Hibernate: QueryException: could not instantiate: (updated)

I was doing some queries today in EJB-QL on JBossAS (which uses Hibernate as persistence provider).
And for a query that was nicely working in the Jopr Gui, I got errors in the test suite:


javax.ejb.EJBTransactionRolledbackException: java.lang.IllegalArgumentException: org.hibernate.QueryException: could not instantiate: com.acme.Foo
at ...... ( a lot ) ....
Caused by: java.lang.IllegalArgumentException: org.hibernate.QueryException: could not instantiate: com.acme.Foo


Ok nice - Hibernate can not instantiate my class because it can not instantiate my class. So what?

After looking some time at this and debugging around I found out that the issue was the following. But before I show you the code, let me assure that IntelliJ flagged it as valid and it did work in the UI.

But now the code:


SELECT new com.acme.FOO(res.name, res.id, ... , parent.name, parent.id)

FROM Resource res

LEFT OUTER JOIN res.parentResource parent

...

WHERE ...


The constructor of com.acme.Foo looked like this:


Foo(String res.name, int res.id, ... , String parentName, int parentId) {}


This is all valid,

but:

It can happen for Resources at the root of the hierarchy to not have a parent (when they are roots). And in this case, parent is null and Hibernate is looking for a constructor to pass NULL in as last argument, which does not fit for 'int'. Thus the failure to instantiate the class.

The correct constrctor looks like this:

Foo(String res.name, int res.id, ... , String parentName, Integer parentId) {}

(of course you need to update the respective property within Foo too).

Conclusion:
Just to get it straight: Hibernate is doing the right thing - it's only not easy to find out what is going wrong.