看完了上一篇的《RocketMQ 拉取消息-通信模塊》，請求進入PullMessageProcessor中，接著

PullMessageProcessor.processRequest(final ChannelHandlerContext ctx, RemotingCommand request)方法中調用了：

 final GetMessageResult getMessageResult =
                this.brokerController.getMessageStore().getMessage(requestHeader.getConsumerGroup(), requestHeader.getTopic(),
                        requestHeader.getQueueId(), requestHeader.getQueueOffset(), requestHeader.getMaxMsgNums(), subscriptionData);
 

來從硬盤中獲取消息體。

接著來看看DefaultMessageStore是如何從消息隊列中獲取消息的：

    /**
     * 獲取消息結果
     * 所有的參數都是從requestheader中獲取的。也就是說從consumer client端傳遞過來的。
     * @param group
     * @param topic
     * @param queueId
     * @param offset
     * @param maxMsgNums
     * @param subscriptionData
     * @return
     */
    public
 
 GetMessageResult getMessage(final String group, final String topic, final int queueId, final long offset, final int maxMsgNums,
                                       final SubscriptionData subscriptionData) {
        if (this.shutdown) {
            log.warn("message store has shutdown, so getMessage is forbidden");
            
 
return null;
        }

        if (!this.runningFlags.isReadable()) {
            log.warn("message store is not readable, so getMessage is forbidden " + this.runningFlags.getFlagBits());
            return null;
        }

        long beginTime = this.getSystemClock().now();

        // 枚舉變量，取消息結果
        GetMessageStatus status = GetMessageStatus.NO_MESSAGE_IN_QUEUE;
        // 當被過濾後，返回下一次開始的Offset
        long nextBeginOffset = offset;
        // 邏輯隊列中的最小Offset
        long minOffset = 0;
        // 邏輯隊列中的最大Offset
        long maxOffset = 0;

        GetMessageResult getResult = new GetMessageResult();


        final long maxOffsetPy = this.commitLog.getMaxOffset();

        //通過topic和queueid查找邏輯隊列對象，相當於字典的目錄，用來指定消息在物理文件commitlog上的位置
        ConsumeQueue consumeQueue = findConsumeQueue(topic, queueId);
        if (consumeQueue != null) {
            minOffset = consumeQueue.getMinOffsetInQuque();
            maxOffset = consumeQueue.getMaxOffsetInQuque();

            if (maxOffset == 0) {
                status = GetMessageStatus.NO_MESSAGE_IN_QUEUE;
                nextBeginOffset = nextOffsetCorrection(offset, 0);
            } else if (offset < minOffset) {
                status = GetMessageStatus.OFFSET_TOO_SMALL;
                nextBeginOffset = nextOffsetCorrection(offset, minOffset);
            } else if (offset == maxOffset) {
                status = GetMessageStatus.OFFSET_OVERFLOW_ONE;
                nextBeginOffset = nextOffsetCorrection(offset, offset);
            } else if (offset > maxOffset) {
                status = GetMessageStatus.OFFSET_OVERFLOW_BADLY;
                if (0 == minOffset) {
                    nextBeginOffset = nextOffsetCorrection(offset, minOffset);
                } else {
                    nextBeginOffset = nextOffsetCorrection(offset, maxOffset);
                }
            } else {
                SelectMapedBufferResult bufferConsumeQueue = consumeQueue.getIndexBuffer(offset);
                if (bufferConsumeQueue != null) {
                    try {
                        status = GetMessageStatus.NO_MATCHED_MESSAGE;

                        long nextPhyFileStartOffset = Long.MIN_VALUE;
                        long maxPhyOffsetPulling = 0;

                        int i = 0;
                        final int MaxFilterMessageCount = 16000;
                        final boolean diskFallRecorded = this.messageStoreConfig.isDiskFallRecorded();
                        for (; i < bufferConsumeQueue.getSize() && i < MaxFilterMessageCount; i += ConsumeQueue.CQStoreUnitSize) {
                            long offsetPy = bufferConsumeQueue.getByteBuffer().getLong();
                            int sizePy = bufferConsumeQueue.getByteBuffer().getInt();
                            long tagsCode = bufferConsumeQueue.getByteBuffer().getLong();

                            maxPhyOffsetPulling = offsetPy;

                            // 說明物理文件正在被刪除
                            if (nextPhyFileStartOffset != Long.MIN_VALUE) {
                                if (offsetPy < nextPhyFileStartOffset)
                                    continue;
                            }

