scala.App

trait App extends DelayedInit

The App trait can be used to quickly turn objects into executable programs. Here is an example:

object Main extends App {
  Console.println("Hello World: " + (args mkString ", "))
}

Here, object Main inherits the main method of App .

args returns the current command line arguments as an array.

Caveats

  • It should be noted that this trait is implemented using the DelayedInit functionality, which means that fields of the object will not have been initialized before the main method has been executed.*

It should also be noted that the main method should not be overridden: the whole class body becomes the “main method”.

Future versions of this trait will no longer extend DelayedInit .

Deprecated Value Members From scala.App

def delayedInit(body: ⇒ Unit): Unit

The init hook. This saves all initialization code for execution within main . This method is normally never called directly from user code. Instead it is called as compiler-generated code for those classes and objects (but not traits) that inherit from the DelayedInit trait and that do not themselves define a delayedInit method.

  • body
    • the initialization code to be stored for later execution
  • Definition Classes
    • App → DelayedInit
  • Annotations
    • @ deprecated
  • Deprecated
    • (Since version 2.11.0) The delayedInit mechanism will disappear.

(defined at scala.App)

Value Members From scala.App

def main(args: Array[String]): Unit

The main method. This stores all arguments so that they can be retrieved with args and then executes all initialization code segments in the order in which they were passed to delayedInit .

  • args
    • the arguments passed to the main method
  • Annotations
    • @ deprecatedOverriding (message = “main should not be overridden”, since = “2.11.0”) (defined at scala.App)

Full Source:

/*                     __                                               *\
**     ________ ___   / /  ___     Scala API                            **
**    / __/ __// _ | / /  / _ |    (c) 2010-2013, LAMP/EPFL             **
**  __\ \/ /__/ __ |/ /__/ __ |    http://scala-lang.org/               **
** /____/\___/_/ |_/____/_/ | |                                         **
**                          |/                                          **
\*                                                                      */

package scala

import scala.compat.Platform.currentTime
import scala.collection.mutable.ListBuffer

/** The `App` trait can be used to quickly turn objects
 *  into executable programs. Here is an example:
 *  {{{
 *  object Main extends App {
 *    Console.println("Hello World: " + (args mkString ", "))
 *  }
 *  }}}
 *  Here, object `Main` inherits the `main` method of `App`.
 *
 *  `args` returns the current command line arguments as an array.
 *
 *  ==Caveats==
 *
 *  '''''It should be noted that this trait is implemented using the [[DelayedInit]]
 *  functionality, which means that fields of the object will not have been initialized
 *  before the main method has been executed.'''''
 *
 *  It should also be noted that the `main` method should not be overridden:
 *  the whole class body becomes the “main method”.
 *
 *  Future versions of this trait will no longer extend `DelayedInit`.
 *
 *  @author  Martin Odersky
 *  @version 2.1, 15/02/2011
 */
trait App extends DelayedInit {

  /** The time when the execution of this program started, in milliseconds since 1
    * January 1970 UTC. */
  @deprecatedOverriding("executionStart should not be overridden", "2.11.0")
  val executionStart: Long = currentTime

  /** The command line arguments passed to the application's `main` method.
   */
  @deprecatedOverriding("args should not be overridden", "2.11.0")
  protected def args: Array[String] = _args

  private var _args: Array[String] = _

  private val initCode = new ListBuffer[() => Unit]

  /** The init hook. This saves all initialization code for execution within `main`.
   *  This method is normally never called directly from user code.
   *  Instead it is called as compiler-generated code for those classes and objects
   *  (but not traits) that inherit from the `DelayedInit` trait and that do not
   *  themselves define a `delayedInit` method.
   *  @param body the initialization code to be stored for later execution
   */
  @deprecated("The delayedInit mechanism will disappear.", "2.11.0")
  override def delayedInit(body: => Unit) {
    initCode += (() => body)
  }

  /** The main method.
   *  This stores all arguments so that they can be retrieved with `args`
   *  and then executes all initialization code segments in the order in which
   *  they were passed to `delayedInit`.
   *  @param args the arguments passed to the main method
   */
  @deprecatedOverriding("main should not be overridden", "2.11.0")
  def main(args: Array[String]) = {
    this._args = args
    for (proc <- initCode) proc()
    if (util.Properties.propIsSet("scala.time")) {
      val total = currentTime - executionStart
      Console.println("[total " + total + "ms]")
    }
  }
}