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Understanding object/relational persistence
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manipulate data, the tables and columns involved must be specified at least three times (insert, update, select), adding to the time required for design and implementation. The distinct dialects for every SQL database management system don t improve the situation. To round out your understanding of object persistence, and before we approach possible solutions, we need to discuss application architecture and the role of a persistence layer in typical application design.
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In a medium- or large-sized application, it usually makes sense to organize classes by concern. Persistence is one concern; others include presentation, workflow, and business logic.1 A typical object-oriented architecture includes layers of code that represent the concerns. It s normal and certainly best practice to group all classes and components responsible for persistence into a separate persistence layer in a layered system architecture. In this section, we first look at the layers of this type of architecture and why we use them. After that, we focus on the layer we re most interested in the persistence layer and some of the ways it can be implemented.
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Layered architecture
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A layered architecture defines interfaces between code that implements the various concerns, allowing changes to be made to the way one concern is implemented without significant disruption to code in the other layers. Layering also determines the kinds of interlayer dependencies that occur. The rules are as follows:
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Layers communicate from top to bottom. A layer is dependent only on the layer directly below it. Each layer is unaware of any other layers except for the layer just below it.
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Different systems group concerns differently, so they define different layers. A typical, proven, high-level application architecture uses three layers: one each for presentation, business logic, and persistence, as shown in figure 1.4. Let s take a closer look at the layers and elements in the diagram:
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There are also the so-called cross-cutting concerns, which may be implemented generically by framework code, for example. Typical cross-cutting concerns include logging, authorization, and transaction demarcation.
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Figure 1.4 A persistence layer is the basis in a layered architecture
Presentation layer The user interface logic is topmost. Code responsible for the presentation and control of page and screen navigation is in the presentation layer. Business layer The exact form of the next layer varies widely between applications. It s generally agreed, however, that the business layer is responsible for implementing any business rules or system requirements that would be understood by users as part of the problem domain. This layer usually includes some kind of controlling component code that knows when to invoke which business rule. In some systems, this layer has its own internal representation of the business domain entities, and in others it reuses the model defined by the persistence layer. We revisit this issue in chapter 3. Persistence layer The persistence layer is a group of classes and components responsible for storing data to, and retrieving it from, one or more data stores. This layer necessarily includes a model of the business domain entities (even if it s only a metadata model). Database The database exists outside the Java application itself. It s the actual, persistent representation of the system state. If an SQL database is used, the database includes the relational schema and possibly stored procedures. Helper and utility classes Every application has a set of infrastructural helper or utility classes that are used in every layer of the application (such as Exception classes for error handling). These infrastructural elements don t form a layer, because they don t obey the rules for interlayer dependency in a layered architecture.
Understanding object/relational persistence
Let s now take a brief look at the various ways the persistence layer can be implemented by Java applications. Don t worry we ll get to ORM and Hibernate soon. There is much to be learned by looking at other approaches.
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