How to Efficiently Convert JPA Native Query Results to POJO Collections?

Solving JPA Collections Query Result Conversion with POJO Classes

When working with JPA, we often encounter scenarios where a native query returns a result set with multiple columns. Converting this result set to a collection of POJO classes is essential for processing the data. In this article, we delve into different approaches to achieve this conversion.

Using Mapped Entities (JPA 2.0 and above)

JPA 2.0 introduces the ability to map native query results to a JPA entity class. This approach is simple and straightforward:

Query query = em.createNativeQuery("SELECT name,age FROM jedi_table", Jedi.class);
List<Jedi> items = (List<Jedi>) query.getResultList();

However, this approach requires Jedi to be a mapped entity class, which may not always be desirable.

Manual Mapping

For scenarios where using mapped entities is not suitable, we can resort to manual mapping. This involves creating a utility method to map the result tuple to a POJO class constructor.

public static <T> T map(Class<T> type, Object[] tuple) {
    Constructor<T> ctor = type.getConstructor(...); // Using reflection to find the constructor
    return ctor.newInstance(tuple);
}

With manual mapping, we can easily convert a list of tuples to a POJO collection:

Query query = em.createNativeQuery("SELECT name,age FROM jedi_table");
@SuppressWarnings("unchecked")
List<Object[]> records = query.getResultList();
List<Jedi> jedis = new LinkedList<>();
for (Object[] record : records) {
    jedis.add(map(Jedi.class, record));
}

JPA 2.1 with @SqlResultSetMapping

JPA 2.1 provides the @SqlResultSetMapping annotation, which offers a more elegant way to map native query results to POJO classes. This annotation is declared in an entity:

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

Using this mapping, we can directly convert the result set to a POJO collection:

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

Using XML Mapping

Alternatively, we can define the @SqlResultSetMapping annotation in the orm.xml file, keeping it out of the entity class:

<named-native-query name="GetAllJedi" result-set-mapping="JediMapping">
    <query>SELECT name,age FROM jedi_table</query>
</named-native-query>

<sql-result-set-mapping name="JediMapping">
    <constructor-result target-class="org.answer.model.Jedi">
        <column name="name" class="java.lang.String" />
        <column name="age" class="java.lang.Integer" />
    </constructor-result>
</sql-result-set-mapping>

Using this XML mapping, we can perform the conversion as follows:

Query query = em.createNativeQuery("GetAllJedi");
@SuppressWarnings("unchecked")
List<Jedi> samples = query.getResultList();

Each of these approaches has its own advantages and drawbacks. Choosing the right method depends on the specific requirements and constraints of the application.

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