Can a JPA Query return results as a Java Map?

We are currently building a Map manually based on the two fields that are returned by a named JPA query because JPA 2.1 only provides a getResultList() method:

@NamedQuery{name="myQuery",query="select c.name, c.number from Client c"}

HashMap<Long,String> myMap = new HashMap<Long,String>();

for(Client c: em.createNamedQuery("myQuery").getResultList() ){
     myMap.put(c.getNumber, c.getName);
}

But, I feel like a custom mapper or similar would be more performant since this list could easily be 30,000+ results.

Any ideas to build a Map without iterating manually.

(I am using OpenJPA, not hibernate)

Returning a Map result using JPA Query getResultStream

Since the JPA 2.2 version, you can use the getResultStream Query method to transform the List<Tuple> result into a Map<Integer, Integer>:

Map<Integer, Integer> postCountByYearMap = entityManager.createQuery("""
    select
       YEAR(p.createdOn) as year,
       count(p) as postCount
    from
       Post p
    group by
       YEAR(p.createdOn)
	""", Tuple.class)
.getResultStream()
.collect(
    Collectors.toMap(
        tuple -> ((Number) tuple.get("year")).intValue(),
        tuple -> ((Number) tuple.get("postCount")).intValue()
    )
);

Returning a Map result using JPA Query getResultList and Java stream

If you're using JPA 2.1 or older versions but your application is running on Java 8 or a newer version, then you can use getResultList and transform the List<Tuple> to a Java 8 stream:

Map<Integer, Integer> postCountByYearMap = entityManager.createQuery("""
    select
       YEAR(p.createdOn) as year,
       count(p) as postCount
    from
       Post p
    group by
       YEAR(p.createdOn)
	""", Tuple.class)
.getResultList()
.stream()
.collect(
    Collectors.toMap(
        tuple -> ((Number) tuple.get("year")).intValue(),
        tuple -> ((Number) tuple.get("postCount")).intValue()
    )
);

Returning a Map result using a Hibernate-specific ResultTransformer

Another option is to use the MapResultTransformer class provided by the Hibernate Types open-source project:

Map<Number, Number> postCountByYearMap = (Map<Number, Number>) entityManager.createQuery("""
    select
       YEAR(p.createdOn) as year,
       count(p) as postCount
    from
       Post p
    group by
       YEAR(p.createdOn)
	""")
.unwrap(org.hibernate.query.Query.class)
.setResultTransformer(
    new MapResultTransformer<Number, Number>()
)
.getSingleResult();

The MapResultTransformer is suitable for projects still running on Java 6 or using older Hibernate versions.

Avoid returning large result sets

The OP said:

> But, I feel like a custom mapper or similar would be more performant > since this list could easily be 30,000+ results.

This is a terrible idea. You never need to select 30k records. How would that fit in the UI? Or, why would you operate on such a large batch of records?

You should use query pagination as this will help you reduce the transaction response time and provide better concurrency.

There is no standard way to get JPA to return a map.

see related question: https://stackoverflow.com/questions/7595328/jpa-2-0-native-query-results-as-map

Iterating manually should be fine. The time to iterate a list/map in memory is going to be small relative to the time to execute/return the query results. I wouldn't try to futz with the JPA internals or customization unless there was conclusive evidence that manual iteration was not workable.

Also, if you have other places where you turn query result Lists into Maps, you probably want to refactor that into a utility method with a parameter to indicate the map key property.

You can retrieve a list of java.util.Map.Entry instead. Therefore the collection in your entity should be modeled as a Map:

@OneToMany
@MapKeyEnumerated(EnumType.STRING)
public Map<PhoneType, PhoneNumber> phones;

In the example PhoneType is a simple enum, PhoneNumber is an entity. In your query use the ENTRY keyword that was introduced in JPA 2.0 for map operations:

public List<Entry> getPersonPhones(){
	return em.createQuery("SELECT ENTRY(pn) FROM Person p JOIN p.phones pn",java.util.Map.Entry.class).getResultList();
}

You are now ready to retrieve the entries and start working with it:

List<java.util.Map.Entry> phoneEntries =   personDao.getPersonPhoneNumbers();
for (java.util.Map.Entry<PhoneType, PhoneNumber> entry: phoneEntries){
    //entry.key(), entry.value()
}

If you still need the entries in a map but don't want to iterate through your list of entries manually, have a look on this post https://stackoverflow.com/questions/16108734/convert-setmap-entryk-v-to-hashmapk-v which works with Java 8.

