Introduction

In this article, we are going to see how you can map a JSON column to a JPA entity attribute using the Hibernate Types open-source project.

While you can create your own custom Hibernate Types, for mapping JSON column types on Oracle, SQL Server, PostgreSQL, or MySQL, you don’t need to implement your own Hibernate Type since the Hibernate Types project already offers this functionality.

Kudos to @vlad_mihalcea, the Hibernate Types library saved me tons of work yesterday by automagically mapping the PostgreSQL jsonb column to a POJO. Brilliant work 💪

— Tomasz Knyziak (@TomaszKnyziak) September 18, 2019

The Domain Model

Assuming we have the following Domain Model:

Location and Ticket are JSON Object(s), while Event and Participant are JPA entities. Our goal is to provide a Hibernate JSON Type that works for any type of JSON Java Object and on any relational database that supports JSON columns.

Maven dependency

The first thing you need to do is to set up the following Maven dependency in your project pom.xml configuration file:

<dependency>
    <groupId>com.vladmihalcea</groupId>
    <artifactId>hibernate-types-52</artifactId>
    <version>${hibernate-types.version}</version>
</dependency>

Declaring the Hibernate Types

To make use of the JSON Hibernate Types, we must declare them using the @TypeDef annotation:

@TypeDefs({
    @TypeDef(name = "json", typeClass = JsonStringType.class),
    @TypeDef(name = "jsonb", typeClass = JsonBinaryType.class)
})
@MappedSuperclass
public class BaseEntity {
    //Code omitted for brevity
}

The `TypeDef` annotations can be applied to a base entity class or in the package-info.java file associated with your current entities package.

MySQL

MySQL 5.7 adds support for JSON types, which, at the JDBC level, need to be materialized as String. For this reason, we are going to use JsonStringType.

The entity mapping looks like this:

@Entity(name = "Event") 
@Table(name = "event")
public class Event extends BaseEntity {

    @Type(type = "json")
    @Column(columnDefinition = "json")
    private Location location;

    public Location getLocation() {
        return location;
    }

    public void setLocation(Location location) {
        this.location = location;
    }
}

@Entity(name = "Participant") 
@Table(name = "participant")
public class Participant extends BaseEntity {

    @Type(type = "json")
    @Column(columnDefinition = "json")
    private Ticket ticket;

    @ManyToOne
    private Event event;

    public Ticket getTicket() {
        return ticket;
    }

    public void setTicket(Ticket ticket) {
        this.ticket = ticket;
    }

    public Event getEvent() {
        return event;
    }

    public void setEvent(Event event) {
        this.event = event;
    }
}

When inserting the following entities:

final AtomicReference<Event> eventHolder = new AtomicReference<>();
final AtomicReference<Participant> participantHolder = new AtomicReference<>();

doInJPA(entityManager -> {
    Event nullEvent = new Event();
    nullEvent.setId(0L);
    entityManager.persist(nullEvent);

    Location location = new Location();
    location.setCountry("Romania");
    location.setCity("Cluj-Napoca");

    Event event = new Event();
    event.setId(1L);
    event.setLocation(location);
    entityManager.persist(event);

    Ticket ticket = new Ticket();
    ticket.setPrice(12.34d);
    ticket.setRegistrationCode("ABC123");

    Participant participant = new Participant();
    participant.setId(1L);
    participant.setTicket(ticket);
    participant.setEvent(event);

    entityManager.persist(participant);

    eventHolder.set(event);
    participantHolder.set(participant);
});

Hibernate generates the following statements:

INSERT INTO event (location, id) 
VALUES (NULL(OTHER), 0)

INSERT INTO event (location, id) 
VALUES ('{"country":"Romania","city":"Cluj-Napoca"}', 1)

INSERT INTO participant (event_id, ticket, id) 
VALUES (1, {"registrationCode":"ABC123","price":12.34}, 1)

The JSON Object(s) are properly materialized into their associated database columns.

Not only that JSON Object(s) are properly transformed from their database representation:

Event event = entityManager.find(Event.class, eventHolder.get().getId());

assertEquals("Cluj-Napoca", event.getLocation().getCity());

Participant participant = entityManager.find(
    Participant.class, participantHolder.get().getId());
assertEquals("ABC123", participant.getTicket().getRegistrationCode());

But we can even issue native JSON-based SQL queries:

List<String> participants = entityManager.createNativeQuery(
    "SELECT p.ticket -> \"$.registrationCode\" " +
    "FROM participant p " +
    "WHERE JSON_EXTRACT(p.ticket, \"$.price\") > 1 ")
.getResultList();

And JSON Object(s) can be modified:

event.getLocation().setCity("Constanța");
entityManager.flush();

Hibernate generating the proper UPDATE statement:

UPDATE event 
SET location = '{"country":"Romania","city":"Constanța"}'
WHERE id = 1

PostgreSQL

PostgreSQL has been supporting JSON types since version 9.2. There are two types that can be used:

• json
• jsonb

Both PostgreSQL JSON types need to be materialized using a binary data format, so we need to use the JsonBinaryType this time.

PostgreSQL JSON column type

For the JSON column type, the two JSON Object(s) mapping must be changed as follows:

@Type(type = "jsonb")
@Column(columnDefinition = "jsonb")
private Location location;

@Type(type = "jsonb")
@Column(columnDefinition = "jsonb")
private Ticket ticket;

The insert works just the same, as well as the entity update, and we can even query the JSON column as follows:

List<String> participants = entityManager.createNativeQuery(
    "SELECT p.ticket ->>'registrationCode' " +
    "FROM participant p " +
    "WHERE p.ticket ->> 'price' > '10'")
.getResultList();

PostgreSQL JSONB column type

For the JSONB column type, we only need to change the columnDefinition attribute:

@Type(type = "jsonb")
@Column(columnDefinition = "jsonb")
private Location location;

@Type(type = "jsonb")
@Column(columnDefinition = "jsonb")
private Ticket ticket;

The insert and the JSON Object update work the same, while the JSONB column type provides more advanced querying capabilities:

List<String> participants = entityManager.createNativeQuery(
    "SELECT jsonb_pretty(p.ticket) " +
    "FROM participant p " +
    "WHERE p.ticket ->> 'price' > '10'")
.getResultList();

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Conclusion

The Hibernate Types support for JSON column mapping is very useful, and you can use it to map entity attributes that are either POJO, String, or even JsonNode.

The best thing about the Hibernate Types project is that it offers support for Oracle, SQL Server, PostgreSQL, or MySQL JSON column types.

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