java - Spring MVC JPA @ManyToMany 双向

Question

希望一切顺利。

我正在尝试使用 Spring Boot、Spring Data JPA、Hibernate、Gradle 和 Spring Data Rest 在 Spring MVC 中实现 @ManyToMany 双向映射。当我实现两 (2) 个实体、事件和标签之间的映射时。一个事件可以有零到多个标签，一个标签可以与零到多个事件相关联。事实上，我能够通过映射实体的 setter 将 POJO 成功保存到 hibernate session ，包括 HashSet<>() 。但是，一旦我尝试通过 CrudRepository 方法(例如 .findAll())查询 POJO 列表，它就会立即崩溃。查看控制台输出，Hibernate 似乎试图附加另一个对象一百多次。由于我将 @RestController 用于列表操作 (.findAll())，当我导航 RESTful 端点时，它不仅显示两个 JSON 对象，还显示大量未闭合的附加困惑JSON 片段。控制台输出还表明 jackson databind 可能在序列化 Hibernate 输出的 .findAll() 结果时遇到问题。

如有任何帮助，我们将不胜感激。对我来说最大的困惑是为什么 Hibernate 会多次尝试附加结果字符串。这可能是以下方面的问题:

1. @ManyToMany 映射配置？需要在 Tag 实体上使用 @JoinColumns？
2. fetch = FetchType.EAGER?
3. 缺少/过多的 GETTER/SETTER？从事件或标签实体中删除/添加 GETTER/SETTER？
4. Spring Boot 和/或 Gradle？我可以在互联网上找到的任何其他示例都是使用 Maven。
5. 依赖项？ spring-starter-boot-starter-data-jpa vs spring-data-jpa？
6. 构造函数？ (Event.java 没有标签的参数构造函数)
7. 事件和/或标签的 HashSet<>() 属性类型？

build.gradle

dependencies {
    compile("org.springframework.boot:spring-boot-starter-data-rest")
    compile("org.springframework.boot:spring-boot-starter-web")
    compile("org.springframework.boot:spring-boot-starter-data-jpa")
    compile("org.hibernate:hibernate-validator")
    compile("org.apache.tomcat.embed:tomcat-embed-el")
    compile("com.h2database:h2")
}

Event.java

package event.model;

import org.hibernate.validator.constraints.Email;
import org.springframework.format.annotation.DateTimeFormat;

import javax.persistence.*;
import javax.validation.constraints.Future;
import javax.validation.constraints.NotNull;
import javax.validation.constraints.Size;
import java.util.*;

@Entity
@Table(uniqueConstraints = {@UniqueConstraint(columnNames = {"title", "date"})})
public class Event {

    @Id
    @GeneratedValue(strategy = GenerationType.AUTO)
    private long id;

    @NotNull
    @Size(min = 2, max = 55)
    private String title;

    // .. some additional private variables

    @ManyToMany(fetch = FetchType.EAGER)
    @JoinTable(
            name = "events_tags",
            joinColumns=@JoinColumn(name = "event_id"),
            inverseJoinColumns = @JoinColumn(name="tag_id")
    )
    private Set<Tag> tags = new HashSet<>();

    public Event() {
    }

    public Event(String title, String description, Date date, String organizer, String email, String location) {
        this.title = title;
        this.description = description;
        this.date = date;
        this.organizer = organizer;
        this.email = email;
        this.location = location;
    }

    public long getId() {
        return id;
    }

    // .. GETTERS and SETTERS for other properties

    public Set<Tag> getTags(){
        return tags;
    }

    public void setTags(Set<Tag> tags){
        this.tags = tags;
    }

    // @Overide toString() method
}

Tag.java

package event.model;

import javax.persistence.*;
import javax.validation.constraints.NotNull;
import javax.validation.constraints.Size;
import java.util.*;

@Entity
public class Tag {

    @Id
    @GeneratedValue(strategy = GenerationType.AUTO)
    private long id;

    @NotNull
    @Size(min = 3, max = 255)
    private String title;

    @ManyToMany(mappedBy = "tags", fetch = FetchType.EAGER)
    private Set<Event> events = new HashSet<>();

    public Tag() {
    }

    public Tag(String title) {
        this.title = title;
    }

    public long getId() {
        return id;
    }

    // .. GETTERS AND SETTERS

    public Set<Event> getEvents() {
        return events;
    }

    public void setEvents(Set<Event> events){
        this.events = events;
    }

    // @Override toString() method

Application.java

@SpringBootApplication
public class Application implements CommandLineRunner {

    @Autowired
    EventRepository eventRepository;

