Related
I have these 3 independent table i.e Student, Teacher and Subject. Independent here refers that there is no relation in these tables.
I want the count of all these tables . SQL query looks like -
SELECT
(SELECT COUNT(*) FROM Student as ST,
(SELECT COUNT(*) FROM Teacher as TE,
(SELECT COUNT(*) FROM Subject as SU
Now I want to map this result into dto .
The DTO looks like
public class CountDto{
Integer student;
Integer teacher;
Integer subject;
}
The repository call looks like -
#Query(value = "SELECT\r\n"
+ " (SELECT COUNT(*) FROM Student) as ST, \r\n"
+ " (SELECT COUNT(*) FROM Teacher) as TE,\r\n"
+ " (SELECT COUNT(*) FROM Subject) as SU", nativeQuery = true)
public CountDto getCount();
While calling this function I get following error stating
"message": "Failed to convert from type [java.lang.Object[]] to type [com.rbl.mdm.dto.CountDto ] for value '{16, 16 , 34}'; nested exception is org.springframework.core.convert.ConverterNotFoundException: No converter found capable of converting from type [java.lang.Integer] to type [com.rbl.mdm.dto.CountDto]"
How should I convert my response to desired DTO ?
You can declare CountDto as a public interface and it should work. It's called a Projection in terms of Spring. Or you can use SqlResultSetMapping or ConstructorResult along with your class.
you don't have to implement it by any entity class, just create it like an independent interface even within the repository file:
public interface StudentRepository extends CrudRepository<Student, Long> {
#Query(value = "SELECT\r\n"
+ " (SELECT COUNT(*) FROM Student) as ST, \r\n"
+ " (SELECT COUNT(*) FROM Teacher) as TE,\r\n"
+ " (SELECT COUNT(*) FROM Subject) as SU", nativeQuery = true)
Counts getCount();
public static interface Counts {
Integer getST();
Integer getTE();
Integer getSU();
}
}
So here is the answer to do -
The DTO will look like
public CountDto{
private Integer studentTotal;
private Integer teacherTota;
private Integer subjectTotal;
}
The repository call -
#Query(value = "SELECT\r\n"
+ " (SELECT COUNT(*) FROM Student) as ST, \r\n"
+ " (SELECT COUNT(*) FROM Teacher) as TE,\r\n"
+ " (SELECT COUNT(*) FROM Subject) as SU", nativeQuery = true)
public Map<String,Integer> getCount();
Finally the serviceImpl--
public CountDto getCount{
CountDto CountValue = new CountDto();
Map<String,Integer> map = repository.getCount();
for (Map.Entry<String,Integer> entry : map.entrySet()) {
if(entry.getKey().equals("ST"))
CountValue.setStudentTotal( entry.getValue());
if(entry.getKey().equals("TE"))
CountValue.setTeacherTotal( entry.getValue());
if(entry.getKey().equals("SU"))
CountValue.setSubjectTotal( entry.getValue());
}
return CountValue ;
}
But the solution seems quite complex to me. Any simpler approach required.
In pure Hibernate/JPA usage this is a simple dynamic-instantiation query (what JPA calls a "constructor result"):
select new CountDTO(
(SELECT COUNT(*) FROM Student) as ST,
...
)
No idea here about Spring, though a word of warning.. in trying to be useful, it often "gets in the way". Not sure that is the case here... Have you tried straight Hibernate/JPA?
I have a many to many relationship in spring boot with User and Role. I have three tables in database (Role_utilisateur, users_roles and utilisateurs). In my repository, ireturn List of my entite called RoleEntite which have two attributes (nom and id).
