Skip to main content

Hibernate Objects,Sessions across threads - Illegal attempt to associate a collection with two open sessions

In the land of Hibernate a concept of Session is important.
it plays the role of tracking changes that happen to (hibernate objects) from the moment you open it.
and you can't actually do much without it, it's your entry to hibernate capabilities

so if you want to load an object you have to obtain a session instance first then load the object:

Session s = sessionFactory.getCurrentSession();
MyObject mo = s.byId(MyObject.class).load(123);

now that object (mo) is associated with that session
if you do something like

mo.setName('new Name');

the session will track that this object is dirty. and it does much more that you can read about.

What I want to get to is that in case you have to do some background work with your object you have to be careful that:
1- the session is not thread safe (i.e. it can't be passed between threads and assume no concurrency issues will happen)
2- the objects associated with one session cannot be associated with another open session

so if you try to do something like this:

// this is psudo code, just to get the point:\
MyObject mo = dao.load(123);
new Thread( () -> {

// Data access methods (usually in a dao)
MyObject load(int id) {
  return   sessionFactory.getCurrentSession().byId(MyObject.class).load(123);

void update(MyObject m) {
   Session s = sessionFactory.getCurrentSession();;

that won't work, and hibernate will complain :
Illegal attempt to associate a collection with two open sessions

because we passed the object between two threads while the first session could still be open when the second other thread starts.
and what makes it worse is that you probably won't see the exception message because the other thread won't stop your main thread execution and you will never know why the update failed.

so as a good practice is to have good error handling in background threads.

I had to learn this the hard way after debugging something similar where my update had to be done in background and spent few hours debugging a ghost issue without clue until I placed some std out /err good old print statements because debugging multiple threads can be hard when there is alot of moving parts.

1- don't pass hibernate object
2- pass the id of the object instead / or anything to reload it fast in the other thread, that way it will not be considered the same object.

// transaction A
MyObject mo = dao.load(123); <-- will work in session 1
//do stuff with mo

new Thread( () -> {
     // transaction B
     // reload the hibernate object and don't use the one from other thread
     MyObject mo2 = dao.load(123); <-- this will be associated with session 2


another solutions involve using evict(); and merge(); 

Cheers !

read more:


Popular posts from this blog

Spring 4 + Hibernate 4 / Java configuration / rest service example with gradle

In this post I'll explain the required work to create a rest API utilizing both spring and hibernate version 4, and the configuration will be using java configuration classes not XML. I'll use gradle to build and for dependency management, it's way easier than maven and keeps you focused on the application, if you are not familiar with gradle and interested in it see my previous post about it. The first part which is dependency management is covered in gradle post mentioned above. I'll skip to explain each tier of the project and its configurations: As you can see we have 4 tiers: 1) DAO tier / data tier In this tier we configure the datasource and hibernate, I used HSQL in memory db it can be easily substituted with other db engine providing the right dependencies The DaoConfig defines the data source, transaction manager, session factory and  hibernate properties The most important part is the annotations : 1)  @Configuration  : to tell spring th

Android RecyclerView - Adding Empty View

So RecyclerView was introduced to replace List view and it's optimized to reuse existing views and so it's faster and more efficient as stated in the documentation: While using it, I faced the issue of missing a useful functionality that is implemented in ListView. that feature is setting an empty view in case there was no records. In ListView it was as simple as this View emptyView = findViewById(; ListView  myList = .... myList.setEmptyView(emptyView); but this method doesn't exist for recycler view so we need a work around until android team fixes this. and here are the screen shots of both list view and recycler view fix List view : Recycler view : here is how I fixed it: here is the content of empty_view, it can be anything. Enjoy.

[PART 5] NuTracker ReactJS app - Add Login & Profile using Router

In the previous part we finished the dashboard read functionality, now we want to add the skeleton for other pages: - Login   In this page the user will be able to login to their account and the dashboard won't show unless the user is logged in. - Profile In this page the user will be able to update their daily nutrition goals that they can track in the dashboard. to be able to have multiple 'pages' in react and navigate from one to one, we need something that can switch the rendered content based on what we want, we can do that with if statements in the App components and store some location state, but why invent the wheel. React Router every major single page app web framework has the routing concept and functionality to interact with the usual browser urls and switch the content based what user should see. for example on the profile page I want the url path to be /profile, and for login to be /login and so on. in more advanced cases you want the users