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Why Hibernate
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Listing 1.3 Setting a newly created Location for the Event
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private Location location = null; private void doMain(String[] args) { // Create location location = createLocation(); // Create Event instance event = createEvent(location); // Create two Speaker instances speakers = createSpeakers(); // and add them to the Event event.setSpeakers(speakers); } private Location createLocation() { Location location = new Location(); location.setName("Grand Hyatt Convention Room A"); return location; } private Event createEvent(Location location) { Event event = new Event(); event.setName("Opening Presentation"); // Assign location to event event.setLocation(location); // Establish bi-directional association location.setEvent(event); // ... set date and duration return event; }
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A persistence layer implemented with direct JDBC must be able to persist all or part of a complex object graph while avoiding foreign key violations. Similar behavior is required when deleting objects and object graphs from a relational database.
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1.2.4 Deleting object graphs
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Suppose that an Event has been cancelled and the associated data should be removed from the database. The direct JDBC persistence
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Using direct JDBC
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layer must handle deleting not only the Event, but also the objects associated to the Event, such as the Speakers and Attendees. The delete operation should not cascade to the Location, however, since multiple Events can be held at a given Location. Assuming that a Location needs to be deleted, each of the corresponding Events held at the Location should also be deleted, being careful to cascade to the child objects of the Event. Deleting, or making a persistent object transient, is the easiest persistence operation when you re using direct JDBC for application persistence. Still, you must be careful to delete only the relevant objects from the graph.
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1.2.5 Querying object graphs
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Although JDBC s Statement and PreparedStatement classes provide the ability to query a database, you must still write the SQL statement and process the results. Most direct JDBC persistence layers have some form of SQL generation for the objects in the domain model, but it s often incomplete and lacks the flexibility needed for an enterprise application. The SQL generation component must also be updated as the application matures and requirements change. Executing a query with the Statement or PreparedStatement class returns a ResultSet object containing the query results. The ResultSet is essentially a list of key-value pairs. Each entry in the list represents one result row: The key is the column name, and the value is the data returned. Now that you have the results, how should you return the data to the application There are two options: You can convert the results either into a list of Event objects or into a list of Map instances. The latter solution may seem more appealing because it s easier to code (you can add the properties as Map.Entry objects), but this method also introduces some problems. A plain old Java object (POJO), such as the Event object, support primitive type properties, such as int, char, and boolean, as well as
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Why Hibernate
objects, such as String and Date. The Java Collections classes only support objects. To return a list of Map instances, you would need to convert each primitive type into its object representation. By using Map instances, you also forfeit any behavior that the Event class provides. Recall that a domain model contains both the data and the behavior for the objects. Clearly, the benefit of the domain model is lost if the Event instances aren t used. To return a list of Event instances from a ResultSet, you must convert each result row into a corresponding Event instance. Data returned for associated objects, like Location, must also be converted and set for the Event. While this may not seem like a huge chore, writing code to convert every object to and from a relational schema quickly becomes tedious and error-prone. There are clearly a number of open issues when persisting objects with direct JDBC. You can certainly try to answer all of them, but this results in a large number of classes, repetitive and complicated code, as well as a huge time commitment in maintenance and testing. Next, we ll look at how to use Hibernate to overcome some of the problems that JDBC presents.
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