java - What does @AttributeOverride mean?

Question

I'm currently coming (back) up to speed with EJB and while I was away it changed drastically (so far for the better). However, I've come across a concept that I am struggling with and would like to understand better as it seems to be used in our (where I work, not me and all the voices in my head) code quite a bit.

Here's the example I've found in a book. It's part of an example showing how to use the @EmbeddedId annotation:

@Entity
public class Employee implements java.io.Serializable
{
    @EmbeddedId
    @AttributeOverrides({
        @AttributeOverride(name="lastName", column=@Column(name="LAST_NAME"),
        @AttributeOverride(name="ssn", column=@Column(name="SSN"))
    })

    private EmbeddedEmployeePK pk;

    ...
}

The EmbeddedEmployeePK class is a fairly straightforward @Embeddable class that defines a pair of @Columns: lastName and ssn.

Oh, and I lifted this example from O'Reilly's Enterprise JavaBeans 3.1 by Rubinger & Burke.

Thanks in advance for any help you can give me.

Answer 1

It's saying that the attributes that make up the embedded id may have predefined (through explicit or implicit mappings) column names. By using the @AttributeOverride you're saying "ignore what other information you have with regard to what column it is stored in, and use the one I specify here".