Hibernate无限循环在多对多关系中的递归
[英]Hibernate infinite loop recursion in many to many relation
问题描述
I want to create bidirectional student-discipline relation. 
我想创建双向的学生-学科关系。 Anything works fine until I've enrolled user to discipline. 在我已将用户注册为纪律人员之前，一切正常。 Now I'm getting infinite recursion. 现在，我得到了无限递归。
Classes looks like this: 类看起来像这样：
//Student.java
@Entity
@Table(name = "students")
public class Student {
@NotNull
@Id
@Column(name = "STUDENT_ID")
private String id;
private String name;
@ManyToMany(cascade = CascadeType.ALL, fetch = FetchType.EAGER)
@JoinTable(name = "students_disciplines", joinColumns = @JoinColumn(name = "STUDENT_ID"), inverseJoinColumns = @JoinColumn(name = "DISCIPLINE_ID"))
@JsonSerialize(using = NestedDisciplineSetSerializer.class)
private Set<Discipline> disciplines = new HashSet<>();
//Getters, Setters
}
//NestedDisciplineSetSerializer.java
public class NestedDisciplineSetSerializer extends JsonSerializer<Set<Discipline>> {
@Override
public void serialize(Set<Discipline> value, JsonGenerator jgen, SerializerProvider p) throws IOException, JsonProcessingException {
jgen.writeStartArray();
for (Discipline s : value) {
jgen.writeStartObject();
jgen.writeStringField("name", s.getName());
jgen.writeNumberField("id", s.getId());
jgen.writeBooleanField("recommended", s.isRecommended());
jgen.writeEndObject();
}
jgen.writeEndArray();
}
}
//Discipline.java
@Entity`enter code here`
@Table(name = "disciplines")
public class Discipline {
@GeneratedValue(strategy = GenerationType.AUTO)
@Id
@Column(name = "DISCIPLINE_ID")
private int id;
@NotNull
private String name;
@NotNull
private int credits;
private String annotation;
private boolean recommended;
@ManyToMany(mappedBy = "disciplines", fetch = FetchType.EAGER)
@JsonSerialize(using = NestedStudentSetSerializer.class)
private Set<Student> students = new HashSet<>();
//Getters, Setters
}
//NestedStudentSetSerializer.java
public class NestedStudentSetSerializer extends JsonSerializer<Set<Student>> {
@Override
public void serialize(Set<Student> value, JsonGenerator jgen, SerializerProvider p) throws IOException, JsonProcessingException {
jgen.writeStartArray();
for (Student s : value) {
jgen.writeStartObject();
jgen.writeStringField("name", s.getName());
jgen.writeStringField("id", s.getId());
jgen.writeEndObject();
}
jgen.writeEndArray();
}
}
Error appears to be while loading Set after Student s = sDao.findOne(id); 在Student s = sDao.findOne(id);之后加载Set时似乎出现错误Student s = sDao.findOne(id);
Tried to search here, but was not able to find right solution. 试图在这里搜索，但找不到正确的解决方案。 Anything made the same result. 一切都取得了相同的结果。
2 个解决方案
解决方案1
3 已采纳 2016-11-19 15:33:23
org.hibernate.collection.internal.PersistentSet.hashCode(PersistentSet.java:430) ~[hibernate-core-5.0.11.Final.jar:5.0.11.Final] at truelecter.practproekt.entity.
I have the impression that the hashcode() method implementations for Discipline and Student entities use both entities. 我的印象是， Discipline和Student实体的hashcode()方法实现同时使用了这两个实体。
Maybe am I wrong, it is not very clear in the stacktrace. 也许我错了，在堆栈跟踪中不是很清楚。
How have you defined equals() and hashcode() methods for your entities ? 您如何为实体定义equals()和hashcode()方法？
Suppose entity A and entity B with a bidirectional relationship. 假设实体A和实体B具有双向关系。
If equals()/hashcode() implementation of entity A invokes equals()/hashcode() on the entity B field that itself invokes equals()/hashcode() on the entity A field, you have a cycle. 如果equals()/hashcode()实现实体的A调用equals()/hashcode()的实体B字段本身调用equals()/hashcode()对实体A领域，你有一个周期。
Whereas your error. 而你的错误。
Besides, even if you cut the cycle by ensuring that only one of both refers the other in equals()/hashcode()/toString() , you should invoke equals()/hashcode()/toString methods of the entity relationships with cautious. 此外，即使通过确保在equals()/hashcode()/toString()中只有一个引用另一个来缩短周期，也应谨慎地调用实体关系的equals()/hashcode()/toString方法。
It may have a real impact on the performance. 它可能会对性能产生实际影响。 If the Hibernate session is opened, it may indeed execute queries that you don't expect to. 如果打开了Hibernate会话，则它实际上可能会执行您不希望执行的查询。
解决方案2
1 2016-11-19 14:52:35
Both of the sides have a Eager fetch type. 双方都有一个渴望获取类型。 Which means that for each Student you load, Hibernate will load every Discipline related, and for each Discipline, it will load its Students. 这意味着，对于您加载的每个学生，Hibernate将加载每个相关的学科，对于每个学科，它将加载其学生。 This is your loop. 这是你的循环。 You could change the fetch type of one side (better both), for FetchType.LAZY . 您可以为FetchType.LAZY更改一侧的获取类型（两者都更好）。 Doing that, the discipline of a student will be load just when you access the list. 这样，仅当您访问列表时，就会加载学生的纪律。
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