Java Reference
In-Depth Information
public List findCustomerByLastName(String someLastName)
{
//code to lookup EntityManager omitted for brevity
Query query =
em.createNamedQuery("Customer.findByLastName");
query.setParameter("lastName", someLastName);
List resultList = query.getResultList();
return resultList;
}
}
Here we see a DAO object containing a method that will return a list of
Customer
entities for customers whose last name equals the one provided in the method's
parameter. In order to implement this, we need to obtain an instance of an object of
type
javax.pesistence.Query
, as we can see in the above code snippet, this can
be accomplished by invoking the
createNamedQuery()
method in
EntityManager
,
passing the query name (as defined in the
@NamedQuery
annotation) as a parameter.
Notice that the named queries generated by the NetBeans wizard contain strings
preceded by a colon (:). These strings are
named parameters
, named parameters are
"placeholders" we can use to substitute for appropriate values.
In our example, we set the
lastName
named parameter in JPQL query with the
someLastName
argument passed to our method.
Once we have populated all parameters in our query, we can obtain a
List
of all
matching entities by invoking the
getResultList()
method in our
Query
object.
Going back to our generated JPA entity, notice that the wizard automatically placed
the
@Id
annotation in the field mapping to the table's primary key. Additionally, each
field is decorated with the
@Column
annotation, which allows us to follow standard
naming conventions in both the relational database and Java worlds. In addition to
allowing us to specify what column each field maps to, the
@Column
annotation has
a
nullable
attribute that allows us to specify if the column accepts null values or
not. As we can see, the wizard automatically sets
nullable
to
false
for the entity's
primary key field.