JPA Specifications by Example

Question

Spring Boot here. I'm trying to wrap my head around JpaRepositories and Specifications when used in the context of implementing complex queries and am struggling to see the "forest through the trees" on several items.

A canonical example of a Specification is as follows:

public class PersonSpecification implements Specification<Person> {
    private Person filter;

    public PersonSpecification(Person filter) {
        super();
        this.filter = filter;
    }

    public Predicate toPredicate(Root<Person> root, CriteriaQuery<?> cq,
            CriteriaBuilder cb) {
        Predicate p = cb.disjunction();

        if (filter.getName() != null) {
            p.getExpressions()
                    .add(cb.equal(root.get("name"), filter.getName()));
        }

        if (filter.getSurname() != null && filter.getAge() != null) {
            p.getExpressions().add(
                    cb.and(cb.equal(root.get("surname"), filter.getSurname()),
                            cb.equal(root.get("age"), filter.getAge())));
        }

        return p;
    }
}

In this toPredicate(...) method, what do the Root<Person> and CriteriaQuery represent? Most importantly, it sounds like you need to create one Specification impl for each type of filter you want to apply, because each spec gets translated into one and only one predicate...so for instance if I wanted to find all people with a surname of "Smeeb" and an age greater than 25, it sounds like I would need to write a LastnameMatchingSpecification<Person> as well as a AgeGreaterThanSpecification<Person>. Can someone confirm or clarify this for me?!

Answer 1

what do the Root<Person> and CriteriaQuery represent?

Root is the root of your query, basically What you are querying for. In a Specification, you might use it to react dynamically on this. This would allow you, for example, to build one OlderThanSpecification to handle Cars with a modelYear and Drivers with a dateOfBirth by detecting the type and using the appropriate property.

Similiar CriteriaQuery is the complete query which you again might use to inspect it and adapt the Predicate you are constructing based on it.

if I wanted to find all people with a surname of "Smeeb" and an age greater than 25, it sounds like I would need to write a LastnameMatchingSpecification<Person> as well as an AgeGreaterThanSpecification<Person>. Can someone confirm or clarify this for me?!

I think you have that wrong. The Spring Data interfaces accepting Specifications only accept a single Specification. So if you want to find all Persons with a certain name and a certain age you would create one Specification. Similar to the example you quote which also combines two constraints.

But you may create separate Specifications and then create another one combining those if you want to use each separately, but also combined.