Related
Would like to hear experts on best practice of editing JPA entities from JSF UI.
So, a couple of words about the problem.
Imagine I have the persisted object MyEntity and I fetch it for editing. In DAO layer I use
return em.find(MyEntity.class, id);
Which returns MyEntity instance with proxies on "parent" entities - imagine one of them is MyParent. MyParent is fetched as the proxy greeting to #Access(AccessType.PROPERTY):
#Entity
public class MyParent {
#Id
#Access(AccessType.PROPERTY)
private Long id;
//...
}
and MyEntity has the reference to it:
#ManyToOne(fetch = FetchType.LAZY)
#LazyToOne(LazyToOneOption.PROXY)
private MyParent myParent;
So far so good. In UI I simply use the fetched object directly without any value objects created and use the parent object in the select list:
<h:selectOneMenu value="#{myEntity.myParent.id}" id="office">
<f:selectItems value="#{parents}"/>
</h:selectOneMenu>
Everything is rendered ok, no LazyInitializationException occurs. But when I save the object I recieve the
LazyInitializationException: could not initialize proxy - no Session
on MyParent proxy setId() method.
I can easily fix the problem if I change the MyParent relation to EAGER
#ManyToOne(fetch = FetchType.EAGER)
private MyParent myParent;
or fetch the object using left join fetch p.myParent (actually that's how I do now). In this case the save operation works ok and the relation is changed to the new MyParent object transparently. No additional actions (manual copies, manual references settings) need to be done. Very simple and convenient.
BUT. If the object references 10 other object - the em.find() will result 10 additional joins, which isn't a good db operation, especially when I don't use references objects state at all. All I need - is links to objects, not their state.
This is a global issue, I would like to know, how JSF specialists deal with JPA entities in their applications, which is the best strategy to avoid both extra joins and LazyInitializationException.
Extended persistence context isn't ok for me.
Thanks!
You should provide exactly the model the view expects.
If the JPA entity happens to match exactly the needed model, then just use it right away.
If the JPA entity happens to have too few or too much properties, then use a DTO (subclass) and/or a constructor expression with a more specific JPQL query, if necessary with an explicit FETCH JOIN. Or perhaps with Hibernate specific fetch profiles, or EclipseLink specific attribute groups. Otherwise, it may either cause lazy initializtion exceptions over all place, or consume more memory than necessary.
The "open session in view" pattern is a poor design. You're basically keeping a single DB transaction open during the entire HTTP request-response processing. Control over whether to start a new DB transaction or not is completely taken away from you. You cannot spawn multiple transactions during the same HTTP request when the business logic requires so. Keep in mind that when a single query fails during a transaction, then the entire transaction is rolled back. See also When is it necessary or convenient to use Spring or EJB3 or all of them together?
In JSF perspective, the "open session in view" pattern also implies that it's possible to perform business logic while rendering the response. This doesn't go very well together with among others exception handling whereby the intent is to show a custom error page to the enduser. If a business exception is thrown halfway rendering the response, whereby the enduser has thus already received the response headers and a part of the HTML, then the server cannot clear out the response anymore in order to show a nice error page. Also, performing business logic in getter methods is a frowned upon practice in JSF as per Why JSF calls getters multiple times.
Just prepare exactly the model the view needs via usual service method calls in managed bean action/listener methods, before render response phase starts. For example, a common situation is having an existing (unmanaged) parent entity at hands with a lazy loaded one-to-many children property, and you'd like to render it in the current view via an ajax action, then you should just let the ajax listener method fetch and initialize it in the service layer.
<f:ajax listener="#{bean.showLazyChildren(parent)}" render="children" />
public void showLazyChildren(Parent parent) {
someParentService.fetchLazyChildren(parent);
}
public void fetchLazyChildren(Parent parent) {
parent.setLazyChildren(em.merge(parent).getLazyChildren()); // Becomes managed.
parent.getLazyChildren().size(); // Triggers lazy initialization.
}
Specifically in JSF UISelectMany components, there's another, completely unexpected, probable cause for a LazyInitializationException: during saving the selected items, JSF needs to recreate the underlying collection before filling it with the selected items, however if it happens to be a persistence layer specific lazy loaded collection implementation, then this exception will also be thrown. The solution is to explicitly set the collectionType attribute of the UISelectMany component to the desired "plain" type.
<h:selectManyCheckbox ... collectionType="java.util.ArrayList">
This is in detail asked and answered in org.hibernate.LazyInitializationException at com.sun.faces.renderkit.html_basic.MenuRenderer.convertSelectManyValuesForModel.
