JPA repositories with lambda expressions

Lambda expression support for JPA (Java persistence API) Criteria API repositories.

Basic Criteria API is hard to write, because its calls cannot be chained. This library solves the problem. The resulting code is short, type safe and easy to read.

Usage

Add the library as a Maven dependency:

<dependency>
    <artifactId>jpa-repositories-with-lambda-expressions</artifactId>
    <groupId>io.github.janhalasa</groupId>
    <version>0.7.3</version>
</dependency>

To create a repository class, extend one of the provided repositories:

• BasicRepository - no public methods - just methods you can use to create your custom repository
• ReadOnlyRepository - extends BasicRepository and makes some querying methods public.
• ReadPersistRepository - extends ReadPersistRepository and makes the persist method public. Intended for repositories that don't support updating or removing of entities.
• CrudRepository - extends ReadPersistRepository and makes merge nad remove methods public.
• VersionAwareCrudRepository - extends CrudRepository and adds load methods with an extra expectedVersion parameter.

Example:

public class CarRepository extends CrudRepository<Car, Long> {
    public CarRepository(EntityManager em) {
        super(em, Car.class, Car_.id);
    }
    // ... custom repository methods
}

Querying

Single

Optional result

Optional<E> optionalEntity = getWhere((cb, root) -> cb.equal(root.get(Car_.color), color));

Required result

If no result is found, jakarta.persistence.NoResultException is thrown.

E Entity = loadWhere((cb, root) -> cb.equal(root.get(Car_.color), color));

Multiple

Find a list

Methods starting with find. They always return a list of entities.

List<E> entities = findWhere(
    (cb, root) -> cb.and(
        cb.equal(root.join(Car_.model).get(CarModel_.vendor), vendor),
        cb.equal(root.get(Car_.color), color))
);

Ordering

Ordering is supported by the find methods using OrderBy class which doesn't depend on EntityManager:

List<E> entities =  findWhereOrdered(
    (cb, root) -> new PredicateAndOrder(
        cb.like(root.get(Vendor_.name), "%" + namePattern + "%"),
        List.of(OrderBy.asc(root.get(Vendor_.name))))
);

Pagination

Pagination requires an order (a stable one) to work and page properties - number and size:

ResultPage<E> page = pageWhere(
    (cb, root) -> new PredicateAndOrder(
        cb.like(root.get(Vendor_.name), "%a%"),
        List.of(OrderBy.asc(
                root.get(Vendor_.name)),
                OrderBy.desc(root.get(Vendor_.id))
        )
    ),
    pageNumber,
    pageSize);

There are no public pagination methods exposed in any of the predefined repository classes, since pagination requires a custom order. The primary key could be used by default (it would also ensure a stable order), but this functionality is not there at the moment.

Lazy assotiation loading

The API supports lazy assotiation loading using entity graphs. Both jakarta.persistence.loadgraph and jakarta.persistence.fetchgraph) are supported by methods with more explicit naming ResultGraph.specifiedAssociationsOnly and ResultGraph.specifiedAndEagerAssociations.

findWhere(
        (cb, root) -> cb.equal(root.get(Vendor_.name), searchedName),
        ResultGraph.specifiedAssociationsOnly(createEntityGraph(List.of(Vendor_.models))));

Modifications

BasicRepository supports basic persist, merge and remove methods analogous to the ones from the JPA EntityManager. It also supports removal of multiple entities by BasicRepository::removeWhere.

removeWhere((cb, root) -> cb.equal(root.get(Car_.color), color));

Versioning and optimistic locking

The library supports optimistic locking by a possibility to load an entity by its primary key and a version. The intended use case is: A service gets a primary key and a version from and tries to load the corresponding entity. If the version of the entity has changed the VersionAwareCrudRepository::loadByPk(pk, expectedVersion) will throw an jakarta.persistence.OptimisticLockException.

E entity = loadByPk(pk, expectedVersion);
// ... modify entity
// modifications will be saved on transaction commit