[五]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）。

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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 one        handleCompleteInboundCommand(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。

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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）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。

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AQMChannel还有些其他的内容，都是边缘性的东西，这里还剩下个RpcContinuation要着重阐述下的：

public interface RpcContinuation {    void handleCommand(AMQCommand command);    void handleShutdownSignal(ShutdownSignalException signal);}public static abstract class BlockingRpcContinuation<T> implements RpcContinuation {    public final BlockingValueOrException<T, ShutdownSignalException> _blocker =        new BlockingValueOrException<T, ShutdownSignalException>();    public void handleCommand(AMQCommand command) {        _blocker.setValue(transformReply(command));    }    public void handleShutdownSignal(ShutdownSignalException signal) {        _blocker.setException(signal);    }    public T getReply() throws ShutdownSignalException    {        return _blocker.uninterruptibleGetValue();    }    public T getReply(int timeout)        throws ShutdownSignalException, TimeoutException    {        return _blocker.uninterruptibleGetValue(timeout);    }    public abstract T transformReply(AMQCommand command);}public static class SimpleBlockingRpcContinuation    extends BlockingRpcContinuation<AMQCommand>{    public AMQCommand transformReply(AMQCommand command) {        return command;    }}

RPCContinuation只是一个接口，而BlockingRpcContinuation这个抽象类缺似乎略有门道。而SimpleBlockingRpcContinuation只是将BlockingRpcContinuation中的handleCommand方法便成为：

_blocker.setValue(command);

BlockingRpcContinuation类主要操纵了BlockingValueOrException _blocker这个成员变量。再接下深究BlockingValueOrException其实是继承了BlockingCell，对其做了一下简单的封装。最后来看下BlockingCell是个什么鬼， 截取部分代码如下：

public class BlockingCell<T> {    private boolean _filled = false;    private T _value;    public synchronized T get() throws InterruptedException {        while (!_filled) {            wait();        }        return _value;    }

其实这个就是capacity为1的BlockingQueue，顾美其名曰BlockingCell，绕了大半圈，原来AMQChannel中的_activeRpc就是个这么玩意儿~

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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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