@OneToMany如果不加@JoinColumn,系統會自動在主從表中增加一箇中間表。
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<>(); }
從表:
@Entity(name = "PostComment") public class PostComment { @Id @GeneratedValue private Long id; private String review; }
如果使用下面代碼添加1條主表記錄以及3條從表記錄:
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);
實際上系統會執行7條SQL語句
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)
這樣如果記錄比較多,將會影響到系統性能。我們可以使用@JoinColumn來避免產生中間表:
@JoinColumn(name = "post_id")
但即使是沒有中間表,系統任然會執行7條SQL語句:
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
如果我們想刪除一條從表記錄
post.getComments().remove(0);
系統任然會執行2條語句:
update post_comment set post_id = null where post_id = 1 and id = 2 delete from post_comment where id=2
要想避免這種情況,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; } }
這樣系統就只會產生4條SQL語句:
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)
刪除一條從表記錄
PostComment comment1 = post.getComments().get( 0 ); post.removeComment(comment1);
系統也只會執行1條SQL語句:
delete from post_comment where id = 2
但是使用這樣同時使用@OneToMany和@ManyToOne要注意以下幾點:
1. 在從表@ManyToOne中要使用FetchType.LAZY,否則會導致性能降低。
2. 主表中增加了2個方法,addComment和removeComment。
3. 從表重載了equals和hashCode方法。
4. 在使用Json來序列化對象時,會產生無限遞歸(Infinite recursion)的錯誤。這裏有2個解決方法:
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標註的屬性在序列化(serialization)時,會被忽略。@JsonManagedReference標註的屬性則會被序列化。在序列化時,@JsonBackReference的作用相當於@JsonIgnore,此時可以沒有@JsonManagedReference。但在反序列化(deserialization)時,如果沒有@JsonManagedReference,則不會自動注入@JsonBackReference標註的屬性;如果有@JsonManagedReference,則會自動注入@JsonBackReference標註的屬性。
@JsonIgnore:直接忽略某個屬性,以斷開無限遞歸,序列化或反序列化均忽略。當然如果標註在get、set方法中,則可以分開控制,序列化對應的是get方法,反序列化對應的是set方法。