Spring Jpa数据存储库使用LinkedEntity for ManyToMany关系进行保存（更新）

Question

There are 2 entities (lets say Rule and Label) with many-to-many relationship using linked entity as per hibernate reference documentation
Rule enity:

@Entity
@Table(name = "rule")
@JsonIdentityInfo(
    generator = ObjectIdGenerators.PropertyGenerator.class,
    property = "name")
public class Rule implements Serializable {

@Id
@GeneratedValue(strategy = GenerationType.IDENTITY)
private Long id;

@NaturalId
@NotBlank
@Column(unique = true)
private String name;

@Lob
@Column(columnDefinition = "TEXT")
private String content;

@OneToMany(mappedBy = "rule", cascade = {CascadeType.PERSIST, 
    CascadeType.MERGE})
private List<RuleLabel> labels = new ArrayList<>();
...

Label entity:

@Entity
@Table(name = "label")
@JsonIdentityInfo(
    generator = ObjectIdGenerators.PropertyGenerator.class,
    property = "id")
public class Label implements Serializable {

@Id
@GeneratedValue(strategy = GenerationType.IDENTITY)
private Long id;

@NotBlank
private String name;

@OneToMany(mappedBy = "label", cascade = {CascadeType.PERSIST, 
    CascadeType.MERGE})
private List<RuleLabel> rules = new ArrayList<>();
...

Link entity:

@Entity
public class RuleLabel implements Serializable {

@Id
@ManyToOne
private Rule rule;

@Id
@ManyToOne
private Label label;
...

Repositories:

@Repository
public interface LabelRepository extends JpaRepository<Label, Long>
...
@Repository
public interface RuleRepository extends JpaRepository<Rule, Long>
...

Creating new entity via RuleRepository.save(Rule) works fine, but when I'm trying to update existing entity (the same method RuleRepository.save(Rule) , but entity to be saved contains id field) it leads to infinite loop of Hibernate: select... queries:

Hibernate: select rule0_.id as id1_7_1_, rule0_.is_active as is_activ2_7_1_, rule0_.content as content3_7_1_, rule0_.is_deleted as is_delet4_7_1_, rule0_.import_section as import_s5_7_1_, rule0_.name as name6_7_1_, rule0_.rule_configuration as rule_con7_7_1_, labels1_.rule_id as rule_id1_8_3_, labels1_.label_id as label_id2_8_3_, labels1_.rule_id as rule_id1_8_0_, labels1_.label_id as label_id2_8_0_ from rule rule0_ left outer join rule_label labels1_ on rule0_.id=labels1_.rule_id where rule0_.id=?

and StackOverflowError as a result

java.lang.StackOverflowError: null
at com.mysql.jdbc.ServerPreparedStatement.getInstance(ServerPreparedStatement.java:332)
...

(LabelRepository acts in the same manner)
How it can be fixed?
Update: After changing fetch strategy to Lazy

@Id
@ManyToOne(fetch = FetchType.LAZY)
private Rule rule;

@Id
@ManyToOne(fetch = FetchType.LAZY)
private Label label;

infinite loop problem has gone, but new one has appeared - related entities are not being populated and when Hibernate is trying to insert values into link table

Hibernate: insert into rule_label (rule_id, label_id) values (?, ?)

we get

org.springframework.dao.DataIntegrityViolationException: could not execute statement; SQL [n/a]; constraint [null]; nested exception is org.hibernate.exception.ConstraintViolationException: could not execute statement
...
Caused by: com.mysql.jdbc.exceptions.jdbc4.MySQLIntegrityConstraintViolationException: Column 'rule_id' cannot be null

Answer 1

Okay, well I've always used an EmbeddableId for link entities with JPA. I haven't tried the hibernate example you refer to in terms of using cascade to do the work for me. It could be interesting but there are some differences between pure JPA and Spring Data Repositories. By using an EmbeddableId you can create a separate spring repository for the link entity. Then you manage the relationships yourself. If you don't want to do that then you should use a ManyToMany annotation, but the link entity allows you to create link entity attributes, not shown here. This code will work for you and get you to point B and you can experiment from there:

@Entity
public class Label {
    @Id @GeneratedValue private Long id;
    @OneToMany(mappedBy = "ruleLabelId.labelId")
    private List<RuleLabel> rules = new ArrayList<>();

@Entity
public class Rule {
    @Id @GeneratedValue private Long id;
    @OneToMany(mappedBy = "ruleLabelId.ruleId")
    private List<RuleLabel> labels = new ArrayList<>();

@Entity
public class RuleLabel {
    @EmbeddedId
    private RuleLabelId ruleLabelId;

@SuppressWarnings("serial")
@Embeddable
public class RuleLabelId implements Serializable {
    private Long ruleId;
    private Long labelId;

public interface RuleRepository extends JpaRepository<Rule, Long> {
    @Query("from Rule r left join fetch r.labels where r.id = :id")
    public Rule getWithLabels(@Param("id") Long id);
}

public interface RuleLabelRepository extends JpaRepository<RuleLabel, RuleLabelId> {}

and to use it:

Rule rule = new Rule();
Label label = new Label();

ruleRepo.save(rule);
labelRepo.save(label);

RuleLabel ruleLabel = new RuleLabel();
RuleLabelId ruleLabelId = new RuleLabelId();
ruleLabelId.setRuleId(rule.getId());
ruleLabelId.setLabelId(label.getId());
ruleLabel.setRuleLabelId(ruleLabelId);

ruleLabelRepo.save(ruleLabel);

rule = ruleRepo.getWithLabels(1L);
System.out.println(rule + Arrays.toString(rule.getLabels().toArray()));

Answer 2

Yes because its what you are telling hibernate to do.

By default, all @ManyToOne and @OneToOne associations are EAGER loaded, so when it querying Rule then its also querying RuleLabel and then inside there is Rule again which is causing infinite select queries. It's better to have them LAZY loaded.

You can do field lazy load like this @ManyToOne(fetch=FetchType.LAZY)

This is what JPA 2.0 spec say about defaults:

OneToMany: LAZY
ManyToOne: EAGER
ManyToMany: LAZY
OneToOne: EAGER

A good read on Lazy and Eager loading