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).
Related
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.
I am following Hibernate video lesson, and there were shown this code:
public class Main {
private static EntityManagerFactory entityManagerFactory;
public static void main(String[] args)
{
entityManagerFactory = Persistence.createEntityManagerFactory("org.hibernate.tutorial.jpa");
addEntities("Client1","Bank1");
entityManagerFactory.close();
}
private static void addEntities(String clientName, String BankName)
{
Client client = new Client();
client.setName(clientName);
Bank bank = new Bank();
bank.setName(BankName);
EntityManager entityManager = entityManagerFactory.createEntityManager();
entityManager.getTransaction().begin();
entityManager.flush();
entityManager.persist(client);
entityManager.persist(bank);
entityManager.getTransaction().commit();
}
}
And I am concerned about this part of code:
EntityManager entityManager = entityManagerFactory.createEntityManager();
entityManager.getTransaction().begin();
entityManager.flush();
We generated new EntityManager. As I understand, it has empty Persistence context, as it was just created, isn't it ?
In that case,why do we call flush() method. What is the purpose ?
EntityManager#flush actually pushes the changes to the database immediately.
In the above code, a transaction has just started entityManager.getTransaction().begin() and there is no change that needs to be pushed to the database so I would say it is not needed there. You may remove it.
Anyways, it is a good practice to let entitymanager take care of when to push the data changes to the database instead of manually taking control over it. There could be use case when different applications or threads are trying to access the same data at same time.
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.
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
Ok, I read bunch of articles/examples how to write Entity Manager Factory in singleton.
One of them easiest for me to understand a bit:
http://javanotepad.blogspot.com/2007/05/jpa-entitymanagerfactory-in-web.html
I learned that EntityManagerFactory (EMF) should only be created once preferably in application scope.
And also make sure to close the EMF once it's used (?)
So I wrote EMF helper class for business methods to use:
public class EmProvider {
private static final String DB_PU = "KogaAlphaPU";
public static final boolean DEBUG = true;
private static final EmProvider singleton = new EmProvider();
private EntityManagerFactory emf;
private EmProvider() {}
public static EmProvider getInstance() {
return singleton;
}
public EntityManagerFactory getEntityManagerFactory() {
if(emf == null) {
emf = Persistence.createEntityManagerFactory(DB_PU);
}
if(DEBUG) {
System.out.println("factory created on: " + new Date());
}
return emf;
}
public void closeEmf() {
if(emf.isOpen() || emf != null) {
emf.close();
}
emf = null;
if(DEBUG) {
System.out.println("EMF closed at: " + new Date());
}
}
}//end class
And my method using EmProvider:
public String foo() {
EntityManager em = null;
List<Object[]> out = null;
try {
em = EmProvider.getInstance().getEntityManagerFactory().createEntityManager();
Query query = em.createNativeQuery(JPQL_JOIN); //just some random query
out = query.getResultList();
}
catch(Exception e) {
//handle error....
}
finally {
if(em != null) {
em.close(); //make sure to close EntityManager
}
//should I not close the EMF itself here?????
EmProvider.getInstance().closeEmf();
}
I made sure to close EntityManager (em) within method level as suggested. But when should EntityManagerFactory be closed then? And why EMF has to be singleton so bad??? I read about concurrency issues but as I am not experienced multi-thread-grammer, I can't really be clear on this idea.
EntityManagerFactory instances are
heavyweight objects. Each factory
might maintain a metadata cache,
object state cache, EntityManager
pool, connection pool, and more. If
your application no longer needs an
EntityManagerFactory, you should
close it to free these resources.
When an EntityManagerFactory closes,
all EntityManagers from that factory,
and by extension all entities managed
by those EntityManagers, become
invalid.
It is much better to keep a factory
open for a long period of time than
to repeatedly create and close new
factories. Thus, most applications
will never close the factory, or only
close it when the application is
exiting.
Only applications that require
multiple factories with different
configurations have an obvious reason
to create and close multiple
EntityManagerFactory instances.
Only one EntityManagerFactory is
permitted to be created for each
deployed persistence unit
configuration. Any number of
EntityManager instances may be
created from a given factory.
More than one entity manager factory
instance may be available
simultaneously in the JVM. Methods of the EntityManagerFactory
interface are threadsafe.