188.8.131.52. Executing JPQL Queries
Query interface is designed to execute JPQL queries. The reference to it may be obtained from the current
EntityManager instance by calling
createQuery() method. If the query is supposed to be used to load entities, we recommend calling
createQuery() with the result type as a parameter. This will create a
The methods of
Query mainly correspond to the methods of the standard JPA javax.persistence.Query interface. Let us have a look at the differences.
setParameter()– sets a value to a query parameter. If the value is an entity instance, implicitly converts the instance into its identifier. For example:
Customer customer = ...; TypedQuery<Order> query = entityManager.createQuery( "select o from sales$Order o where o.customer.id = ?1", Order.class); query.setParameter(1, customer);
Note that the entity is passed as a parameter while comparison in the query is done using identifier.
A variant of the method with
implicitConversions = falsedoes not perform such conversion.
addView()– define a view which is used to load data.
getDelegate()– returns an instance of
javax.persistence.Query, provided by the ORM implementation.
If a view is set for a query, then by default the query has
FlushModeType.AUTO, which affects the case when the current persistence context contains changed entity instances: these instances will be saved to the database prior to the query execution. In other words, ORM first synchronizes the state of entities in the persistence context and in the database, and only after that runs the query. It guarantees that the query results contain all relevant instances, even if they have not been saved to the database explicitly yet. The downside of this is that you will have an implicit flush, i.e. execution of SQL update statements for all currently changed entity instances, which may affect performance.
If a query is executed without a view, then by default the query has
FlushModeType.COMMIT, which means that the query will not cause a flush, and the query results will not respect the contents of the current persistence context.
In most cases ignoring the current persistence context is acceptable, and is a preferred behavior because it doesn’t lead to extra SQL updates. But there is the following issue when using views: if there is a changed entity instance in the persistence context, and you execute a query with a view and
FlushModeType.COMMIT loading the same instance, the changes will be lost. That is why we use
FlushModeType.AUTO by default when running queries with views.
You can also set flush mode explicitly using the
setFlushMode() method of the
Query interface. It will override the default settings described above.