I've setup a spring MVC application for a web application and I'm using Hibernates implementation of JPA 2.1.
I've created my models and am able to interact with the database just fine.
I've also decided to use service classes which will manage returning the entities. What I've done is created a BaseService class, so all other service classes will expand on this, and they'll have access to common functions such as create(), delete(), update() and list().
My problem is I'm unsure as to when I should be creating the EntityManager and when I should be closing it?
Currently, in my controller I'm initiating the required services when the controller loads;
#Controller
#RequestMapping("/mycontroller")
public class TestController {
CarService carService = new CarService();
ShowroomService showroomService = new ShowroomService();
}
}
Here is the BaseService that each other service extends;
public class Service<Ety> {
EntityManager em = null;
public Class<Ety> entityClass;
public Service(Class<Ety> entityClass) {
this.entityClass = entityClass;
em = JPAUtil.getEntityManager();
}
public Ety get(int id) {
Ety object = null;
em.getTransaction().begin();
object = em.find(entityClass, id);
em.getTransaction().commit();
return object;
}
public List list() {
List<Ety> objects;
em.getTransaction().begin();
objects = em.createQuery("SELECT c FROM "+entityClass.getName()+" c").getResultList();
em.getTransaction().commit();
return objects;
}
public void save(Ety object) {
em.getTransaction().begin();
em.persist(object);
em.getTransaction().commit();
}
public void update(Ety object) {
em.getTransaction().begin();
em.merge(object);
em.getTransaction().commit();
}
public void delete(Ety object) {
em.getTransaction().begin();
em.remove(object);
em.getTransaction().commit();
}
}
Here's an example Service which expands the above;
public class CarService extends Service<Car> {
public CarService() {
super(Car.class);
}
}
As you can see, I'm creating an EntityManager when the service is created, but at the moment I'm not closing it anywhere.
I'm I creating the entity manager in the correct place? when should I close it.
I had considered putting the entity manager in a static property and creating it within a filter, and then closing it at the end of the application, however I do believe this wouldn't be thread safe and would cause issues?
Any advice would be appreciated.
your CarService should be a spring bean and the instance is created from spring. NOT from your code. The same with the EntityManager. You can use the entityManager with the #autowired annotation.
You open a new EntityManager for each transaction.
This EntityManager is like a Bag mapped to the database, but with zero entity managed inside when it's just opened.
When you work with it, this Bag will be filled with some entities and Hibernate will work to create the adequate requests.
You will close this Bag to save memory at the end of the transaction.
Of course there is some tricks to have many transactions for a given EntityManager, but you have the most general idea. As always it depends...
If you use a framework like Spring or JavaEE, it will open and close the EntityManager, as well starting and committing transactions for you. You have only your business work to write.
Related
im just working on a project to create, change user in my mysql database. Therefore i have UserService (REST) which creates a user and a GenericDAO class where i can persist users. In my DAO for each user i begin, persist and commit a transaction. Creating single users or find users works perfect.
Now i am facing with the problem to persist or update a list of users. Especially if one user can not be persisted (e.g. duplicates) the hole transaction should be rolled back. It doesnt work in my current setup.
My first idea is to outsource the commit in a separate method. With an loop over all users i only persist them. At the end of the loop i would call my method to commit everything. If a single or more users fails i can catch them with the rollback. Is that a good approach?
AbstractDAO (current)
public abstract class GenericDAO<T> implements IGenericDAO<T>{
#PersistenceContext
protected EntityManager em = null;
private CriteriaBuilder cb = null;
private Class<T> clazz;
public GenericDAO(Class<T> class1) {
this.clazz = class1;
this.em = EntityManagerUtil.getEntityManager();
this.em.getCriteriaBuilder();
}
public final void setClazz(Class<T> clazzToSet) {
this.clazz = clazzToSet;
}
public T create(T entity) {
try {
em.getTransaction().begin();
em.persist(entity);
em.getTransaction().commit();
return entity;
} catch (PersistenceException e) {
em.getTransaction().rollback();
return null;
}
}
public T find(int id) {
return em.find(this.clazz, id);
}
public List<T> findAll() {
return em.createQuery("from "+this.clazz.getName()).getResultList();
}
/** Save changes made to a persistent object. */
public void update(T entity) {
em.getTransaction().begin();
em.merge(entity);
em.getTransaction().commit();
}
/** Remove an object from persistent storage in the database */
public void delete(T entity) {
em.getTransaction().begin();
em.remove(entity);
em.getTransaction().commit();
}
Wouldn't the most convenient solution be to simply add methods like createAll()/updateAll()?
Adding separate public methods for starting and persisting the transaction like start() and commit() creates a whole bunch of problems because it means you suddenly introduce a stateful conversation between the Dao and its clients.
