Scala example source code file (package.scala)

This example Scala source code file (package.scala) is included in my "Source Code Warehouse" project. The intent of this project is to help you more easily find Scala source code examples by using tags.

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Scala tags/keywords

b, file, inputstream, interruptedexception, ioexception, jprocessbuilder, list, system, t, throwable, url

The package.scala Scala example source code

/*                     __                                               *\
**     ________ ___   / /  ___     Scala API                            **
**    / __/ __// _ | / /  / _ |    (c) 2003-2013, LAMP/EPFL             **
**  __\ \/ /__/ __ |/ /__/ __ |    http://scala-lang.org/               **
** /____/\___/_/ |_/____/_/ | |                                         **
**                          |/                                          **
\*                                                                      */

// Developer note:
//   scala -J-Dscala.process.debug
// for process debugging output.
//
package scala.sys {
  /** This package handles the execution of external processes.  The contents of
    * this package can be divided in three groups, according to their
    * responsibilities:
    *
    *   - Indicating what to run and how to run it.
    *   - Handling a process input and output.
    *   - Running the process.
    *
    * For simple uses, the only group that matters is the first one. Running an
    * external command can be as simple as `"ls".!`, or as complex as building a
    * pipeline of commands such as this:
    *
    * {{{
    * import scala.sys.process._
    * "ls" #| "grep .scala" #&& Seq("sh", "-c", "scalac *.scala") #|| "echo nothing found" lines
    * }}}
    *
    * We describe below the general concepts and architecture of the package,
    * and then take a closer look at each of the categories mentioned above.
    *
    * ==Concepts and Architecture==
    *
    * The underlying basis for the whole package is Java's `Process` and
    * `ProcessBuilder` classes. While there's no need to use these Java classes,
    * they impose boundaries on what is possible. One cannot, for instance,
    * retrieve a ''process id'' for whatever is executing.
    *
    * When executing an external process, one can provide a command's name,
    * arguments to it, the directory in which it will be executed and what
    * environment variables will be set. For each executing process, one can
    * feed its standard input through a `java.io.OutputStream`, and read from
    * its standard output and standard error through a pair of
    * `java.io.InputStream`. One can wait until a process finishes execution and
    * then retrieve its return value, or one can kill an executing process.
    * Everything else must be built on those features.
    *
    * This package provides a DSL for running and chaining such processes,
    * mimicking Unix shells ability to pipe output from one process to the input
    * of another, or control the execution of further processes based on the
    * return status of the previous one.
    *
    * In addition to this DSL, this package also provides a few ways of
    * controlling input and output of these processes, going from simple and
    * easy to use to complex and flexible.
    *
    * When processes are composed, a new `ProcessBuilder` is created which, when
    * run, will execute the `ProcessBuilder` instances it is composed of
    * according to the manner of the composition. If piping one process to
    * another, they'll be executed simultaneously, and each will be passed a
    * `ProcessIO` that will copy the output of one to the input of the other.
    *
    * ==What to Run and How==
    *
    * The central component of the process execution DSL is the
    * [[scala.sys.process.ProcessBuilder]] trait. It is `ProcessBuilder` that
    * implements the process execution DSL, that creates the
    * [[scala.sys.process.Process]] that will handle the execution, and return
    * the results of such execution to the caller. We can see that DSL in the
    * introductory example: `#|`, `#&&` and `#!!` are methods on
    * `ProcessBuilder` used to create a new `ProcessBuilder` through
    * composition.
    *
    * One creates a `ProcessBuilder` either through factories on the
    * [[scala.sys.process.Process]]'s companion object, or through implicit
    * conversions available in this package object itself.  Implicitly, each
    * process is created either out of a `String`, with arguments separated by
    * spaces -- no escaping of spaces is possible -- or out of a
    * [[scala.collection.Seq]], where the first element represents the command
    * name, and the remaining elements are arguments to it. In this latter case,
    * arguments may contain spaces.
    *
    * To further control what how the process will be run, such as specifying
    * the directory in which it will be run, see the factories on
    * [[scala.sys.process.Process]]'s object companion.
    *
    * Once the desired `ProcessBuilder` is available, it can be executed in
    * different ways, depending on how one desires to control its I/O, and what
    * kind of result one wishes for:
    *
    *   - Return status of the process (`!` methods)
    *   - Output of the process as a `String` (`!!` methods)
    *   - Continuous output of the process as a `Stream[String]` (`lines` methods)
    *   - The `Process` representing it (`run` methods)
    *
    * Some simple examples of these methods:
    * {{{
    * import scala.sys.process._
    *
    * // This uses ! to get the exit code
    * def fileExists(name: String) = Seq("test", "-f", name).! == 0
    *
    * // This uses !! to get the whole result as a string
    * val dirContents = "ls".!!
    *
    * // This "fire-and-forgets" the method, which can be lazily read through
    * // a Stream[String]
    * def sourceFilesAt(baseDir: String): Stream[String] = {
    *   val cmd = Seq("find", baseDir, "-name", "*.scala", "-type", "f")
    *   cmd.lines
    * }
    * }}}
    *
    * We'll see more details about controlling I/O of the process in the next
    * section.
    *
    * ==Handling Input and Output==
    *
    * In the underlying Java model, once a `Process` has been started, one can
    * get `java.io.InputStream` and `java.io.OutpuStream` representing its
    * output and input respectively. That is, what one writes to an
    * `OutputStream` is turned into input to the process, and the output of a
    * process can be read from an `InputStream` -- of which there are two, one
    * representing normal output, and the other representing error output.
    *
    * This model creates a difficulty, which is that the code responsible for
    * actually running the external processes is the one that has to take
    * decisions about how to handle its I/O.
    *
    * This package presents an alternative model: the I/O of a running process
    * is controlled by a [[scala.sys.process.ProcessIO]] object, which can be
    * passed _to_ the code that runs the external process. A `ProcessIO` will
    * have direct access to the java streams associated with the process I/O. It
    * must, however, close these streams afterwards.
    *
    * Simpler abstractions are available, however. The components of this
    * package that handle I/O are:
    *
    *   - [[scala.sys.process.ProcessIO]]: provides the low level abstraction.
    *   - [[scala.sys.process.ProcessLogger]]: provides a higher level abstraction
    *   for output, and can be created through its object companion
    *   - [[scala.sys.process.BasicIO]]: a library of helper methods for the
    *   creation of `ProcessIO`.
    *   - This package object itself, with a few implicit conversions.
    *
    * Some examples of I/O handling:
    * {{{
    * import scala.sys.process._
    *
    * // An overly complex way of computing size of a compressed file
    * def gzFileSize(name: String) = {
    *   val cat = Seq("zcat", name)
    *   var count = 0
    *   def byteCounter(input: java.io.InputStream) = {
    *     while(input.read() != -1) count += 1
    *     input.close()
    *   }
    *   cat ! new ProcessIO(_.close(), byteCounter, _.close())
    *   count
    * }
    *
    * // This "fire-and-forgets" the method, which can be lazily read through
    * // a Stream[String], and accumulates all errors on a StringBuffer
    * def sourceFilesAt(baseDir: String): (Stream[String], StringBuffer) = {
    *   val buffer = new StringBuffer()
    *   val cmd = Seq("find", baseDir, "-name", "*.scala", "-type", "f")
    *   val lines = cmd lines_! ProcessLogger(buffer append _)
    *   (lines, buffer)
    * }
    * }}}
    *
    * Instances of the java classes `java.io.File` and `java.net.URL` can both
    * be used directly as input to other processes, and `java.io.File` can be
    * used as output as well. One can even pipe one to the other directly
    * without any intervening process, though that's not a design goal or
    * recommended usage. For example, the following code will copy a web page to
    * a file:
    * {{{
    * import java.io.File
    * import java.net.URL
    * import scala.sys.process._
    * new URL("http://www.scala-lang.org/") #> new File("scala-lang.html") !
    * }}}
    *
    * More information about the other ways of controlling I/O can be looked at
    * in the scaladoc for the associated objects, traits and classes.
    *
    * ==Running the Process==
    *
    * Paradoxically, this is the simplest component of all, and the one least
    * likely to be interacted with. It consists solely of
    * [[scala.sys.process.Process]], and it provides only two methods:
    *
    *   - `exitValue()`: blocks until the process exit, and then returns the exit
    *   value. This is what happens when one uses the `!` method of
    *   `ProcessBuilder`.
    *   - `destroy()`: this will kill the external process and close the streams
    *   associated with it.
    */
  package object process extends ProcessImplicits {
    /** The arguments passed to `java` when creating this process */
    def javaVmArguments: List[String] = {
      import scala.collection.JavaConversions._

