Table of Contents
JPA 2.0 Specification introduces a new API to define queries dynamically
via construction of an object-based
javax.persistence.CriteriaQuery instance, rather
than string-based approach used in JPQL (Java Persistence Query Language).
This dynamic query definition capability, referred as Criteria API, is
based on the abstract persistent schema of the entities, their embedded
objects and their relationships. The syntax is designed to construct a
Query Tree whose nodes represent the semantic query
elements such as projections, conditional predicates of WHERE clause or
GROUP BY elements etc.
The CriteriaBuilder interface is the factory for CriteriaQuery. A CriteriaBuilder is obtained from either an EntityManagerFactory or an EntityManager as follows:
EntityManager em = ... ; CriteriaBuilder queryBuilder = em.getCriteriaBuilder(); CriteriaQuery qdef = queryBuilder.createCriteriaQuery();
The first step in constructing a query definition is specification of
query roots. Query roots specify the domain objects on which the query
is evaluated. Query root is an instance of the Root<T> interface. A
query root is added to a CriteriaQuery by
addRoot(Class c) method.
Root<Customer> customer = qdef.from(Customer.class);
A query domain can be further refined by joining to other domain objects.
For example, for the above query definition to operate over customers
and their orders, use
Root<Order> order = customer.join(customer.get(Customer_.orders));
where Customer_.orders represent a field of canonical metamodel class for Customer. These canonical metamodel classes are generated during compilation by processing the persistent annotation in the source code of Customer.java.
The condition of a query definition is set via
where(Predicate p) where the argument
designates a conditional predicate. Conditional predicates are often
composed of one or more comparisons between the attribute values of
the domain objects and some variable. For example, to select the
Customer whose name is "John Doe" and has
orders that are not yet delivered, you can build the predicate and set
it to the query definition as:
qdef.where(customer.get(Customer_.name).equal("John Doe") .and(order.get(Order_.status).equal(OrderStatus.DELIVERED).not()));
select() method defines the result of the
query. If left unspecified, the select projection is assumed to be the
root domain object. However, you can specify the selected projections
explicitly as a list:
An attribute of a domain object can also be specified by navigating via
get(String attr). The attribute
to a valid persistent property of the receiving domain object, however
no such validation is enforced during the construction of the query
definition. All validation is deferred until the query is actually executed.
On the other hand, using canonical metamodel for path navigate enforces
compile type checking.