Proxy instantiation problem from Hibernate Session load

This one had me puzzled for a while yesterday. I was able to retrieve objects from a Session via a query, but I was getting the following error when I was using the load method:

org.hibernate.HibernateException: Javassist Enhancement failed: Thing
        at org.hibernate.proxy.pojo.javassist.JavassistLazyInitializer.getProxy(JavassistLazyInitializer.java:142)
        at org.hibernate.proxy.pojo.javassist.JavassistProxyFactory.getProxy(JavassistProxyFactory.java:72)
        at org.hibernate.tuple.entity.AbstractEntityTuplizer.createProxy(AbstractEntityTuplizer.java:402)
        at org.hibernate.persister.entity.AbstractEntityPersister.createProxy(AbstractEntityPersister.java:3483)
        at org.hibernate.event.def.DefaultLoadEventListener.createProxyIfNecessary(DefaultLoadEventListener.java:298)
        at org.hibernate.event.def.DefaultLoadEventListener.proxyOrLoad(DefaultLoadEventListener.java:219)
        at org.hibernate.event.def.DefaultLoadEventListener.onLoad(DefaultLoadEventListener.java:126)
        at org.hibernate.impl.SessionImpl.fireLoad(SessionImpl.java:905)
        at org.hibernate.impl.SessionImpl.load(SessionImpl.java:822)
        at org.hibernate.impl.SessionImpl.load(SessionImpl.java:815)
...
Caused by: java.lang.InstantiationException: Thing_$$_javassist_0
        at java.lang.Class.newInstance0(Class.java:335)
        at java.lang.Class.newInstance(Class.java:303)
        at org.hibernate.proxy.pojo.javassist.JavassistLazyInitializer.getProxy(JavassistLazyInitializer.java:139)
        ... 49 more

When I changed the code from

  session.load(classOf[Thing], id).asInstanceOf[Thing]

to

  session.createQuery("from Thing where id = " + id).list().asInstanceOf[java.util.List[Thing]].toList(1)

everything worked as expected.

I found a few results from the web, but nothing to indicate what my specific problem was.

The code I was working on was loosely based on another Scala project, which uses an older version of Hibernate. I changed the libraries over, and got a similar problem with CGLIB:

org.hibernate.HibernateException: CGLIB Enhancement failed: Thing
        at org.hibernate.proxy.pojo.cglib.CGLIBLazyInitializer.getProxy(CGLIBLazyInitializer.java:96)
        at org.hibernate.proxy.pojo.cglib.CGLIBProxyFactory.getProxy(CGLIBProxyFactory.java:49)
        at org.hibernate.tuple.entity.AbstractEntityTuplizer.createProxy(AbstractEntityTuplizer.java:379)
        at org.hibernate.persister.entity.AbstractEntityPersister.createProxy(AbstractEntityPersister.java:3455)
        at org.hibernate.event.def.DefaultLoadEventListener.createProxyIfNecessary(DefaultLoadEventListener.java:257)
        at org.hibernate.event.def.DefaultLoadEventListener.proxyOrLoad(DefaultLoadEventListener.java:191)
        at org.hibernate.event.def.DefaultLoadEventListener.onLoad(DefaultLoadEventListener.java:103)
        at org.hibernate.impl.SessionImpl.fireLoad(SessionImpl.java:878)
        at org.hibernate.impl.SessionImpl.load(SessionImpl.java:795)
        at org.hibernate.impl.SessionImpl.load(SessionImpl.java:788)
...
Caused by: java.lang.InstantiationException: Thing$$EnhancerByCGLIB$$e29dbda1
        at java.lang.Class.newInstance0(Class.java:335)
        at java.lang.Class.newInstance(Class.java:303)
        at org.hibernate.proxy.pojo.cglib.CGLIBLazyInitializer.getProxyInstance(CGLIBLazyInitializer.java:107)
        at org.hibernate.proxy.pojo.cglib.CGLIBLazyInitializer.getProxy(CGLIBLazyInitializer.java:93)
        ... 49 more

This time I was luckier with my search results, which eventually lead me to these two pages. Indeed, the problem was due to having a private no-args constructor in my Thing object. As soon as I changed the constructor to be visable, the calls to load() started working as expected.

I was thrown a bit by the InstantiationException, which according to the JavaDoc is

Thrown when an application tries to create an instance of a class using the newInstance method in class Class, but the specified class object cannot be instantiated because it is an interface or is an abstract class.

While I can see that it's related, I don't really think the private constructor issue strictly falls under that description. In any case, I'm sure that a better message, or a different exception, or an update to the documentation, would have helped me isolate the problem a lot quicker.

Oddly, the code from the other project also has a private no-args constructor, yet it seems to work. That part remains a mystery to me, but I thought I'd post my other findings in case it helps anyone else who runs into a similar issue.