Index: src/java/org/apache/lucene/util/AttributeSource.java
===================================================================
--- src/java/org/apache/lucene/util/AttributeSource.java	(revision 894086)
+++ src/java/org/apache/lucene/util/AttributeSource.java	(working copy)
@@ -75,7 +75,10 @@
           Class<? extends AttributeImpl> clazz = attClassImplMap.get(attClass);
           if (clazz == null) {
             try {
-              attClassImplMap.put(attClass, clazz = Class.forName(attClass.getName() + "Impl").asSubclass(AttributeImpl.class));
+              attClassImplMap.put(attClass,
+                clazz = Class.forName(attClass.getName() + "Impl", true, attClass.getClassLoader())
+                .asSubclass(AttributeImpl.class)
+              );
             } catch (ClassNotFoundException e) {
               throw new IllegalArgumentException("Could not find implementing class for " + attClass.getName());
             }