In-Depth

Servlets and EJBs: Friends or foes?

Our return to the columns of ADT after a two-year absence comes after a period of intense change -- the demise of the dot.com phenomenon (which almost overnight became the dot.bomb experience), the introduction of too many buzzwords and standards, a plethora of new products that implement -- or just claim to implement -- the latest buzzwords and standards, and much, much more. As always in this business, vendor marketing dollars moved quickly from yesterday's ''du jour'' technologies a couple of years ago and are now promoting J2EE, .NET and, of course, Web services.

However, though the ever-increasing number of buzzwords does create a degree of confusion, one can oftentimes easily follow the common thread and common sense of what we call ''the buzzword evolution.''

A couple of years ago one of our engineers jokingly declared: ''after the invention of the state machine everything in computing is a hack.'' While this is far from reality, many industry veterans would concur that while the buzzwords and standards do change all the time, the basic principles of distributed computing and middleware remain the same. Just follow the trail of ''buzzword evolution'' in the common vocabulary of magazines. IDL (Interface Definition Language) ''got replaced'' by WSDL (Web Services Definition Language); SII (Static Invocation Interface), DII (Dynamic Invocation Interface) and RMI (Remote Method Invocation) ''were taken over'' by SOAP (Simple Object Access Protocol); and finally, Service Oriented Architecture replaced ''Whatever Oriented Architecture'' was in vogue.

Yes, there are more standards now than ever before (skeptics are referred to the list of XML standards governed by the World Wide Web Consortium, or W3C). But at the end of the day, only a few standards matter to developers.

The ''buzzwords and standards'' evolution begs the question of whether middleware choices are more confusing for IT executives and development managers now than they were two years ago. As a consulting company always trying to stay at the front of the middleware revolution (and evolution), taking advantage of many opportunities to implement important projects using a multitude of middleware technologies, we can report that unfortunately, the answer to the question is not a binary yes or no. The good news is that there are some middleware choices that are becoming close to binary. And at least when it comes to the development of brand-new component-based e-business applications there are really only two middleware platforms to choose from -- J2EE and .NET.

However, the reality is that most companies have long focused less on developing new applications than on integrating existing applications. That reality is even more true today as IT budgets undergo more scrutiny than ever before. According to recent monthly CIO surveys conducted by investment firm Morgan Stanley, application integration was cited consistently as a top priority for 2002 and beyond. The bad news is that no single middleware solution has emerged from the pack to become an obvious choice for projects that are focused on application integration. So the technologies and products that enterprises can evaluate today remain very complex and are even more confusing than they were in earlier years. Today, viable integration technologies include Message Oriented Middleware (MOM), Message Brokers, CORBA, J2EE-based application servers and Microsoft's .NET.

As middleware consultants, important questions we hear regularly include: Are the J2EE-based application servers ready to undertake complex EAI projects? And what are the long-term risks associated with the use of proprietary Message Brokers?

In this and future articles we'll try to answer some of the most asked technical questions, including: What are the differences and similarities between J2EE and .NET frameworks? Are J2EE-based application servers ready to become the technology of choice for EAI projects? And what are the key challenges in the development and deployment of J2EE applications?

In this article we tackle a question that we have been asked repeatedly during the past few years. As J2EE-based application servers like BEA's WebLogic Server and IBM's WebSphere gain more widespread acceptance, should Java servlets or Enterprise JavaBeans (EJBs) be used as a foundation for e-business applications? Most of the early Web-based J2EE applications have been developed using servlets as a base for two main reasons. First, the servlet model offers more simplicity than EJBs and is easier for IT developers and managers to comprehend as they get started with the technology. And second, the majority of the early projects are focused on the so-called ''low-hanging fruit,'' or new e-business applications that focus on presentation-centric applications and do not have high developer demands in terms of complexity, scalability, availability, reliability and extensibility.

Meanwhile, the vendors state that the purpose of J2EE application servers is to support high-end enterprise applications and to serve as the new, strategic middleware platform for all application development and deployment. It is therefore important to understand the difference in capabilities of servlets vs. EJBs so that the suitability of one approach over the other can be determined given the requirements of a particular project, the readiness of IT personnel, budgets and realistic time-to-market.

