code: @Override
      public void visitClassContext(ClassContext classContext) {
          try {
              stack = new OpcodeStack();
              changedAttributes = new HashMap<>();
              savedAttributes = new HashMap<>();
              super.visitClassContext(classContext);
          } finally {
              stack = null;
              changedAttributes = null;
              savedAttributes = null;
          }
      }
docs: implements the visitor to setup the opcode stack and attribute maps
  
  @param classContext
  the currently parsed class