See also:
LazyInitializationException in selectManyCheckbox on #ManyToMany(fetch=LAZY)
What is lazy loading in Hibernate?
For Hibernate >= 4.1.6 read this https://stackoverflow.com/a/11913404/3252285
Using the OpenSessionInView Filter (Design pattern) is very usefull, but in my opinion it dosn't solve the problem completely, here's why :
If we have an Entity stored in Session or handled by a Session Bean or retrieved from the cache, and one of its collections has not been initialized during the same loading request, then we could get the Exception at any time we call it later, even if we use the OSIV desing pattern.
Lets detail the problem:
Any hibernate Proxy need to be attached to an Opened Session to works correctly.
Hibernate is not offering any tool (Listener or Handler) to reatach the proxy in case his session is closed or he's detached from its own session.
Why hibernate dosn't offer that ? :
because its not easy to identify to which Session, the Proxy should be reatached, but in many cases we could.
So how to reattach the proxy when the LazyInitializationException happens ?.
In my ERP, i modify thoses Classes : JavassistLazyInitializer and AbstractPersistentCollection, then i never care about this Exception any more (used since 3 years without any bug) :
class JavassistLazyInitializer{
#Override
public Object invoke(
final Object proxy,
final Method thisMethod,
final Method proceed,
final Object[] args) throws Throwable {
if ( this.constructed ) {
Object result;
try {
result = this.invoke( thisMethod, args, proxy );
}
catch ( Throwable t ) {
throw new Exception( t.getCause() );
}
if ( result == INVOKE_IMPLEMENTATION ) {
Object target = null;
try{
target = getImplementation();
}catch ( LazyInitializationException lze ) {
/* Catching the LazyInitException and reatach the proxy to the right Session */
EntityManager em = ContextConfig.getCurrent().getDAO(
BaseBean.getWcx(),
HibernateProxyHelper.getClassWithoutInitializingProxy(proxy)).
getEm();
((Session)em.getDelegate()).refresh(proxy);// attaching the proxy
}
try{
if (target==null)
target = getImplementation();
.....
}
....
}
and the
class AbstractPersistentCollection{
private <T> T withTemporarySessionIfNeeded(LazyInitializationWork<T> lazyInitializationWork) {
SessionImplementor originalSession = null;
boolean isTempSession = false;
boolean isJTA = false;
if ( session == null ) {
if ( allowLoadOutsideTransaction ) {
session = openTemporarySessionForLoading();
isTempSession = true;
}
else {
/* Let try to reatach the proxy to the right Session */
try{
session = ((SessionImplementor)ContextConfig.getCurrent().getDAO(
BaseBean.getWcx(), HibernateProxyHelper.getClassWithoutInitializingProxy(
owner)).getEm().getDelegate());
SessionFactoryImplementor impl = (SessionFactoryImplementor) ((SessionImpl)session).getSessionFactory();
((SessionImpl)session).getPersistenceContext().addUninitializedDetachedCollection(
impl.getCollectionPersister(role), this);
}catch(Exception e){
e.printStackTrace();
}
if (session==null)
throwLazyInitializationException( "could not initialize proxy - no Session" );
}
}
if (session==null)
throwLazyInitializationException( "could not initialize proxy - no Session" );
....
}
...
}
NB :
I didn't fix all the possiblities like JTA or other cases.
This solution works even better when you activate the cache
A very common approach is to create an open entity manager in view filter. Spring provides one (check here).
I can't see that you're using Spring, but that's not really a problem, you can adapt the code in that class for your needs. You can also check the filter Open Session in View, which does the same, but it keeps a hibernate session open rather than an Entity Manager.
This approach might not be good for your application, there're a few discussions in SO about this pattern or antipattern. Link1. I think that for most applications (smalish, less than 20 concurrent users) this solution works just fine.
Edit
There's a Spring class ties better with FSF here
There is no standard support for open session in view in EJB3, see this answer.
The fetch type of mappings is just a default option, i can be overriden at query time. This is an example:
select g from Group g fetch join g.students
So an alternative in plain EJB3 is to make sure that all the data necessary for rendering the view is loaded before the render starts, by explicitly querying for the needed data.
Lazy Loading is an important feature that can boost performance nicely. However the usability of this is way worse than it should be.
Especially when you start to deal with AJAX-Requests, encountering uninitialized collections, the Annotation ist just usefull to tell Hibernate don't load this right away. Hibernate is not taking care of anything else, but will throw a LazyInitializationException at you - as you experienced.