#Query(value = "SELECT ROLE_UTILISATEUR.NOM, ROLE_UTILISATEUR.ROLE_ID " + "FROM ROLE_UTILISATEUR "
+ "INNER JOIN USERS_ROLES on users_roles.role_id = ROLE_UTILISATEUR.ROLE_ID "
+ "INNER JOIN UTILISATEURS on utilisateurs.utilisateur_id = USERS_ROLES.UTILISATEUR_ID "
+ "WHERE UTILISATEURS.UTILISATEUR_ID = :userId", nativeQuery = true)
List<RoleEntite> findUsersRolesNative(#Param("userId") Long userId);
When i call my function in
private List<GrantedAuthority> getGrantedAuthorities(UserEntite user) {
List<GrantedAuthority> authorities = new ArrayList<GrantedAuthority>();
List<RoleEntite> roleList = userRepository.findUsersRolesNative(user.getId());
for (RoleEntite roleEntite : roleList) {
authorities.add(new SimpleGrantedAuthority(roleEntite.getNom()));
}
return authorities;
}
I get this error in my java eclipse
org.springframework.security.authentication.InternalAuthenticationServiceException: Failed to convert from type [java.lang.Object[]] to type [com.id.firstSpringBoot.entite.RoleEntite] for value '{ADMIN, 1}'; nested exception is org.springframework.core.convert.ConverterNotFoundException: No converter found capable of converting from type [java.lang.String] to type [com.id.firstSpringBoot.entite.RoleEntite]
How can i resolve this error and get the name ADMIN in my function getGrantedAuthorities can do this
The problem is you're selecting two columns and returning a list of entities. You need to either select the entity in your query or return the list of two values in a collection.
To return entities you would need to convert your query to JPQL. The following is a JPQL representation of your native query.
I've had to do some guessing for the joins (I assume you have some JPA related entities):
#Query(value = "SELECT RoleEntite r "
+ "INNER JOIN UserRole r.user u "
+ "INNER JOIN Utilisateurs u.Utilisateur x "
+ "WHERE x.UTILISATEUR_ID = :userId")
List<RoleEntite> findUsersRolesNative(#Param("userId") Long userId);
If you go the native query route:
#Query(value = "SELECT ROLE_UTILISATEUR.NOM, ROLE_UTILISATEUR.ROLE_ID " + "FROM ROLE_UTILISATEUR "
+ "INNER JOIN USERS_ROLES on users_roles.role_id = ROLE_UTILISATEUR.ROLE_ID "
+ "INNER JOIN UTILISATEURS on utilisateurs.utilisateur_id = USERS_ROLES.UTILISATEUR_ID "
+ "WHERE UTILISATEURS.UTILISATEUR_ID = :userId", nativeQuery = true)
List<Object[]> findUsersRolesNative(#Param("userId") Long userId);
The returned list should yield:
for (Object[] obj : list) {
nom = obj[0];
roleId = obj[1];
// ... do what ever you want with the values
}
I'm getting a warning in the Server log "firstResult/maxResults specified with collection fetch; applying in memory!". However everything working fine. But I don't want this warning.
My code is
public employee find(int id) {
return (employee) getEntityManager().createQuery(QUERY).setParameter("id", id).getSingleResult();
}
My query is
QUERY = "from employee as emp left join fetch emp.salary left join fetch emp.department where emp.id = :id"
Although you are getting valid results, the SQL query fetches all data and it's not as efficient as it should.
So, you have two options.
Fixing the issue with two SQL queries that can fetch entities in read-write mode
The easiest way to fix this issue is to execute two queries:
. The first query will fetch the root entity identifiers matching the provided filtering criteria.
. The second query will use the previously extracted root entity identifiers to fetch the parent and the child entities.
This approach is very easy to implement and looks as follows:
List<Long> postIds = entityManager
.createQuery(
"select p.id " +
"from Post p " +
"where p.title like :titlePattern " +
"order by p.createdOn", Long.class)
.setParameter(
"titlePattern",
"High-Performance Java Persistence %"
)
.setMaxResults(5)
.getResultList();
List<Post> posts = entityManager
.createQuery(
"select distinct p " +
"from Post p " +
"left join fetch p.comments " +
"where p.id in (:postIds) " +
"order by p.createdOn", Post.class)
.setParameter("postIds", postIds)
.setHint(
"hibernate.query.passDistinctThrough",
false
)
.getResultList();
Fixing the issue with one SQL query that can only fetch entities in read-only mode
The second approach is to use SDENSE_RANK over the result set of parent and child entities that match our filtering criteria and restrict the output for the first N post entries only.
