embedded class question

[jim li]

hi guys

could you please give me some hints on what is embedded class ?

i do not quite understand the use of the embedded in the hibernate annotation!

thank you

[Cameron Wallace McKenzie]

Well, when we're doing Java class to database table mappings, we always talk about one table to one database table. That's the ideal. But sometimes, you have two classes that must map to ONE database table. But we only mark one class as the entity to be associated with the table. The second class, we say is embedded in the first class, and as such, the embedded class will map to the same database table as the embedded class.

On my website, I use an example where a Thing and a ThingDetail class, two separate classes in a Java domain model must map to a single database table, with the database table being named thing.

Mapping Multiple Java Classes to a Single Database Table with Hibernate and JPA Annotations



So, in this situation, I made the Thing the entity class that maps to the thing table.

 @Entity
public class Thing {
  private long id;  
  private String name;
 
  private ThingDetail thingDetail;
 
 @Embedded
  public ThingDetail getThingDetail(){
    return thingDetail;
  }
  public void setThingDetail(ThingDetail detail) {
    this.thingDetail = detail;
  }
 
  @Id
  @GeneratedValue
  public long getId() {return id;}
  public void setId(long id) {this.id = id;}
 
  public String getName() {return name;}
  public void setName(String name) {this.name = name;}
}
   

But we also want the ThingDetail class to map to the same table, so we mark the ThingDetail class as being embeddable, namely, embeddable in the Thing class:

  @Embeddable
public class ThingDetail {
 
private String alias;
private int count;
 
public String getAlias() {return alias;}
public void setAlias(String alias) {
this.alias = alias;
}
 
public int getCount() {return count;}
public void setCount(int count) {
this.count = count;
}
}
  

This is a simple example. In more complicated scenarios, compound primary keys are defined in a separate class, and those compound primary key classes become embeddable in the class for which they will act as a primary key. That's where you often see the embeddable annotation being used - in compound primary key classes.

Here's a little tutorial on compound primary keys, if you find yourself needing to use the Embeddable annotation in this type of scenario:

Tutorial on Using Hibernate and JPA Annotations to define Compound Primary Key Classes

-Cameron McKenzie
[ June 08, 2008: Message edited by: Cameron Wallace McKenzie ]