My solution to this - which might be not perfect or a nightmare over all - works in any scenario, by applying the following rules (I have to admit, that this was written at the very beginning, but works ever since):
Every Entity that is using fetch = FetchType.LAZY has to extend LazyEntity, and call initializeCollection() in the getter of the collection in question, before it is returned. (A custom validator is taking care of this constraints, reporting missing extensions and/or calls to initializeCollection)
Example-Class (User, which has groups loaded lazy):
public class User extends LazyEntity{
#OneToMany(mappedBy = "user", fetch = FetchType.LAZY)
#BatchSize(size = 5)
List<Group> groups;
public List<Group> getGroups(){
initializeCollection(this.groups);
return this.groups;
}
}
Where the implementation of initializeCollection(Collection collection) looks like the following. The In-Line comments should give you an idea of what is required for which scenario. The method is synchronized to avoid 2 active sessions transfering ownership of an entity while another session is currently fetching data. (Only appears when concurrent Ajax-Requests are going on on the same instance.)
public abstract class LazyEntity {
#SuppressWarnings("rawtypes")
protected synchronized void initializeCollection(Collection collection) {
if (collection instanceof AbstractPersistentCollection) {
//Already loaded?
if (!Hibernate.isInitialized(collection)) {
AbstractPersistentCollection ps = (AbstractPersistentCollection) collection;
//Is current Session closed? Then this is an ajax call, need new session!
//Else, Hibernate will know what to do.
if (ps.getSession() == null) {
//get an OPEN em. This needs to be handled according to your application.
EntityManager em = ContextHelper.getBean(ServiceProvider.class).getEntityManager();
//get any Session to obtain SessionFactory
Session anySession = em.unwrap(Session.class);
SessionFactory sf = anySession.getSessionFactory();
//get a new session
Session newSession = sf.openSession();
//move "this" to the new session.
newSession.update(this);
//let hibernate do its work on the current session.
Hibernate.initialize(collection);
//done, we can abandon the "new Session".
newSession.close();
}
}
}
}
}
But be aware, that this approach needs you to validate IF an Entity is associated to the CURRENT session, whenever you save it - else you have to move the whole Object-Tree to the current session again before calling merge().
Open Session in View design pattern can be easy implemented in Java EE environment (with no dependency to hibernate, spring or something else out side Java EE). It is mostly the same as in OpenSessionInView, but instead of Hibernate session you should use JTA transaction
#WebFilter(urlPatterns = {"*"})
public class JTAFilter implements Filter{
#Resource
private UserTransaction ut;
#Override
public void init(FilterConfig filterConfig) throws ServletException {
}
#Override
public void doFilter(ServletRequest request, ServletResponse response, FilterChain chain) throws IOException, ServletException {
try{
ut.begin();
chain.doFilter(request, response);
}catch(NotSupportedException | SystemException e){
throw new ServletException("", e);
} finally {
try {
if(ut.getStatus()!= Status.STATUS_MARKED_ROLLBACK){
ut.commit();
}
} catch (Exception e) {
throw new ServletException("", e);
}
}
}
#Override
public void destroy() {
}
}
I´m developing a Vaadin webapp using JPAContainer with hibernate 4.3.
I followed the instructions of this link and implemented EntityManager-per-Request pattern successfully. That means the lazy loading works well with JPAContainer
However, every time I need to use pure JPA, it fails to lazy loads.
Here is an example code:
try {
entityManagerProvider.getEntityManager().getTransaction().begin();
List<SubEntity> subEntitiesList = myEntity.getCollectionOfEntities(); //Fails to load
for (SubEntity subEntity : subEntitiesList) {
subEntity.doSomething();
entityManagerProvider.getEntityManager().merge(subEntity);
}
entityManagerProvider.getEntityManager().flush();
entityManagerProvider.getEntityManager().getTransaction().commit();
} catch (Exception ex) {
entityManagerProvider.getEntityManager().getTransaction().rollback();
}
//datasource is JPAContainer<MyEntity> type
datasource.removeItem(index);
I have tried this solutions:
Use hibernate.enable_lazy_load_no_trans
It opened an session automatically, but it gave a serious warning:
"Unable to close temporary session used to load lazy collection associated to no session"
Also, when I remove the Entity from the JPAContainer, I receive an error: org.hibernate.PersistentObjectException: detached entity passed to persist
Can someone give me some hints on how to deal with this problem?
I see two approaches.
Merge the changes of a detached entity to database
Just merge the entity and operate on the managed value.
Entity managed = em.merge(entity);
for (SubEntity sub : managed.getSubEntities()) {
sub.doSomething();
}
Resolve lazy proxies without touching the DB
Find the entity by ID and copy properties to the detached entity.
Entity managed = em.find(entity.getClass(), entity.getId());
entity.setSubEntities(managed.getSubEntities());
For the second option I wrote a generic utility method to initialize detached entities to some depth
that maybe you would find it useful.
