java - 在任何情况下，父级都不会延迟加载子级

Question

我正在开发一个 REST API，我有一个 Question 父级和 Answer(以及与两者关联的 User)子级。我想要实现的是，使用 questionService.getAllQuestions() 仅从数据库加载问题和用户。当我调用 QuestionService.getQuestion(id) 时，我希望从数据库加载该问题以及该问题包含的所有答案和用户(回答者)。所有数据都会被Spring-boot自动转换为JSON对象

但在我目前的情况下，即使使用 getAllQuestions()，所有数据都是从数据库加载的。这对于问题概述请求毫无意义，但对于问题详细信息请求是必需的。

问题父实体:

import com.fasterxml.jackson.annotation.JsonIgnore;
import com.fasterxml.jackson.annotation.JsonManagedReference;

import javax.persistence.*;
import java.sql.Timestamp;
import java.util.Set;

@Entity
@Table(name = "vragen")
public class Question extends BaseEntity {
    @Column(name = "vraag")
    private String question;

    @Column(name = "bericht")
    private String message;

    @ManyToOne
    @JoinColumn(name = "gebruiker")
    private Gebruiker gebruiker;

    @Column(name = "beantwoord", insertable = false)
    private boolean answered;

    @Column(name = "gepost_op", insertable = false)
    private Timestamp postedOn;

    @Column(name = "actief", insertable = false)
    @JsonIgnore
    private boolean active;

    @OneToMany(mappedBy = "question")
    @JsonManagedReference
    private Set<Answer> answers;


    public Question() {}

    public Question(String question, String message, Gebruiker gebruiker) {
        this.question = question;
        this.message = message;
        this.gebruiker = gebruiker;
    }

    public String getQuestion() {
        return question;
    }

    public void setQuestion(String question) {
        this.question = question;
    }

    public String getMessage() {
        return message;
    }

    public void setMessage(String message) {
        this.message = message;
    }

    public Gebruiker getGebruiker() {
        return gebruiker;
    }

    public void setGebruiker(Gebruiker gebruiker) {
        this.gebruiker = gebruiker;
    }

    public boolean isAnswered() {
        return answered;
    }

    public void setAnswered(boolean answered) {
        this.answered = answered;
    }

    public Timestamp getPostedOn() {
        return postedOn;
    }

    public boolean isActive() {
        return active;
    }

    public void setActive(boolean active) {
        this.active = active;
    }

    public Set<Answer> getAnswers() {
        return answers;
    }

    public void setAnswers(Set<Answer> answers) {
        this.answers = answers;
    }
}

应答实体:

import com.fasterxml.jackson.annotation.JsonBackReference;

import javax.persistence.*;
import javax.validation.constraints.Size;
import java.sql.Timestamp;

@Entity
@Table(name = "antwoorden")
public class Answer extends BaseEntity {
    @Size(min = 10)
    @Column(name = "bericht")
    private String message;

    @ManyToOne
    @JoinColumn(name = "gebruiker")
    private Gebruiker gebruiker;

    @ManyToOne
    @JoinColumn(name = "vraag")
    @JsonBackReference
    private Question question;

    @Column(name = "gepost_op", insertable = false)
    private Timestamp postedOn;

    @Column(name = "actief", insertable = false)
    private boolean active;


    public Answer() {}

    public Answer(String message, Gebruiker gebruiker, Question question) {

    }

    public String getMessage() {
        return message;
    }

    public void setMessage(String message) {
        this.message = message;
    }

    public Gebruiker getGebruiker() {
        return gebruiker;
    }

    public void setGebruiker(Gebruiker gebruiker) {
        this.gebruiker = gebruiker;
    }

    public Question getQuestion() {
        return question;
    }

    public void setQuestion(Question question) {
        this.question = question;
    }

    public Timestamp getPostedOn() {
        return postedOn;
    }

    public boolean isActive() {
        return active;
    }

    public void setActive(boolean active) {
        this.active = active;
    }
}

@OneToMany 默认情况下应该延迟加载答案，即使我明确设置该选项，这种情况也不会发生。

是否可以使用 JPA 实现此目的，还是需要在自定义实现中手动编写查询？

Answer 1

我注意到是 Jackson 造成的，特别是 @JsonManagedReference。删除它并添加 @JsonBackReference 可以使其正确延迟加载，尽管该注释在这里并未真正到位。

参见Configure Jackson to omit lazy-loading attributes in Spring Boot真正的解决方案