Popular technologies
Both Java servlets and Enterprise JavaBeans are popular technologies, and both can play a prominent role in enterprise architectures. Frequently, suppliers and users view these technologies as competitive, especially for Web-centric applications. Indeed, on the surface, servlets and EJBs can both be used to allow thin clients, such as browsers, PDAs and the like, to access enterprise data. Two variations on a classical architecture -- a thin client (such as a browser) accessing a servlet that is in turn connected to a back-end system or database; and a second variation that shows a client accessing an EJB through a presentation gateway, such as an HTML servlet -- illustrate this contention.

Indeed, from the browser client perspective, there is not much difference between the two technologies -- both bridge between presentation and back-end functionality. And, in many cases, servlets are considered to be a better choice because their coding and deployment requires simpler skills and shorter times than those required for using EJBs, which deal with a whole spectrum of complex issues, including many additional APIs and rules. Of course, when an enterprise-strength architecture is in question, neither the browser client perspective nor the simplification of the development process is a good enough justification for the choice of technology.

The fundamental difference between servlets and EJBs becomes apparent when such essential design objectives as architectural robustness and business agility are brought into focus. Then, issues like scalability, state management, flexibility of entity relationships and richness of the cooperation metaphor start to play a prominent role in the technology selection process. We intend to show why EJB-based architectures can handle such concerns better than servlets-based technologies, and why a clear understanding and recognition of this conclusion is required when new, complex applications are being developed.

We may be giving readers the impression that we are biased toward EJBs as the preferred vehicle for enterprise architectures. However, our view is that EJBs and servlets are orthogonal technologies that, rather than competing directly, are mostly complementary. To compare EJBs and servlets is not like comparing apples to apples. It is not even like comparing apples to oranges -- it is more like comparing apples and oranges to the crates and barrels that are used to store and transport them. This becomes obvious when the essential differences in intent, focus and, if you like, domain philosophy of the two standards are investigated.

Servlets cater primarily to the delivery of dynamic content to browser-based clients. They are narrowly focused on facilitating presentation, such as programmatic translation and preparation of HTML, and on relief from handling lower-level details of HTTP. This is the extent of the role servlets play in the framework of the J2EE standard.

On the other hand, the EJB standard was devised with the vision of a common pattern for component architectures in mind. It deals principally with enabling the development and deployment of a business application as a collection of components into a framework of powerful and comprehensive infrastructure services (which can include Java Database Connectivity (JDBC), Java Transactional Service (JTS)/ Java Transaction API (JTA), Java Messaging Service (JMS) and the like). Rather than prescribing any specific role (such as HTML rendering or HTTP encapsulation), the Enterprise JavaBean component model is a mold for generic business components whose exact specialization is left to the application designer to establish. In other words, the servlets specification is a somewhat narrow standard specialized on the presentation layer of applications, while the EJB standard is a broad enabling technology.

This distinction becomes more apparent when directly comparing the characteristics of EJBs and servlets to evaluate their respective applicability to enterprise architectures.

* A servlet is a faceless Java object. Beyond some features mandated by its base class, it is amorphous and can have arbitrary internal composition. On the other hand, by contract, an EJB is obliged to implement specific interfaces (such as the home interface, activation and deactivation interfaces) and mandatory properties (such as primary key, for example) that promptly tie it into the cooperation environment (the container).

* A servlet has to implement a rigid set of methods with predefined signatures just as Enterprise JavaBeans need to implement a set of methods imposed by the EJB specification to fulfill the contract with the container. However, the EJB can also define arbitrary methods that best suit its business semantics.

* Servlets have very thin support from the environment they are deployed into, which is mostly limited to HTTP-related matters; EJBs enjoy a wide range of powerful and comprehensive infrastructure services that are provided by the container. Examples include the generation and handling of remote interfaces, component factory services, component instance identification, distributed transactions, automatic persistence for Entity Beans and declarative security.

* Basically, servlets are focused on one task: reacting to HTTP requests with HTML responses in a stateless manner (in fact, state may be maintained via an HttpSession object, but it is not comprehensive and requires explicit client cooperation via session cookies). EJB behavior is defined at a higher level of abstraction: EJB method invocation does not stipulate any syntax or semantics to the invocation arguments or to the maintained state.

* By design, the aim of EJB is to segregate business functionality from infrastructure services (such as life cycle management, transaction and security contexts and persistence). Because servlets are not integrated with the J2EE infrastructure services, they encourage application developers to deal with lower-level J2EE APIs directly, and entangle business functionality with logic related to infrastructure services.

* The servlets specification does not address enterprise concerns such as load balancing and failover. On the contrary, inherent scalability and high availability of services is a declared responsibility of the EJB container. (It should be noted that some servlet engines do support clustering and load balancing. Curiously enough, these are the engines that are embedded into EJB application servers such as WebSphere and WebLogic).