The SQL query can look as follows:
#NamedNativeQuery(
name = "PostWithCommentByRank",
query =
"SELECT * " +
"FROM ( " +
" SELECT *, dense_rank() OVER (ORDER BY \"p.created_on\", \"p.id\") rank " +
" FROM ( " +
" SELECT p.id AS \"p.id\", " +
" p.created_on AS \"p.created_on\", " +
" p.title AS \"p.title\", " +
" pc.id as \"pc.id\", " +
" pc.created_on AS \"pc.created_on\", " +
" pc.review AS \"pc.review\", " +
" pc.post_id AS \"pc.post_id\" " +
" FROM post p " +
" LEFT JOIN post_comment pc ON p.id = pc.post_id " +
" WHERE p.title LIKE :titlePattern " +
" ORDER BY p.created_on " +
" ) p_pc " +
") p_pc_r " +
"WHERE p_pc_r.rank <= :rank ",
resultSetMapping = "PostWithCommentByRankMapping"
)
#SqlResultSetMapping(
name = "PostWithCommentByRankMapping",
entities = {
#EntityResult(
entityClass = Post.class,
fields = {
#FieldResult(name = "id", column = "p.id"),
#FieldResult(name = "createdOn", column = "p.created_on"),
#FieldResult(name = "title", column = "p.title"),
}
),
#EntityResult(
entityClass = PostComment.class,
fields = {
#FieldResult(name = "id", column = "pc.id"),
#FieldResult(name = "createdOn", column = "pc.created_on"),
#FieldResult(name = "review", column = "pc.review"),
#FieldResult(name = "post", column = "pc.post_id"),
}
)
}
)
The #NamedNativeQuery fetches all Post entities matching the provided title along with their associated PostComment child entities. The DENSE_RANK Window Function is used to assign the rank for each Post and PostComment joined record so that we can later filter just the amount of Post records we are interested in fetching.
The SqlResultSetMapping provides the mapping between the SQL-level column aliases and the JPA entity properties that need to be populated.
Now, we can execute the PostWithCommentByRank #NamedNativeQuery like this:
List<Post> posts = entityManager
.createNamedQuery("PostWithCommentByRank")
.setParameter(
"titlePattern",
"High-Performance Java Persistence %"
)
.setParameter(
"rank",
5
)
.unwrap(NativeQuery.class)
.setResultTransformer(
new DistinctPostResultTransformer(entityManager)
)
.getResultList();
Now, by default, a native SQL query like the PostWithCommentByRank one would fetch the Post and the PostComment in the same JDBC row, so we will end up with an Object[] containing both entities.
However, we want to transform the tabular Object[] array into a tree of parent-child entities, and for this reason, we need to use the Hibernate ResultTransformer.
The DistinctPostResultTransformer looks as follows:
public class DistinctPostResultTransformer
extends BasicTransformerAdapter {
private final EntityManager entityManager;
public DistinctPostResultTransformer(
EntityManager entityManager) {
this.entityManager = entityManager;
}
#Override
public List transformList(
List list) {
Map<Serializable, Identifiable> identifiableMap =
new LinkedHashMap<>(list.size());
for (Object entityArray : list) {
if (Object[].class.isAssignableFrom(entityArray.getClass())) {
Post post = null;
PostComment comment = null;
Object[] tuples = (Object[]) entityArray;
for (Object tuple : tuples) {
if(tuple instanceof Identifiable) {
entityManager.detach(tuple);
if (tuple instanceof Post) {
post = (Post) tuple;
}
else if (tuple instanceof PostComment) {
comment = (PostComment) tuple;
}
else {
throw new UnsupportedOperationException(
"Tuple " + tuple.getClass() + " is not supported!"
);
}
}
}
if (post != null) {
if (!identifiableMap.containsKey(post.getId())) {
identifiableMap.put(post.getId(), post);
post.setComments(new ArrayList<>());
}
if (comment != null) {
post.addComment(comment);
}
}
}
}
return new ArrayList<>(identifiableMap.values());
}
}
The DistinctPostResultTransformer must detach the entities being fetched because we are overwriting the child collection and we don’t want that to be propagated as an entity state transition:
post.setComments(new ArrayList<>());
Reason for this warning is that when fetch join is used, order in result sets is defined only by ID of selected entity (and not by join fetched).
If this sorting in memory is causing problems, do not use firsResult/maxResults with JOIN FETCH.
To avoid this WARNING you have to change the call getSingleResult to
getResultList().get(0)
This warning tells you Hibernate is performing in memory java pagination. This can cause high JVM memory consumption.
Since a developer can miss this warning, I contributed to Hibernate by adding a flag allowing to throw an exception instead of logging the warning (https://hibernate.atlassian.net/browse/HHH-9965).
