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How to get a single instance for an associated read-only entity with JPA and Hibernate?

I have this two entities:
#Entity
#Immutable
#Cacheable
#Cache(region = "dosefrequency", usage = CacheConcurrencyStrategy.READ_ONLY)
public class DoseFrequency{
.....
}
#Entity
public class PrescriptionDrug {
.....
#ManyToOne(optional=false)
#JoinColumn(name="doseFrequency")
public DoseFrequency getDoseFrequency() {
return doseFrequency;
}
}
DoseFrequency is a read only entity, and PrescriptionDrug has associated one DoseFreqyency. I want to achive that each time I load one or many PrescriptionDrug the instances of DoseFrequency don't be repeated.
I know that the DoseFrequency instances will be cached in the first level cache of hibernate, but the load is done in several sessions (it is a web app). I used the second level cache, but that cache doesn't store instances only serializes the entity.
I get this behaivor working using Tuplizer in DoseFrequency, but I don't know if there is any other way to achive this.
#Entity
#Immutable
#Cacheable
#Cache(region = "dosefrequency", usage = CacheConcurrencyStrategy.READ_ONLY)
#Tuplizer(impl=DoseFrequencyTuplizer.class)
public class DoseFrequency {
....
}
public class DoseFrequencyTuplizer extends PojoEntityTuplizer {
public DoseFrequencyTuplizer(EntityMetamodel entityMetamodel, PersistentClass mappedEntity) {
super(entityMetamodel, mappedEntity);
}
#Override
protected Instantiator buildInstantiator(EntityMetamodel entityMetamodel, PersistentClass persistentClass) {
return new DoseFrequencyInstantiator(DoseFrequency.class.getName(),entityMetamodel,persistentClass,null);
}
}
public class DoseFrequencyInstantiator implements Instantiator {
private final Class targetClass;
protected PojoEntityInstantiator pojoEntityInstantiator;
public DoseFrequencyInstantiator(String targetClassName, EntityMetamodel entityMetamodel,
PersistentClass persistentClass, ReflectionOptimizer.InstantiationOptimizer optimizer) {
try {
this.targetClass = Class.forName(targetClassName);
} catch (ClassNotFoundException e) {
throw new HibernateException(e);
}
pojoEntityInstantiator = new PojoEntityInstantiator(entityMetamodel, persistentClass, optimizer);
}
#Override
public Object instantiate(Serializable id) {
DoseFrequency df = MedereEMRCache.instance.findDoseFrequencyByID(Long.valueOf(id.toString()));
if (df == null) {
return pojoEntityInstantiator.instantiate(id);
}
return df;
}
#Override
public Object instantiate() {
return instantiate(null);
}
#Override
public boolean isInstance(Object object) {
try {
return targetClass.isInstance(object);
} catch (Throwable t) {
throw new HibernateException("could not get handle to entity as interface : " + t);
}
}
}
I'm aware that the instances will be shared among all the threads of the application, but they are treated as read only, so they should not be modified.
Is this approach right?
Thank you

You can also use a LoadEventListener to always serve the same instance. Nevertheless, this functionality is not needed since the entity is immutable, so, even if you have multiple copies of it, it will still be immutable.
More, even if you implement a Singleton pattern, it will only be enforced per JVM, so I don't see why you'd want to implement this request.
Entities are meant to treated as singletons per EntityManager only.