The Dao methods now need to be called in a certain order and, worse still, the state of the EntityManager transaction is retained. If you forget to commit() at the end of one service call using your Dao, a subsequent call is going to mistakenly assume a transaction was not yet started, and that call is going to fail 'for no apparent reason' (not to mention that the original call will appear completed when in reality the transaction was left hanging). This creates bugs that are hard to debug, and tricky to recover from.
EDIT As I already pointed out in the comment below this answer, getting programmatic transaction management right is tricky in a multi-layer application structure, and so, I would recommend to have a look at declarative transaction management.
However, if you insist on managing transactions yourself, I would probably introduce sth like a TransactionTemplate:
public class TransactionTemplate {
private EntityManager em; //populated in a constructor, for instance
public void executeInTransaction(Runnable action) {
try {
em.getTransaction().begin();
action.run();
em.getTransaction().commit();
} catch (Exception e) {
em.getTransaction().rollback();
} finally {
em.clear(); // since you're using extended persistence context, you might want this line
}
}
}
and use it in a service like so:
public class UserService {
private TransactionTemplate template;
private RoleDao roleDao;
private UserDao userDao; //make sure TransactionTemplate and all Daos use the same EntityManager - for a single transaction, at least
public void saveUsers(Collection<User> users, String roleName) {
template.executeInTransaction(() -> {
Role role = roleDao.findByName(roleName);
users.forEach(user -> {
user.addRole(role);
userDao.create(user);
});
// some other operations
});
}
}
(of course, using the above approach means only one layer - the service layer in this case - is aware of transactions, and so DAOs must always be called from inside a service)
My application is crashing with the error below:
org.hibernate.HibernateException: Flush during cascade is dangerous
I am not flushing unless hibernate is doing it on my behalf.
Specs:
webapp on tomcat
hibernate/jpa for persistence (application managed
entity manager)
This is the code of my util class to manage entity manager:
private static EntityManagerFactory emFactory = Persistence.createEntityManagerFactory("returnit");
private static EntityManager entityManager;
public static EntityManager getEntityManager(){
return entityManager;
}
public static EntityManager initEntityManager(){
if (emFactory == null) {
emFactory = Persistence.createEntityManagerFactory( "returnit" );
}
entityManager = emFactory.createEntityManager();
return entityManager;
}
And this is the method that triggers the error:
#POST
#Consumes(MediaType.APPLICATION_JSON)
public Response post(#HeaderParam(HttpHeaders.AUTHORIZATION) String authHeader, MasterCrossDock mcd) {
EntityManager em = Utils.initEntityManager();
em.getTransaction().begin();
MasterCrossDockDAO.save(mcd);
em.getTransaction().commit();
em.close();
return Response.ok(mcd.getId()).build();
}
public static void save(MasterCrossDock new_mcd) {
List<Receptacle> receptacles = new_mcd.getReceptacles();
List<Long> ids = new ArrayList<Long>();
for (Receptacle r: receptacles) {
ids.add(r.getId());
}
new_mcd.getReceptacles().clear();
EntityManager em = Utils.getEntityManager();
new_mcd.getCountryDestination())
em.createQuery("UPDATE receptacle r"
+ " SET r.masterCrossDock.id = :mcd_id"
+ " WHERE r.id IN :ids")
.setParameter("ids", ids)
.setParameter("mcd_id", new_mcd.getId())
.executeUpdate();
new_mcd.getEreturns());
}
Why am I getting the error above and how to fix it?
Entity manager is not thread safe. Using EntityManager in container managed transatcions is fine, but here you are managing both the EntityManager and the transaction yourself. Also the entity manager is static so you are effectivly re-using it over the different requests you may get from the controller. Incoming call would execute the update query which would invoke a flush.
I noticed that during your initEntityManager you are swapping the static instance of the entityManager with a new one. What about the old reference that may be in use by another thread ?
Do the following:
Delete entirly your method initEntityManager
Delete private static EntityManager entityManager;
Make you method Utils.getEntityManager(); to aways create a new EntityManager
Alternative solution should be to make Spring or your container if you use container manage your transactions. Make a service, annotate it with #Transaction attribute and make Spring/Container inject the EntutyManager in it, or just use spring-data repositories.
The flush operation is called by the EntityTransaction Hibernate implementation, which might be JdbcResourceLocalTransactionCoordinatorImpl in your case, on commit.
Inside SessionImpl, this is what throws the HibernateException.
private void doFlush() {
checkTransactionNeeded();
checkTransactionSynchStatus();
try {
if ( persistenceContext.getCascadeLevel() > 0 ) {
throw new HibernateException( "Flush during cascade is dangerous" );
}
...