The flag is hibernate.query.fail_on_pagination_over_collection_fetch.
I recommend everyone to enable it.
The flag is defined in org.hibernate.cfg.AvailableSettings :
/**
* Raises an exception when in-memory pagination over collection fetch is about to be performed.
* Disabled by default. Set to true to enable.
*
* #since 5.2.13
*/
String FAIL_ON_PAGINATION_OVER_COLLECTION_FETCH = "hibernate.query.fail_on_pagination_over_collection_fetch";
the problem is you will get cartesian product doing JOIN. The offset will cut your recordset without looking if you are still on same root identity class
I guess the emp has many departments which is a One to Many relationship. Hibernate will fetch many rows for this query with fetched department records. So the order of result set can not be decided until it has really fetch the results to the memory. So the pagination will be done in memory.
If you do not want to fetch the departments with emp, but still want to do some query based on the department, you can achieve the result with out warning (without doing ordering in the memory). For that simply you have to remove the "fetch" clause. So something like as follows:
QUERY = "from employee as emp left join emp.salary sal left join emp.department dep where emp.id = :id and dep.name = 'testing' and sal.salary > 5000 "
As others pointed out, you should generally avoid using "JOIN FETCH" and firstResult/maxResults together.
If your query requires it, you can use .stream() to eliminate warning and avoid potential OOM exception.
try (Stream<ENTITY> stream = em.createQuery(QUERY).stream()) {
ENTITY first = stream.findFirst().orElse(null); // equivalents .getSingleResult()
}
// Stream returned is an IO stream that needs to be closed manually.
I'm using Spring Data JPA, and when I use #Query to to define a query WITHOUT Pageable, it works:
public interface UrnMappingRepository extends JpaRepository<UrnMapping, Long> {
#Query(value = "select * from internal_uddi where urn like %?1% or contact like %?1%",
nativeQuery = true)
List<UrnMapping> fullTextSearch(String text);
}
But if I add the second param Pageable, the #Query will NOT work, and Spring will parse the method's name, then throw the exception No property full found. Is this a bug?
public interface UrnMappingRepository extends JpaRepository<UrnMapping, Long> {
#Query(value = "select * from internal_uddi where urn like %?1% or contact like %?1%",
nativeQuery = true)
Page<UrnMapping> fullTextSearch(String text, Pageable pageable);
}
You can use pagination with a native query. It is documented here: Spring Data JPA - Reference Documentation
"You can however use native queries for pagination by specifying the count query yourself:
Example 59. Declare native count queries for pagination at the query method using #Query"
public interface UserRepository extends JpaRepository<User, Long> {
#Query(value = "SELECT * FROM USERS WHERE LASTNAME = ?1",
countQuery = "SELECT count(*) FROM USERS WHERE LASTNAME = ?1",
nativeQuery = true)
Page<User> findByLastname(String lastname, Pageable pageable);
}
A similar question was asked on the Spring forums, where it was pointed out that to apply pagination, a second subquery must be derived. Because the subquery is referring to the same fields, you need to ensure that your query uses aliases for the entities/tables it refers to. This means that where you wrote:
select * from internal_uddi where urn like
You should instead have:
select * from internal_uddi iu where iu.urn like ...
Considering that the UrnMapping class is mapped to the internal_uddi table, I would suggest this:
#Repository
public interface UrnMappingRepository extends JpaRepository<UrnMapping, Long> {
#Query(value = "select iu from UrnMapping iu where iu.urn like %:text% or iu.contact like %:text%")
Page<UrnMapping> fullTextSearch(#Param("text") String text, Pageable pageable);
}
Please note that you might have to turn off native queries with dynamic requests.
