Chapter 11. JPA Criteria
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## Chapter 11. JPA Criteria

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.

## 1. Constructing a CriteriaQuery

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.createQuery();

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 `join(String attribute)`:

    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()));

The `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:

                qdef.select(customer.get(Customer_.name), order.get(Order_.status));

An attribute of a domain object can also be specified by navigating via `get(String attr)`. The attribute should refer 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.

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