八、RabbitMQ-客户端源码之ChannelN

作者:朱小厮 | 出自:https://hiddenpps.blog.csdn.net/column/info/14800

ChannelN是整个RabbitMQ客户端最核心的一个类了,其包含的功能点甚多,这里需要分类阐述。
首先来看看ChannelN的成员变量:

private final Map
   
     
      
     _consumers = Collections.synchronizedMap(new HashMap
    
      
       
     ());
private volatile Consumer defaultConsumer = null;
private final ConsumerDispatcher dispatcher;
private final Collection
     
       
        
       returnListeners = new CopyOnWriteArrayList
      
        
         
       ();
private final Collection
       
         
           flowListeners = new CopyOnWriteArrayList 
          
            (); private volatile CountDownLatch finishedShutdownFlag = null; private final Collection 
           
             confirmListeners = new CopyOnWriteArrayList 
            
              (); private long nextPublishSeqNo = 0L; private final SortedSet 
             
               unconfirmedSet = Collections.synchronizedSortedSet(new TreeSet 
              
                ()); private volatile boolean onlyAcksReceived = true; 
               
              
             
            
           
         
      
        
     
       
    
      
   
     

源代码中有关ChannelN的呈现顺序有所不同,这里博主为了区分开来,重新排了序。


processAsync(Command command)

在AMQChannel这个抽象类中唯一的抽象方法即为此方法,这个方法主要用来针对接受到broker的AMQCommand进行进一步的处理,至于怎么接受Socket,怎么封装成帧,怎么确定一个AMQComand已经封装完毕,都已在调用此方法前完成。此方法可以处理:Channel.Close, Basic.Deliver, Basic.Return, Channel.Flow, Basic.Ack, Basic.Nack, Basic.RecoverOk, Basic.Cancel, Channel.CloseOk等这些从broker端回传的AMQComand.
这个方法也比较长,下面也会涉及到这个方法内的内容。


Confirm.Select & Basic.Publish

在[RabbitMQ之消息确认机制(事务+Confirm)][RabbitMQ_Confirm]这篇文章中,博主就讲到RabbitMQ的producer端确认机制分为事务机制和Confirm机制,这里就来阐述下Confirm机制的内部实现。
和Confirm机制有关的成员变量有:

private final Collection
   
     
      
     confirmListeners = new CopyOnWriteArrayList
    
      
       
     ();
private long nextPublishSeqNo = 0L;
private final SortedSet
     
       
        
       unconfirmedSet = Collections.synchronizedSortedSet(new TreeSet
      
        
         
       ());
private volatile boolean onlyAcksReceived = true;
      
        
     
       
    
      
   
     

在使用Confirm机制的时候,首先要置Channel为Confirm模式,即向broker端发送Confirm.Select。
业务代码(DEMO实例):

channel.confirmSelect();
channel.addConfirmListener(new ConfirmListener() {
    public void handleAck(long deliveryTag, boolean multiple) throws IOException {
        //TODO
    }
    public void handleNack(long deliveryTag, boolean multiple) throws IOException {
        //TODO
    }
});
String message = "RabbitMQ Demo Test:" + System.currentTimeMillis();
channel.basicPublish(EXCHANGE_NAME, routingKey, MessageProperties.PERSISTENT_TEXT_PLAIN, message.getBytes());
channel.waitForConfirms();

在创建完Channel之后调用channel.confirmSelect()方法即可,confirmSelect()代码如下:

public Confirm.SelectOk confirmSelect()
    throws IOException
    if (nextPublishSeqNo == 0) nextPublishSeqNo = 1;
    return (Confirm.SelectOk)
        exnWrappingRpc(new Confirm.Select(false)).getMethod();

