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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:

1 2 3 4

Storage, organization, and retrieval of structured data Concurrency and data integrity Data sharing

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.

Execute public void setUp(). Call a test-prefixed method. Execute public void tearDown(). Repeat these steps for each test method.

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.

Any number of test methods can be added to a TestCase, all beginning with the prefix test. The goal is for each test to be small and simple, and tests will usually require instantiating objects. In order to create some objects and preconfigure their state prior to running each individual test method, override the empty TestCase.setUp method, and store state as member variables to your test case class. Use the TestCase.tearDown method to close any open connections or in some way reset state. Our HtmlDocumentTest takes advantage of setUp and tearDown (see later this chapter) so that all test methods will have implicit access to an HtmlDocument.

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.

The setUp and tearDown methods are called before and after every test method is invoked, preventing one test from affecting the behavior of another. Tests should never make assumptions about the order in which they are called.