whole_func_string: function(key) {
          var str = this.loadVar(key);
          if (typeof (str) != 'string') {
              exitProcess('the value of ' + key + ' is a necessary string, but get ' + str, this.alarm);
              return false;
          }
          return str;
      }
func_code_string: function(key) {
          var str = this.loadVar(key);
          if (typeof (str) != 'string') {
              exitProcess('the value of ' + key + ' is a necessary string, but get ' + str, this.alarm);
              return false;
          }
          return str;
      }
func_documentation_string: Get string value by key, the value must be exist, otherwise the process will exit.
  
  @param {KeyItem} key
  @returns {String}