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Annotations vs. XML in O/R mapping
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The difficulty of choosing between annotations and XML deployment descriptors manifests itself most strikingly in the arena of EJB 3 O/R mapping. XML descriptors are verbose and hard to manage, and most developers find them to be a painpoint for Java EE. While O/R mapping with annotations makes life simpler, you should keep in mind that you are hard-coding your database schema in your code in a way similar to using JDBC. This means that the slightest schema change will result in a recompilation and redeployment cycle as opposed to simple configuration. If you have a stable database design that rarely changes or you are comfortable using JDBC data access objects (DAOs), then there is no issue here. But if you have an environment where the database schema is less stable (subject to change more often), you re probably better off using descriptors. Luckily, you can use XML descriptors to override ORM annotations after deploying to a production environment. As a result, changing your mind in response to the reality on the ground may not be a big deal.
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Mapping entities
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8.2.1 Specifying the table
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@Table specifies the table containing the columns to which the entity is mapped. In listing 8.4, the @Table annotation makes the USERS table s columns available for ORM. In fact, by default all the persistent data for the entity is mapped to the table specified by the annotation s name parameter. As you can see from the annotation s definition here, it contains a few other parameters:
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@Target(TYPE) @Retention(RUNTIME) public @interface Table { String name() default ""; String catalog() default ""; String schema() default ""; UniqueConstraint[] uniqueConstraints() default {}; }
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The @Table annotation itself is optional. If it s omitted, the entity is assumed to be mapped to a table in the default schema with the same name as the entity class. If the name parameter is omitted, the table name is assumed to be the same as the name of the entity. This will be just fine if we are mapping to the USER table.
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Most persistence providers include a great developer-friendly feature known as automatic schema generation. The persistence provider will automatically create database objects for your entities when they do not exist in the database. This behavior is not mandated by specification and is configured using vendor-specific properties. Most of our code examples rely on automatic schema generation. In chapter 11, we ll provide an example configuration to enable automatic schema generation.
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We won t discuss the catalog and schema parameters in depth since they are hardly ever used. In effect, they allow you to fully qualify the mapped table. For example, we could have explicitly specified that the USERS table belongs in the ACTIONBAZAAR schema like so:
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@Table(name="USERS", schema="ACTIONBAZAAR") public class User
NOTE
We ve already discussed what a schema is. For all intents and purposes, you can think of a catalog as a meta-schema or a higher-level abstraction for organizing schemas. Often, a database will only have one common system catalog.
Object-relational mapping
By default, it is assumed that the table belongs in the schema of the data source used. You ll learn how to specify a data source for a persistence module in chapter 11 when we discuss entity packaging. The uniqueConstraints parameter is not used that often either. It specifies unique constraints on table columns and is only used when table autocreation is enabled. Here s an example:
@Table(name="CATEGORIES", uniqueConstraints= {@UniqueConstraint(columnNames={"CATEGORY_ID"})})
If it does not exist and autogeneration is enabled, the code puts a unique constraint on the CATEGORY_ID column of the CATEGORIES table when it is created during deployment time. The uniqueConstraints parameter supports specifying constraints on more than one column. It is important to keep mind, however, that EJB 3 implementations are not mandated to support generation of tables, and it is a bad idea to use automatic table generation beyond simple development databases. Most entities will typically be mapped to a single table. The User object happens to be mapped to two tables, as you might have guessed from the @SecondaryTable annotation used in listing 8.4. We ll come back to this later after we take a look at mapping entity data using the @Column annotation.
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