What is the difference between JPAQuery and JPAQueryFactory? And, When to use which?
According to the querydsl reference documentation:
Both JPAQuery and HibernateQuery implement the JPQLQuery interface.
For the examples of this chapter the queries are created via a JPAQueryFactory instance. JPAQueryFactory should be the preferred option to obtain JPAQuery instances.
But, I could not understand clearly. Can anyone explain it briefly?
What matters is that Hibernates query language (HQL) is a superset of JPA's query language (JPQL). Hibernate also has a special method for result set transformation and being able to iterate over scrollable result sets without the need to keep a reference to all records in memory. In order to take advantage of this extra functionality, the HQLTemplates and the HibernateHandler have to be used. The first is responsible for serializing the additional types of expressions, the second for the integration with Hibernates Query implementation. The HibernateHandler is actually obtained from the HQLTemplates as well, so all that remains is specifying HQLTemplates.
And in fact: a JPAQuery instantiated with HQLTemplates.INSTANCE for the Templates variable, behaves the same as a HibernateQuery. FWIW, if you provide an EntityManager instance with the construction of your JPAQuery, then the appropriate implementation for Templates is deduced for your ORM vendor automatically.
All JPAQueryFactory really is, is a factory method that binds the EntityManager and Templates variables for newly instantiated JPAQueries. This eliminates the need to pass these as a variable individually for each instantiation of a JPAQuery.
There is no need to use the JPAQueryFactory, but it could make your code easier to read. Furthermore, a lot of code examples on the QueryDSL website utilize the query factory, so it might make it easier to use these examples as snippets in your own code.