Scala case class vs object：它是如何实例化的？

Question

Some context so that this question actually makes some sense.

I have a trait from which I extend a case class and an object. I have a method that pattern matches to decide which to pull based on given conditions. When I pattern match and get the case class, everything works fine. But when I pattern match and get the object, an error is thrown.

Within the trait, I have methods that call the DB. And I also have vals that need overriding.

The curious thing is the error that is thrown when it the object is instantiated, is a DB specific error

java.sql.SQLInvalidAuthorizationSpecException: (conn=41) Transaction characteristics can't be changed while a transaction is in progress

Which led me on wild goose chase to hunt down whether the code was opening up another session unknowingly etc. But having checked my code, I knew I was passing around the one DB connection I had started and there wasn't any strange behaviour.

I figured, since my case class instance works, I wanted to see if converting my object into a case class would change anything.

Having converted my object to a case class, I then got an NPE error thrown which pointed me directly to the errant line of code that was causing problems. I then changed my vals to def in the trait and everything works fine.

So my questions is: Why is it that when I converted the object to the case class, it then threw up the error that was the true culprit. Which is why I asked the question: how is a case class vs object instantiated? Because I suspect this was the cause of the misleading error being thrown.

Feel free to correct my assumption if it is wrong.

Answer 1

Here is the crucial line:

I then changed my vals to def in the trait and everything works fine.

The problem was almost certainly caused by an abstract val in the trait being used in the constructor before it had been initialised by the subclass.

As a general rule, abstract values in a trait should always be def .