code: public static final Class resolveTypeVariable(Class invocationClass, Class declaringClass,
                                                    String typeVarName) {
          TypeVariable typeVariable = null;
          for (TypeVariable typeParemeter : declaringClass.getTypeParameters()) {
              if (typeParemeter.getName().equals(typeVarName)) {
                  typeVariable = typeParemeter;
                  break;
              }
          }
          if (typeVariable == null) {
              throw new NullPointerException("not found TypeVariable name " + typeVarName);
          }
          return resolveTypeVariable(invocationClass, typeVariable);
      }
docs: 求declaringClass类中声明的泛型类型变量在invocationClass中真正的值
  
  @param invocationClass 编程时使用的类
  @param declaringClass 声明类型变量typeVarName的类
  @param typeVarName 泛型变量名
  @return