Spring Data 一对多映射

Question

我尝试在 Spring Data 项目中配置 OneToMany 和 ManyToOne 映射，但有一些问题。

所以我有两个实体：Employer 和 Project。一个雇主可以有很多项目。

实体类：

Employer.java

@Entity
@Table (name="Employer")
public class Employer {

    @Id
    @SequenceGenerator(name="my_seq", sequenceName="GLOBAL_SEQUENCE")
    @GeneratedValue(strategy = GenerationType.SEQUENCE ,generator="my_seq")
    @Column (name="employer_id")
    private int id;

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

    @JoinColumn (name="employer_id")
    @OneToMany(mappedBy = "employer", cascade = CascadeType.ALL, fetch = FetchType.EAGER)
    private Set<Project> project = new HashSet<Project>();

    ......................
}   

项目.java

@Entity
@Table (name="Project")
public class Project {

    @Id
    @SequenceGenerator(name="my_seq", sequenceName="GLOBAL_SEQUENCE")
    @GeneratedValue(strategy = GenerationType.SEQUENCE ,generator="my_seq")
    @Column (name="project_id")
    private int id;

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

    @ManyToOne
    @JoinColumn(name="employer_id", nullable = false)
    private Employer employer;
    ......................
}

存储库类：

public interface EmployerRepository extends JpaRepository<Employer, Integer> {
}

public interface ProjectRepository extends JpaRepository<Project, Integer> {
}

服务：

@Service
public class EmployerServiceImpl implements EmployerService {

    @Autowired
    private EmployerRepository employerRepository;

    @Override
    public List<Employer> getAllEmployers() {
        return employerRepository.findAll();
    }   
}

@Service
public class ProjectServiceImpl implements ProjectService {

    @Autowired
    private ProjectRepository projectRepository;

    @Override
    public List<Project> getAllProjects() {
        return projectRepository.findAll();
    }
}

控制器：

@Controller
public class MainController {

    private EmployerService employerService;

    private ProjectService projectService;

    @Autowired(required = true)
    @Qualifier(value = "employerService")
    public void setEmployerService(EmployerService employerService) {
        this.employerService = employerService;
    }

    @Autowired(required = true)
    @Qualifier(value = "projectService")
    public void setProjectService(ProjectService projectService) {
        this.projectService = projectService;
    }


    @RequestMapping(value="/employers", method=RequestMethod.GET)
    public @ResponseBody List<Employer> getEmployers(@RequestParam(value = "name", required = false) String name) {
        return employerService.getAllEmployers();        
    }
    ..............................
}

雇主表：

EMPLOYER_ID  .  NAME
......................
1            .  Google
2            .  Oracle
3            .  Facebook 

项目表：

PROJECT_ID .    NAME            . EMPLOYER_ID
.......................................
1          . Create web site    .  1
2          . Create reporting   .  2
3          . Create web service .  3
4          . Fixing web site    .  1        

我期待这样的事情：

[{"id":1,"name":"Google","project":[{"id":1,"name":"Create web site"}, {"id":4,"name":"Fixing web site"}},
 {"id":2,"name":"Oracle","project":{"id":2,"name":"Create reporting"}},
 {"id":3,"name":"Facebook","project":{"id":3,"name":"Create web service"}}]

但是来自controller 类的getEmployers 方法返回这个：

[{"id":1,"name":"Oracle","project":[{"id":4,"name":"Fixing web site",
"employer":{"id":1,"name":"Oracle","project":[{"id":4,"name":"Fixing web site",
"employer":{"id":1,"name":"Oracle","project":[{"id":4,"name":"Fixing web site",
"employer":{"id":1,"name":"Oracle","project":[{"id":4,"name":"Fixing web site",
"employer":{"id":1,"name":"Oracle","project":[{"id":4,"name":"Fixing web site",
"employer":{"id":1,"name":"Oracle","project":[{"id":4,"name":"Fixing web site","employer":
...................

如果这个问题讨论了很多次但我没有找到合适的答案，请见谅。

提前致谢！

Answer 1

Employers 和 Projects 之间存在双向关联。此外，您已将关联的双方配置为加载EAGERly（@ManyToOne 关联默认获取EAGERly，并且您已强制获取@OneToMany 一边EAGERly）。由于这种配置，当序列化框架加载Employers 时，它会找到有效的Projects，这些Projects 具有指向Employers 的反向链接并最终陷入循环循环。

为了获得您想要的结果，您必须将要获取的@ManyToOne 一侧（在Project 实体上）标记为LAZYly 为@ManyTone(fetch = FetchType.LAZY)。

Answer 2

在关系的前向部分添加@JsonManagedReference（即User.java类）：

@Entity
public class User implements Serializable {

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

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

    @ManyToMany
    @JoinTable(name="users_roles",joinColumns=@JoinColumn(name = "user_fk"), inverseJoinColumns=@JoinColumn(name = "role_fk"))
    @JsonManagedReference
    private Set<Role> roles = new HashSet<Role>();

    ...

在关系的后面部分添加@JsonBackReference（即Role.java类）：

@Entity
public class Role implements Serializable {

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

    @ManyToMany(mappedBy="roles")
    @JsonBackReference
    private Set<User> users = new HashSet<User>();

    ...

这对我来说非常有效。 :-)