Always execute code at beginning and end of method

I'm connecting to MongoDb using following code :

   def insert() = {

    val mc = new com.mongodb.MongoClient("localhost", 27017);
    val db = mc.getDatabase("MyDb");

    //My insert code

    mc.close();

  }                                               //> insert: ()Unit

I have various methods which open and close the connection. Can the lines :

  val mc = new com.mongodb.MongoClient("localhost", 27017);
val db = mc.getDatabase("MyDb");
mc.close();

be extracted so that they are implicitly called at beginning and end of method. Does Scala implicits cater for this scenario or is reflection required ?

Solution

A common pattern would be to use a call-by-name method where you can pass a function that accepts a DB and does something with it. The call-by-name method can facilitate the creation of the client, etc, and execute the code within.

def withDB[A](block: DB => A): A = {
  val mc = new com.mongodb.MongoClient("localhost", 27017);
  val db = mc.getDatabase("MyDb");
  try block(db) finally mc.close()
}

And use it:

def insert() = withDB { db =>
   // do something with `db`
}

However, a look at the documentation says:

A MongoDB client with internal connection pooling. For most applications, you should have one MongoClient instance for the entire JVM.

Which makes the above approach look like a bad idea, assuming that is the version you are using. I can definitely see some concurrency issues trying to do this and having too many connections open.

But, you can follow the same pattern, stuffing the connection creating into a singleton object. You would need to manage the closing of the client when your application is shutdown, however.

object Mongo {
  lazy val mc = new com.mongodb.MongoClient("localhost", 27017);
  lazy val db = mc.getDatabase("MyDb");

  def withDB[A](block: DB => A): A = block(db)

  def close(): Unit = mc.close()
}