@OneToMany如果不加@JoinColumn,系统会自动在主从表中增加一个中间表。
主表：
@Entity(name = "Post")
public class Post {
@Id
@GeneratedValue
private Long id;
private String title;
@OneToMany(
cascade = CascadeType.ALL,
orphanRemoval = true
)
private List<PostComment> comments = new ArrayList<>();
}从表：
@Entity(name = "PostComment")
public class PostComment {
@Id
@GeneratedValue
private Long id;
private String review;
}如果使用下面代码添加1条主表记录以及3条从表记录：
Post post = new Post("First post");
post.getComments().add(
new PostComment("My first review")
);
post.getComments().add(
new PostComment("My second review")
);
post.getComments().add(
new PostComment("My third review")
);
entityManager.persist(post);实际上系统会执行7条SQL语句
insert into post (title, id) values (‘First post‘, 1) insert into post_comment (review, id) values (‘My first review‘, 2) insert into post_comment (review, id) values (‘My second review‘, 3) insert into post_comment (review, id) values (‘My third review‘, 4) insert into post_post_comment (Post_id, comments_id) values (1, 2) insert into post_post_comment (Post_id, comments_id) values (1, 3) insert into post_post_comment (Post_id, comments_id) values (1, 4)
这样如果记录比较多，将会影响到系统性能。我们可以使用@JoinColumn来避免产生中间表：
@JoinColumn(name = "post_id")
但即使是没有中间表，系统任然会执行7条SQL语句：
insert into post (title, id) values (‘First post‘, 1) insert into post_comment (review, id) values (‘My first review‘, 2) insert into post_comment (review, id) values (‘My second review‘, 3) insert into post_comment (review, id) values (‘My third review‘, 4) update post_comment set post_id = 1 where id = 2 update post_comment set post_id = 1 where id = 3 update post_comment set post_id = 1 where id = 4
如果我们想删除一条从表记录
post.getComments().remove(0);
系统任然会执行2条语句：
update post_comment set post_id = null where post_id = 1 and id = 2 delete from post_comment where id=2
要想避免这种情况，就要使用@ManyToOne
@Entity(name = "Post")
@Table(name = "post")
public class Post {
@Id
@GeneratedValue
private Long id;
private String title;
@OneToMany(
mappedBy = "post",
cascade = CascadeType.ALL,
orphanRemoval = true
)
private List<PostComment> comments = new ArrayList<>();
public void addComment(PostComment comment) {
comments.add(comment);
comment.setPost(this);
}
public void removeComment(PostComment comment) {
comments.remove(comment);
comment.setPost(null);
}
}
@Entity(name = "PostComment")
@Table(name = "post_comment")
public class PostComment {
@Id
@GeneratedValue
private Long id;
private String review;
@ManyToOne(fetch = FetchType.LAZY)
@JoinColumn(name = "post_id")
private Post post;
@Override
public boolean equals(Object o) {
if (this == o) return true;
if (!(o instanceof PostComment )) return false;
return id != null && id.equals(((PostComment) o).id);
}
@Override
public int hashCode() {
return 31;
}
}这样系统就只会产生4条SQL语句：
insert into post (title, id) values (‘First post‘, 1) insert into post_comment (post_id, review, id) values (1, ‘My first review‘, 2) insert into post_comment (post_id, review, id) values (1, ‘My second review‘, 3) insert into post_comment (post_id, review, id) values (1, ‘My third review‘, 4)
删除一条从表记录
PostComment comment1 = post.getComments().get( 0 ); post.removeComment(comment1);
系统也只会执行1条SQL语句：
delete from post_comment where id = 2
但是使用这样同时使用@OneToMany和@ManyToOne要注意以下几点：
1. 在从表@ManyToOne中要使用FetchType.LAZY，否则会导致性能降低。
2. 主表中增加了2个方法，addComment和removeComment。
3. 从表重载了equals和hashCode方法。
4. 在使用Json来序列化对象时，会产生无限递归（Infinite recursion）的错误。这里有2个解决方法：
a. 在@ManyToOne下面使用@JsonIgnore.
b. 在@OneToMany下面使用@JsonManagedReference,在@ManyToOne下面使用@JsonBackReference
@JsonBackReference和@JsonManagedReference：@JsonBackReference标注的属性在序列化（serialization)时，会被忽略。@JsonManagedReference标注的属性则会被序列化。在序列化时，@JsonBackReference的作用相当于@JsonIgnore，此时可以没有@JsonManagedReference。但在反序列化（deserialization）时，如果没有@JsonManagedReference，则不会自动注入@JsonBackReference标注的属性；如果有@JsonManagedReference，则会自动注入@JsonBackReference标注的属性。
@JsonIgnore：直接忽略某个属性，以断开无限递归，序列化或反序列化均忽略。当然如果标注在get、set方法中，则可以分开控制，序列化对应的是get方法，反序列化对应的是set方法。