private SendResult sendDefaultImpl(
    Message msg,
    final CommunicationMode communicationMode,
    final SendCallback sendCallback,
    final long timeout
) throws MQClientException, RemotingException, MQBrokerException, InterruptedException {
    // 确保producer已经启动
    this.makeSureStateOK();
    // 校验message，例如topic不能为空
    Validators.checkMessage(msg, this.defaultMQProducer);
    // 目前来看，仅在日志输出时标示此次调用
    final long invokeID = random.nextLong();
    long beginTimestampFirst = System.currentTimeMillis();
    long beginTimestampPrev = beginTimestampFirst;
    long endTimestamp = beginTimestampFirst;
    // 获取topic发布路由信息 -- 会先尝试缓存中获取，其次从namesrv中获取
    TopicPublishInfo topicPublishInfo = this.tryToFindTopicPublishInfo(msg.getTopic());
    if (topicPublishInfo != null && topicPublishInfo.ok()) {
        MessageQueue mq = null;
        Exception exception = null;
        SendResult sendResult = null;
        // 同步发送默认重试3次
        int timesTotal = communicationMode == CommunicationMode.SYNC ? 1 + this.defaultMQProducer.getRetryTimesWhenSendFailed() : 1;
        int times = 0;
        String[] brokersSent = new String[timesTotal];
        for (; times < timesTotal; times++) {
            String lastBrokerName = null == mq ? null : mq.getBrokerName();
            /*
                * 选择一个消息队列
                * 规则：1）默认情况，不开启发送的延迟容错策略时，RoundRobin形式的选择一个不属于上一次发送的broker队列
                *      2）开启延迟容错策略时：
                *          2.1）优先从上一次的发送的broker中RoundRobin形式选择一个可用队列
                *          2.2）其次按照可用性排名（是否可用>延迟时间>开始时间）从前半数中RoundRobin选
                *          2.3）最次，啥都不管，RoundRobin选
                */
            MessageQueue mqSelected = this.selectOneMessageQueue(topicPublishInfo, lastBrokerName);
            if (mqSelected != null) {
                mq = mqSelected;
                brokersSent[times] = mq.getBrokerName();
                try {
                    beginTimestampPrev = System.currentTimeMillis();
                    // 核心消息投递方法
                    sendResult = this.sendKernelImpl(msg, mq, communicationMode, sendCallback, topicPublishInfo, timeout);
                    endTimestamp = System.currentTimeMillis();
                    // 更新延迟容错信息
                    this.updateFaultItem(mq.getBrokerName(), endTimestamp - beginTimestampPrev, false);
                    switch (communicationMode) {
                        case ASYNC:
                            return null;
                        case ONEWAY:
                            return null;
                        case SYNC:
                            if (sendResult.getSendStatus() != SendStatus.SEND_OK) {
                                if (this.defaultMQProducer.isRetryAnotherBrokerWhenNotStoreOK()) {
                                    continue;
                                }
                            }
                            return sendResult;
                        default:
                            break;
                    }
                } catch (RemotingException e) {
                    // ...省略部分代码
                }
            } else {
                break;
            }
        }
        // ...省略部分代码
    }
    // ...省略部分代码
}