public static Boolean isWxClient(HttpServletRequest request){
    String header = request.getHeader("USER-AGENT");
    return header != null && header.contains("MicroMessenger");
}
            

// 微信授权页面
private static final String authorizeUrl = "https://open.weixin.qq.com/connect/oauth2/authorize";
// 获取微信用户openId
private static final String getOpenIdUrl = "https://api.weixin.qq.com/sns/oauth2/access_token";
// 获取访问token
private static final String getTokenUrl = "https://api.weixin.qq.com/cgi-bin/token?grant_type=client_credential";
        

String requestUri = req.getRequestURI();
if (WxConstants.isWxClient(req)
    && filterUri(requestUri)){ // 过滤一些业务中不需要拦截的URI, 比如登入登出等(若登入登出拦截了, 用户将无法登入登出)
    Object openId = req.getSession().getAttribute(WxConstants.OPEN_ID);
    if (openId == null){
        String redirectUrl = "http://" + ThreadVars.getHost() + requestUri;
        // 未登录
        Object code = req.getParameter(WxConstants.CODE);
        if (code == null){
            // 重定向到微信认证
            wxRequestor.toAuthorize(redirectUrl, resp);
            return Boolean.FALSE;
            } else {
                log.info("[weixin]: get code({}) from authorized.", code);
                // 获取openId
                Map<String, Object> mapResp = wxRequestor.getOpenId(String.valueOf(code));
                if (mapResp.get("errcode") != null){
                    log.error("[weixin]: failed to get openid by code({}) cause: {}", code, mapResp);
                } else {
                    String openIdStr = String.valueOf(mapResp.get(WxConstants.OPEN_ID));
                    // TODO 这里可以从数据源中通过openIdStr查询对应的系统用户是否存在, 存在则可以让用户自动登录
                }
            }
        }
    }
}
            

public final class WxConstants {
    // 微信浏览器USER-AGENT标识
    public static final String WEIXIN_USER_AGENT = "MicroMessenger";
    // openid变量
    public static final String OPEN_ID = "openid";
    // code变量
    public static final String CODE = "code";

    /**
     * 判断当前请求是否来自微信浏览器
    **/
    public static Boolean isWxClient(HttpServletRequest request){
        String header = request.getHeader("USER-AGENT");
        return header != null && header.contains(WEIXIN_USER_AGENT);
    }
}

public class WxRequestor{

    private String appId;   //微信公众号ID

    private String secret;  //微信公众号密钥

    ...

    /**
    * 重定向微信认证页面
    */
    public void toAuthorize(String redirectUrl, HttpServletResponse response) throws IOException {
        toAuthorize(redirectUrl, "123", response);
    }

    /**
    * 重定向微信认证页面
    */
    public void toAuthorize(String redirectUrl, String state, HttpServletResponse response) throws IOException {
        String encodedUrl = URLEncoder.encode(redirectUrl, "utf-8");
        StringBuilder redirect = new StringBuilder(authorizeUrl);
        redirect.append("?appid=").append(appId)
                .append("&redirect_uri=").append(encodedUrl)
                .append("&response_type=code&scope=snsapi_base") //其中scope分两种: snsapi_base和snsapi_userinfo, 可见http://mp.weixin.qq.com/wiki/17/c0f37d5704f0b64713d5d2c37b468d75.html
                .append("&state=").append(state)
                .append("#wechat_redirect");
        log.info("[weixin]: go to authorize page : {}.", redirect);
        response.sendRedirect(redirect.toString());
    }
    /**
    * 获取微信用户的openid
    * @param code 认证后的code值
    * @return 获取成功openid存放在map中, 反之errcode为错误码
    */
    public Map<String, Object> getOpenId(String code){
        StringBuilder url = new StringBuilder(getOpenIdUrl);
        url.append("?appid=").append(appId)
            .append("&secret=").append(secret)
            .append("&code=").append(code)
            .append("&grant_type=authorization_code");
        return get(url.toString());
    }

    /**
    * 发送微信GET请求(默认不带上access_token)
    * @param url 请求url
    * @return map结果集
    */
    public Map<String, Object> get(String url){
        return get(url, Boolean.FALSE);
    }

    /**
    * 发送微信GET请求
    * @param url 请求url
    * @param withAccessToken 是否要带上access_token
    * @return map结果集
    */
    public Map<String, Object> get(String url, boolean withAccessToken){
        Map<String, Object> mapResp = doGet(url, withAccessToken);
        log.info("[weixin]: do get request({}), and get response({}).", url, mapResp);
        Object errcode = mapResp.get("errcode");
        if (errcode != null && withAccessToken){
            if (Objects.equal("40003", errcode)){
                // token失效
                doGetAccessToken();
                return doGet(url, withAccessToken);
            }
            log.error("[weixin]: failed to do wx request({}), cause: {}", url, mapResp);
        }
        return mapResp;
    }

    private Map<String, Object> doGet(String url, boolean withAccessToken){
        String requestUrl = withAccessToken ? addAccessToken(url) : url;
        String jsonResp = HttpRequest.get(requestUrl).body();
        Map<String, Object> mapResp = toJson(jsonResp, Map.class); //将json字符串转换为map对象
        return mapResp;
    }
}