这里的成员变量nextPublishSeqNo是用来为Confirm机制服务的,当Channel开启Confirm模式的时候,nextPublishSeqNo=1,标记第一条publish的序号,当Publish时:

public void basicPublish(String exchange, String routingKey,  boolean mandatory, boolean immediate, BasicProperties props, byte[] body) throws IOException
    if (nextPublishSeqNo > 0) {
        unconfirmedSet.add(getNextPublishSeqNo());
        nextPublishSeqNo++;
    }
    BasicProperties useProps = props;
    if (props == null) {
        useProps = MessageProperties.MINIMAL_BASIC;
    }
    transmit(new AMQCommand(new Basic.Publish.Builder()
                                .exchange(exchange)
                                .routingKey(routingKey)
                                .mandatory(mandatory)
                                .immediate(immediate)
                            .build(),
                            useProps, body));

client端向broker端Basic.Pubish发送消息并将当前的序号加入到unconfirmedSet中,并自加nextPublishSeqNo++等待下一个消息的发送。

有关Confirm.Select的详细用法可以参考:[RabbitMQ之消息确认机制(事务+Confirm)][RabbitMQ_Confirm]

之后等待broker的确认回复(Basic.Ack/.Nack):channel.waitForConfirms()

public boolean waitForConfirms(long timeout)
        throws InterruptedException, TimeoutException {
    if (nextPublishSeqNo == 0L)
        throw new IllegalStateException("Confirms not selected");
    long startTime = System.currentTimeMillis();
    synchronized (unconfirmedSet) {
        while (true) {
            if (getCloseReason() != null) {
                throw Utility.fixStackTrace(getCloseReason());
            }
            if (unconfirmedSet.isEmpty()) {
                boolean aux = onlyAcksReceived;
                onlyAcksReceived = true;
                return aux;
            }
            if (timeout == 0L) {
                unconfirmedSet.wait();
            } else {
                long elapsed = System.currentTimeMillis() - startTime;
                if (timeout > elapsed) {
                    unconfirmedSet.wait(timeout - elapsed);
                } else {
                    throw new TimeoutException();
                }
            }
        }
    }

可以看到waitForConfirms其实本质上是在等待unconfirmedSet变成empty,否则就线程wait()。

当接收到broker端的ACK/NACK回复时,一步步的经过处理到达processAsync(Command command)方法,然后进而处理Basic.Ack/.Nack帧。

else if (method instanceof Basic.Ack) {
    Basic.Ack ack = (Basic.Ack) method;
    callConfirmListeners(command, ack);
    handleAckNack(ack.getDeliveryTag(), ack.getMultiple(), false);
    return true;
} else if (method instanceof Basic.Nack) {
    Basic.Nack nack = (Basic.Nack) method;
    callConfirmListeners(command, nack);
    handleAckNack(nack.getDeliveryTag(), nack.getMultiple(), true);
    return true;
} 

首先是将相应的Method做一下转换,之后callConfirmListeners(),这个方法是调用成员变量confirmListeners这个list里的所有的ConfirmListener:

private final Collection
   
     
      
     confirmListeners = new CopyOnWriteArrayList
    
      
       
     ();
    
      
   
     

这个ConfirmListener的list就需要在channel.basicPushlish()调用之前先:

channel.confirmSelect();
channel.addConfirmListener(new ConfirmListener() {
    public void handleAck(long deliveryTag, boolean multiple) throws IOException {
        //TODO
    }
    public void handleNack(long deliveryTag, boolean multiple) throws IOException {
        //TODO
    }
});

在调用完ConfirmListener之后继续调用handleAckNack方法:

private void handleAckNack(long seqNo, boolean multiple, boolean nack) {
    if (multiple) {
        unconfirmedSet.headSet(seqNo + 1).clear();
    } else {
        unconfirmedSet.remove(seqNo);
    }
    synchronized (unconfirmedSet) {
        onlyAcksReceived = onlyAcksReceived && !nack;
        if (unconfirmedSet.isEmpty())
            unconfirmedSet.notifyAll();
    }

