How to manage DB connection in Scala using functional programming style? - scala

Question

I have a piece of Scala code using DB connection:
def getAllProviderCodes()(implicit conf : Configuration) : List[String] = {
var conn: java.sql.Connection = null
try {
conn = DriverManager.getConnection(DBInfo.dbUrl(conf), DBInfo.dbUserName(conf), DBInfo.dbPassword(conf))
return ResultSetIterator.create(
conn.prepareStatement("SELECT pcode FROM providers").executeQuery()
){_.getString("pcode")}.toList
} catch {
case e: Exception =>
logger.warn("Something went wrong with creating the connection: " + e.getStackTrace)
} finally {
if (conn != null) {
conn.close()
}
}
List()
}
It's very OOP-Java-like style, so I'd like to know is there a way to write it in more functional way? I tried to succeed in applying Try monad, but failed: my biggest concern is that we have state here, as well as finally block. Maybe there's some kind of pattern for such cases?
Thank you in advance.
UPD: Here's the example from here of what IMHO the solution will look like:
val connection = database.getConnection()
val data: Seq[Data] = Try{
val results = connection.query("select whatever")
results.map(convertToWhatIneed)
} recover {
case t: Throwable =>
Seq.empty[Data]
} get
connection.close()
But as I've mentioned in the comment, I have to close the connection, then I have to place all the things regarding to connection inside Try to keep it pure... and then I to the variant with "try-catch-finally" inside Try block.

Answer 1

I've never played around with the Java SQL Connection library so the syntax of my answer has been written as pseudocode, but if I understand your question correctly here is how I would implement what you have done:
def getAllProviderCodes()(implicit conf : Configuration): List[String] = {
val conn: Connection = DriverManager.getConnection(???) // replace ??? with parameters
val result: List[String] = Try {
??? // ResultSetIterator stuff
} match {
case Success(output) => output // or whatever .toList thing
case Failure(_) => List.empty // add logging here
}
if(conn != null) conn.close()
result // will be whatever List you make (or an empty List if Try fails)
}
Instead of a Java-like try-catch-finally block, one Scala-like way of doing things would be to put the stuff which could explode in a Try block and assigning the response to a value using case Success(out) and case Failure(ex).

Answer 2

Just pull the connection outside of the try:
val conn = getConnection()
try {
doStuff(conn)
} finally {
conn.close
}
If you want the result of whole thing to be a Try, just wrap it into a Try:
def doDBStuff = Try {
val conn = getConnection()
try {
doStuff(conn)
} finally {
conn.close
}
}
Or with a bit less nesting (but this will throw connection exceptions):
def doDBStuff = {
val conn = getConnection()
val result = Try { doStuff(conn) }
conn.close
result
}