休眠，获取，HQL和hashCode（）

Question

I have a HQL query something ala'

SELECT myclass 
FROM 
    MyClass myclass JOIN FETCH 
    myclass.anotherset sub JOIN FETCH 
    sub.yetanotherset
...

So, class MyClass has a property "anotherset" , which is a set containing instance of another class, lets call it MyClassTwo. And, class MyClassTwo has a property yetanotherset which is a set of a third type of class (with no further associations on it).

In this scenario, I'm having trouble with the hashCode implementation. Basically, the hashCode implementation of MyClassTwo, uses the "yetanotherset" property, and on the exact line it accesses the yetanothertest property, it fails with a LazyInitializationException.

org.hibernate.LazyInitializationException: illegal access to loading collection 

I'm guessing, this is because the data from "yetanotherset" hasn't been fetched yet, but how do I fix this? I don't particularly like the idea of dumbing down the hashCode to ignore the property.

Additional question, does HQL ignore fetch=FetchType.EAGER as defined in XML or annotations, it seems like it does. But I cant verify this anywhere.

Answer 1

Implementing hashCode() using a mutable field is a bad idea: it makes storing the entity in a HashSet and modifying the mutable property impossible.

Implementing it in terms of a collection of other entities is an even worse idea: it forces the loading of the collection to compute the hashCode.

Choose a unique, immutable property (or set of properties) in your entity, and implement hashCode based on that. On last resort, you have the option of using the ID, but if it's autogenerated, you must not put it in a Set before the ID is generated.

Answer 2

This is hibernate's most famous exception and it is exactly as you described it. The session has been disconnected, transaction closed, and you are attempting to access this collection. JOIN FETCH in your HQL should force EAGER loading to occur regardless of whether than annotation is present.

I suspect that your annotations are malformed, you have missing or out of date jars, or some other problem of that type.

Bump your Hibernate logging level up to generate the SQL hibernate.SQL=debug and investigate exactly what SQL is being executed up to where you see this exception. This should indicate to you whether your hibernate configuration is behaving the way you think its configured.

Post more of your code and the logs and someone might be able to help you spot the error.