// initialize lazy collections and collection elements
JpaUtils.initialize(em, entity, 2);
See JpaUtils for source code.
I'm reading some entities with Hibernate:
Criteria criteria = session.createCriteria(…);
List<Entity> list = (List<Entity>) criteria.list();
Now I'm iterating over this list and want to send every object inside a Runnable to an Executor. I therefore use a RunnableBean.
for (Entity entity : list) {
IRunnableBean runnableBean = (IRunnableBean)
applicationContext.getBean("myRunnableBean", IRunnableBean.class);
runnableBean.setEntity(entity);
executor.execute(runnableBean);
}
The RunnableBean looks like this:
RunnableBean implements IRunnableBean {
// Setter
#Transactional
void run() {
entity.getMyCollection();
}
}
When I'm accessing the collection, I'm getting a org.hibernate.LazyInitializationException (no session or session was closed).
In Spring's log I see that the transactional method run() is correctly added. What am I doing wrong?
I guess you are using Spring's OpenSessionInViewFilter. If so, this behaviour is expected. Filter puts the database connection in the thread local context which is not available in your RunnableBean.
As myCollection isn't loaded eagerly, Spring does not have access to the database connection inside RunnableBean and can't load it. You need to:
create an enclosing session wrapper in your RunnableBean;
pass the id of your collection to the RunnableBean instead of passing object and load the collection inside RunnableBean
Alternatively, you can make your entity to load myCollection eagerly but this will make the overall loading process slower.
Just add the following line within your already written for loop:
Hibernate.initialize(entity.getMyCollection());
This is load the collection eagerly instead of lazily: no LazyInitializationException anymore.
I would also guess (like #mindas) that the transaction is not available in your bean because it runs in a different thread than the one that holds the transaction. As far as my experience goes spring also uses thread locals to resolve scoped proxies, so these won't work either in a bean that is run asynchronously.
Basically I would try to avoid running logic that requires a transaction in an asynchronous fashion, since asynchronous calls run for a longer time (otherwise, why use async calls?) and this will block the transaction and/or lead to timeouts.
The criteria api from jpa offers ways to fetch a relation eagerly only for a specific query. Maybe that could be a choice? Otherwise accessing the size() method of a collection will initialize it.
I have a situation in which I need to re-attach detached objects to a hibernate session, although an object of the same identity MAY already exist in the session, which will cause errors.
Right now, I can do one of two things.
getHibernateTemplate().update( obj )
This works if and only if an object doesn't already exist in the hibernate session. Exceptions are thrown stating an object with the given identifier already exists in the session when I need it later.
getHibernateTemplate().merge( obj )
This works if and only if an object exists in the hibernate session. Exceptions are thrown when I need the object to be in a session later if I use this.
Given these two scenarios, how can I generically attach sessions to objects? I don't want to use exceptions to control the flow of this problem's solution, as there must be a more elegant solution...
So it seems that there is no way to reattach a stale detached entity in JPA.
merge() will push the stale state to the DB,
and overwrite any intervening updates.
refresh() cannot be called on a detached entity.
lock() cannot be called on a detached entity,
and even if it could, and it did reattach the entity,
calling 'lock' with argument 'LockMode.NONE'
implying that you are locking, but not locking,
is the most counterintuitive piece of API design I've ever seen.
So you are stuck.
There's an detach() method, but no attach() or reattach().
An obvious step in the object lifecycle is not available to you.
Judging by the number of similar questions about JPA,
it seems that even if JPA does claim to have a coherent model,
it most certainly does not match the mental model of most programmers,
who have been cursed to waste many hours trying understand
how to get JPA to do the simplest things, and end up with cache
management code all over their applications.
It seems the only way to do it is discard your stale detached entity
and do a find query with the same id, that will hit the L2 or the DB.
Mik
All of these answers miss an important distinction. update() is used to (re)attach your object graph to a Session. The objects you pass it are the ones that are made managed.
merge() is actually not a (re)attachment API. Notice merge() has a return value? That's because it returns you the managed graph, which may not be the graph you passed it. merge() is a JPA API and its behavior is governed by the JPA spec. If the object you pass in to merge() is already managed (already associated with the Session) then that's the graph Hibernate works with; the object passed in is the same object returned from merge(). If, however, the object you pass into merge() is detached, Hibernate creates a new object graph that is managed and it copies the state from your detached graph onto the new managed graph. Again, this is all dictated and governed by the JPA spec.
