Add toString methods to classloaders, for debugging.

* Add toString methods to all classloader classes.

util/classpath/src/main/scala/sbt/classpath/ClassLoaders.scala

@@ -46,14 +46,26 @@ final class SelfFirstLoader(classpath: Seq[URL], parent: ClassLoader) extends Lo
 
 /** Doesn't load any classes itself, but instead verifies that all classes loaded through `parent` either come from `root` or `classpath`.*/
 final class ClasspathFilter(parent: ClassLoader, root: ClassLoader, classpath: Set[File]) extends ClassLoader(parent) {
+  override def toString =
+    s"""|ClasspathFilter(
+        |  parent = $parent
+        |  root = $root
+        |  cp = $classpath
+        |)""".stripMargin
+
   private[this] val directories: Seq[File] = classpath.toSeq.filter(_.isDirectory)
   override def loadClass(className: String, resolve: Boolean): Class[_] =
     {
       val c = super.loadClass(className, resolve)
       if (includeLoader(c.getClassLoader, root) || fromClasspath(c))
         c
-      else
+      else {
+        System.err.println(s"DEBUGME: Failing to load class $className because it was not in expected parent chain.")
+        System.err.println(s"DEBUGME: Found: ${c.getClassLoader}")
+        System.err.println(s"DEBUGME: Expected: ${root}")
+        System.err.println(s"DEBUGME: onClasspath: ${fromClasspath(c)}")
         throw new ClassNotFoundException(className)
+      }
     }
   private[this] def fromClasspath(c: Class[_]): Boolean =
     {

util/classpath/src/main/scala/sbt/classpath/ClasspathUtilities.scala

@@ -26,6 +26,13 @@ object ClasspathUtilities {
     new URLClassLoader(Path.toURLs(paths), parent) with RawResources with NativeCopyLoader {
       override def resources = resourceMap
       override val config = new NativeCopyConfig(nativeTemp, paths, javaLibraryPaths)
+      override def toString =
+        s"""|URLClassLoader with NativeCopyLoader with RawResources(
+            |  urls = $paths,
+            |  parent = $parent,
+            |  resourceMap = ${resourceMap.keySet},
+            |  nativeTemp = $nativeTemp
+            |)""".stripMargin
     }
 
   def javaLibraryPaths: Seq[File] = IO.parseClasspath(System.getProperty("java.library.path"))

util/classpath/src/main/scala/sbt/classpath/DualLoader.scala

@@ -12,6 +12,7 @@ final class NullLoader extends ClassLoader {
   override final def loadClass(className: String, resolve: Boolean): Class[_] = throw new ClassNotFoundException("No classes can be loaded from the null loader")
   override def getResource(name: String): URL = null
   override def getResources(name: String): Enumeration[URL] = null
+  override def toString = "NullLoader"
 }
 
 /** Exception thrown when `loaderA` and `loaderB` load a different Class for the same name. */
@@ -84,6 +85,8 @@ class DualLoader(parentA: ClassLoader, aOnlyClasses: String => Boolean, aOnlyRes
           new DualEnumeration(urlsA, urlsB)
       }
     }
+
+  override def toString = s"DualLoader(a = $parentA, b = $parentB)"
 }
 
 /** Concatenates `a` and `b` into a single `Enumeration`.*/