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This chapter examines two important concepts: persisting collections and arrays, and creating custom value types. As before, we ll present this information in the context of our event management application. Once this chapter is complete, we will have accomplished the following:
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Created mapping definitions for collections and custom value types Examined the different types of collection associations Converted the Address instance introduced in chapter 4 from a component to a custom value type
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Since we re building on the lessons in the previous chapter, you should be able to
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Create a basic mapping document with properties, many-to-one associations, and components Use Ant to create and update the application database
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Persisting collections and arrays
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5.1 Persisting collections and arrays
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Persisting collections with Hibernate is straightforward, but some details can cause you problems if you re not aware of them. We start out this section discussing how Hibernate manages persistent collections, including the mapping definitions for one-to-many and many-tomany associations. After that, we ll give an example for each of the collection types and address some of the infrequently used components, such as idbags. When a collection is persisted by Hibernate, it retains all of the semantics of the Java collection interface. For example, when a java.util.List is persisted, the index order of each element will also be persisted. The index order is persisted so that the list can be re-created when retrieved from the database. behavior of collections doesn t stop at the interface. For instance, a persistent Set cannot contain duplicate elements, and is naturally unordered. In the case of a java.util.Map, the keys used must be unique.
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java.util.List
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Persisting
Because Hibernate enforces the semantics of the collection class, how can you just store a collection of objects without worrying about the semantics of the underlying collection Hibernate supports another type of collection called a Bag. Bags are basically unordered and unindexed Lists that can contain duplicate elements. The notion of a Bag is Hibernate specific; there isn t a Java class or interface representing the Bag. In fact, there isn t even a specific class for a Bag collection. Persistent objects wishing to have a Bag collection can simply use a java.util.List. Hibernate handles the persistence details for you. We ll explain Bag usage later in the chapter. To avoid confusing the Java collection classes with their Hibernate counterparts, table 5.1 summarizes the persistent collection types with their Java collection class.
Collections and custom types
Table 5.1 Hibernate persistent collections compared with Java collections Hibernate collection type set map list bag array primitive-array idbag Java collection type java.util.Set java.util.Map java.util.List java.util.List N/A N/A java.util.List Description Persists an unordered, unique collection of values or objects. Persists a collection of key/value pairs. Persists an ordered, non-unique collection of values or objects. Persists an unordered, non-unique collection of values or objects. Persists an indexed, non-unique collection of values or objects. Persists an indexed, non-unique collection of primitive values. Persists an unordered, non-unique, many-to-many collection using a surrogate key.
If you use a collection class that adds additional behavior to the implemented interface, like a LinkedList (LinkedList implements List), keep in mind that the additional behavior is not persisted. Behind the scenes, Hibernate uses its own implementation of the core Collections interfaces, primarily to support lazy collections. (We ll discuss lazy collections in section 5.1.4.) The custom interface implementations allow Hibernate to intercept calls to the persistent collection and populate it when needed.
5.1.1 Using interfaces
Hibernate s custom collection implementations have another impact on your persistent classes. When you re creating the accessor methods for the collection classes, it s important to declare the collection interface, from the java.util package, instead of having a class implement the
Persisting collections and arrays
interface. To illustrate, suppose you have a class with the following accessors:
public void setGroups(ArrayList groups) { } public ArrayList getGroups() { }
Since your accessor uses a java.util.ArrayList, you ll have problems at runtime when Hibernate tries to populate the collection. Instead, you should use a java.util.List. This is because Hibernate provides its own implementation of the Collections interfaces, partially illustrated in figure 5.1. By examining figure 5.1, you can see that Hibernate simply implements the Collections interfaces in the java.util package. When Hibernate populates a collection, such as a java.util.List, the implementing class is actually org.hibernate.collection.List. We ll get started by looking at mapping definitions for a collection. For demonstration purposes, we ll use collections of type java.util.Set. Despite the different collection types supported by Hibernate, managing persistent collections is similar regardless of the underlying collection type. We ll point out some subtle configuration and usage differences as we encounter them.
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