Create a JPA Criteria fully dynamically

问题:

Usually I'm a Hibernate user and for my new project we use JPA 2.0.

My DAO receives a Container with a generic.

public class Container<T> {
  private String fieldId;    // example "id"
  private T value;           // example new Long(100) T is a Long
  private String operation;  // example ">"

  // getter/setter
}

The following lines won't compile:

if (">".equals(container.getOperation()) {
  criteriaBuilder.greaterThan(root.get(container.getFieldId()), container.getValue());
}

Because I must specify the type like this:

if (">".equals(container.getOperation()) {
  criteriaBuilder.greaterThan(root.<Long>get(container.getFieldId()), (Long)container.getValue());
}

But I don't want to do that! Because I use a generic in my container! Have you an idea?

回答1:

As long as your T is Comparable (it's required for greaterThan), you should be able to do something like the following:

public class Container<T extends Comparable<T>> { 
    ...
    public <R> Predicate toPredicate(CriteriaBuilder cb, Root<R> root) {
        ...
        if (">".equals(operation) {
            return cb.greaterThan(root.<T>get(fieldId), value);
        } 
        ...
    }
    ...
}