With #Query , we can use pagination as well where you need to pass object of Pageable class at end of JPA method
For example:
Pageable pageableRequest = new PageRequest(page, size, Sort.Direction.DESC, rollNo);
Where,
page = index of page (index start from zero)
size = No. of records
Sort.Direction = Sorting as per rollNo
rollNo = Field in User class
UserRepository repo
repo.findByFirstname("John", pageableRequest);
public interface UserRepository extends JpaRepository<User, Long> {
#Query(value = "SELECT * FROM USER WHERE FIRSTNAME = :firstname)
Page<User> findByLastname(#Param("firstname") String firstname, Pageable pageable);
}
Please reference :Spring Data JPA #Query, if you are using Spring Data JPA version 2.0.4 and later. Sample like below:
#Query(value = "SELECT u FROM User u ORDER BY id")
Page<User> findAllUsersWithPagination(Pageable pageable);
Declare native count queries for pagination at the query method by using #Query
public interface UserRepository extends JpaRepository<User, Long> {
#Query(value = "SELECT * FROM USERS WHERE LASTNAME = ?1",
countQuery = "SELECT count(*) FROM USERS WHERE LASTNAME = ?1",
nativeQuery = true)
Page<User> findByLastname(String lastname, Pageable pageable);
}
Hope this helps
https://docs.spring.io/spring-data/jpa/docs/current/reference/html/#jpa.query-methods
Rewrite your query to:
select iu from internal_uddi iu where iu.urn....
description: http://forum.spring.io/forum/spring-projects/data/126415-is-it-possible-to-use-query-and-pageable?p=611398#post611398
I found it works different among different jpa versions, for debug, you'd better add this configurations to show generated sql, it will save your time a lot !
spring.jpa.show-sql=true
spring.jpa.properties.hibernate.format_sql=true
for spring boot 2.1.6.RELEASE, it works good!
Sort sort = new Sort(Sort.Direction.DESC, "column_name");
int pageNumber = 3, pageSize = 5;
Pageable pageable = PageRequest.of(pageNumber - 1, pageSize, sort);
#Query(value = "select * from integrity_score_view " +
"where (?1 is null or data_hour >= ?1 ) " +
"and (?2 is null or data_hour <= ?2 ) " +
"and (?3 is null or ?3 = '' or park_no = ?3 ) " +
"group by park_name, data_hour ",
countQuery = "select count(*) from integrity_score_view " +
"where (?1 is null or data_hour >= ?1 ) " +
"and (?2 is null or data_hour <= ?2 ) " +
"and (?3 is null or ?3 = '' or park_no = ?3 ) " +
"group by park_name, data_hour",
nativeQuery = true
)
Page<IntegrityScoreView> queryParkView(Date from, Date to, String parkNo, Pageable pageable);
you DO NOT write order by and limit, it generates the right sql
I had the same issue - without Pageable method works fine.
When added as method parameter - doesn't work.
After playing with DB console and native query support came up to decision that method works like it should. However, only for upper case letters.
Logic of my application was that all names of entity starts from upper case letters.
Playing a little bit with it. And discover that IgnoreCase at method name do the "magic" and here is working solution:
public interface EmployeeRepository
extends PagingAndSortingRepository<Employee, Integer> {
Page<Employee> findAllByNameIgnoreCaseStartsWith(String name, Pageable pageable);
}
Where entity looks like:
#Data
#Entity
#Table(name = "tblEmployees")
public class Employee {
#Id
#Column(name = "empID")
#GeneratedValue(strategy = GenerationType.IDENTITY)
private Integer id;
#NotEmpty
#Size(min = 2, max = 20)
#Column(name = "empName", length = 25)
private String name;
#Column(name = "empActive")
private Boolean active;
#ManyToOne
#JoinColumn(name = "emp_dpID")
private Department department;
}
When using nativeQuery that is having (nativeQuery = true), you may do the pagination yourself in the query by adding (LIMIT :sizeValue OFFSET :page)
Note:
Your page value passed to this method should be offset * size
Example
#Query(value = "SELECT * FROM person " +
"LIMIT ?1 OFFSET ?2", nativeQuery = true)
Optional<List<TDriverJob>> findPersons(int size, int page);
I tried all above solution and non worked , finally I removed the Sorting from Pagination and it worked
the following tutorial helped me
-> https://www.baeldung.com/spring-data-jpa-query
At this point 4.3. Spring Data JPA Versions Prior to 2.0.4
VERY IMPORTANT to add \ n-- #pageable \ n
Without this I was wrong
Also the pagination setting must be without ordering
PageRequest paginaConf = new PageRequest ((param1 - 1)
, param2);
Finally to convert the Page <Object []>
Page <Object []> list = myQueryofRepo ();
List <XXXModel> lstReturn = myConversor (list.getContent ());
Page <XXXModel> ret = new PageImpl <XXXModel> (lstReturn, pageConf, param2);
This bugged me for a while but I managed with a very smooth solution.
