Hibernate Query API

API of org.hibernate.query.Query<T>

• This interface is specified in javax.persistence specification

1. Iterator iterate() throws HibernateException

   • This api is deprecated since 5.2 version

   • Return the query results as an Iterator.

   • If the query contains multiple results per row, the results are returned Object[].

   • Entities are returned in lazy manner.

   • Read more here for difference between list() and iterator().

2.     List<T> getResultList()

   • Execute query to get List of selected PERSISTENT POJOs

   • API of org.hibernate.query.Query interface, taken from javax.persistence.TypedQuery<T>.

   • Execute a SELECT query and return the query results as a generic List<T>.

   • T is the type of POJO/Result.

         String hql = "select b from BookPOJO b";
         try {
             List<Book> l1 = hs.createQuery(jpql, Book.class)
                             .getResultList();
         }

   • Passing IN parameters to query & execute it.

     • Objective : Display all books from specified author, with price less than the specified price.

     • API from org.hibernate.query.Query interface

           Query<R> setParameter(String name, Object value)

     • Bind a named query parameter using its inferred Type.

       • name : query parameter name

       • value : parameter value

     • How to set IN parameters ?

       • org.hibernate.query.Query<T> API

       • public Query<T> setParameter(String pName, Object val);

     • Example,

           String hql="select b from BookPOJO b where b.price < :sp_price and b.author = :sp_auth";
           List<Book> l1 = hibSession.createQuery(hql,Book.class).setParameter("sp_price", user_price).setParameter("sp_auth", user_auth).getResultList();

3. Updating POJOs

   • Can be done either with select, followed by update or ONLY with update queries. (following is eg of 2nd option commonly known as bulk update).

   • Objective : Decrease price of all books with author=specified author.

         String jpql = "update BookPOJO b set b.price = b.price - :disc where b.author = :au and b.publishDate < :dt ";
         int updateCount= hs.createQuery(jpql).setParameter("disc", disc).setParameter("dt", d1).executeUpdate();

   • This approach is typically not recommended often, since it bypasses L1 cache. Interesting read here

   • Cascading is not supported. Doesn't support optimistic locking directly.

   • Use case, for bulk updations to be performed on a standalone unrelated table.

Pagination API

1. Query setMaxResults(int maxResults)

   • Set the maximum number of rows to retrieve.

   • If not set, there is no limit to the number of rows retrieved.

2. Query setFirstResult(int firstResult)

   • Set the first row to retrieve.

   • If not set, rows will be retrieved beginnning from row 0. (NOTE row num starts from 0).

       List<CustomerPOJO> l1=session
           .createQuery("select c from CustomerPOJO c")
           .setFirstResult(30).setMaxResults(10).list();

   • How to count rows & use it in pagination techniques?

       int pageSize = 10;
       String countQ = "Select count (f.id) from Foo f";
       Query countQuery = session.createQuery(countQ);
       Long countResults = (Long) countQuery.uniqueResult();
       int lastPageNumber = (int) ((countResults / pageSize) + 1);
       Query selectQuery = session.createQuery("From Foo");
       selectQuery.setFirstResult((lastPageNumber - 1) * pageSize);
       selectQuery.setMaxResults(pageSize);
       List<Foo> lastPage = selectQuery.list();

3. <T> T getSingleResult()

   • Executes a SELECT query that returns a single result.

   • Returns a single instance (persistent object) that matches the query.

   • It throws:

     • NoResultException, if there is no result.

     • NonUniqueResultException, if more than one result.

     • IllegalStateException, if called for a JPQL UPDATE or DELETE statement.

4. How to get Scrollable Result from Query?

   • ScrollableResults scroll(ScrollMode scrollMode) throws HibernateException

   • The scrollability of the returned results depends upon JDBC driver support for scrollable ResultSets.

   • Return the query results as ScrollableResults. Then, one can use methods of ScrollableResults first(), next(), last(), scroll(n).

5. How to create NamedQuery from Session interface?

   • What is a named query ?

     • It's a technique to group the HQL statements in single location (typically in POJOS) and lately refer them by some name whenever need to use them. It helps largely in code cleanup because these HQL statements are no longer scattered in whole code.

   • It is Fail fast: Their syntax is checked when the session factory is created, making the application fail fast in case of an error.

   • Reusable: They can be accessed and used from several places which increases re-usability.

   • Example, In POJO class, at class level, one can declare Named queries.

       @Entity
       @NamedQueries({
               @NamedQuery(name = "DepartmentEntity.GET_DEPARTMENT_BY_ID", query="select d from DepartmentEntity d where d.id = :id")
       })
       public class Department{....}
   
       //Usage
       Department d1 = (Department) session.getNamedQuery("DepartmentEntity.GET_DEPARTMENT_BY_ID").setInteger("id", 1);

6. How to invoke native sql from hibernate?

       Query q = hs.createSQLQuery("select * from books").addEntity(BookPOJO.class);
      List<Object[]> l1 = q.list();

Inheritance Hierarchy

Hibernate Criteria API

• A powerful and elegent alternative to HQL. Well adapted for dynamic search functionalities where complex Hibernate queries have to be generated 'on-the-fly'.

• Typical steps are,

  • Create a criteria for POJO

  • Add restrictions, projections, add order

  • Fire query (via list() or uniqueResult())

Composite primary key

• Use annotation @Embeddable (NOT @Entity).

• Must be Serializable.

• Must implement hashCode() & equals() as per general contract.

• In owning Entity class add usual annotation @Id.