    @Autowired
    TagRepository tagRepository;

    public static void main(String[] args) throws Exception {
        SpringApplication.run(Application.class, args);
    }

    @Override
    public void run(String... strings) throws Exception {
        SimpleDateFormat formatter = new SimpleDateFormat("MM-dd-yyyy HH:mm a");
        String[] dateStrings = {"07-25-2015 12:00 PM", "08-25-2015 12:00 PM"};

        Date date1 = formatter.parse(dateStrings[0]);
        Date date2 = formatter.parse(dateStrings[1]);

        Event event1 = eventRepository.save(new Event("Bacon Day", "This day is about bacon. I just need so much bacon all the time", date1, "John Smith", "john.smith@test.com", "W523"));
        Event event2 = eventRepository.save(new Event("Salad Day", "This day is about salad. I just need so much salad all the time", date2, "Jane Smith", "jane.smith@test.com", "N423"));

        Tag tag1 = tagRepository.save(new Tag("Savory"));
        Tag tag2 = tagRepository.save(new Tag("Vegetarian"));

        event1.getTags().add(tag1);
        event1.getTags().add(tag2);
        event1 = eventRepository.save(event1);
        System.out.println(event1);
        System.out.println(eventRepository.findAll());
    }
}

控制台错误输出

Hibernate: select events0_.tag_id as tag_id2_2_0_, events0_.event_id as event_id1_1_0_, event1_.id as id1_0_1_, event1_.date as date2_0_1_, event1_.description as descript3_0_1_, event1_.email as email4_0_1_, event1_.location as location5_0_1_, event1_.organizer as organize6_0_1_, event1_.title as title7_0_1_ from events_tags events0_ inner join event event1_ on events0_.event_id=event1_.id where events0_.tag_id=?
2015-07-13 18:35:28.084 ERROR 4088 --- [nio-8080-exec-1] o.a.c.c.C.[.[.[/].[dispatcherServlet]    : Servlet.service() for servlet [dispatcherServlet] in context with path [] threw exception [Handler processing failed; nested exception is java.lang.StackOverflowError] with root cause

java.lang.StackOverflowError: null
    at java.util.Date.getYear(Date.java:651)
    at java.sql.Timestamp.toString(Timestamp.java:279)
    at java.lang.String.valueOf(String.java:2982)
    at java.lang.StringBuilder.append(StringBuilder.java:131)
    at event.model.Event.toString(Event.java:128)
    at java.lang.String.valueOf(String.java:2982)
    at java.lang.StringBuilder.append(StringBuilder.java:131)
    at java.util.AbstractCollection.toString(AbstractCollection.java:462)
    at org.hibernate.collection.internal.PersistentSet.toString(PersistentSet.java:317)
    at java.lang.String.valueOf(String.java:2982)
    at java.lang.StringBuilder.append(StringBuilder.java:131)
    at event.model.Tag.toString(Tag.java:51)
    at java.lang.String.valueOf(String.java:2982)
    at java.lang.StringBuilder.append(StringBuilder.java:131)
    at java.util.AbstractCollection.toString(AbstractCollection.java:462)
    at org.hibernate.collection.internal.PersistentSet.toString(PersistentSet.java:317)
    at java.lang.String.valueOf(String.java:2982)
    at java.lang.StringBuilder.append(StringBuilder.java:131)
    at event.model.Event.toString(Event.java:128)
    at java.lang.String.valueOf(String.java:2982)
    at java.lang.StringBuilder.append(StringBuilder.java:131)
    at java.util.AbstractCollection.toString(AbstractCollection.java:462)
    at org.hibernate.collection.internal.PersistentSet.toString(PersistentSet.java:317)
    at java.lang.String.valueOf(String.java:2982)
    at java.lang.StringBuilder.append(StringBuilder.java:131)
    at event.model.Tag.toString(Tag.java:51)
    at java.lang.String.valueOf(String.java:2982)
    at java.lang.StringBuilder.append(StringBuilder.java:131)
    at java.util.AbstractCollection.toString(AbstractCollection.java:462)
    at org.hibernate.collection.internal.PersistentSet.toString(PersistentSet.java:317)
    at java.lang.String.valueOf(String.java:2982)
    at java.lang.StringBuilder.append(StringBuilder.java:131)

这会持续数百行。

Answer 1

尝试在父集合的子类中使用 @JsonIgnore。它正在尝试递归序列化。

Follow此链接用于处理此问题的其他方法。