Solve Hibernate Lazy-Init issue with hibernate.enable_lazy_load_no_trans

Question

I have been suffering from infamous hibernate exception

org.hibernate.LazyInitializationException: could not initialize proxy - no Session 

Now the community is cheering over

<property name="hibernate.enable_lazy_load_no_trans" value="true"/>

saying it solves the problem but USE IT WITH CAUTION.

What they mean by use it with caution? What this property actually does?

Please give me any insights. Thanks in advance.

Answer 1

The problem with this approach is that you can have the N+1 effect.

Imagine that you have the following entity:

public class Person{     @OneToMany // default to lazy     private List<Order> orderList; } 

If you have a report that returns 10K of persons, and if in this report you execute the code person.getOrderList() the JPA/Hibernate will execute 10K of queries. This is the N+1 effect, you will have no control about all the queries that will be executed.

Imagine now that Order is like below:

public class Order{     @OneToMany // default to lazy     private List<EmailSent> emailSentList; } 

Imagine now that you have a iteration with the person.getOrderList() and for every Order orderyou will do a order.getEmailSentList(). Can you see the problem now?

For LazyInitializationException you can have some solutions:

• Use the OpenInSessionInView approach. You will need to create a WebFilter that will open and close the transaction. The problem with is the N+1 effect.
• Use the hibernate.enable_lazy_load_no_trans configuration, that is a hibernate and you will not be able to port your project to other JPA provider if needed. You also can have the N+1 effect.
• Use the EJB feature named PersistenceContext Extended. With this you will keep the context opened of several transactions. The problems are: N+1 effect can happen, use a lot of server memory (entities will stay managed)
• Use the FETCH in the query. With this approach you could do a JPQL/HQL like: select p from Person p join fetch p.orderList. With this query you will have your list loaded from the database and will not have the N+1 effect. The problem is that you will need to write a JPQL for each case.

If you still have any problem, check these links:

• Part 1
• Part 2

Answer 2

This goes against how we can take advantage of Hibernate's enforcement of repeatable read semantics with the Session concept. When an object is first loaded and if the object is referenced again within the life of the session, then the same object is returned IRRESPECTIVE of whether this object has changed in the DB. This is the repeatable read semantics provided automatically by hibernate.

With this setting, you have no session providing this guarantee, so if you now access this data you will be getting the latest version of the data.

This might be fine. But consider the scenario where this object is held in some place for a long time and the data has changed considerably, so that the lazily fetched data is much different that the data already loaded when the session was alive. This is what you need to be concerned about.

To put it simple you can safely use this setting if your program is not affected by: How stale the data that was already fetched when in session to the data that will be fetched lazily out of session

But if this (your program is exposed to timing issues, when it might work fine one time and fail another time) is a concern, then fetch all the necessary data while in session.