Spring repository custom methods

I am trying to add some custom functionality to a spring data repository.
Using this as my starting point http://docs.spring.io/spring-data/jpa/docs/current/reference/html/#repositories.single-repository-behaviour I have created the following code:
public interface TableLock<T> {
void checkout(T entity);
void checkin(T entity, boolean cmpltBatch);
}
public interface BatchTableLock extends TableLock<MyEntity> {
}
public class BatchTableLockImpl implements BatchTableLock {
private static final Logger logger = LoggerFactory.getLogger(BatchTableLockImpl.class);
#PersistenceContext(unitName = "mysql")
private EntityManager em;
#Override
public void checkout(MyEntity batch) {
Long id = batch.getMyEntityId();
try {
MyEntity p = em.find(MyEntity.class, id, LockModeType.PESSIMISTIC_WRITE);
if (p == null) {
logger.error("checkout : MyEntity id {} must be valid", id);
throw new PessimisticLockException();
}
if (myCondition is true) {
return;
}
} catch (LockTimeoutException | PessimisticLockException e) {
logger.error("checkout : Unable to get write lock on MyEntity id {}", id, e);
}
throw new PessimisticLockException();
}
#Override
public void checkin(MyEntity batch, boolean cmplt) {
Long id = batch.getMyEntityId();
try {
MyEntity p = em.find(MyEntity.class, id, LockModeType.PESSIMISTIC_WRITE);
if (p == null) {
logger.error("complete : MyEntity id {} must be valid", id);
return;
}
if (this is true) {
if (cmplt) {
yep;
} else {
nope;
}
} else if (cmplt) {
logger.error("complete: Unable to complete MyEntity {} with status.", id);
}
} catch (LockTimeoutException | PessimisticLockException e) {
logger.error("complete : Unable to get write lock on MyEntity id {}", id, e);
}
}
}
#Repository
public interface MyDao extends CrudRepository<MyEntity, BigInteger>, BatchTableLock {
... My queries ...
}
unfortunately I am getting the following error:
org.springframework.data.mapping.PropertyReferenceException: No property checkin found for type MyEntity!
This if I'm not mistaken means that spring is trying to generate a query based on the method 'checkin' and it can't find a field in MyEntity with the name 'checkin'. which is correct there is no such field. how do I make it stop doing this? based on the link above I don't think it should be trying to generate a query for this method, but it seems to be doing it anyway. I may be missing something, that is usually the case, but I don't see what it is.

As stated in the reference documentation section you linked to, you need a MyDaoImpl class that implements the custom methods. I guess the easiest way is to either rename BatchTableLockImpl to that or just create an empty MyDaoImpl extending that class.

What Service should receives - a whole entity or an id?

Imagine that we have some service:
public class SomeService
{
public void someWorkByEntity(Entity entity)
{
int id = entity.getId();
// here is some work...
}
public void someWorkByEntityId(int id)
{
// here is some work...
}
}
In fact Service require only entity id.
But write two versions of each method it's too tedious.
So what is better: Service that receives a model or an id?

JPA and generics, why my class doesn't work?

