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1.3.6 Other options
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Of course, there are other kinds of persistence layers. XML persistence is a variation on the serialization theme; this approach addresses some of the limitations of byte-stream serialization by allowing tools to access the data structure easily (but is itself subject to an object/hierarchical impedance mismatch). Furthermore, there is no additional benefit from the XML, because it s just another text file format. You can use stored procedures (even write them in Java using SQLJ) and move the problem into the database tier. We re sure there are plenty of other examples, but none of them are likely to become popular in the immediate future. Political constraints (long-term investments in SQL databases) and the requirement for access to valuable legacy data call for a different approach. ORM may be the most practical solution to our problems.
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1.4 Object/relational mapping
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Now that we ve looked at the alternative techniques for object persistence, it s time to introduce the solution we feel is the best, and the one we use with Hibernate: ORM. Despite its long history (the first research papers were published in the late 1980s), the terms for ORM used by developers vary. Some call it object
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Object/relational mapping
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relational mapping, others prefer the simple object mapping. We exclusively use the term object/relational mapping and its acronym, ORM. The slash stresses the mismatch problem that occurs when the two worlds collide. In this section, we first look at what ORM is. Then we enumerate the problems that a good ORM solution needs to solve. Finally, we discuss the general benefits that ORM provides and why we recommend this solution.
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1.4.1 What is ORM
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In a nutshell, object/relational mapping is the automated (and transparent) persistence of objects in a Java application to the tables in a relational database, using metadata that describes the mapping between the objects and the database. ORM, in essence, works by (reversibly) transforming data from one representation to another. This implies certain performance penalties. However, if ORM is implemented as middleware, there are many opportunities for optimization that wouldn t exist for a hand-coded persistence layer. A further overhead (at development time) is the provision and management of metadata that governs the transformation. But again, the cost is less than equivalent costs involved in maintaining a hand-coded solution. And even ODMG-compliant object databases require significant classlevel metadata.
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Isn t ORM a Visio plugin The acronym ORM can also mean object role modeling, and this term was invented before object/relational mapping became relevant. It describes a method for information analysis, used in database modeling, and is primarily supported by Microsoft Visio, a graphical modeling tool. Database specialists use it as a replacement or as an addition to the more popular entity-relationship modeling. However, if you talk to Java developers about ORM, it s usually in the context of object/relational mapping.
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An ORM solution consists of the following four pieces:
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An API for performing basic CRUD operations on objects of persistent classes A language or API for specifying queries that refer to classes and properties of classes A facility for specifying mapping metadata A technique for the ORM implementation to interact with transactional objects to perform dirty checking, lazy association fetching, and other optimization functions
Understanding object/relational persistence
We re using the term ORM to include any persistence layer where SQL is autogenerated from a metadata-based description. We aren t including persistence layers where the object/relational mapping problem is solved manually by developers hand-coding SQL and using JDBC. With ORM, the application interacts with the ORM APIs and the domain model classes and is abstracted from the underlying SQL /JDBC. Depending on the features or the particular implementation, the ORM runtime may also take on responsibility for issues such as optimistic locking and caching, relieving the application of these concerns entirely. Let s look at the various ways ORM can be implemented. Mark Fussel [Fussel 1997], a researcher in the field of ORM, defined the following four levels of ORM quality. Pure relational The whole application, including the user interface, is designed around the relational model and SQL-based relational operations. This approach, despite its deficiencies for large systems, can be an excellent solution for simple applications where a low level of code reuse is tolerable. Direct SQL can be fine-tuned in every aspect, but the drawbacks, such as lack of portability and maintainability, are significant, especially in the long run. Applications in this category often make heavy use of stored procedures, shifting some of the work out of the business layer and into the database. Light object mapping Entities are represented as classes that are mapped manually to the relational tables. Hand-coded SQL / JDBC is hidden from the business logic using wellknown design patterns. This approach is extremely widespread and is successful for applications with a small number of entities, or applications with generic, metadata-driven data models. Stored procedures might have a place in this kind of application. Medium object mapping The application is designed around an object model. SQL is generated at build time using a code generation tool, or at runtime by framework code. Associations between objects are supported by the persistence mechanism, and queries may be specified using an object-oriented expression language. Objects are cached by the persistence layer. A great many ORM products and homegrown persistence layers support at least this level of functionality. It s well suited to medium-sized applications with some complex transactions, particularly when portability between
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