Is there a way to reduce the amount of boiler-plate code associated with a CriteriaQuery (in JPA 2.0)?

问题

I love the type safety CriteriaQuery brings ing JPA 2.0 but it also brings a bit of boiler-plate code. For example, let say I have an entity called NamedEntity, which simply has an id and a String field called "name" (assume it has the unique constraint set to true). Here's what the NamedEntityManager might look like:

public class NamedEntityManager
{
    //inject using your framework
    EntityManager entityManager;

    //retrieve all existing entities of type NamedEntity from DB
    public Iterable<NamedEntity> queryAll()
    {
        CriteriaBuilder builder = entityManager.getCriteriaBuilder();
        CriteriaQuery<NamedEntity> query = builder.createQuery(NamedEntity.class);
        return entityManager.createQuery(query).getResultList();
    }

    //retrieve a single entity of type NamedEntity from DB using specified name
    public NamedEntity queryByName(String name)
    {
        CriteriaBuilder builder = entityManager.getCriteriaBuilder();
        CriteriaQuery<NamedEntity> query = builder.createQuery(NamedEntity.class);
        Root<NamedEntity> root = query.from(NamedEntity.class);
        query = query.where(root.<NamedEntity>get("name").in(name));

        //skipped the try/catch block for the sake of brevity
        return entityManager.createQuery(query).getSingleResult();
    }
}

Is there a way to condense the code in order to avoid copying/pasting the same lines of code into each query method? Perhaps somehow reuse the CriteriaQuery object?

回答1:

I was looking for something like that, you could take a look at Querydsl (LGPL licensed) which can have JPA as backend.

Im still reading into it, but from their examples, it looks pretty clean.

HQLQuery q = new HibernateQuery(session);
QCat cat = new QCat("cat"); // query type
List<Cat> cats = q.from(cat).where(cat.name.between("A", "B")).list(cat);

回答2:

In JPA 2.1, it will most probably be possible to mix JPQL and Criterias. With such an approach you could define a base query with JPQL and then use the Criteria API to dynamically add small parts.

I figure the API will be less verbose then, since you only need to use small parts of it.

回答3:

Then Use JPA-2.0 MetaData model. http://docs.jboss.org/hibernate/jpamodelgen/1.0/reference/en-US/html_single/

回答4:

It seems there's no way to reduce the amount of code. I guess something had to be sacrificed to gain type safety.

回答5:

Way outdated, this post, but I want to add what I recently built for simple queries

    public static class Jpa2Whatsoever {

    private final EntityManager em;

    public class Jpa2WhatsoeverProgress<T> {

        private CriteriaQuery<T> cq;
        private List<Predicate> predicates = new ArrayList<>();
        private Root<T> root;

        public Jpa2WhatsoeverProgress(Class<T> type) {
            this.cq = em.getCriteriaBuilder().createQuery(type);
            this.root = cq.from(type);

        }

        public Jpa2WhatsoeverProgress<T> where(String attributeName, Object value) {

            Predicate equal = em.getCriteriaBuilder().equal(root.get(attributeName), value);

            predicates.add(equal);
            return this;
        }

        public List<T> getResultList() {
            Predicate[] predicatesArray = new Predicate[predicates.size()];
            TypedQuery<T> typedQuery = em.createQuery(cq.select(root).where(predicates.toArray(predicatesArray)));

            List<T> resultList = typedQuery.getResultList();

            return Collections.unmodifiableList(resultList);
        }

    }

    public Jpa2Whatsoever(EntityManager entityManager) {
        this.em = entityManager;
    }

    public <T> Jpa2WhatsoeverProgress<T> select(Class<T> type) {
        return new Jpa2WhatsoeverProgress<T>(type);
    }
}

You can use it like this

List<MyEntity> matchingEntities = new Jpa2Whatsoever(entityManager).select(MyEntity.class).where("id", id).where("due", new Date()).getResultList();

In the end I stopped this. Mainly because I saw that I had only two queries and I would have to extend the DSL to get the required query characteristics into it, such as

greater than, less than
Metamodel support
QueryBuilder.currentDate() and alike.

Further, I find it ugly to always call where while it actually corresponds to a more SQLly and. Anyway, if someone is interested in a very simple query API, it is still worth a try.

BTW: Forget about the names, this was a prototype, nothing more.

来源：https://stackoverflow.com/questions/3037478/is-there-a-way-to-reduce-the-amount-of-boiler-plate-code-associated-with-a-crite

标签

java

jpa

jpa-2.0

criteriaquery