code: public AccessToken obtainAccessTokenByClientCredentials() throws TokenExpiredException{
          String key = "obtainAccessTokenByClientCredentials:"+config.getClientId();
          AccessToken accessToken = getAccessTokenFromCache(key);
          if (accessToken != null){
              return accessToken;
          }
  
          accessToken = tp().obtainAccessTokenByClientCredentials();
          cp().put(key,accessToken,accessToken.getExpires());
          return accessToken;
      }
docs: 使用<code>client_id</code>和<code>client_secret</code>通过<code>grant_type=client_credentials</code>的方式
  获取access token。
  
  返回SSO颁发给当前应用的access token，代表当前应用的身份。
  
  @see <a href="http://openid.net/specs/openid-connect-core-1_0.html#ClientAuthentication">ClientAuthentication</a>
  
  @since 3.0.1