Querying in JPA 2: Typesafe and Object Oriented

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Query Projection

The result of a criteria query is the same as what is specified during the criteria query creation. The result can also be explicitly specified by passing a query root to the CriteriaQuery.select. The criteria query also gives the developer the strength to project the result in various types, as discussed in the following sections.

Using the construct()

Using this approach, the results of a query can be made to be of a non entity type. In the following code snippet, a criteria query object is created for the EmployeeDetails class, which is a non entity type. This criteria query object is necessary for the EmployeeDetails class to have a constructor that accepts a String and an Integer type argument. The CriteriaBuilder.construct method is used to create the instance of the EmployeeDetails type.

CriteriaQuery<EmployeeDetails> criteriaQuery = criteriaBuilder.createQuery(EmployeeDetails.class);Root<Employee> employee = criteriaQuery.from(Employee.class);criteriaQuery.select(criteriaBuilder.construct(EmployeeDetails.class, employee.get(Employee_.name), employee.get(Employee_.age)));em.createQuery(criteriaQuery).getResultList();Corresponding SQL: SELECT name, age FROM employee

Query Returning Object[]

Criteria queries can also return the Object[] type by setting the values to the CriteriaBuilder.array method. In the following code snippet, the array size is 2 (composed of a String and an Integer value).

CriteriaQuery<Object[]> criteriaQuery = criteriaBuilder.createQuery(Object[].class);Root<Employee> employee = criteriaQuery.from(Employee.class);criteriaQuery.select(criteriaBuilder.array(employee.get(Employee_.name), employee.get(Employee_.age)));em.createQuery(criteriaQuery).getResultList();Corresponding SQL: SELECT name, age FROM employee

Query Returning a Tuple

A row of data or a single record in the database is generally referred to as a tuple. Queries can be made on the tuple by invoking the CriteriaBuilder.createTupleQuery() method. A CriteriaQuery.multiselect method is passed with the parameters, which must be returned from the query in a single record of data.

CriteriaQuery<Tuple> criteriaQuery = criteriaBuilder.createTupleQuery();Root<Employee> employee = criteriaQuery.from(Employee.class);criteriaQuery.multiselect(employee.get(Employee_.name).alias("name"), employee.get(Employee_.age).alias("age"));em.createQuery(criteriaQuery).getResultList();Corresponding SQL: SELECT name, age FROM employee

Conclusion

Criteria queries are a means of querying the database in a more object-oriented fashion. In this article I discussed typesafe criteria queries in JPA 2 as well as the concept of metamodel, which is vital for understanding criteria queries. The various APIs for criteria queries were also discussed.

Acknowledgements

The author would like to sincerely thank Mr. Subrahmanya SV (VP, ECOM Research Group, E&R) for his constant encouragement and Ms. Sangeetha S for providing ideas, guidance and valuable comments, as well as kindly reviewing this article.

About the Author

Nitin KL works at the E-Commerce Research Labs at Infosys Technologies. He is involved in design and development of Java EE applications using Hibernate, iBATIS, and JPA.

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