Map JSON string column of a JPA entity to Java object automatically

Question

I have a JPA entity object with following structure:

@Table(name="item_info")
class Item(){
    @Id
    @GeneratedValue(strategy=GenerationType.IDENTITY)
    private Long id;

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

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

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

    @Transient
    private ItemJson itemJson;

    //Getters and setters

}

The itemJsonString field contains a json string value such as '{"key1":"value1","key2":"value2"}'

And the itemJson field contains the corresponding object which maps to the json string.

I get this entity object from database as follows:

Item item = itemRepository.findOne(1L);    // Returns item with id 1

Now, the itemJson field is null since it is a transient field. And I have to set it manually using Jackson's ObjectMapper as follows:

itemJson = objectMapper.readValue(item.getItemJsonString(), ItemJson.class);

How can I make it such that when I do itemRepository.findOne(), it returns an Item object with the itemJson field mapped to the json String automatically?

Answer 1

Your best bet would be to implement a javax.persistence.Converter. It would look something like:

import javax.persistence.AttributeConverter;
import javax.persistence.Converter;

@Converter(autoApply = true)
public class LocalDateAttributeConverter implements AttributeConverter<ItemJson, String> {

    @Override
    public String convertToDatabaseColumn(ItemJson entityValue) {
        if( entityValue == null )
            return null;

        ObjectMapper mapper = new ObjectMapper();

        return mapper.writeValueAsString(entityValue);
    }

    @Override
    public ItemJson convertToEntityAttribute(String databaseValue) {
        if( databaseValue == null )
            return null;

        ObjectMapper mapper = new ObjectMapper();

        return mapper.readValue(databaseValue, ItemJson.class);

    }
}

I've used this with WildFly and didn't have to do anything except have it be in the war file I was deploying.

Answer 2

Here is the full working version of AttributeConverter + JPA + Kotlin.

Entity Class

In my case, database was mysql (8.x), which supports JSON as the underlying data type for column definition, and we can apply a custom converter using @Convert annotation.

@Entity
data class Monitor (
    @Id
    val id: Long? = null,

    @Column(columnDefinition = "JSON")
    @Convert(converter = AlertConverter::class)
    var alerts: List<Alert> = emptyList(),

    var active: Boolean = false
)

Converter Definition Attribute converter needs to specify the conversion mechanism from data to db and reverse. We are using Jackson to convert a java object into String format and vice versa.

@Converter(autoApply = true)
class AlertConverter : AttributeConverter<List<Alert>, String> {

private val objectMapper = ObjectMapper()

 override fun convertToDatabaseColumn(data: List<Alert>?): String {
    return if (data != null && !data.isEmpty())
        objectMapper.writeValueAsString(data)
    else ""
 }

 override fun convertToEntityAttribute(dbData: String?): List<Alert> {
    if (StringUtils.isEmpty(dbData)) {
        return emptyList()
    }
    return objectMapper.readValue(dbData, object : TypeReference<List<Alert>>() {})
 }
}