code: public static Class getFieldType(ClassNode node, String fieldName) {
          while (node != null) {
              for (FieldNode field: node.getFields()) {
                  if (field.getName().equals(fieldName)) {
                      return getFieldType(field);
                  }
              }
              node = node.getOuterClass();
          }
          return null;
      }
docs: Supports discovering many common JDK types, but not all.