Trouble catching IllegalArgumentException thrown from Java method

[Stuart Sierra]

Hi there,

I want to catch a java.lang.IllegalArgumentException thrown by a
library method.

This works:
user=> (try (throw (new java.lang.IllegalArgumentException))
(catch java.lang.IllegalArgumentException e
"recovered"))
"recovered"

But calling the method doesn't:
user=> (try (. *value-factory* (createURI "this thing"))
(catch java.lang.IllegalArgumentException e
"recovered"))
java.lang.reflect.InvocationTargetException
....
Caused by: java.lang.IllegalArgumentException: Not a valid
(absolute) URI: this thing

Unless I catch the base Exception class:
user=> (try (. *value-factory* (createURI "this thing"))
(catch java.lang.Exception e "recovered"))
"recovered"

Any idea what's going on?
Thanks,
Stuart

[Rich Hickey]

Sure - if you look at the exception that flows through -
java.lang.reflect.InvocationTargetException - that tells you the call
was made via reflection. Any exceptions thrown in reflection calls get
wrapped by the reflection API in InvocationTargetExceptions.

Your options are:

- put a type hint on *value-factory* so the call becomes non-
reflective.

- change your expectations :)

There are lots of situations where exceptions can get wrapped -
reflection, threading APIs, etc. I'm not sure of the viability of
catching exceptions by type, vs catching base exceptions and examining
the cause (using getCause).

Rich

[John Cowan]

When a method called by reflection throws an exception, Java wraps it
in an InvocationTargetException to deal with the fact that it might be
a checked exception, which can't be thrown by a method that doesn't
declare it. So either you have to catch InvocationTargetException, or
you should lobby to have Clojure modified to unwrap the exception.

--
GMail doesn't have rotating .sigs, but you can see mine at
http://www.ccil.org/~cowan/signatures

[Stuart Sierra]

Ok, now I understand. I didn't realize that reflection would change
the type of the exception. I added a type hint on *value-factory*,
per John Cowan's suggestion, and the catch works. But I'll consider
just catching Exception.

Thanks to you both,
-Stuart