Convert JPA query.getResultList() to MY Objects

Question

I'm performing a Query to my DB in JPA. The Query "queries" 4 tables, and the result aggregates columns from that different tables.

My Query is something like:

Query query = em.createQuery("SELECT o.A, o.B, o.C, e.D, c.E FROM Table1 o, 
Table2 i, Table3 e, Table4 c WHERE o.X = i.X AND i.Y = e.Y AND i.Z = c.Z");

How can I get the query result and extract the different fields?

I created a class (MyObject) that represents each item of the result list, and I want to convert the query.getResultList() into a List< MyObject>.

How can I do it?

Answer 1

This kind of query returns a List<Object[]>. So you just need a loop to convert each array into an instance of your class:

List<Object[]> rows = query.getResultList();
List<MyObject> result = new ArrayList<>(rows.size());
for (Object[] row : rows) {
    result.add(new MyObject((String) row[0],
                            (Long) row[1],
                            ...));
}

Answer 2

You're looking for the SELECT NEW construct, explained in this answer:

"SELECT NEW your.package.MyObject(o.A, o.B, o.C, e.D, c.E) 
   FROM Table1 o, Table2 i, Table4 c
   WHERE o.X = i.X AND i.Y = e.Y AND i.Z = c.Z"

Of course, your MyObject must have a matching constructor.

Answer 3

I faced the same issue after a lot of research found this working solution. For my HQL Query, I wanted to get the output of the POJO List, but I was getting a List of Object. This is how you can get the list of your POJO class.

Can achieve this in two ways:

Query query = em.createNativeQuery(sqlQuery, Picking.class);
OR
Query query = em.createQuery(sqlQuery);

Working Example of NativeQuery:

import javax.persistence.EntityManager;
import javax.persistence.PersistenceContext;
import javax.persistence.Query;

@PersistenceContext
protected EntityManager em;

private <T> List getPickingQuery(Map<String, String> searchFields, Map<String, String> orderByFields,
                                                      Pageable pageable, String status) {
    List resultList = new ArrayList();
    try {
        String sql = "select * from picking where status = '" + status + "'";
        String sqlQuery = createQuery(sql, searchFields, orderByFields);
        Query query = em.createNativeQuery(sqlQuery, Picking.class);

        setQueryParam(sqlQuery, searchFields);
        query.setMaxResults(pageable.getPageSize());
        query.setFirstResult((int) pageable.getOffset());
        log.info("Query " + query);
        resultList = query.getResultList();
    } catch (Exception e) {
        log.error("Error " + e.getMessage());
    }
    return resultList;
}

private String createQuery(String sql, Map<String, String> searchFields, Map<String, String> orderByFields) {
    for (String paramName : searchFields.keySet()) {
        sql = sql.concat(" and " + paramName + " = ?");
    }
    if (orderByFields != null) {
        if (orderByFields.containsKey("key") && orderByFields.containsKey("order"))
            sql = sql.concat(" order by " + orderByFields.get("key") + " " + orderByFields.get("order"));
        else if (orderByFields.containsKey("key"))
            sql = sql.concat(" order by " + orderByFields.get("key"));
    }
    return sql;
}

private void setQueryParam(String sqlQuery, Map<String, String> searchFields) {
    int param = 1;
    for (String paramName : searchFields.keySet()) {
        query.setParameter(param++, searchFields.get(paramName));
    }
}

You can also refer to this link for other cases: https://www.tabnine.com/code/java/methods/javax.persistence.Query/getResultList