思路

1.第一次获取放设置过期时间放map
2.下次从map中获取，如果如果为空就请求微信api. 如果不为空，就拿上次存放的过期时间和现在时间做比对，如果过期时间早于当前时间，就重新获取。

代码 
    protected synchronized AccessToken getAccessToken() {
        AccessToken accessToken = (AccessToken)tokenMap.get(this.appid);
        Long now = (new Date()).getTime();
        if (accessToken == null || now > accessToken.getExpiresTimestemp()) {
            accessToken = WechatUtils.getAccessToken(this.appid, this.secret);
            tokenMap.put(this.appid, accessToken);
            logger.info("new accessToken");
        }

        return accessToken;
    }
    public static AccessToken getAccessToken(String appid, String secret) {
        if (appid != null && secret != null) {
            ClientConfig config = new DefaultClientConfig();
            Client client = Client.create(config);
            WebResource cosmsservice = client.resource("https://api.weixin.qq.com/cgi-bin/token");
            ClientResponse clientResponse = (ClientResponse)cosmsservice.queryParam("grant_type", "client_credential").queryParam("appid", appid).queryParam("secret", secret).get(ClientResponse.class);
            if (clientResponse.getStatus() != 200) {
                throw new IllegalStateException("status error:" + clientResponse.getStatus());
            } else {
                Map result = (Map)(new Gson()).fromJson((String)clientResponse.getEntity(String.class), Map.class);
                if (result.get("errcode") != null) {
                    throw new WechatAccessException(result.get("errcode") + "", result.get("errmsg") + "");
                } else {
                    long now = (new Date()).getTime();
                    return new AccessToken((String)result.get("access_token"), ((Double)result.get("expires_in")).longValue() * 1000L + now);
                }
            }
        } else {
            throw new IllegalArgumentException("appid or secret was null.");
        }
    }