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different database products is important. These applications usually don t use stored procedures. Full object mapping Full object mapping supports sophisticated object modeling: composition, inheritance, polymorphism, and persistence by reachability. The persistence layer implements transparent persistence; persistent classes do not inherit any special base class or have to implement a special interface. Efficient fetching strategies (lazy and eager fetching) and caching strategies are implemented transparently to the application. This level of functionality can hardly be achieved by a homegrown persistence layer it s equivalent to months or years of development time. A number of commercial and open source Java ORM tools have achieved this level of quality. This level meets the definition of ORM we re using in this book. Let s look at the problems we expect to be solved by a tool that achieves full object mapping.
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1.4.2 Generic ORM problems
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The following list of issues, which we ll call the O/R mapping problems, are the fundamental problems solved by a full object/relational mapping tool in a Java environment. Particular ORM tools may provide extra functionality (for example, aggressive caching), but this is a reasonably exhaustive list of the conceptual issues that are specific to object/relational mapping:
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What do persistent classes look like Are they fine-grained JavaBeans Or are they instances of some (coarser granularity) component model like EJB How transparent is the persistence tool Do we have to adopt a programming model and conventions for classes of the business domain How is mapping metadata defined Since the object/relational transformation is governed entirely by metadata, the format and definition of this metadata is a centrally important issue. Should an ORM tool provide a GUI to manipulate the metadata graphically Or are there better approaches to metadata definition How should we map class inheritance hierarchies There are several standard strategies. What about polymorphic associations, abstract classes, and interfaces How do object identity and equality relate to database (primary key) identity How do we map instances of particular classes to particular table rows
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Understanding object/relational persistence
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How does the persistence logic interact at runtime with the objects of the business domain This is a problem of generic programming, and there are a number of solutions including source generation, runtime reflection, runtime bytecode generation, and buildtime bytecode enhancement. The solution to this problem might affect your build process (but, preferably, shouldn t otherwise affect you as a user). What is the lifecyle of a persistent object Does the lifecycle of some objects depend upon the lifecycle of other associated objects How do we translate the lifecyle of an object to the lifecycle of a database row What facilities are provided for sorting, searching, and aggregating The application could do some of these things in memory. But efficient use of relational technology requires that this work sometimes be performed by the database. How do we efficiently retrieve data with associations Efficient access to relational data is usually accomplished via table joins. Object-oriented applications usually access data by navigating an object graph. Two data access patterns should be avoided when possible: the n+1 selects problem, and its complement, the Cartesian product problem (fetching too much data in a single select).
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In addition, two issues are common to any data-access technology. They also impose fundamental constraints on the design and architecture of an ORM:
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Transactions and concurrency Cache management (and concurrency)
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As you can see, a full object-mapping tool needs to address quite a long list of issues. We discuss the way Hibernate manages these problems and data-access issues in chapters 3, 4, and 5, and we broaden the subject later in the book. By now, you should be starting to see the value of ORM. In the next section, we look at some of the other benefits you gain when you use an ORM solution.
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