public class Message implements Serializable {
    private static final long serialVersionUID = 8445773977080406428L;
    //主題名字
    private String topic;
    //消息擴(kuò)展信息，Tag,keys,延遲級(jí)別都存在這里
    private Map<String, String> properties;
    //消息體，字節(jié)數(shù)組
    private byte[] body;
    //設(shè)置消息的key，
    public void setKeys(String keys) {}
    //設(shè)置topic
    public void setTopic(String topic) {}
    //延遲級(jí)別
    public int setDelayTimeLevel(int level) {}
    //消息過濾的標(biāo)記
    public void setTags(String tags) {}
    //擴(kuò)展信息存放在此
    public void putUserProperty(final String name, final String value) {}
}

//這里取到了重試的次數(shù)
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();
    //獲取消息隊(duì)列
    MessageQueue mqSelected = this.selectOneMessageQueue(topicPublishInfo, lastBrokerName);
    if (mqSelected != null) {
        mq = mqSelected;
        brokersSent[times] = mq.getBrokerName();
        try {
            beginTimestampPrev = System.currentTimeMillis();
            if (times > 0) {
                //Reset topic with namespace during resend.
                msg.setTopic(this.defaultMQProducer.withNamespace(msg.getTopic()));
            }
            long costTime = beginTimestampPrev - beginTimestampFirst;
            if (timeout < costTime) {
                callTimeout = true;
                break;
            }
            //發(fā)送消息
            sendResult = this.sendKernelImpl(msg, mq, communicationMode, sendCallback, topicPublishInfo, timeout - costTime);
            ...
        } catch (RemotingException e) {
            ...
            continue;
        } catch (MQClientException e) {
            ...
            continue;
        } catch (MQBrokerException e) {
            ...
            continue;
        } catch (InterruptedException e) {
            //可以看到只有InterruptedException拋出了異常，其他的exception都會(huì)繼續(xù)重試
            throw e;
        }
    } else {
        break;
    }
}

public MessageQueue selectOneMessageQueue(final TopicPublishInfo tpInfo, final String lastBrokerName) {
    //判斷發(fā)送延遲容錯(cuò)開關(guān)是否開啟
    if (this.sendLatencyFaultEnable) {
        try {
            //選擇一個(gè)延遲上可以接受，并且和上次發(fā)送相同的Broker
            int index = tpInfo.getSendWhichQueue().incrementAndGet();
            for (int i = 0; i < tpInfo.getMessageQueueList().size(); i++) {
                int pos = Math.abs(index++) % tpInfo.getMessageQueueList().size();
                if (pos < 0)
                    pos = 0;
                MessageQueue mq = tpInfo.getMessageQueueList().get(pos);
                //若是Broker的延遲時(shí)間可以接受，則返回這個(gè)Broker
                if (latencyFaultTolerance.isAvailable(mq.getBrokerName()))
                    return mq;
            }
            //若是第一步?jīng)]能選中一個(gè)Broker，就選擇一個(gè)延遲較低的Broker
            final String notBestBroker = latencyFaultTolerance.pickOneAtLeast();
            int writeQueueNums = tpInfo.getQueueIdByBroker(notBestBroker);
            if (writeQueueNums > 0) {
                final MessageQueue mq = tpInfo.selectOneMessageQueue();
                if (notBestBroker != null) {
                    mq.setBrokerName(notBestBroker);
                    mq.setQueueId(tpInfo.getSendWhichQueue().incrementAndGet() % writeQueueNums);
                }
                return mq;
            } else {
                latencyFaultTolerance.remove(notBestBroker);
            }
        } catch (Exception e) {
            log.error("Error occurred when selecting message queue", e);
        }
        //若是前邊都沒選中一個(gè)Broker，就隨機(jī)選一個(gè)Broker
        return tpInfo.selectOneMessageQueue();
    }
    return tpInfo.selectOneMessageQueue(lastBrokerName);
}

public MessageQueue selectOneMessageQueue(final String lastBrokerName) {
    //若是沒有上次的Brokername做參考，就隨機(jī)選一個(gè)
    if (lastBrokerName == null) {
        return selectOneMessageQueue();
    } else {
        //如果有，那么就選一個(gè)其他的Broker
        for (int i = 0; i < this.messageQueueList.size(); i++) {
            int index = this.sendWhichQueue.incrementAndGet();
            int pos = Math.abs(index) % this.messageQueueList.size();
            if (pos < 0)
                pos = 0;
            MessageQueue mq = this.messageQueueList.get(pos);
            //這里判斷遇上一個(gè)使用的Broker不是同一個(gè)
            if (!mq.getBrokerName().equals(lastBrokerName)) {
                return mq;
            }
        }
        //若是上邊的都沒選中，那么就隨機(jī)選一個(gè)
        return selectOneMessageQueue();
    }
}