classes and Jar archives can be hosted in all of these tiers very well, apart from the J2EE-specific components listed above. In many cases, we will find application codes spilling outside the J2EE components into normal Java classes.
Components are not available for the Enterprise Information System (EIS) tier because EIS encompasses all enterprise backend resources such as databases, BackOffice/legacy systems, and ERP implementationsin the J2EE realm. Obviously, all we need is access to these resources from our application components, which are taken care of by J2EE enterprise services. Thus, there is no scope for application components in the EIS tier.
Applications are to be coded by the programmer, using one or more of these components, and deployed in respective containers.
Arriving at a proper mix of application components for a given business scenario is the crafty job of a J2EE architect. Because there is a wide variety of components available, it requires considerable knowledge and experience to choose the right mixture of components that will result in optimized code and performance-rich applications.
By assigning specific and well-defined roles to each application component, J2EE aims to provide room for logical application architecture, design, and development. The component interfaces ensure that the components adhere to certain standards and exhibit common behavior, which provides the base for interoperable J2EE server environments.
These application components are loosely coupled across the tiers to ensure flexibility and reusability while interacting with each other. A whole range of enterprise servicessuch as mailing, database connectivity, messaging, and transaction processingare available at the disposal of application components.
On the flip side, unlike .Net framework, which is Microsoft's middleware architecture, all J2EE application components are to be coded only in the Java language!
J2EE Enterprise Services
The nutrients for the blossoming of enterprise applicationssuch as mailing, database connectivity, messaging, and transaction processingare readily catered to by the J2EE environment as enterprise services.
These essential services are made available in the form of interfaces, class libraries, drivers, and adapters. As in the case of operating systems, J2EE provides a common Java interface over these services, so that the application codes are shielded from the subtleties of specific service implementation provided by the application vendors.
For example, JDBC is one service that provides connectivity to a wide variety of databases available in the market. Connectivity to different databases (such as Oracle, MySQL, Informix, and SQL Server) is achieved through specific JDBC drivers that are provided by the database manufacturers themselves or by third-party vendors. Application developers need not worry about the intricacies of specific databases; all they need to do is to make use of standard JDBC calls to access these resources, and the underlying JDBC driver takes care of implementing them.
J2EE enterprise services include the following:
Java Database Connectivity (JDBC): provides database connectivity services
Java Connector Architecture (JCA): provides connectivity to legacy systems
Java Messaging Service (JMS): provides messaging services across the tiers and components
Java Activation Framework/Java Mailing services (JAF/Javamail): provides e-mail services
Java Interface Definition Language/RMIIIOP: enable CORBA communication with J2EE
Java XML APIs (JAX): provides XML parsing/binding services
Java Naming and Directory Interface (JNDI): provides distributed naming and directory services
Java Transaction Service (JTS/JTA): provides transaction-monitoring services
Java Authentication and Authorization service (JAAS): provides access control services
Specific drivers and adapters for selected services usually come bundled with the application server itself. For example, Web logic comes with a JDBC driver for connecting J2EE applications to Informix. This becomes an important selection criterion when evaluating different application servers for a given enterprise infrastructure.
J2EE Runtime Environment (Containers)
J2EE application components in different tiers come to life in their runtime environments, which are called containers in J2EE terminology. These containers are vendor-specific products that adhere to certain common interfaces, and provide the much-required low-level infrastructure facilities for J2EE middleware components.
After being coded to J2EE standards, application components need to be deployed in the respective containers by using vendor-specific deployment tools.
J2EE standard defines four different containers:
Applet container: hosts applets
Application client container: hosts standard Java application clients (including swing windows applications)
Web container: hosts servlets and JSPs in the presentation logic tier
EJB container: hosts Enterprise Javabeans in the business logic tier
It is typical to include an HTTP Web server along with the Web containers to host static HTML Web pages, and almost all popular J2EE application servers have built-in HTTP Web servers.
It is worthwhile to note that one of the principal visions of J2EE architecture was that developers should be writing codes for their business logic, and should not worry about system-level capabilities. This vision is achieved only through the concept of containers.
The infrastructure facilities provided by the containers typically include memory management, synchronization/threading, garbage collection, availability, scaling, load balancing, and fault tolerance.
The basic interfaces and facilities that should be implemented by the containers are defined in J2EE specifications, but the means by which they are to be accomplished, is left to the container vendor. Thus, J2EE provides the scope for vendor-specific codes while ensuring compatibility across different application servers. In fact, it is mainly with the help of containers, apart from other facilities, that vendors are able to distinguish themselves in the middleware industry.