Scala example source code file (ScalaClassLoader.scala)

This example Scala source code file (ScalaClassLoader.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.

All credit for the original source code belongs to scala-lang.org; I'm just trying to make examples easier to find. (For my Scala work, see my Scala examples and tutorials.)

Scala tags/keywords

anyref, boolean, hasclasspath, jclassloader, option, reflection, scalaclassloader, seq, string, t, urlclassloader

The ScalaClassLoader.scala Scala example source code

/* NSC -- new Scala compiler
 * Copyright 2005-2013 LAMP/EPFL
 * @author  Paul Phillips
 */

package scala
package reflect.internal.util

import java.lang.{ ClassLoader => JClassLoader }
import java.lang.reflect.{ Constructor, Modifier, Method }
import java.io.{ File => JFile }
import java.net.{ URLClassLoader => JURLClassLoader }
import java.net.URL
import scala.reflect.runtime.ReflectionUtils.unwrapHandler
import ScalaClassLoader._
import scala.util.control.Exception.{ catching }
import scala.language.implicitConversions
import scala.reflect.{ ClassTag, classTag }

trait HasClassPath {
  def classPathURLs: Seq[URL]
}

/** A wrapper around java.lang.ClassLoader to lower the annoyance
 *  of java reflection.
 */
trait ScalaClassLoader extends JClassLoader {
  /** Executing an action with this classloader as context classloader */
  def asContext[T](action: => T): T = {
    val saved = contextLoader
    try { setContext(this) ; action }
    finally setContext(saved)
  }
  def setAsContext() { setContext(this) }

  /** Load and link a class with this classloader */
  def tryToLoadClass[T <: AnyRef](path: String): Option[Class[T]] = tryClass(path, initialize = false)
  /** Load, link and initialize a class with this classloader */
  def tryToInitializeClass[T <: AnyRef](path: String): Option[Class[T]] = tryClass(path, initialize = true)

  private def tryClass[T <: AnyRef](path: String, initialize: Boolean): Option[Class[T]] =
    catching(classOf[ClassNotFoundException], classOf[SecurityException]) opt
      Class.forName(path, initialize, this).asInstanceOf[Class[T]]

  /** Create an instance of a class with this classloader */
  def create(path: String): AnyRef =
    tryToInitializeClass[AnyRef](path).map(_.newInstance()).orNull

  /** The actual bytes for a class file, or an empty array if it can't be found. */
  def classBytes(className: String): Array[Byte] = classAsStream(className) match {
    case null   => Array()
    case stream => scala.reflect.io.Streamable.bytes(stream)
  }

  /** An InputStream representing the given class name, or null if not found. */
  def classAsStream(className: String) =
    getResourceAsStream(className.replaceAll("""\.""", "/") + ".class")

  /** Run the main method of a class to be loaded by this classloader */
  def run(objectName: String, arguments: Seq[String]) {
    val clsToRun = tryToInitializeClass(objectName) getOrElse (
      throw new ClassNotFoundException(objectName)
    )
    val method = clsToRun.getMethod("main", classOf[Array[String]])
    if (!Modifier.isStatic(method.getModifiers))
      throw new NoSuchMethodException(objectName + ".main is not static")

    try asContext(method.invoke(null, Array(arguments.toArray: AnyRef): _*)) // !!! : AnyRef shouldn't be necessary
    catch unwrapHandler({ case ex => throw ex })
  }
}

/** Methods for obtaining various classloaders.
 *      appLoader: the application classloader.  (Also called the java system classloader.)
 *      extLoader: the extension classloader.
 *     bootLoader: the boot classloader.
 *  contextLoader: the context classloader.
 */
object ScalaClassLoader {
  /** Returns loaders which are already ScalaClassLoaders unaltered,
   *  and translates java.net.URLClassLoaders into scala URLClassLoaders.
   *  Otherwise creates a new wrapper.
   */
  implicit def apply(cl: JClassLoader): ScalaClassLoader = cl match {
    case cl: ScalaClassLoader => cl
    case cl: JURLClassLoader  => new URLClassLoader(cl.getURLs.toSeq, cl.getParent)
    case _                    => new JClassLoader(cl) with ScalaClassLoader
  }
  def contextLoader = apply(Thread.currentThread.getContextClassLoader)
  def appLoader     = apply(JClassLoader.getSystemClassLoader)
  def setContext(cl: JClassLoader) =
    Thread.currentThread.setContextClassLoader(cl)
  def savingContextLoader[T](body: => T): T = {
    val saved = contextLoader
    try body
    finally setContext(saved)
  }

  class URLClassLoader(urls: Seq[URL], parent: JClassLoader)
      extends JURLClassLoader(urls.toArray, parent)
         with ScalaClassLoader
         with HasClassPath {

    private var classloaderURLs: Seq[URL] = urls
    def classPathURLs: Seq[URL] = classloaderURLs

    /** Override to widen to public */
    override def addURL(url: URL) = {
      classloaderURLs :+= url
      super.addURL(url)
    }
  }

  def fromURLs(urls: Seq[URL], parent: ClassLoader = null): URLClassLoader =
    new URLClassLoader(urls, parent)

  /** True if supplied class exists in supplied path */
  def classExists(urls: Seq[URL], name: String): Boolean =
    (fromURLs(urls) tryToLoadClass name).isDefined

  /** Finding what jar a clazz or instance came from */
  def originOfClass(x: Class[_]): Option[URL] =
    Option(x.getProtectionDomain.getCodeSource) flatMap (x => Option(x.getLocation))
}