How to add logging to try/catch with the borrowing pattern - scala

Question

How can I add logging in the case of an exception in the code below?
I tried to add a catch, but then I get an error saying that Unit does not conform to the type T.
Is there a trick around this somehow?
private def withClient[T](body: Jedis => T): T = {
var jedis: Jedis = null
try {
jedis = pool.getResource
body(jedis)
}
catch {
case _ => println("hllo")
}
finally {
if(jedis != null) jedis.close()
}
}

Answer 1

The problem is that in the case of an exception you are returning the value of println("hllo") which is Unit
You need to change the return type to something that represents the fact that you may not have a value, I'd suggest Try[T] or if you don't care to carry the exception data out of the function you could use Option[T]
private def withClient[T](body: Jedis => T): Try[T] = {
var jedis: Jedis = null
Try(
try {
jedis = pool.getResource
body(jedis)
}
catch {
case e =>
println("hllo")
throw e
}
finally {
if(jedis != null) jedis.close()
}
)
}
or
private def withClient[T](body: Jedis => T): Option[T] = {
var jedis: Jedis = null
try {
jedis = pool.getResource
Some(body(jedis))
}
catch {
case e =>
println("hllo")
None
}
finally {
if(jedis != null) jedis.close()
}
}

Answer 2

If your body function throws an exception instead of returning a T,
there's pretty much nothing you can do--you have to propagate the
exception forward instead of returning a T from withClient. Try is
the right approach to handle this because it safely captures the
exception and allows you to do something only if there was an exception
thrown. E.g.,
private def withClient[T](body: Jedis => T): T = {
val jedis = pool.getResource
val tryT = scala.util.Try(body(jedis))
jedis.close()
tryT.failed foreach { e => println(e.getMessage) }
tryT.get
}
The following points are notable:
We got rid of null and var
We don't need to worry about pool.getResource throwing an exception
because if it does, we can't go ahead anyway and we don't have to
worry about closing the resource because the exception means it was
never opened in the first place.
We make sure we close the jedis resource regardless of exception
because if there was an exception, it would've been safely caught in
the Try
We print a message only if there is any exception
We 'force' a value using Try#get only if there was no exception, and
if there was an exception, it will get rethrown here.