@ManyToMany 保存关系在一个方向上只能使用一次

Question

我将尝试在我的 SpringBoot 应用程序中创建 @ManyToMany 关系。 我使用 JPARespository，我创建了 2 个模型：

用户

@Entity(name = "User")
@Table(name = "users")
public class User {

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

    private String name;

    @ManyToMany//(cascade = {CascadeType.MERGE, CascadeType.PERSIST})
    @LazyCollection(value = LazyCollectionOption.FALSE)
    @JoinTable(
            name = "users_groups",
            joinColumns = @JoinColumn(name = "users_id"),
            inverseJoinColumns = @JoinColumn(name = "groups_id")
    )
    private Set<UGroup> groups;

    public User() {
        this.groups = new HashSet<>();
    }

    public void addGroup(UGroup uGroup) {
        this.groups.add(uGroup);
        uGroup.users.add(this);
    }

    public void removeGroup(UGroup uGroup) {
        this.groups.remove(uGroup);
        uGroup.users.remove(this);
    }
}

UGroups

@Data
@Entity(name = "UGroup")
@Table(name = "groups")
public class UGroup {

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

    private String name;

    @ManyToMany(mappedBy = "groups", cascade = {CascadeType.MERGE, CascadeType.PERSIST})
    @LazyCollection(value = LazyCollectionOption.FALSE)
    public Set<User> users;

    public UGroup() {
        this.users = new HashSet<>();
    }

    public void addUser(User user) {
        this.users.add(user);
        user.getGroups().add(this);
    }

    public void removeUser(User user) {
        this.users.remove(user);
        user.getGroups().remove(this);
    }
}

我通过 DataConfiguration 类加载启动数据：

    @Override
    public void onApplicationEvent(ContextRefreshedEvent contextRefreshedEvent) {
        User user = new User();
        user.setName("Damian");
        userRepository.save(user);

        user = new User();
        user.setName("Marta");
        userRepository.save(user);

        user = new User();
        user.setName("Natalia");
        userRepository.save(user);

        UGroup uGroup = new UGroup();
        uGroup.setName("Mieszkanie");
        uGroup = uGroupRepository.save(uGroup);

        user = userRepository.findById(1).get();
        user.addGroup(uGroup);
        user = userRepository.save(user);

        // unable to execute
        /*
        user = userRepository.findById(2).get();
        user.addGroup(uGroup);
        user = userRepository.save(user);
         */

        // unable to execute 2
        /*
        uGroup = uGroupRepository.findById(4).get();
        uGroup.addUser(userRepository.findById(3).get());
        uGroup = uGroupRepository.save(uGroup);
         */
    }

当我只为 1 个用户执行插入组时，一切都很好。 虽然当我尝试为 1 和 2 个用户执行插入组或为组插入 3 个用户时，它会导致 java.lang.StackOverflowError: null 异常。

我不知道哪里出错了。谁能帮助我并解释我应该在哪里以及为什么要更改某些内容？ 提前致谢。

Answer 1

通过Join Table，可以映射ManyToMany双向关系。这些是修改后的实体类

import org.hibernate.annotations.LazyCollection;
import org.hibernate.annotations.LazyCollectionOption;

import javax.persistence.*;
import java.util.HashSet;
import java.util.Set;

@Entity(name = "User")
@Table(name = "users")
public class User {

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

    private String name;

    public Integer getId() {
        return id;
    }

    public String getName() {
        return name;
    }

    public Set<UGroup> getGroups() {
        return groups;
    }

    public void setGroups(Set<UGroup> groups) {
        this.groups = groups;
    }

    public void setName(String name) {
        this.name = name;
    }

    @ManyToMany(mappedBy = "users",cascade = CascadeType.ALL)
    @LazyCollection(value = LazyCollectionOption.FALSE)
    private Set<UGroup> groups=new HashSet<UGroup>();

    public User() {
    }

    public void addGroup(UGroup uGroup) {
        this.groups.add(uGroup);
        uGroup.users.add(this);
    }

    public void removeGroup(UGroup uGroup) {
        this.groups.remove(uGroup);
        uGroup.users.remove(this);
    }
}

import javax.persistence.*;
import java.util.HashSet;
import java.util.Set;

@Entity(name = "UGroup")
@Table(name = "groups")
public class UGroup {

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

    private String name;

    @ManyToMany(cascade = CascadeType.ALL)
    @JoinTable(name = "USER_GROUP_MAP" ,joinColumns = @JoinColumn(name="GROUP_ID",referencedColumnName = "ID")
            ,inverseJoinColumns = @JoinColumn(name="USER_ID",referencedColumnName = "ID"))
    @LazyCollection(value = LazyCollectionOption.FALSE)
    public Set<User> users=new HashSet<>();

    public UGroup() {
    }

    public void addUser(User user) {
        this.users.add(user);
        user.addGroup(this);
    }

    public void removeUser(User user) {
        this.users.remove(user);
        user.removeGroup(this);
    }

    public Integer getId() {
        return id;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public Set<User> getUsers() {
        return users;
    }

    public void setUsers(Set<User> users) {
        this.users = users;
    }
}

对于多对多关系，连接表是必要的。

SELECT * FROM USER_GROUP_MAP;
GROUP_ID    USER_ID  
4            1
4            2
4            3

这是您发布的代码的结果。您应该覆盖两个实体类中的 hashcode 和 equals 方法，以避免表中的数据重复。

用于测试上述关系映射的代码。

@Override
public void onApplicationEvent(ContextRefreshedEvent applicationEvent) {
    User user = new User();
    user.setName("Damian");
    userRepository.save(user);

    user = new User();
    user.setName("Marta");
    userRepository.save(user);

    user = new User();
    user.setName("Natalia");
    userRepository.save(user);

    UGroup uGroup = new UGroup();
    uGroup.setName("Mieszkanie");
    uGroup = uGroupRepository.save(uGroup);

    user = userRepository.findById(1).get();
    user.addGroup(uGroup);
    user = userRepository.save(user);

    // unable to execute

    user = userRepository.findById(2).get();
    user.addGroup(uGroupRepository.findById(4).get());
    user = userRepository.save(user);


    // unable to execute 2

    uGroup = uGroupRepository.findById(4).get();
    uGroup.addUser(userRepository.findById(3).get());
    uGroup = uGroupRepository.save(uGroup);

}