使用Hibernate annotation 注解,在添加实体时发生的错误

•  1: 注解的实体

 @Entity
@Table(name="category")
@DiscriminatorValue("category")
public class Category implements Serializable {

 private static final long serialVersionUID = 1L;
 private int id;
 private String name;
 @Id
    @GeneratedValue(strategy = GenerationType.AUTO)
 public int getId() {
  return id;
 }

 public void setId(int id) {
  this.id = id;
 }

 public String getName() {
  return name;
 }

 public void setName(String name) {
  this.name = name;
 }
}

• 添加时报错:
  2007-11-27 08:45:29,890 ERROR [/saber/music/category.do] - <No row with the given identifier exists: [saber.domain.music.Category#0]>
  javax.servlet.ServletException: No row with the given identifier exists: [saber.domain.music.Category#0]
•  思考:
  将以上的实体主键类型由int 改成Integer,就不报错了.  我不禁要问了  why?