1，MessageDispatch消息分发信息

    public static final byte DATA_STRUCTURE_TYPE = CommandTypes.MESSAGE_DISPATCH;

    protected ConsumerId consumerId;
    protected ActiveMQDestination destination;
    protected Message message;
    protected int redeliveryCounter;

    protected transient long deliverySequenceId;
    protected transient Object consumer;
    protected transient Runnable transmitCallback;
    protected transient Throwable rollbackCause;

2,自动确认，是再接受到消息，反馈给上层应用之前就给确认

在afterMessageIsConsumed方法中
   先deliveredMessages.clear();接着 session.sendAck(ack);

3,在从tcp取到消息后放到unconsumedMessages等待消费

4,在从unconsumedMessages取出消息预处理后，在beforeMessageIsConsumed方法加消息加到deliveredMessages 里面。 unconsumedMessages; 消费者从mq接受到消息存储的位置，还没有消费

5,receive内部消费之前

 private void beforeMessageIsConsumed(MessageDispatch md) throws JMSException {
        md.setDeliverySequenceId(session.getNextDeliveryId());
        lastDeliveredSequenceId = md.getMessage().getMessageId().getBrokerSequenceId();
        if (!isAutoAcknowledgeBatch()) {
            synchronized(deliveredMessages) {
                deliveredMessages.addFirst(md);
            }
            if (session.getTransacted()) {
                if (transactedIndividualAck) {
                    immediateIndividualTransactedAck(md);
                } else {
                    ackLater(md, MessageAck.DELIVERED_ACK_TYPE);
                }
            }
        }
    }

6,receive内部消费之后

private void afterMessageIsConsumed(MessageDispatch md, boolean messageExpired) throws JMSException {
        if (unconsumedMessages.isClosed()) {
            return;
        }
        if (messageExpired) {
            acknowledge(md, MessageAck.DELIVERED_ACK_TYPE);
            stats.getExpiredMessageCount().increment();
        } else {
            stats.onMessage();
            if (session.getTransacted()) {
                // Do nothing.
            } else if (isAutoAcknowledgeEach()) {
                if (deliveryingAcknowledgements.compareAndSet(false, true)) {
                    synchronized (deliveredMessages) {
                        if (!deliveredMessages.isEmpty()) {
                            if (optimizeAcknowledge) {
                                ackCounter++;

                                // AMQ-3956 evaluate both expired and normal msgs as
                                // otherwise consumer may get stalled
                                if (ackCounter + deliveredCounter >= (info.getPrefetchSize() * .65) || (optimizeAcknowledgeTimeOut > 0 && System.currentTimeMillis() >= (optimizeAckTimestamp + optimizeAcknowledgeTimeOut))) {
                                    MessageAck ack = makeAckForAllDeliveredMessages(MessageAck.STANDARD_ACK_TYPE);
                                    if (ack != null) {
                                        deliveredMessages.clear();
                                        ackCounter = 0;
                                        session.sendAck(ack);
                                        optimizeAckTimestamp = System.currentTimeMillis();
                                    }
                                    // AMQ-3956 - as further optimization send
                                    // ack for expired msgs when there are any.
                                    // This resets the deliveredCounter to 0 so that
                                    // we won't sent standard acks with every msg just
                                    // because the deliveredCounter just below
                                    // 0.5 * prefetch as used in ackLater()
                                    if (pendingAck != null && deliveredCounter > 0) {
                                        session.sendAck(pendingAck);
                                        pendingAck = null;
                                        deliveredCounter = 0;
                                    }
                                }
                            } else {
                                MessageAck ack = makeAckForAllDeliveredMessages(MessageAck.STANDARD_ACK_TYPE);
                                if (ack!=null) {
                                    deliveredMessages.clear();
                                    session.sendAck(ack);
                                }
                            }
                        }
                    }
                    deliveryingAcknowledgements.set(false);
                }
            } else if (isAutoAcknowledgeBatch()) {
                ackLater(md, MessageAck.STANDARD_ACK_TYPE);
            } else if (session.isClientAcknowledge()||session.isIndividualAcknowledge()) {
                boolean messageUnackedByConsumer = false;
                synchronized (deliveredMessages) {
                    messageUnackedByConsumer = deliveredMessages.contains(md);
                }
                if (messageUnackedByConsumer) {
                    ackLater(md, MessageAck.DELIVERED_ACK_TYPE);
                }
            }
            else {
                throw new IllegalStateException("Invalid session state.");
            }
        }
    }

apollo 消息分发源码分析