JSON serializing and deserializing with hibernate jpa to have parent object into child in JSON response

Question

I am developing rest web app with spring framework, Hibernate and JSON. Please Assume that I have two entities like below:

BaseEntity.java

@MappedSuperclass
@JsonIdentityInfo(generator = ObjectIdGenerators.PropertyGenerator.class,property = "id" )
public abstract class BaseEntity implements Serializable {

    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private long id;
    public long getId() {
        return id;
    }
}

University.java

 public class University extends BaseEntity {

      private String uniName;

       @OneToMany(cascade = CascadeType.ALL, fetch = FetchType.EAGER,orphanRemoval = true)
      @JoinColumn(name = "university_id")
        private List<Student> students=new ArrayList<>();
    // setter an getter
    }

Student.java

    public class Student extends BaseEntity{

        private String stuName;

        @ManyToOne(fetch = FetchType.EAGER)
        @JoinColumn(name = "university_id",updatable = false,insertable = false)   
        private University university;

    // setter an getter
        }

when I call my rest api to list University every things work fine as I expect, but when I call my rest api to list student eagerly my JSON response is

[
   {
    "id": 1,
    "stuName": "st1",
    "university": {
        "id": 1,
        "uniName": "uni1"
                 }
    },
    {
        "id": 2,
        "stuName": "st2",
        "university": 1
    }
]

but my desired response is:

[
    {
        "id": 1,
        "stutName": "st1",
        "university": 
        {
         "id": 1,
        "uniName": "uni1"
        }
    },
    {
        "id": 2,
        "stutName": "st2",
        "university": 
        {
         "id": 1,
        "uniName": "uni1"
        }
    }

Update 1: my hibernate annotation working fine I have JSON issue

Requirements :

1. I need both side fetch eagerly(the university side is Ok)

2. I need university object in student side for every student(when I fetching student eagerly)

What kind of serialization or JSON config I need to do that for matching my desired response?

Update 2:

by removing @JsonIdentityInfo and editing student side like below:

@ManyToOne(fetch = FetchType.EAGER)
@JoinColumn(name = "university_id",updatable = false,insertable = false)
@JsonIgnoreProperties(value = "students", allowSetters = true)
private University university;

the json response still same I need my desired response that is mentioned above.

thanks

Answer 1

I had the same problem. Hibernate (or eclipselink) are not the problem. The only constraint in JPA is the FetchType.EAGER .

In the BaseEntity I have added a standard method

public String getLabel(){
   return "id:"+this.getId();
}

this method would be abstract, but I had a lot of class and i didn't want to change it all so I added a default value.

In parent entity, in this case University, override the method

@Override
public String getLabel{
    return this.uniName;
}

For each parent class, use a particular field as a label for your entity

Define a MyStandardSerializer:

public class StandardJsonSerializer extends JsonSerializer<EntityInterface> {

@Override
public void serializeWithType(EntityInterface value, JsonGenerator jgen, SerializerProvider provider, TypeSerializer typeSer) throws IOException, JsonProcessingException {
        serialize(value, jgen, provider); 
}

@Override
public void serialize(EntityInterface value, JsonGenerator jgen, SerializerProvider provider) 
  throws IOException, JsonProcessingException {
    jgen.writeStartObject();
    jgen.writeNumberField("id", (long) value.getId());
    jgen.writeStringField("label", value.getLabel());
    jgen.writeEndObject();
}

}

In the student class, on univrsity add:

@ManyToOne(fetch = FetchType.EAGER)
@JoinColumn(name = "university_id",updatable = false,insertable = false)   
@JsonSerialize(using=StandardJsonSerializer.class)
private University university;

Now you have resolved circularity. When you need a label, override the method in the parent entity. When you need some particular fields, create a specific Serializer.

Answer 2

You might want to try using @JsonRawValue as an annotation for your university property. The behavior you're encountering is due to reference collapsing - since it's the same University twice, the serializer tries to be smart and just return a reference the second time it's encountered.

EDIT: The toString():

@Override
public String toString() {
    ObjectMapper mapper = new ObjectMapper();
    return mapper.writeValueAsString(this);
}