In my Spring web app I'm using a generic dao class:
public abstract class GenericDaoImpl<T> implements GenericDao<T> {
#Override
public T create(final T t) {
this.getEntityManager().persist(t);
return t;
}
#Override
public void delete(final Object id) {
this.getEntityManager().remove(
this.getEntityManager().getReference(getEntityType(), id));
}
#Override
public T find(final Object id) {
return (T) this.getEntityManager().find(getEntityType(), id);
}
#Override
public T update(final T t) {
return this.getEntityManager().merge(t);
}
}
I implement this class for every entity in my model and it works perfectly. For example:
#Repository
public class GruppoDaoImpl extends GenericDaoImpl<Gruppo> implements GruppoDao {
}
I use these dao classes in my service layer. I have a service layer for every entity in my model, but methods for most of these classes, are the same, so I tried to create a generic service class that I can extend in the same way I do for the generic dao:
public abstract class GenericAdminServiceImpl<ENTITY extends AbstractEntity, DTO extends AbstractDto>
implements GenericAdminService<ENTITY, DTO> {
private GenericDao<ENTITY> dao;
private Class<ENTITY> entityClass;
private Class<DTO> dtoClass;
#SuppressWarnings({ "unchecked", "rawtypes" })
protected GenericAdminServiceImpl(GenericDao<ENTITY> dao) {
this.dao = dao;
//
Type t = getClass().getGenericSuperclass();
ParameterizedType pt = (ParameterizedType) t;
this.entityClass = (Class) pt.getActualTypeArguments()[0];
this.dtoClass = (Class) pt.getActualTypeArguments()[1];
}
public DTO getById(Object id) {
DTO dto = null;
ENTITY entity = dao.find(id);
if (entity != null) {
try {
dto = dtoClass.newInstance();
initDto(entity, dto);
} catch (Exception e) {
}
}
return dto;
}
public void create(DTO dto) throws ServiceOperationException {
ENTITY entity;
try {
entity = entityClass.newInstance();
initEntity(dto, entity);
Date dt = new Date();
entity.setDataUltimoAggiornamento(dt);
entity.setUtenteUltimoAggiornamento(dto.getLoggedUser());
entity.setDataInserimento(dt);
entity.setUtenteInserimento(dto.getLoggedUser());
dao.create(entity);
} catch (Exception e) {
throw new ServiceOperationException("impossibile creare entity ["
+ entityClass.getSimpleName() + "]", e);
}
}
public void update(DTO dto) throws ServiceOperationException {
ENTITY entity = dao.find(dto.getId());
if (!entityExists(entity)) {
throw new ServiceOperationException("entity non esistente ["
+ entityClass.getSimpleName() + "#" + dto.getId() + "]");
}
initEntity(dto, entity);
Date dt = new Date();
entity.setDataUltimoAggiornamento(dt);
entity.setUtenteUltimoAggiornamento(dto.getLoggedUser());
dao.update(entity);
}
public void delete(Object id) throws ServiceOperationException {
try {
dao.delete((int) id);
} catch (Exception e) {
throw new ServiceOperationException(
"impossibile eliminare entity ["
+ entityClass.getSimpleName() + "#" + id + "]", e); // TODO
}
}
protected abstract void initDto(ENTITY entity, DTO outDto);
protected abstract void initEntity(DTO dto, ENTITY outEntity);
protected abstract boolean entityExists(ENTITY entity);
}
Extending this class I just have to implement specific parts for every entity, leaving all the common stuff in the abstract/generic class.
The problem is that using the generic service, merge, persist and delete don't work. Only select seems to work and I cannot understand why...
When I run debug mode in Eclipse all seems correct. A consistent entity is passed to merge/persist methods, so why they don't work? can you help me?
UPDATE #1
This is an example of implementation:
#Service
#Transactional(propagation = Propagation.REQUIRES_NEW)
public class GruppoServiceImplG extends
GenericAdminServiceImpl<Gruppo, GruppoDto> implements GruppoServiceG {
#Autowired
protected GruppoServiceImplG(GruppoDao gruppoDao) {
super(gruppoDao);
}
#Override
protected void initDto(Gruppo entity, GruppoDto outDto) {
outDto.setId(entity.getId());
outDto.setNome(entity.getNome());
outDto.setDescrizione(entity.getDescrizione());
outDto.setDataInizioValidita(entity.getDataInizioValidita());
outDto.setDataFineValidita(entity.getDataFineValidita());
}
#Override
protected void initEntity(GruppoDto dto, Gruppo outEntity) {
outEntity.setId(dto.getId());
outEntity.setNome(dto.getNome());
outEntity.setDescrizione(dto.getDescrizione());
outEntity.setDataInizioValidita(dto.getDataInizioValidita());
outEntity.setDataFineValidita(dto.getDataFineValidita());
}
#Override
protected boolean entityExists(Gruppo entity) {
return entity != null && entity.getId() > 0;
}
}
UPDATE #2
Following Łukasz L. suggestion, I added to all my crud methods a flush(). Now I get this exception javax.persistence.TransactionRequiredException: no transaction is in progress. What's wrong with my transaction declaration? it works fine with non-generic serices...

