How to map a JSON collection using JPA and Hibernate

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Introduction

The open-source hibernate-types project allows you to map Java objects or Jackson JsonNode as JPA or Hibernate entity properties, and, thanks to our awesome contributors, we have added support for storing type-safe JSON collections.

In this article, you are going to see how to achieve this goal.

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

Maven dependency

First of all, you need 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>

If you’re using older versions of Hibernate, check out the hibernate-types GitHub repository for more info about the matching dependency for your current Hibernate version.

Domain Model

Let’s assume we have the following Location Java object type.

public class Location implements Serializable {

    private String country;

    private String city;

    //Getters and setters omitted for brevity

    @Override
    public String toString() {
        return "Location{" +
                "country='" + country + ''' +
                ", city='" + city + ''' +
                '}';
    }
}

And, one Event entity:

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

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

    @Type(type = "jsonb")
    @Column(columnDefinition = "jsonb")
    private List<Location> alternativeLocations = new ArrayList<Location>();

    //Getters and setters omitted for brevity
}

The BaseEntity defines some basic properties (e.g. @Id, @Version) and several custom Hibernate types, among which, we are interested in the JsonBinaryType one.

@TypeDefs({
    @TypeDef(name = "string-array", typeClass = StringArrayType.class),
    @TypeDef(name = "int-array", typeClass = IntArrayType.class),
    @TypeDef(name = "json", typeClass = JsonStringType.class),
    @TypeDef(name = "jsonb", typeClass = JsonBinaryType.class),
    @TypeDef(name = "jsonb-node", typeClass = JsonNodeBinaryType.class),
    @TypeDef(name = "json-node", typeClass = JsonNodeStringType.class),
})
@MappedSuperclass
public class BaseEntity {

    @Id
    private Long id;

    @Version
    private Integer version;

    //Getters and setters omitted for brevity
}

For more details about using @MappedSuperclass, check out this article.

To store both the Location object or the List<Location> in a jsonb PostgreSQL column, we just need to annotate the location property with @Type(type = "jsonb").

That’s it!

Testing time

When saving the following Event entity:

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

Location newYork = new Location();
newYork.setCountry("US");
newYork.setCity("New-York");

Location london = new Location();
london.setCountry("UK");
london.setCity("London");

Event event = new Event();
event.setId(1L);
event.setLocation(cluj);
event.setAlternativeLocations(
    Arrays.asList(newYork, london)
);

entityManager.persist(event);

Hibernate will generate the following SQL INSERT statement:

INSERT INTO event (
    version, 
    alternativeLocations, 
    location, 
    id
) 
VALUES (
    0, 
    [
        {"country":"US","city":"New-York"},
        {"country":"UK","city":"London"}
    ], 
    {"country":"Romania","city":"Cluj-Napoca"}, 
    1
)

Also, when retrieving back the Event entity, both the location and thealternativeLocations` properties are properly fetched:

Event event = entityManager.find(Event.class, eventId);

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

assertEquals(2, event.getAlternativeLocations().size());

assertEquals(
    "New-York", 
    event.getAlternativeLocations().get(0).getCity()
);
assertEquals(
    "London", 
    event.getAlternativeLocations().get(1).getCity()
);

Cool, right?

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Conclusion

The hibernate-types project supports more than JSON types. You can map PostgreSQL ARRAY types or PostgreSQL-specific Enums, nullable Character, or even provide your own immutable Hibernate custom Types.