[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：

        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：

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

之后调用processDelivery方法：

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：

@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：

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。

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附：本系列全集

1. [Conclusion]RabbitMQ-客户端源码之总结
2. [一]RabbitMQ-客户端源码之ConnectionFactory
3. [二]RabbitMQ-客户端源码之AMQConnection
4. [三]RabbitMQ-客户端源码之ChannelManager
5. [四]RabbitMQ-客户端源码之Frame
6. [五]RabbitMQ-客户端源码之AMQChannel
7. [六]RabbitMQ-客户端源码之AMQCommand
8. [七]RabbitMQ-客户端源码之AMQPImpl+Method
9. [八]RabbitMQ-客户端源码之ChannelN
10. [九]RabbitMQ-客户端源码之Consumer

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