Maybe, and I say maybe, some other thread got a hold on the Session object and is operating on your entities.
Is it secure to pass a Injected EntityManager created on an EJB, to a method that will return an Object, and after, persist that Object on a Web Session for web clients use it?
Like in this example:
the EJB
#Stateless(mappedName = "MyService")
#LocalBean
public class MyService implements MyServiceLocal {
#PersistenceContext(unitName="primary")
private EntityManager em;
/**
* Default constructor.
*/
public MyService() {
}
#Override
public Service newServiceX(User user) {
return new ServiceX(user,em); // here, passing the EntityManager
}
}
After, I persist this Service in a web client (using struts):
The base action
public class YAction extends ActionSupport implements SessionAware{
#Inject
private MyServiceLocal service;
public String execute(){
Service x = service.newServiceX();
persistInCookie("ServiceX",x);
}
public void persistInCookie(String, Object){
// persist
}
}
And after, using another Action:
// another Action that
class XAction{
public String useService(){
getService().doSomething();
}
protected Service getService(){
Service service = (Service) getSessionMap().get("ServiceX");
return service;
}
}
the POJO class ServiceX using the EntityManager:
public class ServiceX extends Service{
EntityManager em;
public ServiceX(User user, EntityManager em){
this.em = em;
}
public void doSomething(){
// do something with the EntityManager passed by the EJB
}
}
First, the action that would be call is the Y action to persist the Service on the Session, next, the X action will return the Service persisted on the Session and try to use it.
I believe that the EJB Stateless Session Bean can close My EntityManager and this ServiceX POJO class can't use it. This can happen? I found similar question HERE, but in this question, the EntityManager is passed to a helper class. In my case is different because I want to persist this Object on a session cookie, and use later.
I don't think It is a good idea to store a EntityManager in SessionMap. What is more, I don't even think that it is a good idea to perform EntityManager operations outside the EJB container.
Have read about transaction-boundaries in JPA?
By default, EJB container is using CMT (Container Managed Transactions). In this case, container uses entitymanager-per-request pattern which means that the transaction begins and ends when one of the business methods of MyService starts and ends (transaction is committed or rollbacked in case of RuntimeException). For whole transaction time, EntityManager is connected with the same PersistenceContext. After the transaction is ended the container closes EntityManager which means that the EntityManager is disconnected with recent PersistenceContext:
// transaction begins
Service x = service.newServiceX();
// transaction ends
This might be crucial if you were going to do some update/insert operations outside the transaction.
Now, when you call EntityManager operation (like find) outside the transaction, for every each operation the EntityManager will create new PersistentContext. This may cause some issues, as two entities that represent the same record will be treated as different entities:
// each operation occurs in a separate persistence context, and returns
// a new detached instance
Magazine mag1 = em.find(Magazine.class, magId);
Magazine mag2 = em.find(Magazine.class, magId);
assertTrue(mag2 != mag1);
Some more articles to read:
Persistent Context
Transactions and Concurrency
Entity Lifecycle Management
What I want is to implement the Repository pattern in a JPA/Hibernate application. I have a generic interface that describes the basic contract of my repositories:
public interface EntityRepository<Entity extends Object, EntityId> {
Entity add(Entity entity);
Entity byId(EntityId id);
void remove(Entity entity);
void removeById(EntityId id);
void save();
List<Entity> toList();
}
And here is an implementation of such an interface:
public class EntityRepositoryHibernate<Entity extends Object, EntityId>
implements Serializable,
EntityRepository<Entity, EntityId> {
private static final long serialVersionUID = 1L;
#Inject
protected EntityManager entityManager;
protected Class<Entity> entityClass;
public EntityRepositoryHibernate(Class<Entity> entityClass) {
this.entityClass = entityClass;
}
public EntityManager getEntityManager() {
return entityManager;
}
#Override
public Entity add(Entity entity) {
entityManager.persist(entity);
return entity;
}
#SuppressWarnings("unchecked")
#Override
public Entity byId(EntityId id) {
DetachedCriteria criteria = criteriaDAO.createDetachedCriteria(entityClass);
criteria.add(Restrictions.eq("id", id));
return (Entity)criteriaDAO.executeCriteriaUniqueResult(criteria);
}
#Override
public void remove(Entity entity) {
if(entity==null)
return;
entityManager.remove(entity);
}
#Override
public void removeById(EntityId id) {
remove(byId(id));
}
#Override
public List<Entity> toList() {
throw new UnsupportedOperationException("toList() not implemented in "+entityClass.getName());
}
#Override
public void save() {
entityManager.flush();
}
}
All methods are working fine, except save(), so this is the focus here.
