休眠中的复合键

Question

Java Persistence with Hibernate shows how to create a composite key:

@Entity
@Table(name = "CATEGORIZED_ITEM")
  public class CategorizedItem {

  @Embeddable
  public static class Id implements Serializable {

      @Column(name = "CATEGORY_ID")
      private Long categoryId;

      @Column(name = "ITEM_ID")
      private Long itemId;

      public Id() {}
      public Id(Long categoryId, Long itemId) {
          this.categoryId = categoryId;
          this.itemId = itemId;
      }

      public boolean equals(Object o) {
          if (o != null && o instanceof Id) {
              Id that = (Id)o;
              return this.categoryId.equals(that.categoryId) &&
                  this.itemId.equals(that.itemId);
          } else {
              return false;
          }
      }
      public int hashCode() {
          return categoryId.hashCode() + itemId.hashCode();
      }
  }

@EmbeddedId
private Id id = new Id();

@Column(name = "ADDED_BY_USER")
private String username;

@Column(name = "ADDED_ON")
private Date dateAdded = new Date();

Is the approach of making the Id static common when making a composite key? If so, why?

Why is it necessary to instantiate Id in CategorizedItem ?

private Id id = new Id();

Answer 1

There are actually two ways you can specify composite keys, one being @EmbeddedId and the other being an ID class: @IdClass . This is a pretty good tutorial showing you the options and providing suggestions on how to work on both styles of composite key specifier: http://www.objectdb.com/java/jpa/entity/id

I haven't personally seen the primary key class be embedded inside the class using that key, but if the key class is not required to be used anywhere else, than it makes sense. A public static nested class is basically the same as a root level class, it just shows the intent that the class has a tight association with its enclosing class.

As for creating an instance of the class, I think most examples do it via the constructor. You must obviously provide values for the primary key components before trying to persist the entity to the database. Here's a typical example of that: http://www.java2s.com/Tutorial/Java/0355__JPA/EmbeddedCompoundPrimaryKey.htm