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If we blindly used POJOs (plain-old Java objects) and lightweight frameworks, we would be repeating the mistake the enterprise Java community made with EJB (Enterprise JavaBeans). Every technology has both strengths and weaknesses, and it's important to know how to choose the most appropriate one for a given situation.
This book is about implementing enterprise applications using design patterns and lightweight frameworks. To enable you to use them effectively in your application, it provides a decision-making framework that consists of five key questions that must be answered when designing an application or implementing the business logic for an individual use-case. By consciously addressing each of these design issues and understanding the consequences of your decisions, you will vastly improve the quality of your application.
In this article you will get an overview of those five design decisions. I briefly describe each design decision's options as well as its respective benefits and drawbacks.
There are two quite different ways to design an enterprise Java application. One option is to use the classic EJB 2 approach, which I will refer to as the heavyweight approach. When using the heavyweight approach, you use session beans and message-driven beans to implement the business logic. You use either DAOs (data access objects) or entity beans to access the business logic.
The other option is to use POJOs and lightweight frameworks, which I'll refer to as the POJO approach. When using the POJO approach, your business logic consists entirely of POJOs. You use a persistence framework (a.k.a. object/relational mapping framework) such as Hibernate or JDO (Java Data Objects) to access the database, and you use Spring AOP (aspect-oriented programming) to provide enterprise services such as transaction management and security.
EJB 3 somewhat blurs the distinction between the two approaches because it has embraced POJOs and some lightweight concepts. For example, entity beans are POJOs that can be run both inside and outside the EJB container. However, while session beans and message-driven beans are POJOs, they also have heavyweight behavior since they can only run inside the EJB container. So as you can see, EJB 3 has both heavyweight and POJO characteristics. EJB 3 entity beans are part of the lightweight approach, whereas session beans and message-driven beans are part of the heavyweight approach.
Choosing between the heavyweight approach and the POJO approach is one of the first of myriad design decisions that you must make during development. It's a decision that affects several aspects of the application, including business logic organization and the database access mechanism. To help decide between the two approaches, let's look at the architecture of a typical enterprise application, which is shown in Figure 1, and examine the kinds of decisions that must be made when developing it.
Figure 1. A typical application architecture and the key business logic and database access design decisions. Click on thumbnail to view full-sized image.
The design of the presentation tier is outside the scope of this article, but let's look at the rest of the diagram. We need to decide the structure of the business tier and the interface that it exposes to the presentation tier and its other clients. We also need to decide how the persistence tier accesses databases, which is the main source of data for many applications. We must also decide how to handle concurrency in short transactions and long-running transactions. That adds up to five decisions that any designer/architect must make and that any developer must know in order to understand the big picture.
These decisions determine key characteristics of the design of the application's business and the persistence tiers. There are, of course, many other important decisions that you must make—such as how to handle transactions, security, and caching, and how to assemble the application—but answering those five questions often addresses these other issues as well.
Each of the five decisions shown in Figure 1 has multiple options. Each option has benefits and drawbacks that determine its applicability to a given situation. As you will see in this article, each one makes different trade-offs in terms of one or more areas, including functionality, ease of development, maintainability, and usability. Even though I'm a big fan of the POJO approach, it is important to know these benefits and drawbacks so that you can make the best choices for your application.
Let's now take a brief look at each decision and its options.
These days a lot of attention is focused on the benefits and drawbacks of particular technologies. Although this is certainly very important, it is also essential to think about how your business logic is structured. It is quite easy to write code without giving much thought to how it is organized. For example, it is too easy to add yet more code to a session bean instead of carefully deciding which domain model class should be responsible for the new functionality. Ideally, however, you should consciously organize your business logic in the way that's the most appropriate for your application. After all, I'm sure you've experienced the frustration of having to maintain someone else's badly structured code.
The key decision you must make is whether to use an object-oriented approach or a procedural approach. This isn't a decision about technologies, but your choice of technologies can potentially constrain the organization of the business logic. Using EJB 2 firmly pushes you toward a procedural design, whereas POJOs and lightweight frameworks enable you to choose the best approach for your particular application. Let's examine the options.
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