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）。

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.

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。

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。

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~