使用entityManager.createNativeQuery（query，foo.class）

Question

I would like to return a List of Integers from a

javax.persistence.EntityManager.createNativeQuery call

Why is the following incorrect?

entityManager.createNativeQuery("Select P.AppID From P", Integer.class);

specifically why do I get "...Unknown entity: java.lang.Integer"

Would I have to create an entity class that has a single field that is an Integer ?

Thanks

Answer 1

What you do is called a projection . That's when you return only a scalar value that belongs to one entity. You can do this with JPA. See scalar value .

I think in this case, omitting the entity type altogether is possible:

   Query query = em.createNativeQuery(  "select id from users where username = ?");  
   query.setParameter(1, "lt");  
   BigDecimal val = (BigDecimal) query.getSingleResult(); 

Example taken from here .

Answer 2

That doesn't work because the second parameter should be a mapped entity and of course Integer is not a persistent class (since it doesn't have the @Entity annotation on it).

for you you should do the following:

Query q = em.createNativeQuery("select id from users where username = :username");
q.setParameter("username", "lt");
List<BigDecimal> values = q.getResultList();

or if you want to use HQL you can do something like this:

Query q = em.createQuery("select new Integer(id) from users where username = :username");
q.setParameter("username", "lt");
List<Integer> values = q.getResultList();

Regards.

Answer 3

Here is a DB2 Stored Procidure that receive a parameter

SQL

CREATE PROCEDURE getStateByName (IN StateName VARCHAR(128))
DYNAMIC RESULT SETS 1
P1: BEGIN
    -- Declare cursor
    DECLARE State_Cursor CURSOR WITH RETURN for
    -- #######################################################################
    -- # Replace the SQL statement with your statement.
    -- # Note: Be sure to end statements with the terminator character (usually ';')
    -- #
    -- # The example SQL statement SELECT NAME FROM SYSIBM.SYSTABLES
    -- # returns all names from SYSIBM.SYSTABLES.
    -- ######################################################################
    SELECT * FROM COUNTRY.STATE
    WHERE PROVINCE_NAME LIKE UPPER(stateName);
    -- Cursor left open for client application
    OPEN Province_Cursor;
END P1

Java

//Country is a db2 scheme

//Now here is a java Entity bean Method

public List<Province> getStateByName(String stateName) throws Exception {

    EntityManager em = this.em;
    List<State> states= null;
    try {
        Query query = em.createNativeQuery("call NGB.getStateByName(?1)", Province.class);
        query.setParameter(1, provinceName);
        states= (List<Province>) query.getResultList();
    } catch (Exception ex) {
        throw ex;
    }

    return states;
}

Answer 4

Suppose your query is " select id,name from users where rollNo = 1001 ".

Here query will return a object with id and name column. Your Response class is like bellow:

public class UserObject{
        int id;
        String name;
        String rollNo;

        public UserObject(Object[] columns) {
            this.id = (columns[0] != null)?((BigDecimal)columns[0]).intValue():0;
            this.name = (String) columns[1];
        }

        public int getId() {
            return id;
        }

        public void setId(int id) {
            this.id = id;
        }

        public String getName() {
            return name;
        }

        public void setName(String name) {
            this.name = name;
        }

        public String getRollNo() {
            return rollNo;
        }

        public void setRollNo(String rollNo) {
            this.rollNo = rollNo;
        }
    }

here UserObject constructor will get a Object Array and set data with object.

public UserObject(Object[] columns) {
            this.id = (columns[0] != null)?((BigDecimal)columns[0]).intValue():0;
            this.name = (String) columns[1];
        }

Your query executing function is like bellow :

public UserObject getUserByRoll(EntityManager entityManager,String rollNo) {

        String queryStr = "select id,name from users where rollNo = ?1";
        try {
            Query query = entityManager.createNativeQuery(queryStr);
            query.setParameter(1, rollNo);

            return new UserObject((Object[]) query.getSingleResult());
        } catch (Exception e) {
            e.printStackTrace();
            throw e;
        }
    }

Here you have to import bellow packages:

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

Now your main class, you have to call this function. First you have to get EntityManager and call this getUserByRoll(EntityManager entityManager,String rollNo) function. Calling procedure is given bellow:

@PersistenceContext
private EntityManager entityManager;

UserObject userObject = getUserByRoll(entityManager,"1001");

Now you have data in this userObject.

Here is Imports

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

Note:

query.getSingleResult() return a array. You have to maintain the column position and data type.

select id,name from users where rollNo = ?1 

query return a array and it's [0] --> id and [1] -> name .

For more info, visit this Answer

Thanks :)

Answer 5

JPA was designed to provide an automatic mapping between Objects and a relational database. Since Integer is not a persistant entity, why do you need to use JPA ? A simple JDBC request will work fine.