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When you work with an SQL database in a Java application, the Java code issues SQL statements to the database via the Java Database Connectivity (JDBC) API. Whether the SQL was written by hand and embedded in the Java code, or generated on the fly by Java code, you use the JDBC API to bind arguments to prepare query parameters, execute the query, scroll through the query result table, retrieve values from the result set, and so on. These are low-level data access tasks; as application developers, we re more interested in the business problem that requires this data access. What we d really like to write is code that saves and
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Understanding object/relational persistence
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retrieves objects the instances of our classes to and from the database, relieving us of this low-level drudgery. Because the data access tasks are often so tedious, we have to ask: Are the relational data model and (especially) SQL the right choices for persistence in objectoriented applications We answer this question immediately: Yes! There are many reasons why SQL databases dominate the computing industry relational database management systems are the only proven data management technology, and they re almost always a requirement in any Java project. However, for the last 15 years, developers have spoken of a paradigm mismatch. This mismatch explains why so much effort is expended on persistence-related concerns in every enterprise project. The paradigms referred to are object modeling and relational modeling, or perhaps object-oriented programming and SQL. Let s begin our exploration of the mismatch problem by asking what persistence means in the context of object-oriented application development. First we ll widen the simplistic definition of persistence stated at the beginning of this section to a broader, more mature understanding of what is involved in maintaining and using persistent data.
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Persistence in object-oriented applications
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In an object-oriented application, persistence allows an object to outlive the process that created it. The state of the object can be stored to disk, and an object with the same state can be re-created at some point in the future. This isn t limited to single objects entire networks of interconnected objects can be made persistent and later re-created in a new process. Most objects aren t persistent; a transient object has a limited lifetime that is bounded by the life of the process that instantiated it. Almost all Java applications contain a mix of persistent and transient objects; hence, we need a subsystem that manages our persistent data. Modern relational databases provide a structured representation of persistent data, enabling the manipulating, sorting, searching, and aggregating of data. Database management systems are responsible for managing concurrency and data integrity; they re responsible for sharing data between multiple users and multiple applications. They guarantee the integrity of the data through integrity rules that have been implemented with constraints. A database management system provides data-level security. When we discuss persistence in this book, we re thinking of all these things:
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What is persistence
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Storage, organization, and retrieval of structured data Concurrency and data integrity Data sharing
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And, in particular, we re thinking of these problems in the context of an objectoriented application that uses a domain model. An application with a domain model doesn t work directly with the tabular representation of the business entities; the application has its own object-oriented model of the business entities. If the database of an online auction system has ITEM and BID tables, for example, the Java application defines Item and Bid classes. Then, instead of directly working with the rows and columns of an SQL result set, the business logic interacts with this object-oriented domain model and its runtime realization as a network of interconnected objects. Each instance of a Bid has a reference to an auction Item, and each Item may have a collection of references to Bid instances. The business logic isn t executed in the database (as an SQL stored procedure); it s implemented in Java in the application tier. This allows business logic to make use of sophisticated object-oriented concepts such as inheritance and polymorphism. For example, we could use well-known design patterns such as Strategy, Mediator, and Composite (Gamma and others, 1995), all of which depend on polymorphic method calls. Now a caveat: Not all Java applications are designed this way, nor should they be. Simple applications may be much better off without a domain model. Complex applications may have to reuse existing stored procedures. SQL and the JDBC API are perfectly serviceable for dealing with pure tabular data, and the JDBC RowSet makes CRUD operations even easier. Working with a tabular representation of persistent data is straightforward and well understood. However, in the case of applications with nontrivial business logic, the domain model approach helps to improve code reuse and maintainability significantly. In practice, both strategies are common and needed. Many applications need to execute procedures that modify large sets of data, close to the data. At the same time, other application modules could benefit from an object-oriented domain model that executes regular online transaction processing logic in the application tier. An efficient way to bring persistent data closer to the application code is required. If we consider SQL and relational databases again, we finally observe the mismatch between the two paradigms. SQL operations such as projection and join always result in a tabular representation of the resulting data. (This is known as
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