You might find this information, from my website, to be interesting on the topic:
A Quick Discussion: What is Hibernate and Annotations? and How Does it Work?
JPA annotations greatly simplify persistence programming with Hibernate, but to understand why they're so great, it helps to understand what we needed to do before the introduction of annotations.
Back to the Future: This Historical hibernate-mapping.xml File
Hibernate makes persisting the state of your Java objects incredibly simple. However, in order for Hibernate to know where to story your JavaBeans, or how to map the property of a JavaBean to a database column, the developer has to provide a bit of direction to the Hibernate framework. As such, people developing Hibernate based applications had to maintain an unweildly, monolithic mapping file that described how to save a given Java object to the database.
So, for example, if you had a class named Event that had three properties, one called id, one called birthday, and another property called title, you would have to add the following segment to a hibernate-mapping file:
Code:
<hibernate-mapping>
<class name="events.Event" table="EVENTS">
<id name="id" column="id">
<generator class="native"/>
</id>
<property name="birthday" type="timestamp"/>
<property name="title"/>
</class>
</hibernate-mapping>
What's wrong with an XML mapping file?
There is nothing inherently wrong, with a mapping file, and in fact, thousands of very salacious hibernate applications that are in production use an XML mappings file, but having a big XML mapping file presents a variety of non-lethal, but certainly annoying problems, including the following:
* information about the Java class must be maintained in an external file
* XML isn't always easy to write
* with lots of classes, the XML file can become unweildly and massive
* errors in one part of the XML file can ricochet all over your Java program
Anyways, Java 5 introducted a new Java based artifact - that annotation. Basically, an annotation allows you to add detail an information about a Java class, without damaging, disturbing or changing any of the code that is actually found inside a Java class or a Java method. So, instead of using a monolithic mappings file, Hibernate with JPA annotations allows you to completely rid applications of a mapping file, and instead, you can annotate your Java classes like so:
Code:
@Entity
public class Event {
private Long id;
private String title;
private Date date;
@Id
@GeneratedValue
public Long getId() { return id; }
private void setId(Long id) {this.id = id;}
public Date getDate() {return date;}
public void setDate(Date date) {this.date = date;}
public String getTitle() {return title;}
public void setTitle(String title) {this.title = title;}
}
The @Entity, @Id and @GeneratedValue tags you see in the Event class are the JPA annotations, and they replace the neeed to describe how to persist your Java classes in an external hibernate-mappings.xml file. Instead of using an external file, each Java class maintains its own mapping information, which is much more natural, and much easier to maintain on a class by class basis. Furthermore, it makes introducing new classes, or even removing persistent classes from your domain model, much much easier.
If you're using Hibernate, and you have the ability to choose between using annotations or using a hibernate-mapping file, well, it's really not much of a choice. Always use JPA annotations if you have a choice. JPA annotations are much easier to use, easy to maintain, and will help to make your Hibernate development experience a real pleasure. :)
A Quick Discussion: What is Hibernate and Annotations? and How Does it Work?
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