How to Convert Native Query Results to POJO Class Collections in JPA?

How to Convert Native Query Result to POJO Class Collection in JPA

When working with JPA, it is sometimes necessary to execute native queries to access data that cannot be efficiently retrieved using JPA criteria queries. If the result of a native query is a collection of values, there may be a need to convert each result object to a POJO (Plain Old Java Object) class for further processing.

Direct Casting Using Mapped Entities

In JPA 2.0 and earlier, it was not possible to directly cast a native query result to a POJO class. However, JPA 2.1 introduced the @SqlResultSetMapping annotation, which allows the mapping of native query results to POJOs.

To use this feature, a @SqlResultSetMapping annotation must be defined in an entity class, specifying the class to which the result should be mapped and the columns that correspond to the properties of the class. For example:

@SqlResultSetMapping(
        name = "JediResult",
        classes = {
                @ConstructorResult(targetClass = Jedi.class,
                        columns = {@ColumnResult(name = "name"), @ColumnResult(name = "age")})
        }
)

With this annotation in place, you can execute a native query and use the @SqlResultSetMapping annotation to specify the mapping to the POJO class:

Query query = em.createNativeQuery("SELECT name, age FROM jedi_table", "JediResult");
@SuppressWarnings("unchecked")
List<Jedi> jedis = query.getResultList();

Indirect Casting Using Manual Mapping

If JPA 2.1 is not available, it is possible to perform manual mapping of native query results to POJOs using reflection. This involves creating a method that takes a tuple array (returned by native queries) and a POJO class as input and uses reflection to create an instance of the POJO class with the values from the tuple. This method can then be used to map each result to a POJO object.

Using Named Native Queries

Another option for converting native query results to POJOs is to use named native queries. Named native queries allow you to define a native query in an entity class and specify the result class for the query. This result class can be a POJO or an entity class. By using a named native query, you can avoid using reflection and directly map the result to the POJO class.

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