In terms of a generic strategy for "make sure this entity is managed, or make it managed", it kind of depends on if you want to account for not-yet-inserted data as well. Assuming you do, use something like
if ( session.contains( myEntity ) ) {
// nothing to do... myEntity is already associated with the session
}
else {
session.saveOrUpdate( myEntity );
}
Notice I used saveOrUpdate() rather than update(). If you do not want not-yet-inserted data handled here, use update() instead...
Entity states
JPA defines the following entity states:
New (Transient)
A newly created object that hasn’t ever been associated with a Hibernate Session (a.k.a Persistence Context) and is not mapped to any database table row is considered to be in the New (Transient) state.
To become persisted we need to either explicitly call the EntityManager#persist method or make use of the transitive persistence mechanism.
Persistent (Managed)
A persistent entity has been associated with a database table row and it’s being managed by the currently running Persistence Context. Any change made to such an entity is going to be detected and propagated to the database (during the Session flush-time).
With Hibernate, we no longer have to execute INSERT/UPDATE/DELETE statements. Hibernate employs a transactional write-behind working style and changes are synchronized at the very last responsible moment, during the current Session flush-time.
Detached
Once the currently running Persistence Context is closed all the previously managed entities become detached. Successive changes will no longer be tracked and no automatic database synchronization is going to happen.
Entity state transitions
You can change the entity state using various methods defined by the EntityManager interface.
To understand the JPA entity state transitions better, consider the following diagram:
When using JPA, to reassociate a detached entity to an active EntityManager, you can use the merge operation.
When using the native Hibernate API, apart from merge, you can reattach a detached entity to an active Hibernate Sessionusing the update methods, as demonstrated by the following diagram:
Merging a detached entity
The merge is going to copy the detached entity state (source) to a managed entity instance (destination).
Consider we have persisted the following Book entity, and now the entity is detached as the EntityManager that was used to persist the entity got closed:
Book _book = doInJPA(entityManager -> {
Book book = new Book()
.setIsbn("978-9730228236")
.setTitle("High-Performance Java Persistence")
.setAuthor("Vlad Mihalcea");
entityManager.persist(book);
return book;
});
While the entity is in the detached state, we modify it as follows:
_book.setTitle(
"High-Performance Java Persistence, 2nd edition"
);
Now, we want to propagate the changes to the database, so we can call the merge method:
doInJPA(entityManager -> {
Book book = entityManager.merge(_book);
LOGGER.info("Merging the Book entity");
assertFalse(book == _book);
});
And Hibernate is going to execute the following SQL statements:
SELECT
b.id,
b.author AS author2_0_,
b.isbn AS isbn3_0_,
b.title AS title4_0_
FROM
book b
WHERE
b.id = 1
-- Merging the Book entity
UPDATE
book
SET
author = 'Vlad Mihalcea',
isbn = '978-9730228236',
title = 'High-Performance Java Persistence, 2nd edition'
WHERE
id = 1
If the merging entity has no equivalent in the current EntityManager, a fresh entity snapshot will be fetched from the database.
Once there is a managed entity, JPA copies the state of the detached entity onto the one that is currently managed, and during the Persistence Context flush, an UPDATE will be generated if the dirty checking mechanism finds that the managed entity has changed.
So, when using merge, the detached object instance will continue to remain detached even after the merge operation.
Reattaching a detached entity
Hibernate, but not JPA supports reattaching through the update method.
A Hibernate Session can only associate one entity object for a given database row. This is because the Persistence Context acts as an in-memory cache (first level cache) and only one value (entity) is associated with a given key (entity type and database identifier).
An entity can be reattached only if there is no other JVM object (matching the same database row) already associated with the current Hibernate Session.
Considering we have persisted the Book entity and that we modified it when the Book entity was in the detached state:
Book _book = doInJPA(entityManager -> {
Book book = new Book()
.setIsbn("978-9730228236")
.setTitle("High-Performance Java Persistence")
.setAuthor("Vlad Mihalcea");
entityManager.persist(book);
return book;
});
_book.setTitle(
"High-Performance Java Persistence, 2nd edition"
);
We can reattach the detached entity like this:
doInJPA(entityManager -> {
Session session = entityManager.unwrap(Session.class);
session.update(_book);
LOGGER.info("Updating the Book entity");
});
And Hibernate will execute the following SQL statement:
-- Updating the Book entity
UPDATE
book
SET
author = 'Vlad Mihalcea',
isbn = '978-9730228236',
title = 'High-Performance Java Persistence, 2nd edition'
WHERE
id = 1
The update method requires you to unwrap the EntityManager to a Hibernate Session.
Unlike merge, the provided detached entity is going to be reassociated with the current Persistence Context and an UPDATE is scheduled during flush whether the entity has modified or not.