If you read that question about Spring and Hibernate flush behaviour, it's not easy that commiting your transaction will make also the EntityManager to save all changes. Spring and JPA (Hibernate&CO) are designed to work quite nice (from the Spring side) but nevertheless, you must assert that your entity manager will write all queries to database before commiting transaction.
The problem: JPAs like to cache. It means, they tend to avoid issuing queries. If you do SELECT, they have no choice - they must fetch some data (as long as that data portion was not fetched - like when getting single entity by ID). By INSERTs and UPDATEs - well, they CAN cache. It means, that create, merge or remove will usually not issue a query to RDBMS until you call flush() on EntityManager.
If you leave transactional block without calling flush, and entity manager is delaying operations, you'll commit transactions, by which the modifying queries were not issued!
Just make sure to call EntityManager.flush() at least at the end of the transactional method. You can also call it after each DML operation, it's your choice (I prefer that way because it gives me full control in which order the DML queries are issued by JPA, if you heavily uses DB constraints/triggers, it can be essential).
#Transactional
public void myTransactionalMethod() {
getEntityManager().persist(a); // I've made some JPA change, that is not issued to DB yet
...
// I'm doing something more
...
getEntityManager().flush(); // the last moment to flush, after that instruction I leave transactional context
}

Following Łukasz L. suggestion I discovered the actual issue in my generic class.
Transaction declaration was wrong. I set #Transactional(propagation = Propagation.REQUIRES_NEW) only in concrete service class.
I solved this way:
#Transactional(propagation = Propagation.REQUIRES_NEW)
public abstract class GenericAdminServiceImpl<ENTITY extends AbstractEntity, DTO extends AbstractDto>
implements GenericAdminService<ENTITY, DTO> {
// ...
}
And (in concrete implementation):
#Service
#Transactional
public class GruppoServiceImplG extends
GenericAdminServiceImpl<Gruppo, GruppoDto> implements GruppoServiceG {
// ...
}

Making datastore and blobstore work transactional

I'm usnig Google App Engine with Java, and using Datastore via JPA.
I want to save profile image for each user:
#Entity
public class User implements Serializable {
#Id
private String userId;
private String imageKey; // saves BlobKey.getKeyString()
// ... other fields and getter/setter methods ...
}
If user wants to change their profile image, I (should) update imageKey field AND delete old image associated with old imageKey.
But, the document of Blobstore for Python says:
Deleting a Blobstore value occurs separately from datastore transactions. If you call this method during a datastore transaction, it takes effect immediately, regardless of whether the transaction commits successfully or is retried.
This seems that I can't make updating imageKey and deleting old image as one atomic action, and it also would affect to Java.
This is my attempt to do this work:
public class Engine implements ServletContextListener {
private EntityManagerFactory emf;
private BlobstoreService blobstoreService;
// Servlet will call getUser, modify returned object, and call updateUser with that object
public User getUser(final String userId) throws EngineException {
return doTransaction(new EngineFunc<User>() { // Utility method for create Entity manager and start transaction
#Override
public User run(EntityManager em) throws EngineException {
return em.find(User.class, key);
}
});
}
public void updateUser(final User user) throws EngineException {
doTransaction(new EngineFunc<Void>() {
#Override
public Void run(EntityManager em) throws EngineException {
em.merge(user);
return null;
}
});
user.purgeOldImages(blobstoreService);
}
// ... Other methods ...
}
public class User {
#Transient
private transient Set<String> oldImageList = new CopyOnWriteArraySet<>();
public void setImageKey(String imageKey) {
if (this.imageKey != null) {
oldImageList.add(this.imageKey);
}
this.imageKey = imageKey;
if (imageKey != null) {
oldImageList.remove(imageKey);
}
}
public void purgeOldImages(BlobstoreService blobService) {
Set<BlobKey> toDelete = new HashSet<>();
for (String s : oldImageList) {
toDelete.add(new BlobKey(s));
oldImageList.remove(s);
}
blobService.delete(toDelete.toArray(new BlobKey[0]));
}
// ... Other methods ...
}
I think this is neither "beautiful" nor correct code.
Is there the right way to do this?