String code = request.getParameter("code")

/**
 * 常量类
 * @author rory.wu
 *
 */
public class Constants {
 // 第三方用户唯一凭证
 public static String appid = "";
 // 第三方用户唯一凭证密钥
 public static String appsecret = "";
 //商户ID
 public static String mch_id="";
 //获取openId
 public static String oauth_url = "https://api.weixin.qq.com/sns/oauth/access_token?appid=APPID&secret=SECRET&code=CODE&grant_type=authorization_code";
}

/**
 * 通用工具类
 * @author rory.wu
 * @version .
 * @since 年月日
 */
 public class CommonUtil {
 private static Logger log = Logger.getLogger(CommonUtil.class);
 public static JSONObject httpsRequestToJsonObject(String requestUrl, String requestMethod, String outputStr) {
  JSONObject jsonObject = null;
  try {
  StringBuffer buffer = httpsRequest(requestUrl, requestMethod, outputStr);
  jsonObject = JSONObject.fromObject(buffer.toString());
  } catch (ConnectException ce) {
  log.error("连接超时："+ce.getMessage());
  } catch (Exception e) {
  log.error("https请求异常："+e.getMessage());
  }
  return jsonObject;
 }
 
 private static StringBuffer httpsRequest(String requestUrl, String requestMethod, String output)
  throws NoSuchAlgorithmException, NoSuchProviderException, KeyManagementException, MalformedURLException,
  IOException, ProtocolException, UnsupportedEncodingException {
  URL url = new URL(requestUrl);
  HttpsURLConnection connection = (HttpsURLConnection) url.openConnection();
  connection.setDoOutput(true);
  connection.setDoInput(true);
  connection.setUseCaches(false);
  connection.setRequestMethod(requestMethod);
  if (null != output) {
  OutputStream outputStream = connection.getOutputStream();
  outputStream.write(output.getBytes("UTF-"));
  outputStream.close();
  }
  // 从输入流读取返回内容
  InputStream inputStream = connection.getInputStream();
  InputStreamReader inputStreamReader = new InputStreamReader(inputStream, "utf-");
  BufferedReader bufferedReader = new BufferedReader(inputStreamReader);
  String str = null;
  StringBuffer buffer = new StringBuffer();
  while ((str = bufferedReader.readLine()) != null) {
  buffer.append(str);
  }
  bufferedReader.close();
  inputStreamReader.close();
  inputStream.close();
  inputStream = null;
  connection.disconnect();
  return buffer;
 } }

 　　/**
 * 获取用户的openId，并放入session
 * @param code 微信返回的code
 */
 private void setOpenId(String code) {
  session.put("code", code);
  String oauth_url = Constants.oauth_url.replace("APPID", Constants.appid).replace("SECRET", Constants.appsecret).replace("CODE", String.valueOf(session.get("code")));
  log.info("oauth_url:"+oauth_url);
  JSONObject jsonObject = CommonUtil.httpsRequestToJsonObject(oauth_url, "POST", null);
  log.info("jsonObject:"+jsonObject);
  Object errorCode = jsonObject.get("errcode");
  if(errorCode != null) {
  log.info("code不合法");
  }else{
  String openId = jsonObject.getString("openid");
  log.info("openId:"+openId);
  session.put("openId", openId);
  }
 }
oauth_url返回的格式是：
　　{ 
　　　"access_token":"ACCESS_TOKEN", 
　　　"expires_in":, 
 "refresh_token":"REFRESH_TOKEN", 
 "openid":"OPENID", "scope":"SCOPE", 
 "unionid": "o_bmasdasdsad_sgVthMZOPfL" 
 }
Code无效时：
　　{
　　 "errcode":
　　　,"errmsg":"invalid code"
 }