This works fine.
Repository code :

@Repository
public interface BookRepository extends CrudRepository<Book,Id> {

    @Query("SELECT b.name, b.author from Book b")
    List<Object[]> findBooks();
}

service.java

      List<Object[]> list = bookRepository.findBooks();
                for (Object[] ob : list){
                    String key = (String)ob[0];
                    String value = (String)ob[1];
}

link https://codereview.stackexchange.com/questions/1409/jpa-query-to-return-a-map

Map<String,Object> map = null;
    try {
        EntityManager entityManager = getEntityManager();
        Query query = entityManager.createNativeQuery(sql);
            query.setHint(QueryHints.RESULT_TYPE, ResultType.Map);
        map = (Map<String,Object>) query.getSingleResult();
    }catch (Exception e){ }

 List<Map<String,Object>> list = null;
    try {
        EntityManager entityManager = getEntityManager();
        Query query = entityManager.createNativeQuery(sql);
            query.setHint(QueryHints.RESULT_TYPE, ResultType.Map);
            list = query.getResultList();
    }catch (Exception e){  }

JPA v2.2

Though I am late here, but if someone reaches here for solution, here is my custom working solution for multiple selected columns with multiple rows:

Query query = this.entityManager.createNativeQuery("SELECT abc, xyz, pqr,...FROM...", Tuple.class);
.
.
.
List<Tuple> lst = query.getResultList();
List<Map<String, Object>> result = convertTuplesToMap(lst);

Implementation of convertTuplesToMap():

public static List<Map<String, Object>> convertTuplesToMap(List<Tuple> tuples) {
	List<Map<String, Object>> result = new ArrayList<>();
	for (Tuple single : tuples) {
		Map<String, Object> tempMap = new HashMap<>();
		for (TupleElement<?> key : single.getElements()) {
			tempMap.put(key.getAlias(), single.get(key));
		}
		result.add(tempMap);
	}
	return result;
}

> in case java 8 there built in entry "CustomEntryClass"

• since return is stream, then caller function (repoistory layer) must have @Transactional(readonly=true|false) annotation, otherwithe exception will be thrown

• make sure you will use full qualified name of class CustomEntryClass...

  > @Query("select new CustomEntryClass(config.propertyName, config.propertyValue) " + > "from ClientConfigBO config where config.clientCode =:clientCode ") > Stream> getByClientCodeMap(@Param("clientCode") String clientCode);

With custom result class and a bit of Guava, this is my approach which works quite well:

public static class SlugPair {
	String canonicalSlug;
	String slug;
	
	public SlugPair(String canonicalSlug, String slug) {
		super();
		this.canonicalSlug = canonicalSlug;
		this.slug = slug;
	}
	
}

...

final TypedQuery<SlugPair> query = em.createQuery(
    "SELECT NEW com.quikdo.core.impl.JpaPlaceRepository$SlugPair(e.canonicalSlug, e.slug) FROM "
      + entityClass.getName() + " e WHERE e.canonicalSlug IN :canonicalSlugs",
    SlugPair.class);

query.setParameter("canonicalSlugs", canonicalSlugs);

final Map<String, SlugPair> existingSlugs = 
    FluentIterable.from(query.getResultList()).uniqueIndex(
		new Function<SlugPair, String>() {
	@Override @Nullable
	public String apply(@Nullable SlugPair input) {
		return input.canonicalSlug;
	}
});

> using java 8 (+) you can get results as a list of array object (each column will from select will have same index on results array) by hibernate entity manger, and then from results list into stream, map results into entry (key, value), then collect them into map of same type.

 final String sql = "SELECT ID, MODE FROM MODES";
     List<Object[]> result = entityManager.createNativeQuery(sql).getResultList();
        return result.stream()
                .map(o -> new AbstractMap.SimpleEntry<>(Long.valueOf(o[0].toString()), String.valueOf(o[1])))
                .collect(Collectors.toMap(Map.Entry::getKey, Map.Entry::getValue));

Please refer, https://stackoverflow.com/questions/7595328/jpa-2-0-native-query-results-as-map/46190527#46190527

In your case in Postgres, it would be something like,

List<String> list = em.createNativeQuery("select cast(json_object_agg(c.number, c.name) as text) from schema.client c")
                   .getResultList();

//handle exception here, this is just sample
Map map = new ObjectMapper().readValue(list.get(0), Map.class);

Kindly note, I am just sharing my workaround with Postgres.

How about this ?

@NamedNativeQueries({
@NamedNativeQuery(
  name="myQuery",
  query="select c.name, c.number from Client c",
  resultClass=RegularClient.class
)
})

and

     public static List<RegularClient> runMyQuery() {
     return entityManager().createNamedQuery("myQuery").getResultList();
 }