      java.lang.management.ManagementFactory.getRuntimeMXBean().getInputArguments().toList
    }
    /** The input stream of this process */
    def stdin  = java.lang.System.in
    /** The output stream of this process */
    def stdout = java.lang.System.out
    /** The error stream of this process */
    def stderr = java.lang.System.err
  }
  // private val shell: String => Array[String] =
  //   if (isWin) Array("cmd.exe", "/C", _)
  //   else Array("sh", "-c", _)

  package process {
    // These are in a nested object instead of at the package level
    // due to the issues described in tickets #3160 and #3836.
    private[process] object processInternal {
      final val processDebug = props contains "scala.process.debug"
      dbg("Initializing process package.")

      type =?>[-A, +B]     = PartialFunction[A, B]
      type Closeable       = java.io.Closeable
      type File            = java.io.File
      type IOException     = java.io.IOException
      type InputStream     = java.io.InputStream
      type JProcess        = java.lang.Process
      type JProcessBuilder = java.lang.ProcessBuilder
      type OutputStream    = java.io.OutputStream
      type SyncVar[T]      = scala.concurrent.SyncVar[T]
      type URL             = java.net.URL

      def onInterrupt[T](handler: => T): Throwable =?> T = {
        case _: InterruptedException => handler
      }

      def ioFailure[T](handler: IOException => T): Throwable =?> T = {
        case e: IOException => handler(e)
      }

      def dbg(msgs: Any*) = if (processDebug) {
        Console.println("[process] " + (msgs mkString " "))
      }
    }
  }
}