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Domain models and metadata
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has better defaults. However, JDK 5.0 annotations are sometimes more difficult to read than XDoclet annotations, because they aren t inside regular comment blocks; you should use an IDE that supports configurable syntax highlighting of annotations. Other than that, we found no serious disadvantage in working with annotations in our daily work in the past years, and we consider annotation-metadata support to be one of the most important features of JDK 5.0. We ll now introduce mapping annotations and use JDK 5.0. If you have to work with JDK 1.4 but like to use annotation-based metadata, consider XDoclet, which we ll show afterwards. Defining and using annotations Before you annotate the first persistent class, let s see how annotations are created. Naturally, you ll usually use predefined annotations. However, knowing how to extend the existing metadata format or how to write your own annotations is a useful skill. The following code example shows the definition of an Entity annotation:
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package javax.persistence; @Target(TYPE) @Retention(RUNTIME) public @interface Entity { String name() default ""; }
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The first line defines the package, as always. This annotation is in the package javax.persistence, the Java Persistence API as defined by EJB 3.0. It s one of the most important annotations of the specification you can apply it on a POJO to make it a persistent entity class. The next line is an annotation that adds metainformation to the @Entity annotation (metadata about metadata). It specifies that the @Entity annotation can only be put on type declarations; in other words, you can only mark up classes with the @Entity annotation, not fields or methods. The retention policy chosen for this annotation is RUNTIME; other options (for other use cases) include removal of the annotation metadata during compilation, or only inclusion in byte-code without possible runtime reflectivity. You want to preserve all entity meta-information even at runtime, so Hibernate can read it on startup through Java Reflection. What follows in the example is the actual declaration of the annotation, including its interface name and its attributes (just one in this case, name, with an empty string default). Let s use this annotation to make a POJO persistent class a Java Persistence entity:
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Object/relational mapping metadata
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package auction.model; import javax.persistence.*; @Entity @Table(name = "ITEM") public class Item { ... }
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This public class, Item, has been declared as a persistent entity. All of its properties are now automatically persistent with a default strategy. Also shown is a second annotation that declares the name of the table in the database schema this persistent class is mapped to. If you omit this information, the JPA provider defaults to the unqualified class name (just as Hibernate will if you omit the table name in an XML mapping file). All of this is type-safe, and declared annotations are read with Java Reflection when Hibernate starts up. You don t need to write any XML mapping files, Hibernate doesn t need to parse any XML, and startup is faster. Your IDE can also easily validate and highlight annotations they are regular Java types, after all. One of the clear benefits of annotations is their flexibility for agile development. If you refactor your code, you rename, delete, or move classes and properties all the time. Most development tools and editors can t refactor XML element and attribute values, but annotations are part of the Java language and are included in all refactoring operations. Which annotations should you apply You have the choice among several standardized and vendor-specific packages. Considering standards Annotation-based metadata has a significant impact on how you write Java applications. Other programming environments, like C# and .NET, had this kind of support for quite a while, and developers adopted the metadata attributes quickly. In the Java world, the big rollout of annotations is happening with Java EE 5.0. All specifications that are considered part of Java EE, like EJB, JMS, JMX, and even the servlet specification, will be updated and use JDK 5.0 annotations for metadata needs. For example, web services in J2EE 1.4 usually require significant metadata in XML files, so we expect to see real productivity improvements with annotations. Or, you can let the web container inject an EJB handle into your servlet, by adding an annotation on a field. Sun initiated a specification effort (JSR 250) to take care of the annotations across specifications, defining common annotations for the
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