code: private Class getTargetClass(Method setMethod) {
          Class[] params = setMethod.getParameterTypes();
          if (params == null || params.length != 1) {
              throw new BeanMappingException("illegal set method[" + setMethod.getName() + "] for ParameterType");
          }
          return params[0];
      }
docs: 根据{@linkplain SetExecutor}获取对应的目标targetClass