java - entityManager.find 和 entityManager.createQuery 有什么区别？

Question

好的，我正在使用 EJB 3.0 和 hibernate，我们将 .ear 文件放入嵌入 Apache Tomcat 6.0.18 的 Easy-Beans 1.0.1(带有 Hibernate)部署目录。所以我的数据库必须保留这样的东西:

@Entity
@Table(name="AUTHOR")
public class Author implements Serializable {

//////////////////////////////////////////////////////////////////
// Fields
//////////////////////////////////////////////////////////////////

@Id
@GeneratedValue(strategy = IDENTITY)
@Column(name = "A_ID", unique=true, nullable = false)
private Integer id;

@Column (name = "A_NAME", unique = false, nullable = false)
private String name;

@Column (name = "A_LASTNAME", unique = false, nullable = false)
private String lastname;

@OneToMany(cascade = {ALL}, fetch = EAGER, mappedBy = "author")
private Set<BookAuthor> bookAuthors = new HashSet<BookAuthor>();

  @Override
  public boolean equals(Object o) {
      if (this == o) return true;
      if (o == null || getClass() != o.getClass()) return false;

      Author author = (Author) o;

      if (id != null ? !id.equals(author.id) : author.id != null) return false;

      return true;
  }

  @Override
  public int hashCode() {
      return id != null ? id.hashCode() : 0;
  }  
}

@Entity
@Table(name = "BOOK" )
public class Book implements Serializable {
//////////////////////////////////////////////////////////////////
// Fields
//////////////////////////////////////////////////////////////////
@Id
@GeneratedValue (strategy = IDENTITY)
@Column(name = "B_ID", unique = true, nullable = false)
private Integer bid;

@Column(name = "B_YEAR", unique = false, nullable = true)
private Integer year;

@Column(name = "B_ISBN", unique = false, nullable = false)
private String isbn;

@Column(name = "B_TITLE", unique = false, nullable = false)
private String title;

@OneToMany(cascade = {ALL}, fetch = EAGER, mappedBy = "book")
private Set<BookAuthor> bookAuthors = new HashSet<BookAuthor>();

  @Override
  public boolean equals(Object o) {
    if (this == o) return true;
    if (o == null || getClass() != o.getClass()) return false;

    Book book = (Book) o;

    if (isbn != null ? !isbn.equals(book.isbn) : book.isbn != null) return false;
    if (bid != null ? !kid.equals(book.bid) : book.bid != null) return false;

    return true;
  }

  @Override
  public int hashCode() {
    int result = bid != null ? bid.hashCode() : 0;
    result = 31 * result + (isbn != null ? isbn.hashCode() : 0);
    return result;
  }

}

@Entity
@Table(name = "BOOK_AUTHOR")
public class BookAuthor implements Serializable {

  //////////////////////////////////////////////////////////////////
  // Fields
  //////////////////////////////////////////////////////////////////
  @Id
  @GeneratedValue(strategy = IDENTITY)
  @Column(name = "BA_ID", unique=true, nullable = false)
  private Integer id;

  @Column(name = "BA_ROLE", unique = false, nullable = true)
  private String role;

  @ManyToOne
  @JoinColumn (name = "A_ID", referencedColumnName = "A_ID", nullable = false)
  private Author author;

  @ManyToOne
  @JoinColumn (name = "B_ID", referencedColumnName = "B_ID", nullable = false)
  private Book book;

  @Override
  public boolean equals(Object o) {
    if (this == o) return true;
    if (o == null || getClass() != o.getClass()) return false;

    BookAuthor that = (BookAuthor) o;

    if (auhtor != null ? !author.equals(that.author) : that.author != null) return false;
    if (book != null ? !book.equals(that.book) : that.book!= null) return false;
    if (id != null ? !id.equals(that.id) : that.id != null) return false;

    return true;
  }

  @Override
  public int hashCode() {
    int result = id != null ? id.hashCode() : 0;
    result = 31 * result + (author != null ? author.hashCode() : 0);
    result = 31 * result + (book != null ? book.hashCode() : 0);
    return result;
  }

}

所以当删除项目时，我有一个实体 bean，它是这样的:

@Stateless
@Local(DeleteBookAuthor.class)
public class DeleteBookAuthorBean implements DeleteBookAuthor
{

   @PersistenceContext(unitName="Library")
   protected EntityManager em;

   @Override
   public void removeById(Integer id) {
      try{

        Query q = em.createQuery("SELECT ba FROM BookAuthor ba WHERE id = ?1");
        q.setParameter(1,id);

        BookAuthor ba = (BookAuthor) q.getSingleResult();

        ba.getAuthor().getBookAuthors().remove(ba);
        ba.getBook().getBookAuthors().remove(ba);

        em.remove(ba);
      }catch (Exception e){
         e.printStackTrace();
      }
   }
}

不幸的是，当我的 Servlet 调用这个 bean 时，它返回一个“已删除的实体传递给持久化”异常；但是改变行:

Query q = em.createQuery("SELECT ba FROM BookAuthor ba WHERE id = ?1");
q.setParameter(1,id);        
BookAuthor ba = (BookAuthor) q.getSingleResult(); 

到

BookAuthor ba = em.find(BookAuthor.class, id)

让问题消失。我问的问题是为什么？在类似的情况下，我使用 em.createQuery 来检索和删除多个实体并且它工作顺利。那为什么它现在不起作用了？

更新:调用查询 q=...，然后删除 BookAuthors 会从 Books 中删除 BookAuthors，但不会从 Authors 中删除。在第二种情况下，它被从两个集合中移除。当使用 baQuery.equals(baFind) 进行比较时，两个 ba 都具有相同的哈希并返回 true。

为什么两个函数会返回相同的对象，但调用 remove 会根据是否调用 query/find 表现不同，是否有任何说明？

Answer 1

可能与BookAuthor中缺少equals()/hashCode()有关。如果是这样，在查询的情况下，您似乎有多个具有相同状态的 BookAuthor 实例，因此它们不会从 Author 和 书。