Java 14 Records and JPA

Introduction

This small project tests what is currently possible wrt using records with JPA. Only Hibernate has been tested for now.

Records used as projections

Inside queries

Projections seem to be supported without any modification to existing JPA providers.

select new my.company.MyRecord(e.x, sum(c.y)) from MyEntity e join e.children c

Outside queries

If in-query projections are not powerful enough, they can be done outside of the query in standard java code or by using a library (mapstruct, beanutils, dozer...).

Records used as attributes

@Embeddable/@Embedded records

Records cannot be used as @Embeddable/@Embedded/@EmbeddedId as they don't define a default constructor. Hibernate won't even persist them.

It would be really nice if a records could be used and annotated like any other embeddable object:

@Embeddable
public record Fraction(@Basic(optional = false) BigInteger numerator, @Basic(optional = false) BigInteger denominator) {}

Alternatively, the record could be declared in orm.xml:

<entity-mappings xmlns="http://xmlns.jcp.org/xml/ns/persistence/orm" version="2.1">
    <embeddable class="com.mycompany.Fraction">
        <attributes>
            <basic name="numerator" optional="false" />
            <basic name="denominator" optional="false" />
        </attributes>
    </embeddable>
</entity-mappings>

With AttributeConverter

Single-valued records could be used as attributes but require a standard JPA AttributeConverter.

Multi-valued records could be used as attributes but require a vendor-specific adapter (like Hibernate UserType) as standard AttributeConverters do not support multiple columns.

Records used as entities

Records cannot be used as @Entity as they don't define a default constructor: they can be persisted but not read back. Using records as entities implies that those entities are immutable: identifiers cannot be generated (@GeneratedValue) and must be passed in the constructor.

Question: What if an immutable record entity contains mutable attributes? Won't hashCode()/equals() be impacted?

Conclusions

The biggest problem for JPA providers seems to be that records do not have a default constructor. If JPA providers could add some flexibility in how objects are instantiated/initialized, records could easily be supported. Supporting initialization through non-default constructors could also be beneficial for Object-Oriented Design: entities could have a single constructor and fields could be made final (JPA could even infer that @Column.updatable = false for final fields). Some inspiration could be found from JSON-B which allows non-default constructors to be used (see @JsonbCreator).

Another track to keep an eye on is the (potential) new serialization mechanism which would allow better/easier state extraction and object creation from that state.

kalgon / jpa-records