In other words, servlets do one thing and do it well -- they provide a shell for the presentation layer of a Web-based application; EJB is a complex framework for implementing widely dissimilar business functionality in a coherent and comprehensive fashion.

Servlets expose; EJBs encapsulate
The other point of distinction between servlets and EJBs is that, in our experience, in the majority of cases, servlets are used to expose business data, whereas EJBs are used to encapsulate business functionality.

A classic use of a servlet would be to parse an HTTP request, access a database for inquiry or update, and to compose an outgoing HTML reply. (A less common use of a servlet would involve passing a serialized Java object between itself and a browser-side applet. This scenario will be discussed later.) For example, a browser may submit a request for an airline schedule, and the servlet will fetch the data from the database and format the reply as an HTML table. Also, the browser may submit a filled form whose elements will be stored by the servlet in a database table. In either case, it is the data, not the behavior, the client is interested in.

Arguably, HTML data-level cooperation achieves a great level of independence between the presentation at a client (in this case the browser) and the processing at the server (the servlet). The interface between the client and the server is primarily concerned with passing data; there is no API specific to any particular business function. The browser client displays HTML returns unconditionally (subject to its validity, of course). Thus, if the servlet implementation is changed to render richer replies (such as embedding sound clips, for example), the client will benefit from this extended business functionality automatically and transparently. However, if the servlet is changed to display, for example, the latest cricket results instead of stock quotes, all the same data will be unconditionally rendered by the client, much to the user's dismay.

In other words, the client's horizon extends all the way to the data and the servlet acts merely as a rendering agent that enables the client to access the database.

Separating the presentation by the browser client and the processing in servlets achieves a higher degree of independence compared to an EJB-based architecture because it does not rely on the strong coupling that the interface of an EJB with rigid business semantics imposes. However, this comes at the cost of compromising business process integrity. This might not be a cause for concern for simple Web browsing interactions, but from an enterprise-strength B2B/B2C perspective, cohesiveness of the end-to-end business process flow must be protected. This objective is best achieved by encapsulating business behavior in services, and service granulation and exposition is precisely the domain of the EJB.

With a typical EJB implementation, a client gains access to business functionality through a presentation gateway, whereby the latter provides a rendering agent for business functionality as opposed to data. Thus, a typical use of an EJB would be for the client to remotely invoke the server-side service, which may or may not result in any data being returned to the client, but which would definitely result in some meaningful processing having taken place at the server side.

Two-tier vs. three-plus tier
Another way to contrast servlets and EJBs is to assert that servlets by intent serve to accommodate a classical two-tier application (where the presentation and data access/business logic tiers are commingled within the servlet and the data store). On the other hand, EJBs tend to three-tier or multitier complex cooperative environments.

Apparently, the flexibility of interpretation built into the servlets standard permits us to break this data-only paradigm easily. Nothing prevents a servlet from being engineered in such a way that it encapsulates a business service. Practically, this can be achieved in a number of ways. For example, instead of only accessing a database, a servlet implementation may invoke back-end functionality that has been written in Java via an RMI call, or a servlet may exchange serialized Java objects with a browser-resident applet. These and other design patterns that turn servlets into business logic capsules are well known and quite popular.

In practice, it has become a common architecture approach to allow servlets to benefit from their amorphous ''anything goes'' internal structure -- or, better yet, their lack of any internal structure. Nothing prevents servlet implementations from internally integrating with a variety of technologies like RMI/CORBA, JNDI, JMS, JTS/JTA and so on; just as no out-of-the-box transparency exists for JDBC -- a main vehicle to access databases from within a servlet -- the constructs within a servlet for all of the above technologies have to be manually coded on a case-by-case basis. Nevertheless, the ability to embed the whole spectrum of Java platform solutions on the client's behalf turns servlets into a powerful middleware mechanism that extends the Java APIs to the Web.

It seems that both the servlets and the EJB technology are suitable platforms for an enterprise application. Furthermore, they both seem suitable for presentation-centric applications that do not require support for ''hard-core'' distributed services such as transactions, load balancing, persistence, state management and message queuing, among others. The use of servlets might be even more appropriate and certainly more straightforward. However, even though it is possible to use servlets for truly distributed applications, such use would be sub-optimal. The reason for this far-reaching statement will be clarified in Part II of this article, in which we will compare these two technologies in more detail. Stay tuned.

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