code: public static String extractAccessToken(HttpServletRequest request) {
          String accessToken = request.getHeader(AUTHORIZATION_HEADER);
          if(accessToken ==null || accessToken.trim().isEmpty()){
          	accessToken=request.getParameter(ACCESS_TOKEN_PARAM_NAME);
          }else{
          	accessToken = accessToken.trim();
              if(accessToken.startsWith(BEARER)){
                  accessToken = accessToken.substring(BEARER.length());
                  accessToken= accessToken.trim();
              }
          }
          return accessToken;
      }
docs: 从{@link HttpServletRequest}对象中解析accessToken，这里的accessToken是放在名为Authorization的请求头里。
  
  如果没有名为Authorization的请求头，则返回null
  
  <p/>
  示例：
  <pre>
  Authorization: Bearer {accessToken}
  </pre>