The challenge is JPA did not automatically detect the count query so I resolved to use the countName which according JPA docs Returns the name of the javax.persistence.NamedQuery to be used to execute count queries when pagination is used. Will default to the named query name configured suffixed by .count.
So I created a named query
#NamedNativeQuery(
name = "[queryName].count",
query = [your count query],
resultSetMapping = "[query resultSetMapping name]"
)
}
As indicated, the count query should be suffixed with .count
Count query returns Long so add the resultSetMapping as
#SqlResultSetMapping(
name="[query resultSetMapping name]",
columns={#ColumnResult(name="count", type = Long.class)})
Then in your repository, use the count query as indicated below
#Query(countName ="[queryName].count" , nativeQuery = true)
Page<Object> [mainQuery](...params)
Hope this helps!
I'm getting a warning in the Server log "firstResult/maxResults specified with collection fetch; applying in memory!". However everything working fine. But I don't want this warning.
My code is
public employee find(int id) {
return (employee) getEntityManager().createQuery(QUERY).setParameter("id", id).getSingleResult();
}
My query is
QUERY = "from employee as emp left join fetch emp.salary left join fetch emp.department where emp.id = :id"
Although you are getting valid results, the SQL query fetches all data and it's not as efficient as it should.
So, you have two options.
Fixing the issue with two SQL queries that can fetch entities in read-write mode
The easiest way to fix this issue is to execute two queries:
. The first query will fetch the root entity identifiers matching the provided filtering criteria.
. The second query will use the previously extracted root entity identifiers to fetch the parent and the child entities.
This approach is very easy to implement and looks as follows:
List<Long> postIds = entityManager
.createQuery(
"select p.id " +
"from Post p " +
"where p.title like :titlePattern " +
"order by p.createdOn", Long.class)
.setParameter(
"titlePattern",
"High-Performance Java Persistence %"
)
.setMaxResults(5)
.getResultList();
List<Post> posts = entityManager
.createQuery(
"select distinct p " +
"from Post p " +
"left join fetch p.comments " +
"where p.id in (:postIds) " +
"order by p.createdOn", Post.class)
.setParameter("postIds", postIds)
.setHint(
"hibernate.query.passDistinctThrough",
false
)
.getResultList();
Fixing the issue with one SQL query that can only fetch entities in read-only mode
The second approach is to use SDENSE_RANK over the result set of parent and child entities that match our filtering criteria and restrict the output for the first N post entries only.
The SQL query can look as follows:
#NamedNativeQuery(
name = "PostWithCommentByRank",
query =
"SELECT * " +
"FROM ( " +
" SELECT *, dense_rank() OVER (ORDER BY \"p.created_on\", \"p.id\") rank " +
" FROM ( " +
" SELECT p.id AS \"p.id\", " +
" p.created_on AS \"p.created_on\", " +
" p.title AS \"p.title\", " +
" pc.id as \"pc.id\", " +
" pc.created_on AS \"pc.created_on\", " +
" pc.review AS \"pc.review\", " +
" pc.post_id AS \"pc.post_id\" " +
" FROM post p " +
" LEFT JOIN post_comment pc ON p.id = pc.post_id " +
" WHERE p.title LIKE :titlePattern " +
" ORDER BY p.created_on " +
" ) p_pc " +
") p_pc_r " +
"WHERE p_pc_r.rank <= :rank ",
resultSetMapping = "PostWithCommentByRankMapping"
)
#SqlResultSetMapping(
name = "PostWithCommentByRankMapping",
entities = {
#EntityResult(
entityClass = Post.class,
fields = {
#FieldResult(name = "id", column = "p.id"),
#FieldResult(name = "createdOn", column = "p.created_on"),
#FieldResult(name = "title", column = "p.title"),
}
),
#EntityResult(
entityClass = PostComment.class,
fields = {
#FieldResult(name = "id", column = "pc.id"),
#FieldResult(name = "createdOn", column = "pc.created_on"),
#FieldResult(name = "review", column = "pc.review"),
#FieldResult(name = "post", column = "pc.post_id"),
}
)
}
)
The #NamedNativeQuery fetches all Post entities matching the provided title along with their associated PostComment child entities. The DENSE_RANK Window Function is used to assign the rank for each Post and PostComment joined record so that we can later filter just the amount of Post records we are interested in fetching.
The SqlResultSetMapping provides the mapping between the SQL-level column aliases and the JPA entity properties that need to be populated.
