Update Classes.getClass() to never use the bootstrap class loader. (junit-team#1404)

Previously, if there was no context class loader then the
bootstrap class loader is used. Now, the class loader for
Classes is used (or for the passed in class, if the new overloaded
version of getClass() is used).

Author: kcooney

src/main/java/org/junit/experimental/categories/CategoryFilterFactory.java

@@ -37,7 +37,11 @@ private List<Class<?>> parseCategories(String categories) throws ClassNotFoundEx
         List<Class<?>> categoryClasses = new ArrayList<Class<?>>();
 
         for (String category : categories.split(",")) {
-            Class<?> categoryClass = Classes.getClass(category);
+            /*
+             * Load the category class using the context class loader.
+             * If there is no context class loader, use the class loader for this class.
+             */
+            Class<?> categoryClass = Classes.getClass(category, getClass());
 
             categoryClasses.add(categoryClass);
         }

src/main/java/org/junit/internal/Classes.java

@@ -17,11 +17,28 @@ public Classes() {
 
     /**
      * Returns Class.forName for {@code className} using the current thread's class loader.
+     * If the current thread does not have a class loader, falls back to the class loader for
+     * {@link Classes}.
      *
      * @param className Name of the class.
      * @throws ClassNotFoundException
      */
     public static Class<?> getClass(String className) throws ClassNotFoundException {
-        return Class.forName(className, true, currentThread().getContextClassLoader());
+        return getClass(className, Classes.class);
+    }
+
+    /**
+     * Returns Class.forName for {@code className} using the current thread's class loader.
+     * If the current thread does not have a class loader, falls back to the class loader for the
+     * passed-in class.
+     *
+     * @param className Name of the class.
+     * @param callingClass Class that is requesting a the class
+     * @throws ClassNotFoundException
+     * @since 4.13
+     */
+    public static Class<?> getClass(String className, Class<?> callingClass) throws ClassNotFoundException {
+        ClassLoader classLoader = currentThread().getContextClassLoader();
+        return Class.forName(className, true, classLoader == null ? callingClass.getClassLoader() : classLoader);
     }
 }