RabbitMQ-客户端源码之AMQChannel

AMQChannel是一个抽象类，是ChannelN的父类。其中包含唯一的抽象方法：

/*** Protected API - called by nextCommand to check possibly handle an incoming Command before it is returned to the caller of nextCommand. If this method* returns true, the command is considered handled and is not passed back to nextCommand's caller; if it returns false, nextCommand returns the command as* usual. This is used in subclasses to implement handling of Basic.Return and Basic.Deliver messages, as well as Channel.Close and Connection.Close.* @param command the command to handle asynchronously* @return true if we handled the command; otherwise the caller should consider it "unhandled"*/
public abstract boolean processAsync(Command command) throws IOException;

有关processAsync()这个方法的会在介绍ChannelN类的时候详细阐述（[[八]RabbitMQ-客户端源码之ChannelN][RabbitMQ-_ChannelN]）。

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首先来说下AMQChannel的成员变量：

protected final Object _channelMutex = new Object();
/** The connection this channel is associated with. */
private final AMQConnection _connection;
/** This channel's channel number. */
private final int _channelNumber;
/** Command being assembled */
private AMQCommand _command = new AMQCommand();
/** The current outstanding RPC request, if any. (Could become a queue in future.) */
private RpcContinuation _activeRpc = null;
/** Whether transmission of content-bearing methods should be blocked */
public volatile boolean _blockContent = false;

• _channelMutex这个是内部用来当对象锁的，没有实际的意义，可忽略
• _connection是指AMQConnection这个对象。
• _channelNumber是指channel number, 这个应该不用多解释了吧。通道编号为０的代表全局连接中的所有帧，1-65535代表特定通道的帧．
• _command是内部处理使用的对象，调用AMQCommand的方法来处理一些东西。
• _activeRpc是指当前未处理完的rpc请求（the current outstanding rpc request）。
• _blockContent 是在Channel.Flow里用到的，其余情况都是false
  在AMQChannel的构造函数中，只有两个参数：AMQConnection connection以及int channelNumber.

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AMQChannel中有个handleFrame方法：

/*** Private API - When the Connection receives a Frame for this* channel, it passes it to this method.* @param frame the incoming frame* @throws IOException if an error is encountered*/
public void handleFrame(Frame frame) throws IOException {AMQCommand command = _command;if (command.handleFrame(frame)) { // a complete command has rolled off the assembly line_command = new AMQCommand(); // prepare for the next onehandleCompleteInboundCommand(command);}
}/*** Private API - handle a command which has been assembled* @throws IOException if there's any problem** @param command the incoming command* @throws IOException*/
public void handleCompleteInboundCommand(AMQCommand command) throws IOException {// First, offer the command to the asynchronous-command// handling mechanism, which gets to act as a filter on the// incoming command stream.  If processAsync() returns true,// the command has been dealt with by the filter and so should// not be processed further.  It will return true for// asynchronous commands (deliveries/returns/other events),// and false for commands that should be passed on to some// waiting RPC continuation.if (!processAsync(command)) {// The filter decided not to handle/consume the command,// so it must be some reply to an earlier RPC.nextOutstandingRpc().handleCommand(command);markRpcFinished();}
}

这个在[[六]RabbitMQ-客户端源码之AMQCommand][RabbitMQ-_AMQCommand]有所介绍，主要是用来处理Frame帧的，当调用AMQCommand的handleFrame处理之后返回为true是，即处理完毕时继续调用handleCompleteInboundCommand方法。这其中也牵涉到AMQConnection的MainLoop内部类，具体可以看看：[[六]RabbitMQ-客户端源码之AMQCommand][RabbitMQ-_AMQCommand]。

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AMQChannel中有很多方法带有rpc的字样，这来做一个整理。
首先是：

public void enqueueRpc(RpcContinuation k)
{synchronized (_channelMutex) {boolean waitClearedInterruptStatus = false;while (_activeRpc != null) {try {_channelMutex.wait();} catch (InterruptedException e) {waitClearedInterruptStatus = true;}}if (waitClearedInterruptStatus) {Thread.currentThread().interrupt();}_activeRpc = k;}
}

这个方法在AMQConnection.start()方法中有过使用：_channel0.enqueueRpc(conStartBroker)。这个方法就是将参数付给成员变量_activeRpc，至于这个RpcContinuation到底是个什么gui，我们下面再讲。
继续下一个方法：

public boolean isOutstandingRpc()
{synchronized (_channelMutex) {return (_activeRpc != null);}
}