As far as I understand, Hibernate is able to track all changes in any instance returned by a query (the byId() method). So, the idea of the save() method is to save any instances that where retrieved and changed, that's why the method does not receives any parameters, it is supposed to save everything that has to be saved (which means, any persistent instance that was retrived and somehow updated while the repository lives.
In a possible scenario, I could call byId() 10 times to retrieve 10 different instances and change only 4 of them. The idea is that by calling save() once, those 4 instances would be saved in the data server.
Problem is when I call flush() I receive an exception stating that there is no transaction active. Since I'm using a JTA persistence unit, it's illegal to open the transation programatically by calling entityManager.getTransaction().
Considering that, what to do to fix the code?
First of all, it seems that your are missunderstanding the purpose of EntityManager.flush method. It doesn't commit any changes managed by persistence context, just sends SQL instructuions to the database. I mean, for the same JTA transaction, when you retrieve and modify some entity instances the changes/SQL instructions are cached waiting to be sent to the database. If the underlying transaction is commited this changes are flushed to the database along with the commit instruction. If you invoke flush before transaction is commited, only flush the changes until the invokation point (well, some SQL instruction could have been flushed previously by reasons out of this matter) but not the commit instruction is sent.
How to fixed?
I suggest you to don't mix Repository Pattern with Transaction manipulation.
Looks like you are using Container Managed Transactions (javaee tutorial) so just erase the save method and let container to manage the transactions. This will change your focus, you now have to care about rolling back transactions (throwing exception or invoking setRollbackOnly) but you don't need to explicit commmit.
The usual idiom I see for creating the EntityManager is something like this:
public class BaseDao {
private static final String PERSISTENCE_UNIT_NAME = "Employee";
EntityManagerFactory factory = Persistence.createEntityManagerFactory(PERSISTENCE_UNIT_NAME);
public EntityManager getEntityManager() {
return factory.createEntityManager();
}
}
Then it is used like this:
Employee emp = new Employee();
emp.setName("Joe M");
getEntityManager().persist(emp);
Question is why not do it this way:
public class BaseDao{
private static final String PERSISTENCE_UNIT_NAME = "Employee";
EntityManagerFactory factory = Persistence.createEntityManagerFactory(PERSISTENCE_UNIT_NAME);
private EntityManager entityManager = null;
public void setEntityManger() {
EntityManagerFactory factory = Persistence.createEntityManagerFactory(PERSISTENCE_UNIT_NAME);
this.entityManager = factory.createEntityManager();
}
public EntityManager getEntityManager() {
return this.entityManager;
}
}
In other words is there a need to always get the entity manager through factory.createEntityManager()? or can it be created as an instance (or even static) variable and retrieved like that?
To clarify, I am talking about an environment that doesn't use EJB or Spring containers.
Thanks.
There are two ways to create EntityManager instances.
One way is for SDK applications, and I use this way a lot in unit testing. This is what you have in your example:
EntityManagerFactory factory =
Persistence.createEntityManagerFactory(PERSISTENCE_UNIT_NAME);
In Enterprise applications you let the container create them for you and inject them when needed.
EntityManager is just a wrapper around a JDBC connection. It's very light weight and can be created and destroyed without performance penalty.
Keep in mind that the EntityManager is not thread safe, so if you have one instance, you may need to synchronize access to it. See transaction basics for details.
Here's how I would do it (roughly):
public class BaseDao{
private static final String PERSISTENCE_UNIT_NAME = "Employee";
private static EntityManagerFactory factory =
Persistence.createEntityManagerFactory(PERSISTENCE_UNIT_NAME);
public void create(MyEntiy person){
EntityManager em = factory.createEntityManager();
em.getTransaction().begin();
// do what ever you need
em.getTransaction().commit();
em.close();
}
// add more methods to the dao.
}
Once you get this protoyped and ready, you can use a generic DAO.
Today you should probably look at sometime like spring-data and #PersistanceUnit for managing your EntityManager.
An EntityManager is more than just a wrapper a wrapper for a JDBC connection. It defines the scope of a persistence context, which defines the unit of work that should be performed when a transaction is committed (of when you flush queries to the database). Within a persistence context you are also guaranteed that a given entity in the database will result in the same Java object, regardless if you load it directly, or access it through a OneToMany relation of another entity.
With regards to the original question about obtaining an EntityManagerFactory in a non-spring setting. You simply call
Persistence.createEntityManagerFactory(PERSISTENCE_UNIT_NAME);
This method is a static factory method, depending on your JPA implementation you either get the same instance for the same PU, or a shallow wrapper that wraps the underlying persistence session (of which there is one per PU).