                            // 判斷是否拉磁盤數據
                            boolean isInDisk = checkInDiskByCommitOffset(offsetPy, maxOffsetPy);
                            // 此批消息達到上限了
                            if (this.isTheBatchFull(sizePy, maxMsgNums, getResult.getBufferTotalSize(), getResult.getMessageCount(),
                                    isInDisk)) {
                                break;
                            }

                            // 消息過濾
                            if (this.messageFilter.isMessageMatched(subscriptionData, tagsCode)) {
                                SelectMapedBufferResult selectResult = this.commitLog.getMessage(offsetPy, sizePy);
                                if (selectResult != null) {
                                    this.storeStatsService.getGetMessageTransferedMsgCount().incrementAndGet();
                                    getResult.addMessage(selectResult);
                                    status = GetMessageStatus.FOUND;
                                    nextPhyFileStartOffset = Long.MIN_VALUE;
                                } else {
                                    if (getResult.getBufferTotalSize() == 0) {
                                        status = GetMessageStatus.MESSAGE_WAS_REMOVING;
                                    }


                                    nextPhyFileStartOffset = this.commitLog.rollNextFile(offsetPy);
                                }
                            } else {
                                if (getResult.getBufferTotalSize() == 0) {
                                    status = GetMessageStatus.NO_MATCHED_MESSAGE;
                                }

                                if (log.isDebugEnabled()) {
                                    log.debug("message type not matched, client: " + subscriptionData + " server: " + tagsCode);
                                }
                            }
                        }


                        if (diskFallRecorded) {
                            long fallBehind = maxOffsetPy - maxPhyOffsetPulling;
                            brokerStatsManager.recordDiskFallBehindSize(group, topic, queueId, fallBehind);
                        }

                        nextBeginOffset = offset + (i / ConsumeQueue.CQStoreUnitSize);


                        long diff = maxOffsetPy - maxPhyOffsetPulling;
                        long memory = (long) (StoreUtil.TotalPhysicalMemorySize
                                * (this.messageStoreConfig.getAccessMessageInMemoryMaxRatio() / 100.0));
                        getResult.setSuggestPullingFromSlave(diff > memory);
                    } finally {
                        // 必須釋放資源
                        bufferConsumeQueue.release();
                    }
                } else {
                    status = GetMessageStatus.OFFSET_FOUND_NULL;
                    nextBeginOffset = nextOffsetCorrection(offset, consumeQueue.rollNextFile(offset));
                    log.warn("consumer request topic: " + topic + "offset: " + offset + " minOffset: " + minOffset + " maxOffset: "
                            + maxOffset + ", but access logic queue failed.");
                }
            }
        }
        // 請求的隊列Id沒有
        else {
            status = GetMessageStatus.NO_MATCHED_LOGIC_QUEUE;
            nextBeginOffset = nextOffsetCorrection(offset, 0);
        }

        if (GetMessageStatus.FOUND == status) {
            this.storeStatsService.getGetMessageTimesTotalFound().incrementAndGet();
        } else {
            this.storeStatsService.getGetMessageTimesTotalMiss().incrementAndGet();
        }
        long eclipseTime = this.getSystemClock().now() - beginTime;
        this.storeStatsService.setGetMessageEntireTimeMax(eclipseTime);

        getResult.setStatus(status);
        getResult.setNextBeginOffset(nextBeginOffset);
        getResult.setMaxOffset(maxOffset);
        getResult.setMinOffset(minOffset);
        return getResult;
    }

先來看一下獲取消息隊列的方法：

    public ConsumeQueue findConsumeQueue(String topic, int queueId) {
        ConcurrentHashMap<Integer, ConsumeQueue> map = consumeQueueTable.get(topic);
        if (null == map) {
            ConcurrentHashMap<Integer, ConsumeQueue> newMap = new ConcurrentHashMap<Integer, ConsumeQueue>(128);
            ConcurrentHashMap<Integer, ConsumeQueue> oldMap = consumeQueueTable.putIfAbsent(topic, newMap);
            if (oldMap != null) {
                map = oldMap;
            } else {
                map = newMap;
            }
        }

        ConsumeQueue logic = map.get(queueId);
        if (null == logic) {
            ConsumeQueue newLogic = new ConsumeQueue(//
                    topic, //
                    queueId, //
                    StorePathConfigHelper.getStorePathConsumeQueue(this.messageStoreConfig.getStorePathRootDir()), //
                    this.getMessageStoreConfig().getMapedFileSizeConsumeQueue(), //
                    this);
            ConsumeQueue oldLogic = map.putIfAbsent(queueId, newLogic);
            if (oldLogic != null) {
                logic = oldLogic;
            } else {
                logic = newLogic;
            }
        }

        return logic;
    }

RocketMQ 拉取消息-文件獲取