code: public static InputStream unpackFilterInputStream(FilterInputStream is) throws IllegalAccessException {
      try {
        Field field = FilterInputStream.class.getDeclaredField("in");
        field.setAccessible(true);
        return (InputStream) field.get(is);
      } catch (NoSuchFieldException nsfe) {
        throw new RuntimeException(nsfe);
      }
    }
docs: Finds the underlying {@link InputStream} to a {@link FilterInputStream}. Note this is not always possible due
  to security restrictions of the JVM.
  @throws IllegalAccessException If security policies of the JVM prevent unpacking of the {@link FilterInputStream}.