When to Use EJB

One of the most important design decisions when designing a J2EE app is whether to use EJB. EJB is often perceived to be the core of J2EE. This is a misconception; EJB is merely one of the choices J2EE offers. It's ideally suited to solving some problems, but adds little value in many apps. When requirements dictate a distributed architecture and RMI/IIOP is the natural remoting protocol, EJB gives us a standard implementation. We can code our business objects as EJBs with remote interfaces and can use the EJB container to manage their lifecycle and handle remote invocation. This is far superior to a custom solution using RMI, which requires us to manage the lifecycle of server-side objects. If requirements don't dictate a distributed architecture or if RMI/IIOP isn't the natural remoting protocol, the decision as to whether to use EJB is much tougher. EJB is the most complex technology in J2EE and is the biggest J2EE buzzword. This can lead to developers using EJBs for the wrong reasons: because EJB experience looks good on a resume; because there is a widespread belief that using EJB is a best practice; because EJB is perceived to be the only way to write scalable Java apps; or just because EJB exists. EJB is a high-end technology. It solves certain problems very well, but should not be used without good reason. In this section we'll take a dispassionate look at the implications of using EJB, and important considerations influencing the decision of whether to use EJB.

Implications of Using EJB

One of the key goals of the EJB specification is to simplify app code. The EJB 2.0 specification (§2.1) states that "The EJB architecture will make it easy to write apps: app developers will not have to understand low-level transaction and state management details, multi-threading, connection pooling, and other complex low-level APIs." In theory, by deferring all low-level issues to the EJB container, developers are free to devote all their effort to business logic. Unfortunately, experience shows that this is not often realized in practice. Using EJB often adds at least as much complexity to an app as it removes. Moreover, it may be dangerous for developers to "not have to understand" the enterprise software issues that their apps face. Introducing EJB technology has the following practical implications, which should be weighed carefully:

Using EJB makes apps harder to test
Distributed apps are always harder to test than apps that run in a single JVM. EJB apps - whether they use remote or local interfaces - are hard to test, as they are heavily dependent on container services.
Using EJB makes apps harder to deploy
Using EJB introduces many deployment issues. For example:
- Complex classloader issues. An enterprise app that involves EJB JAR files and web apps will involve many classloaders. The details vary between servers, but avoiding class loading problems such as inability to find classes or incompatible class versions is a nontrivial problem, and requires understanding of app server design.
- Complex deployment descriptors. While some of the complexity of EJB deployment descriptors reduces complexity in EJB code (with respect to transaction management, for example), other complexity is gratuitous. Tools can help here, but it's preferable to avoid complexity rather than rely on tools to manage it.
- Slower development-deployment-test cycles. Deploying EJBs is usually slower than deploying J2EE web apps. Thus, using EJB can reduce developer productivity.
Most practical frustrations with J2EE relate to EJB. This is no trivial concern; it costs time and money if EJB doesn't deliver compensating benefits.
Using EJB with remote interfaces may hamper practicing OO design
This is a serious issue. Using EJB - a technology, which should really be an implementation choice - to drive overall design is risky. In EJB Design Patterns from Wiley(), for example, four of the six "EJB layer architectural patterns" are not true patterns, but workarounds for problems that are introduced by using EJB with remote interfaces. (The Session Façade pattern strives to minimize the number of network round trips, the result being a session bean with a coarse-grained interface. Interface granularity should really be dictated by normal object design considerations. The EJB Command pattern is another attempt to minimize the number of network round trips in EJB remote invocation, although its consequences are more benign. The Data Transfer Object Factory pattern addresses the problems of passing data from the EJB tier to a remote client, while the Generic Attribute Access patterns attempt to reduce the overhead of working with entity beans.)

The pernicious effects of unnecessarily using EJBs with remote interfaces include:

Interface granularity and method signatures dictated by the desire to minimize the number of remote method calls. If business objects are naturally fine-grained (as is often the case), this results in unnatural design.
The need for serialization determining the design of objects that will be communicated over RMI. For example, we must decide how much data should be returned with each serializable object - should we traverse associations and, if so, to what depth? We are also forced to write additional code to extract data needed by remote clients from any objects that are not serializable.
A discontinuity in an app's business objects at the point of remote invocation.

These objections don't apply when we genuinely need distributed semantics. In this case, EJB isn't the cause of the problem but an excellent infrastructure for distributed apps. But if we don't need distributed semantics, using EJB has a purely harmful effect if it makes an app distributed. As we've discussed, distributed apps are much more complex than apps that run on a single server. EJB also adds some additional problems we may wish to avoid:

Using EJB may make simple things hard
Some simple things are surprisingly difficult in EJB (with remote or local interfaces). For example, it's hard to implement the Singleton design pattern and to cache read-only data. EJB is a heavyweight technology, and makes heavy work of some simple problems.
Reduced choice of app servers
There are more web containers than EJB containers, and web containers tend to be easier to use than EJB containers. Thus, a web app can run on a wider choice of servers - or cheaper versions of the same servers - compared to an EJB app, with simpler configuration and deployment (however, if we have a license for an integrated J2EE server, cost isn't a concern, and the EJB container may already be familiar through use in other projects).

These are important considerations. Most tutorials ignore them, concentrating on theory rather than real-world experience.

