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Specifications
JPA 2 introduces a criteria API that you can use to build queries programmatically. By writing a criteria, you define the where clause of a query for a domain class. Taking another step back, these criteria can be regarded as a predicate over the entity that is described by the JPA criteria API constraints.
Spring Data JPA takes the concept of a specification from Eric Evans' book, “Domain Driven Design”, following the same semantics and providing an API to define such specifications with the JPA criteria API. To support specifications, you can extend your repository interface with the JpaSpecificationExecutor interface, as follows:
public interface CustomerRepository extends CrudRepository<Customer, Long>, JpaSpecificationExecutor<Customer> {
…
}The additional interface has methods that let you run specifications in a variety of ways. For example, the findAll method returns all entities that match the specification, as shown in the following example:
List<T> findAll(Specification<T> spec);The Specification interface is defined as follows:
public interface Specification<T> {
Predicate toPredicate(Root<T> root, CriteriaQuery<?> query,
CriteriaBuilder builder);
}Specifications can easily be used to build an extensible set of predicates on top of an entity that then can be combined and used with JpaRepository without the need to declare a query (method) for every needed combination, as shown in the following example:
public class CustomerSpecs {
public static Specification<Customer> isLongTermCustomer() {
return (root, query, builder) -> {
LocalDate date = LocalDate.now().minusYears(2);
return builder.lessThan(root.get(Customer_.createdAt), date);
};
}
public static Specification<Customer> hasSalesOfMoreThan(MonetaryAmount value) {
return (root, query, builder) -> {
// build query here
};
}
}The Customer_ type is a metamodel type generated using the JPA Metamodel generator (see the Hibernate implementation’s documentation for an example).
So the expression, Customer_.createdAt, assumes the Customer has a createdAt attribute of type Date.
Besides that, we have expressed some criteria on a business requirement abstraction level and created executable Specifications.
So a client might use a Specification as follows:
List<Customer> customers = customerRepository.findAll(isLongTermCustomer());Why not create a query for this kind of data access? Using a single Specification does not gain a lot of benefit over a plain query declaration. The power of specifications really shines when you combine them to create new Specification objects. You can achieve this through the default methods of Specification we provide to build expressions similar to the following:
MonetaryAmount amount = new MonetaryAmount(200.0, Currencies.DOLLAR);
List<Customer> customers = customerRepository.findAll(
isLongTermCustomer().or(hasSalesOfMoreThan(amount)));Specification offers some “glue-code” default methods to chain and combine Specification instances. These methods let you extend your data access layer by creating new Specification implementations and combining them with already existing implementations.
And with JPA 2.1, the CriteriaBuilder API introduced CriteriaDelete. This is provided through JpaSpecificationExecutor’s `delete(Specification) API.
Specification to delete entries.Specification<User> ageLessThan18 = (root, query, cb) -> cb.lessThan(root.get("age").as(Integer.class), 18)
userRepository.delete(ageLessThan18);The Specification builds up a criteria where the age field (cast as an integer) is less than 18.
Passed on to the userRepository, it will use JPA’s CriteriaDelete feature to generate the right DELETE operation.
It then returns the number of entities deleted.