Now, we can execute the PostWithCommentByRank #NamedNativeQuery like this:
List<Post> posts = entityManager
.createNamedQuery("PostWithCommentByRank")
.setParameter(
"titlePattern",
"High-Performance Java Persistence %"
)
.setParameter(
"rank",
5
)
.unwrap(NativeQuery.class)
.setResultTransformer(
new DistinctPostResultTransformer(entityManager)
)
.getResultList();
Now, by default, a native SQL query like the PostWithCommentByRank one would fetch the Post and the PostComment in the same JDBC row, so we will end up with an Object[] containing both entities.
However, we want to transform the tabular Object[] array into a tree of parent-child entities, and for this reason, we need to use the Hibernate ResultTransformer.
The DistinctPostResultTransformer looks as follows:
public class DistinctPostResultTransformer
extends BasicTransformerAdapter {
private final EntityManager entityManager;
public DistinctPostResultTransformer(
EntityManager entityManager) {
this.entityManager = entityManager;
}
#Override
public List transformList(
List list) {
Map<Serializable, Identifiable> identifiableMap =
new LinkedHashMap<>(list.size());
for (Object entityArray : list) {
if (Object[].class.isAssignableFrom(entityArray.getClass())) {
Post post = null;
PostComment comment = null;
Object[] tuples = (Object[]) entityArray;
for (Object tuple : tuples) {
if(tuple instanceof Identifiable) {
entityManager.detach(tuple);
if (tuple instanceof Post) {
post = (Post) tuple;
}
else if (tuple instanceof PostComment) {
comment = (PostComment) tuple;
}
else {
throw new UnsupportedOperationException(
"Tuple " + tuple.getClass() + " is not supported!"
);
}
}
}
if (post != null) {
if (!identifiableMap.containsKey(post.getId())) {
identifiableMap.put(post.getId(), post);
post.setComments(new ArrayList<>());
}
if (comment != null) {
post.addComment(comment);
}
}
}
}
return new ArrayList<>(identifiableMap.values());
}
}
The DistinctPostResultTransformer must detach the entities being fetched because we are overwriting the child collection and we don’t want that to be propagated as an entity state transition:
post.setComments(new ArrayList<>());
Reason for this warning is that when fetch join is used, order in result sets is defined only by ID of selected entity (and not by join fetched).
If this sorting in memory is causing problems, do not use firsResult/maxResults with JOIN FETCH.
To avoid this WARNING you have to change the call getSingleResult to
getResultList().get(0)
This warning tells you Hibernate is performing in memory java pagination. This can cause high JVM memory consumption.
Since a developer can miss this warning, I contributed to Hibernate by adding a flag allowing to throw an exception instead of logging the warning (https://hibernate.atlassian.net/browse/HHH-9965).
The flag is hibernate.query.fail_on_pagination_over_collection_fetch.
I recommend everyone to enable it.
The flag is defined in org.hibernate.cfg.AvailableSettings :
/**
* Raises an exception when in-memory pagination over collection fetch is about to be performed.
* Disabled by default. Set to true to enable.
*
* #since 5.2.13
*/
String FAIL_ON_PAGINATION_OVER_COLLECTION_FETCH = "hibernate.query.fail_on_pagination_over_collection_fetch";
the problem is you will get cartesian product doing JOIN. The offset will cut your recordset without looking if you are still on same root identity class
I guess the emp has many departments which is a One to Many relationship. Hibernate will fetch many rows for this query with fetched department records. So the order of result set can not be decided until it has really fetch the results to the memory. So the pagination will be done in memory.
If you do not want to fetch the departments with emp, but still want to do some query based on the department, you can achieve the result with out warning (without doing ordering in the memory). For that simply you have to remove the "fetch" clause. So something like as follows:
QUERY = "from employee as emp left join emp.salary sal left join emp.department dep where emp.id = :id and dep.name = 'testing' and sal.salary > 5000 "
As others pointed out, you should generally avoid using "JOIN FETCH" and firstResult/maxResults together.
If your query requires it, you can use .stream() to eliminate warning and avoid potential OOM exception.
try (Stream<ENTITY> stream = em.createQuery(QUERY).stream()) {
ENTITY first = stream.findFirst().orElse(null); // equivalents .getSingleResult()
}
// Stream returned is an IO stream that needs to be closed manually.