Question

Using JPA 2 with EclipseLink implementation.

I’m trying to build a dynamic query which should bring me some records persisted after a given date.

CriteriaBuilder builder = em.getCriteriaBuilder();
CriteriaQuery<Event> criteria = builder.createQuery(Event.class);
Root<Event> root = criteria.from(Event.class);
criteria.select(root);
criteria.distinct(true);
List<Predicate> predicates = new ArrayList<Predicate>();
//...
if (dateLimit != null){
    ParameterExpression<Date> param = builder.parameter(Date.class, "dateLimit");
    predicates.add(builder.lessThanOrEqualTo(root.get("dateCreated"), param));
}

lessThanOrEqualTo() and le() are the only two methods in the API which look like may help me in this case. This warning is thrown by the eclipse though:

Bound mismatch: The generic method lessThanOrEqualTo(Expression<? extends Y>, Expression<? extends Y>)
of type CriteriaBuilder is not applicable for the arguments (Path<Object>, ParameterExpression<Date>).
The inferred type Object is not a valid substitute for the bounded parameter
<Y extends Comparable<? super Y>>

I can imagine that I’m not taking the correct approach for this problem but I can’t find anywhere some tips or pointers for a possible solution.

Answer 1

The problem is that with the string-based API it cannot infer the type for the result value of the get-Operation. This is explained for example in Javadoc for Path.

If you use

predicates.add(builder.lessThanOrEqualTo(root.<Date>get("dateCreated"), param));

instead, it will work fine, because it can figure out the return type from the type argument and will find out that it is comparable. Note, the use of a parameterised method invocation root.<Date>get(...) (see, e.g., When is a parameterized method call useful?).

Another (in my opinion better) solution is to use the metamodel based API instead of the string-based one. A simple example about canonical metamodel is given for example here. If you have more time to invest, this is a good article about static metamodel: Dynamic, typesafe queries in JPA 2.0