Let's now review some of the arguments - good and bad - for using EJB in a J2EE app.

Questionable Arguments for Using EJB

Here are a few unconvincing arguments for using EJB:

To ensure clean architecture by exposing business objects as EJBs
EJB promotes good design practice in that it results in a distinct layer of business objects (session EJBs). However, the same result can be achieved with ordinary Java objects. If we use EJBs with remote interfaces, we are forced to use coarse-grained access to business objects to minimize the number of remote method calls, which forces unnatural design choices in business object interfaces.
To permit the use of entity beans for data access
I regard this as a poor reason for using EJB. Although entity beans have generated much interest, they have a poor track record. We'll discuss data access options for J2EE apps in more detail later.
To develop scalable, robust apps
Well-designed EJB apps scale well - but so do web apps. Also, EJB allows greater potential to get it wrong: inexperienced architects are more likely to develop a slow, unscalable system using EJB than without it. Only when a remote EJB interface is based on stateless session EJBs is a distributed EJB system likely to offer greater scalability than a web app, at the cost of greater runtime overhead. (In this approach, business objects can be run on as many servers as required.)

Compelling Arguments for Using EJB

Here are a few arguments that strongly suggest EJB use:

To allow remote access to app components
This is a compelling argument if remote access over RMI/IIOP is required. However, if web services style remoting is required, there's no need to use EJB.
To allow app components to be spread across multiple physical servers
EJBs offer excellent support for distributed apps. If we are building a distributed app, rather than adding web services to an app that isn't necessarily distributed internally, EJB is the obvious choice.
To support multiple Java or CORBA client types
If we need to develop a Java GUI client (using Swing or another windowing technology), EJB is a very good solution. EJB is interoperable with CORBA's IIOP and thus is a good solution for serving CORBA clients. As there is no web tier in such apps, the EJB tier provides the necessary middle tier. Otherwise, we return to the days of client-server apps and limited scalability because of inability to pool resources such as database connections on behalf of multiple clients.
To implement message consumers when an asynchronous model is appropriate
Message-driven beans make particularly simple JMS message consumers. This is a rare case in which EJB is "the simplest thing that could possibly work".

Arguments for Using EJB to Consider on a Case-by-Case Basis

The following arguments for using EJB should be assessed on a case-by-case basis:

To free app developers from writing complex multi-threaded code
EJB moves the burden of synchronization from app developers to the EJB container. (EJB code is written as if it is single-threaded.) This is a boon, but whether it justifies the less desirable implications of using EJB depends on the individual app.
There's a lot of FUD (Fear, Uncertainty, and Doubt) revolving around the supposed necessity of using EJB to take care of threading issues. Writing threadsafe code isn't beyond a professional enterprise developer. We have to write threadsafe code in servlets and other web tier classes, regardless of whether we use EJB. Moreover, EJB isn't the sole way of simplifying concurrent programming. We don't need to implement our own threading solution from the ground up; we can use a standard package such as Doug Lea's util.concurrent. See http://gee.cs.oswego.edu/dl/classes/EDU/oswego/cs/dl/util/concurrent/intro.html for an overview of this package, which provides solutions to many common concurrency problems.

Important
EJB's simplification of multi-threaded code is a strong, but not decisive, argument for using EJB.
To use the EJB container's transparent transaction management
EJBs may use Container-Managed Transactions (CMT). This enables transaction management to be largely moved out of Java code and be handled declaratively in EJB deployment descriptors. app code only needs to concern itself with transaction management if it wants to roll back a transaction. The actual rollback can be done with a single method call.
CMT is one of the major benefits of using EJB. Since enterprise apps are almost always transactional, without EJB CMT we will normally need to use the Java Transaction API (JTA). JTA is a moderately complex API and it's thus advisable (but not essential) to avoid using it directly. As with threading, it's possible to use helper classes to simplify JTA coding and reduce the likelihood of errors.
Note that the J2EE transaction management infrastructure (for example, the ability to coordinate transactions across different enterprise resources) is available to all code running within a J2EE server, not merely EJBs; the issue is merely the API we use to control it.

Important
The availability of declarative transaction management via CMT is the most compelling reason for using EJB.
To use EJB declarative support for role-based security
J2EE offers both programmatic and declarative security. While any code running in a J2EE server can find the user's security role and limit access accordingly, EJBs offer the ability to limit access declaratively (in deployment descriptors), down to individual business methods. Access permissions can thus be manipulated at deployment time, without any need to modify EJB code. If we don't use EJB, only the programmatic approach is available.
The EJB infrastructure is familiar
If the alternative to using EJB is to develop a substantial subset of EJB's capabilities, use of EJB is preferable even if our own solution appears simpler. For example, any competent J2EE developer will be familiar with the EJB approach to multi-threading, but not with a complex homegrown approach, meaning that maintenance costs will probably be higher. It's also better strategy to let J2EE server vendors maintain complex infrastructure code than to maintain it in-house.

Important

EJBs are a good solution to problems of distributed apps and complex transaction management. However, many apps don't encounter these problems. EJBs add unnecessary complexity in such apps. An EJB solution can be likened to a truck and a web app to a car. When we need to perform certain tasks, such as moving large objects, a truck will be far more effective than a car, but when a truck and a car can do the same job, the car will be faster, cheaper to run, more maneuverable and more fun to drive.

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