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EclipseLink/UserGuide/JPA/Basic JPA Development/Configuration/JPA/Annotations


Annotations

An annotation is a simple, expressive means of decorating Java source code with metadata that is compiled into the corresponding Java class files for interpretation at run time by a JPA persistence provider to manage persistent behavior.

A metadata annotation represents a Java language feature that lets you attach structured and typed metadata to the source code. Annotations alone are sufficient for the metadata specification -- you do not need to use XML. Annotations for object relational mapping are in the javax.persistence package.

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For more information, see Chapter 10 "Metadata Annotations" in the JPA Specification.

An object relational mapping XML file is optional. If you choose to provide one, then it should contain mapping information for the classes listed in it. The persistence provider loads an orm.xml file (or other mapping file) as a resource. If you provide a mapping file, the classes and mapping information specified in the mapping file will be used. The XML mapping metadata may combine with and override annotation metadata.

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For more information, see Section 12.2 "XML Overriding Rules" in the JPA Specification.


Using Metadata Annotations

Use annotations to configure the persistent behavior of your entities. For example, to designate a Java class as a JPA entity, use the @Entity annotation as shown in this example:

 @Entity
 public class Employee implements Serializable {
     ...
 }

You can apply annotations at the following levels:

  • Class
  • Method
  • Field

EclipseLink defines a set of proprietary annotations to take advantage of EclipseLink-specific features. You can find them in the org.eclipselink.annotations package. These annotations are not available through the use of JPA metadata.

Advantages and Disadvantages of Using Annotations

Using annotations provides several advantages:

  • They are relatively simple to use and understand.
  • They provide in-line metadata within with the code that it describes; you do not need to replicate the source code context of where the metadata applies.

The primary disadvantage of annotations is that the metatdata becomes unnecessarily coupled to the code; changes to metadata require changing the source code.



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