To prevent this, you can use the #SelectBeforeUpdate Hibernate annotation which will trigger a SELECT statement that fetched loaded state which is then used by the dirty checking mechanism.
#Entity(name = "Book")
#Table(name = "book")
#SelectBeforeUpdate
public class Book {
//Code omitted for brevity
}
Beware of the NonUniqueObjectException
One problem that can occur with update is if the Persistence Context already contains an entity reference with the same id and of the same type as in the following example:
Book _book = doInJPA(entityManager -> {
Book book = new Book()
.setIsbn("978-9730228236")
.setTitle("High-Performance Java Persistence")
.setAuthor("Vlad Mihalcea");
Session session = entityManager.unwrap(Session.class);
session.saveOrUpdate(book);
return book;
});
_book.setTitle(
"High-Performance Java Persistence, 2nd edition"
);
try {
doInJPA(entityManager -> {
Book book = entityManager.find(
Book.class,
_book.getId()
);
Session session = entityManager.unwrap(Session.class);
session.saveOrUpdate(_book);
});
} catch (NonUniqueObjectException e) {
LOGGER.error(
"The Persistence Context cannot hold " +
"two representations of the same entity",
e
);
}
Now, when executing the test case above, Hibernate is going to throw a NonUniqueObjectException because the second EntityManager already contains a Book entity with the same identifier as the one we pass to update, and the Persistence Context cannot hold two representations of the same entity.
org.hibernate.NonUniqueObjectException:
A different object with the same identifier value was already associated with the session : [com.vladmihalcea.book.hpjp.hibernate.pc.Book#1]
at org.hibernate.engine.internal.StatefulPersistenceContext.checkUniqueness(StatefulPersistenceContext.java:651)
at org.hibernate.event.internal.DefaultSaveOrUpdateEventListener.performUpdate(DefaultSaveOrUpdateEventListener.java:284)
at org.hibernate.event.internal.DefaultSaveOrUpdateEventListener.entityIsDetached(DefaultSaveOrUpdateEventListener.java:227)
at org.hibernate.event.internal.DefaultSaveOrUpdateEventListener.performSaveOrUpdate(DefaultSaveOrUpdateEventListener.java:92)
at org.hibernate.event.internal.DefaultSaveOrUpdateEventListener.onSaveOrUpdate(DefaultSaveOrUpdateEventListener.java:73)
at org.hibernate.internal.SessionImpl.fireSaveOrUpdate(SessionImpl.java:682)
at org.hibernate.internal.SessionImpl.saveOrUpdate(SessionImpl.java:674)
Conclusion
The merge method is to be preferred if you are using optimistic locking as it allows you to prevent lost updates.
The update is good for batch updates as it can prevent the additional SELECT statement generated by the merge operation, therefore reducing the batch update execution time.
Undiplomatic answer: You're probably looking for an extended persistence context. This is one of the main reasons behind the Seam Framework... If you're struggling to use Hibernate in Spring in particular, check out this piece of Seam's docs.
Diplomatic answer: This is described in the Hibernate docs. If you need more clarification, have a look at Section 9.3.2 of Java Persistence with Hibernate called "Working with Detached Objects." I'd strongly recommend you get this book if you're doing anything more than CRUD with Hibernate.
If you are sure that your entity has not been modified (or if you agree any modification will be lost), then you may reattach it to the session with lock.
session.lock(entity, LockMode.NONE);
It will lock nothing, but it will get the entity from the session cache or (if not found there) read it from the DB.
It's very useful to prevent LazyInitException when you are navigating relations from an "old" (from the HttpSession for example) entities. You first "re-attach" the entity.
Using get may also work, except when you get inheritance mapped (which will already throw an exception on the getId()).
entity = session.get(entity.getClass(), entity.getId());
I went back to the JavaDoc for org.hibernate.Session and found the following:
Transient instances may be made persistent by calling save(), persist() or
saveOrUpdate(). Persistent instances may be made transient by calling delete(). Any instance returned by a get() or load() method is persistent. Detached instances may be made persistent by calling update(), saveOrUpdate(), lock() or replicate(). The state of a transient or detached instance may also be made persistent as a new persistent instance by calling merge().
Thus update(), saveOrUpdate(), lock(), replicate() and merge() are the candidate options.
update(): Will throw an exception if there is a persistent instance with the same identifier.
saveOrUpdate(): Either save or update
lock(): Deprecated
replicate(): Persist the state of the given detached instance, reusing the current identifier value.
merge(): Returns a persistent object with the same identifier. The given instance does not become associated with the session.
Hence, lock() should not be used straightway and based on the functional requirement one or more of them can be chosen.