这个方法本意上是对收到某条消息的ACK或者NACK的处理,发送消息时Basic.Publish的nextPublishNo对应于相应的ACK/NACK的deliveryTag,将其从unconfirmedSet中删除即可,如果有NACK帧,则将其相应的标识onlyAcksReceived设置为false,判断此时unconfirmedSet是否为空,如果条件成立则notifyAll(),将waitForConfirm唤起,返回onlyAcksReceived的状态。

如果channel.waitForConfirm()返回为false,则说明broker没有接受client发送的消息,此时需要在业务代码中做进一步处理,比如重发。


Basic.Qos

消费者在开启ACK的情况下,对接受到的消息可以根据业务的需要异步对消息进行确认。
然而在实际使用过程中,由于消费者自身处理能力有限,从RabbitMQ获取一定数量的消息好厚,希望rabbitmq不再将队列中的消息推送过来,当对消息处理完后(即对消息进行了ack,并且有能力处理更多的消息)再接受来自队列的消息。在这种场景下,我们可以设置Basic.Qos中的prefetch_count来达到这个效果。


Basic.Consume

与消费有关的成员变量:

private final Map
   
     
      
     _consumers =
    Collections.synchronizedMap(new HashMap
    
      
       
     ());
private volatile Consumer defaultConsumer = null;
private final ConsumerDispatcher dispatcher;
    
      
   
     

源码如下:

/**
 * Start a consumer. Calls the consumer's {@link Consumer#handleConsumeOk}
 * method.
 * @param queue the name of the queue
 * @param autoAck true if the server should consider messages
 * acknowledged once delivered; false if the server should expect
 * explicit acknowledgements
 * @param consumerTag a client-generated consumer tag to establish context
 * @param noLocal true if the server should not deliver to this consumer
 * messages published on this channel's connection
 * @param exclusive true if this is an exclusive consumer
 * @param callback an interface to the consumer object
 * @param arguments a set of arguments for the consume
 * @return the consumerTag associated with the new consumer
 * @throws java.io.IOException if an error is encountered
 * @see com.rabbitmq.client.AMQP.Basic.Consume
 * @see com.rabbitmq.client.AMQP.Basic.ConsumeOk
 */
public String basicConsume(String queue, boolean autoAck, String consumerTag,
                           boolean noLocal, boolean exclusive, Map
   
     
      
     arguments,
                           final Consumer callback)
    throws IOException
    BlockingRpcContinuation
    
      
       
      k = new BlockingRpcContinuation
     
       
        
      () {
        public String transformReply(AMQCommand replyCommand) {
            String actualConsumerTag = ((Basic.ConsumeOk) replyCommand.getMethod()).getConsumerTag();
            _consumers.put(actualConsumerTag, callback);
            dispatcher.handleConsumeOk(callback, actualConsumerTag);
            return actualConsumerTag;
        }
    };
    rpc(new Basic.Consume.Builder()
         .queue(queue)
         .consumerTag(consumerTag)
         .noLocal(noLocal)
         .noAck(autoAck)
         .exclusive(exclusive)
         .arguments(arguments)
        .build(),
        k);
    try {
        return k.getReply();
    } catch(ShutdownSignalException ex) {
        throw wrap(ex);
    }
     
       
    
      
   
     

这个方法最精简的只要两个参数,即String queue和Consumer callback:public String basicConsume(String queue, Consumer callback)。

方法主要是发送Basic.Consume帧,然后等待Basic.ConsumeOk帧。待收到broker端的Basic.ConsumeOk帧之后,触发BlockingRpcContinuation中的transformReply()方法。有关BlockingRpcContinuation在[[五]RabbitMQ-客户端源码之AMQChannel][RabbitMQ-_AMQChannel]中有陈述。transformReply()方法先是提取consumerTag,这个consumerTag是在channel.basicConsume()方法中设置的,是其中的一个参数,如果设置了此参数,那么consumerTag就是这个参数的值;如果没有设置这个consumerTag,Broker会返回一个consumerTag,类似:amq.ctag-Mg0eSv2GgfG6UzfncD8E9g。然后作为key和Consumer这个回调函数一起放置到_consumer这个回调函数中以备后面检索调用。这个consumerTag还作为transformReply()方法的返回值,存入到BlockingRpcContinuation对象中,既而在basicConsume这个方法最后调用k.getReply()方法是获取其值,也就是说basicConsume方法的返回值就是consumerTag。