这个方法是判断一下当前的_activeRpc是否为null，为null则为false，否则为true。看方法的名字应该猜出大半。
下面一个方法：

public RpcContinuation nextOutstandingRpc()
{synchronized (_channelMutex) {RpcContinuation result = _activeRpc;_activeRpc = null;_channelMutex.notifyAll();return result;}
}

方法将当前的_activeRpc返回，并置AQMChannel的_activeRpc为null。

接下来几个方法联系性很强：

/*** Protected API - sends a {@link Method} to the broker and waits for the* next in-bound Command from the broker: only for use from* non-connection-MainLoop threads!*/
public AMQCommand rpc(Method m)throws IOException, ShutdownSignalException
{return privateRpc(m);
}public AMQCommand rpc(Method m, int timeout)throws IOException, ShutdownSignalException, TimeoutException {return privateRpc(m, timeout);
}private AMQCommand privateRpc(Method m)throws IOException, ShutdownSignalException
{SimpleBlockingRpcContinuation k = new SimpleBlockingRpcContinuation();rpc(m, k);// At this point, the request method has been sent, and we// should wait for the reply to arrive.//// Calling getReply() on the continuation puts us to sleep// until the connection's reader-thread throws the reply over// the fence.return k.getReply();
}private AMQCommand privateRpc(Method m, int timeout)throws IOException, ShutdownSignalException, TimeoutException {SimpleBlockingRpcContinuation k = new SimpleBlockingRpcContinuation();rpc(m, k);return k.getReply(timeout);
}public void rpc(Method m, RpcContinuation k)throws IOException
{synchronized (_channelMutex) {ensureIsOpen();quiescingRpc(m, k);}
}public void quiescingRpc(Method m, RpcContinuation k)throws IOException
{synchronized (_channelMutex) {enqueueRpc(k);quiescingTransmit(m);}
}

主要是看最后一个方法——quiescingRpc.这个方法说白就两行代码：
enqueueRpc(k);是将由privateRpc等方法内部创建的SimpleBlockingRpcContinuation对象附给当前的AQMChannel对象的成员变量_activeRpc
关于quiescingTransmit(m)就要接下去看了：

public void quiescingTransmit(Method m) throws IOException {synchronized (_channelMutex) {quiescingTransmit(new AMQCommand(m));}
}
public void quiescingTransmit(AMQCommand c) throws IOException {synchronized (_channelMutex) {if (c.getMethod().hasContent()) {while (_blockContent) {try {_channelMutex.wait();} catch (InterruptedException e) {}// This is to catch a situation when the thread wakes up during// shutdown. Currently, no command that has content is allowed// to send anything in a closing state.ensureIsOpen();}}c.transmit(this);}
}

上面代码只需要看： c.transmit(this);这一句，其余的都是摆设。看到这里，就调用了AMQCommand的transmit方法，这个transmit方法就是讲AMQChannel中封装的内容发给broker，然后等待broker返回，进而通过之前附值的_activeRpc来处理回传的帧。

虽然之前在AMQConnection（[[二]RabbitMQ-客户端源码之AMQConnection][RabbitMQ-_AMQConnection]）中详细讲述了start()方法，但是这里还是要来拿这个来举例这个AMQChannel中的rpc怎么使用
在AMQConnection中有这么一段代码：

Method method = (challenge == null)? new AMQP.Connection.StartOk.Builder().clientProperties(_clientProperties).mechanism(sm.getName()).response(response).build(): new AMQP.Connection.SecureOk.Builder().response(response).build();try {Method serverResponse = _channel0.rpc(method, HANDSHAKE_TIMEOUT/2).getMethod();if (serverResponse instanceof AMQP.Connection.Tune) {connTune = (AMQP.Connection.Tune) serverResponse;} else {challenge = ((AMQP.Connection.Secure) serverResponse).getChallenge();response = sm.handleChallenge(challenge, this.username, this.password);}

客户端将Method封装成Connection.StartOk帧之后等待broker返回Connection.Tune帧。
此时调用了AMQChannel的rpc(Method m, int timeout)方法，其间接调用了AMQChannel的privateRpc(Method, int timeout)方法。代码详情上面已经罗列出来。

注意privateRpc(Method, int timeout)方法的最有一句返回：return k.getReply(timeout);这句代码的意思是SimpleBlockingRpcContinuation对象在等待broker的返回，确切的来说是MainLoop线程处理之后返回，即AMQChannel类中handleCompleteInboundCommand方法的nextOutstandingRpc().handleCommand(command)这行代码。