I did it that way in C# with NHibernate, but it should work the same way in Java:
public virtual void Attach()
{
if (!HibernateSessionManager.Instance.GetSession().Contains(this))
{
ISession session = HibernateSessionManager.Instance.GetSession();
using (ITransaction t = session.BeginTransaction())
{
session.Lock(this, NHibernate.LockMode.None);
t.Commit();
}
}
}
First Lock was called on every object because Contains was always false. The problem is that NHibernate compares objects by database id and type. Contains uses the equals method, which compares by reference if it's not overwritten. With that equals method it works without any Exceptions:
public override bool Equals(object obj)
{
if (this == obj) {
return true;
}
if (GetType() != obj.GetType()) {
return false;
}
if (Id != ((BaseObject)obj).Id)
{
return false;
}
return true;
}
Session.contains(Object obj) checks the reference and will not detect a different instance that represents the same row and is already attached to it.
Here my generic solution for Entities with an identifier property.
public static void update(final Session session, final Object entity)
{
// if the given instance is in session, nothing to do
if (session.contains(entity))
return;
// check if there is already a different attached instance representing the same row
final ClassMetadata classMetadata = session.getSessionFactory().getClassMetadata(entity.getClass());
final Serializable identifier = classMetadata.getIdentifier(entity, (SessionImplementor) session);
final Object sessionEntity = session.load(entity.getClass(), identifier);
// override changes, last call to update wins
if (sessionEntity != null)
session.evict(sessionEntity);
session.update(entity);
}
This is one of the few aspects of .Net EntityFramework I like, the different attach options regarding changed entities and their properties.
I came up with a solution to "refresh" an object from the persistence store that will account for other objects which may already be attached to the session:
public void refreshDetached(T entity, Long id)
{
// Check for any OTHER instances already attached to the session since
// refresh will not work if there are any.
T attached = (T) session.load(getPersistentClass(), id);
if (attached != entity)
{
session.evict(attached);
session.lock(entity, LockMode.NONE);
}
session.refresh(entity);
}
Sorry, cannot seem to add comments (yet?).
Using Hibernate 3.5.0-Final
Whereas the Session#lock method this deprecated, the javadoc does suggest using Session#buildLockRequest(LockOptions)#lock(entity)and if you make sure your associations have cascade=lock, the lazy-loading isn't an issue either.
So, my attach method looks a bit like
MyEntity attach(MyEntity entity) {
if(getSession().contains(entity)) return entity;
getSession().buildLockRequest(LockOptions.NONE).lock(entity);
return entity;
Initial tests suggest it works a treat.
Perhaps it behaves slightly different on Eclipselink. To re-attach detached objects without getting stale data, I usually do:
Object obj = em.find(obj.getClass(), id);
and as an optional a second step (to get caches invalidated):
em.refresh(obj)
try getHibernateTemplate().replicate(entity,ReplicationMode.LATEST_VERSION)
In the original post, there are two methods, update(obj) and merge(obj) that are mentioned to work, but in opposite circumstances. If this is really true, then why not test to see if the object is already in the session first, and then call update(obj) if it is, otherwise call merge(obj).
The test for existence in the session is session.contains(obj). Therefore, I would think the following pseudo-code would work:
if (session.contains(obj))
{
session.update(obj);
}
else
{
session.merge(obj);
}
to reattach this object, you must use merge();
this methode accept in parameter your entity detached and return an entity will be attached and reloaded from Database.
Example :
Lot objAttach = em.merge(oldObjDetached);
objAttach.setEtat(...);
em.persist(objAttach);
calling first merge() (to update persistent instance), then lock(LockMode.NONE) (to attach the current instance, not the one returned by merge()) seems to work for some use cases.
Property hibernate.allow_refresh_detached_entity did the trick for me. But it is a general rule, so it is not very suitable if you want to do it only in some cases. I hope it helps.
Tested on Hibernate 5.4.9
SessionFactoryOptionsBuilder
try getHibernateTemplate().saveOrUpdate()
What would be the easiest way to detach a specific JPA Entity Bean that was acquired through an EntityManager. Alternatively, could I have a query return detached objects in the first place so they would essentially act as 'read only'?
The reason why I want to do this is becuase I want to modify the data within the bean - with in my application only, but not ever have it persisted to the database. In my program, I eventually have to call flush() on the EntityManager, which would persist all changes from attached entities to the underyling database, but I want to exclude specific objects.
(may be too late to answer, but can be useful for others)
I'm developing my first system with JPA right now. Unfortunately I'm faced with this problem when this system is almost complete.
