九、RabbitMQ-客户端源码之Consumer

在[八]RabbitMQ-客户端源码之ChannelN中讲述basicConsume的方法时设计到Consumer这个回调函数，Consumer其实是一个接口，真正实现它的是QueueingConsumer和DefaultConsumer，且DefaultConsumer是QueueingConsumer的父类，里面都是空方法。在用户使用时可以简单的采用QueueingConsumer或者采用DefaultConsumer来重写某些方法。

这里先来看下消费者客户端的关键代码：

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      QueueingConsumer consumer = new QueueingConsumer(channel);
      channel.basicQos(32);
channel.basicConsume(QUEUE_NAME, false, "consumer_zzh",consumer)

      while (true) {
          QueueingConsumer.Delivery delivery = consumer.nextDelivery();
          String message = new String(delivery.getBody());
          System.out.println(" [X] Received '" + message + "'");
          channel.basicAck(delivery.getEnvelope().getDeliveryTag(),false);
      }

可以看到QueueingConsumer作为channel.basicConsume的回调函数，之后再进行处理。

在AMQConnection中有关MainLoop的主线程，专门用来”第一线”的处理Broker发送回客户端从帧。当Basic.Consume/.ConsumeOk开启消费模式之后，Broker主动的向客户端发送Basic.Delivery帧，MainLoop线程一步步的调用，最后到ChannelN的processAsync()方法中有：

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if (method instanceof Basic.Deliver) {
    processDelivery(command, (Basic.Deliver) method);
    return true;
}

之后调用processDelivery方法：

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protected void processDelivery(Command command, Basic.Deliver method) {
    Basic.Deliver m = method;

    Consumer callback = _consumers.get(m.getConsumerTag());
    if (callback == null) {
        if (defaultConsumer == null) {
            throw new IllegalStateException("Unsolicited delivery -" + " see Channel.setDefaultConsumer to handle this" + " case.");
        }
        else {
            callback = defaultConsumer;
        }
    }

    Envelope envelope = new Envelope(m.getDeliveryTag(), m.getRedelivered(),m.getExchange(),m.getRoutingKey());
    try {
        this.dispatcher.handleDelivery(callback, m.getConsumerTag(),envelope, (BasicProperties) command.getContentHeader(),command.getContentBody());
    } catch (Throwable ex) {
        getConnection().getExceptionHandler().handleConsumerException(this, ex,callback,m.getConsumerTag(), "handleDelivery");
    }
}

这个方法首先根据consumerTag从ChannelN中的_consumer这个HashMap中获取相应的Consumer回调函数，然后调用这个回调函数的handleDeliver()方法进行处理，这里有些同学会有疑问，明明是调用ConsumerDispatcher dispatcher的handleDeliver()方法，其实这里只是包了一层皮，ConsumerDispatcher的handleDeliver()方法就是调用了Consumer的handleDeliver()方法。

我们接下去看看QueueingConsumer这个实现Consumer接口的类是怎么处理的：

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@Override public void handleDelivery(String consumerTag,
                           Envelope envelope,
                           AMQP.BasicProperties properties,
                           byte[] body)
    throws IOException
{
    checkShutdown();
    this._queue.add(new Delivery(envelope, properties, body));
}

这里的queue就是一个LinkedBlockingQueue，客户端程序通过调用nextDelivery()方法来获取数据：

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public Delivery nextDelivery()
    throws InterruptedException, ShutdownSignalException, ConsumerCancelledException
{
    return handle(_queue.take());
}

private Delivery handle(Delivery delivery) {
    if (delivery == POISON ||
        delivery == null && (_shutdown != null || _cancelled != null)) {
        if (delivery == POISON) {
            _queue.add(POISON);
            if (_shutdown == null && _cancelled == null) {
                throw new IllegalStateException(
                    "POISON in queue, but null _shutdown and null _cancelled. " +
                    "This should never happen, please report as a BUG");
            }
        }
        if (null != _shutdown)
            throw Utility.fixStackTrace(_shutdown);
        if (null != _cancelled)
            throw Utility.fixStackTrace(_cancelled);
    }
    return delivery;
}

这个nextDelivery方法说白就是一个LinkedBlockingQueue的take()操作，也就是一个可能会阻塞等待的操作。