test case demonstrating two ways of augmenting sbt's classpath. addExtra1 is the recommended way

sbt/src/sbt-test/project/extra/project/B.scala

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+import sbt._
+import Keys._
+
+object B extends Build
+{
+	lazy val root = Project("root", file(".")) settings(
+		autoScalaLibrary := false,
+		libraryDependencies += "log4j" % "log4j" % "1.2.16",
+		retrieveManaged := true,
+		commands ++= Seq(
+			addExtra("add1", addExtra1),
+			addExtra("add2", addExtra2),
+			checkExtra
+		)
+	)
+	def addExtra(name: String, f: (State, Seq[File]) => State) = Command.command(name) { s =>
+		f(s, (file("lib_managed") ** "*.jar").get)		
+	}
+	def checkExtra = Command.command("check") { s =>
+		val loader = Class.forName("org.apache.log4j.Level").getClassLoader
+		val sbtLoader = classOf[sbt.Build].getClassLoader
+		assert(loader eq sbtLoader, "Different loader for sbt and extra: " + sbtLoader + " and " + loader)
+		s
+	}
+	def addExtra1(s: State, extra: Seq[File]): State =
+	{
+		val cs = s.configuration.provider.components()
+		val copied = cs.addToComponent("extra", extra.toArray)
+		if(copied) s.reload else s
+	}
+	def addExtra2(s: State, extra: Seq[File]): State =
+	{
+		val reload = State.defaultReload(s)
+		val currentID = reload.app
+		val currentExtra = currentID.classpathExtra
+		val newExtra = (currentExtra ++ extra).distinct
+		if(newExtra.length == currentExtra.length)
+			s
+		else
+		{
+			val newID = ApplicationID(currentID).copy(extra = extra)
+			s.setResult(Some(reload.copy(app = newID)))
+		}
+	}
+}

sbt/src/sbt-test/project/extra/test

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+-> check
+> update
+> add1
+> check
+
+> reboot full
+-> check
+
+> add2
+> check