Hibernate无限递归和双向关系

Question

I have two entities. I want a bidirectional relationship between projects and tasks.

• A project has one or more tasks
• A task is associated to only one project

There are my entities:

---

ProjectEntity.java

@Entity
@Table(name = "projects")
public class ProjectEntity {
    @Id
    @GeneratedValue
    @Column(name = "pr_id")
    private long id;

    @Column(name = "pr_name", nullable = false)
    private String name;

    @OneToMany(cascade=CascadeType.ALL, mappedBy="project",orphanRemoval=true)
    @JsonBackReference
    private Set<TaskEntity> tasks = new HashSet<TaskEntity>();

   // Getters, constructors, setters

---

TaskEntity.java

@Entity
@Table(name = "tasks")
public class TaskEntity {
    @Id
    @GeneratedValue
    @Column(name = "ta_id")
    private long id;

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

    @ManyToOne
    @JoinColumn(name="pr_id")
    @JsonManagedReference
    private ProjectEntity project;

    // Getters, constructors, setters

I would like to have the list of tasks in each ProjectEntity , and in each TaskEntity the project associated.

Here, I'm using @JsonManagedReference and @JsonBackReference to stop the infinite recursion it generated, but in my ProjectEntity , I don't have the tasks list (because of the @JsonBackReference ) ...

Could you help me to get back tasks list in ProjectEntity ? I heard about @JsonIdentifyInfo , but I have not managed to do with.

Hope I'm understandable :)

Answer 1

You could opt for implement a custom serializer for the list ProjectEntity.tasks. But you must controll/stop the serialization cycle at some point; this will depend on your requirements. Here is an example of what I mean.

@Entity
@Table(name = "projects")
public class ProjectEntity {
    ...
    @OneToMany(cascade=CascadeType.ALL, mappedBy="project", orphanRemoval=true)
    @JsonSerialize(using = CustomListSerializer.class)
    private Set<TaskEntity> tasks = new HashSet<TaskEntity>();
}

And the CustomListSerializer could be something like this,

public class CustomListSerializer extends JsonSerializer<List<TaskEntity>>{

    @Override
    public void serialize(List<TaskEntity> tasks, JsonGenerator generator, SerializerProvider provider)
          throws IOException, JsonProcessingException {

        generator.writeStartArray();

        for (TaskEntity task : tasks) {
            generator.writeStartObject("taskEntity")
                .write("id", task.id)
                .write("name", task.name)
                .write("project", task.project.id) //<- here you stop the cycle 
            .writeEnd();
        }
        generator.writeEnd();
    }
}

Note that it is basically the example of the mentioned mini guide but it serialize information of the elements of project's tasks list instead of the tasks' id only.

Answer 2

The solution that I know is really using '@JsonIdentityInfo' in instead of '@JsonBackReference' and '@JsonManagedReference'. You have to remove the '@JsonBackReference' and '@JsonManagedReference' to use '@JsonIdentityInfo'.

@JsonIdentityInfo(generator = ObjectIdGenerators.PropertyGenerator.class, property="id")
@Entity
@Table(name = "projects")
public class ProjectEntity {
    @Id
    @GeneratedValue
    @Column(name = "pr_id")
    private long id;

    @Column(name = "pr_name", nullable = false)
    private String name;

    @OneToMany(cascade=CascadeType.ALL, mappedBy="project",orphanRemoval=true)
    private Set<TaskEntity> tasks = new HashSet<TaskEntity>();
}


@JsonIdentityInfo(generator = ObjectIdGenerators.PropertyGenerator.class, property="id")
@Entity
@Table(name = "tasks")
public class TaskEntity {
    @Id
    @GeneratedValue
    @Column(name = "ta_id")
    private long id;

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

    @ManyToOne
    @JoinColumn(name="pr_id")
    private ProjectEntity project;
}