Simply put. Use Hibernate, or wait for JPA 2.0.
In Hibernate, you can use 'session.evict(object)' to remove one object from session. In JPA 2.0, in draft right now, there is the 'EntityManager.detach(object)' method to detach one object from persistence context.
No matter which JPA implementation you use, Just use entityManager.detach(object) it's now in JPA 2.0 and part of JEE6.
If you need to detach an object from the EntityManager and you are using Hibernate as your underlying ORM layer you can get access to the Hibernate Session object and use the Session.evict(Object) method that Mauricio Kanada mentioned above.
public void detach(Object entity) {
org.hibernate.Session session = (Session) entityManager.getDelegate();
session.evict(entity);
}
Of course this would break if you switched to another ORM provider but I think this is preferably to trying to make a deep copy.
Unfortunately, there's no way to disconnect one object from the entity manager in the current JPA implementation, AFAIR.
EntityManager.clear() will disconnect all the JPA objects, so that might not be an appropriate solution in all the cases, if you have other objects you do plan to keep connected.
So your best bet would be to clone the objects and pass the clones to the code that changes the objects. Since primitive and immutable object fields are taken care of by the default cloning mechanism in a proper way, you won't have to write a lot of plumbing code (apart from deep cloning any aggregated structures you might have).
As far as I know, the only direct ways to do it are:
Commit the txn - Probably not a reasonable option
Clear the Persistence Context - EntityManager.clear() - This is brutal, but would clear it out
Copy the object - Most of the time your JPA objects are serializable, so this should be easy (if not particularly efficient).
If using EclipseLink you also have the options,
Use the Query hint, eclipselink.maintain-cache"="false - all returned objects will be detached.
Use the EclipseLink JpaEntityManager copy() API to copy the object to the desired depth.
If there aren't too many properties in the bean, you might just create a new instance and set all of its properties manually from the persisted bean.
This could be implemented as a copy constructor, for example:
public Thing(Thing oldBean) {
this.setPropertyOne(oldBean.getPropertyOne());
// and so on
}
Then:
Thing newBean = new Thing(oldBean);
this is quick and dirty, but you can also serialize and deserialize the object.
Since I am using SEAM and JPA 1.0 and my system has a fuctinality that needs to log all fields changes, i have created an value object or data transfer object if same fields of the entity that needs to be logged. The constructor of the new pojo is:
public DocumentoAntigoDTO(Documento documentoAtual) {
Method[] metodosDocumento = Documento.class.getMethods();
for(Method metodo:metodosDocumento){
if(metodo.getName().contains("get")){
try {
Object resultadoInvoke = metodo.invoke(documentoAtual,null);
Method[] metodosDocumentoAntigo = DocumentoAntigoDTO.class.getMethods();
for(Method metodoAntigo : metodosDocumentoAntigo){
String metodSetName = "set" + metodo.getName().substring(3);
if(metodoAntigo.getName().equals(metodSetName)){
metodoAntigo.invoke(this, resultadoInvoke);
}
}
} catch (IllegalArgumentException e) {
e.printStackTrace();
} catch (IllegalAccessException e) {
e.printStackTrace();
} catch (InvocationTargetException e) {
e.printStackTrace();
}
}
}
}
In JPA 1.0 (tested using EclipseLink) you could retrieve the entity outside of a transaction. For example, with container managed transactions you could do:
public MyEntity myMethod(long id) {
final MyEntity myEntity = retrieve(id);
// myEntity is detached here
}
#TransactionAttribute(TransactionAttributeType.NOT_SUPPORTED)
public MyEntity retrieve(long id) {
return entityManager.find(MyEntity.class, id);
}
Do deal with a similar case I have created a DTO object that extends the persistent entity object as follows:
class MyEntity
{
public static class MyEntityDO extends MyEntity {}
}
Finally, an scalar query will retrieve the desired non managed attributes:
(Hibernate) select p.id, p.name from MyEntity P
(JPA) select new MyEntity(p.id, p.name) from myEntity P
If you get here because you actually want to pass an entity across a remote boundary then you just put some code in to fool the hibernazi.
for(RssItem i : result.getChannel().getItem()){
}
Cloneable wont work because it actually copies the PersistantBag across.
And forget about using serializable and bytearray streams and piped streams. creating threads to avoid deadlocks kills the entire concept.
I think there is a way to evict a single entity from EntityManager by calling this
EntityManagerFactory emf;
emf.getCache().evict(Entity);
This will remove particular entity from cache.
Im using entityManager.detach(returnObject);
which worked for me.
I think you can also use method EntityManager.refresh(Object o) if primary key of the entity has not been changed. This method will restore original state of the entity.