当发送Basic.Consume帧之后,由broker返回的是Basic.ConsumeOk帧+Basic.Deliver帧,Basic.ConsumerOk帧由上面方法处理,Basic.Deliver帧由processAsync处理。

说到basicConsume方法,还有一个重要的就是设置Consumer这个回调函数。一般为了方便直接使用RabbitMQ客户端自带的QueueingConsumer来处理,当然也可以实现一个自定义的Consumer,当然了需要实现Consumer这个接口,可以参考QueueingConsumer的父类DefaultConsumer, 有关Consumer相关的更多细节,可以参考:[[九]RabbitMQ-客户端源码之Consumer][RabbitMQ-_Consumer]。

dispatcher.handleConsumeOk(callback, actualConsumerTag);这段代码实际上就是:callback.handleConsumeOk(actualConsumerTag),这个还是调用到Consumer的方法处理。


Basic.Get

上面的Basic.Consume是基于push模式的,而Basic.Get是基于pull模式的。相关的代码如下:

public GetResponse basicGet(String queue, boolean autoAck)
    throws IOException
    AMQCommand replyCommand = exnWrappingRpc(new Basic.Get.Builder()
                                              .queue(queue)
                                              .noAck(autoAck)
                                             .build());
    Method method = replyCommand.getMethod();
    if (method instanceof Basic.GetOk) {
        Basic.GetOk getOk = (Basic.GetOk)method;
        Envelope envelope = new Envelope(getOk.getDeliveryTag(),
                                         getOk.getRedelivered(),
                                         getOk.getExchange(),
                                         getOk.getRoutingKey());
        BasicProperties props = (BasicProperties)replyCommand.getContentHeader();
        byte[] body = replyCommand.getContentBody();
        int messageCount = getOk.getMessageCount();
        return new GetResponse(envelope, props, body, messageCount);
    } else if (method instanceof Basic.GetEmpty) {
        return null;
    } else {
        throw new UnexpectedMethodError(method);
    }

基本上就是客户端发送Basic.Get至Broker,Broker返回Basic.GetOK并携带数据。注意方法最后返回GetResponse对象,这个对象就是包装了一下数据。


事务

和事务有关的代码:

/** Public API - {@inheritDoc} */
public Tx.SelectOk txSelect()
    throws IOException
    return (Tx.SelectOk) exnWrappingRpc(new Tx.Select()).getMethod();
/** Public API - {@inheritDoc} */
public Tx.CommitOk txCommit()
    throws IOException
    return (Tx.CommitOk) exnWrappingRpc(new Tx.Commit()).getMethod();
/** Public API - {@inheritDoc} */
public Tx.RollbackOk txRollback()
    throws IOException
    return (Tx.RollbackOk) exnWrappingRpc(new Tx.Rollback()).getMethod();
}

这里可以看到基本对于事务的处理是采用rpc的方法一对一的进行交互,有关RabbitMQ的事务机制可以参考:[RabbitMQ之消息确认机制(事务+Confirm)][RabbitMQ_Confirm]。


其余

ChannelN还有:

  • 关于Exchange,Queue的申明创建,删除,绑定,解绑
  • 关闭处理
  • Basic.Return
  • Basic.Flow
  • Basic.Recover
  • Basic.Cancel
  • Basic.Ack/.Nack/.Reject

这些就不做详细介绍了。有兴趣的同学可以继续翻阅源码,这些都比较简单。