code: public static Object getElValue(JspContext context, String elName) {
          VariableResolver variableResolver = context.getVariableResolver();
          Object var = null;
          try {
              elName = TagUtils.getElName(elName);
              var    = variableResolver.resolveVariable(elName);
              logger.debug("ExpressionLanguage variable {} is [{}]", elName, var);
          } catch (ELException e) {
              logger.error("ExpressionLanguage Error", e);
          }
  
          return var;
      }
docs: "${...}" EL 명을 Object 값으로 변환한다.
  
  @param context
  @param elName
  @return