延迟加载（一对一使用 FK）问题

Question

当执行session.createQuery("from Author").list()时；即使关系标记为 optional=false,fetch type = lazy from Author->book 也会根据下面生成的 sql 急切地加载关联的书籍实例信息。我将这种关系视为一对一只是为了理解目的，目前不是一对多。是否可以从作者书中让它变得懒惰，如果不确定我错过了什么？

Hibernate：select author0_.AUTHOR_ID 作为 AUTHOR_I1_0_，author0_.email 作为 email2_0_，author0_.name 作为 name3_0_ 来自作者 author0_

休眠：select book0_.BOOK_ID 作为 BOOK_ID1_1_0_，book0_.AUTHOR_ID 作为 AUTHOR_I5_1_0_，book0_.description 作为 descript2_1_0_，book0_.PUBLISHED as PUBLISHE3_1_0_, book0_.title as title4_1_0_ from BOOK book0_ where book0_.AUTHOR_ID=?

@Entity
@BatchSize(size=3)
@Table(name = "AUTHOR")
public class Author {
    private long id;
    private String name;
    private String email;
    private Book book;


    public Author() {
    }

    public Author(String name, String email) {
        this.name = name;
        this.email = email;
    }

    @Id
    @Column(name = "AUTHOR_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;
    }

    public String getEmail() {
        return email;
    }

    public void setEmail(String email) {
        this.email = email;
    }

    @OneToOne(cascade=CascadeType.ALL,fetch=FetchType.LAZY,optional=false,mappedBy="author")
    public Book getBook() {
        return book;
    }

    public void setBook(Book book) {
        this.book = book;
    }


@Entity
@BatchSize(size=3)
@Table(name = "BOOK")
public class Book {

     private long id;
        private String title;
        private String description;
        private Date publishedDate;

        private Author author;

        public Book() {
        }

        @Id
        @Column(name = "BOOK_ID")
        @GeneratedValue
        public long getId() {
            return id;
        }

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

        public String getTitle() {
            return title;
        }

        public void setTitle(String title) {
            this.title = title;
        }

        public String getDescription() {
            return description;
        }

        public void setDescription(String description) {
            this.description = description;
        }

        @Temporal(TemporalType.DATE)
        @Column(name = "PUBLISHED")
        public Date getPublishedDate() {
            return publishedDate;
        }

        public void setPublishedDate(Date publishedDate) {
            this.publishedDate = publishedDate;
        }

        @OneToOne(cascade=CascadeType.ALL,fetch=FetchType.LAZY,optional=false)
        @JoinColumn(name ="AUTHOR_ID",unique=true)
        public Author getAuthor() {
            return author;
        }

        public void setAuthor(Author author) {
            this.author = author;
        }
    }

Answer 1

你错过了从 Author 到 Book 的 link 不是双向关联的定义所有者，我的意思是它被定义为 mappedBy 并且 FK 位于Book table。因此，当检索到作者时，它需要查询 Book table 以检查 FK 并找出是否有书以及每个作者关联的书。这就是为什么您在书 table 上看到的 select 语句与您拥有的作者一样多（这被称为 N+1 查询问题）。

避免这种情况的一种方法是切换关联的所有者方（在作者 table 中定义书籍的外键），但您会看到类似的查询书籍的行为。

如果您关心附属查询，一种选择是在查询中应用提取策略，强制检索与作者关联的书籍。查询将类似于， session.createQuery("from Author a left join fetch a.book").list();

我告诉了你原因和基本的 solution/workaround 但还有更复杂的选项，你肯定可以在 Whosebug 中找到它们。

延迟加载（一对一使用 FK）问题

Lazy loading (one to one using FK) issue

hibernate

jpa

persistence