[三]RabbitMQ-客户端源码之ChannelManager

关于ChannelManager，官方注解：Manages a set of channels, indexed by channel number (1.. _channelMax)。

ChannelManager类的代码量不是很多，主要用来管理Channel的，channelNumber=0的除外，应为channelNumber=0是留给Connection的特殊的channelNumber。

下面是ChannelManager的成员变量：

/** Monitor for <code>_channelMap</code> and <code>channelNumberAllocator</code> */
private final Object monitor = new Object();
    /** Mapping from <code><b>1.._channelMax</b></code> to {@link ChannelN} instance */
    private final Map<Integer, ChannelN> _channelMap = new HashMap<Integer, ChannelN>();
    private final IntAllocator channelNumberAllocator;

private final ConsumerWorkService workService;

private final Set<CountDownLatch> shutdownSet = new HashSet<CountDownLatch>();

/** Maximum channel number available on this connection. */
private final int _channelMax;
private final ThreadFactory threadFactory;

这上面的成员变量下面会有涉及。

对于ChannelManager的使用，是AMQConnection中的成员变量：

/** Object that manages a set of channels */
private volatile ChannelManager _channelManager;

AMQConnection中start()的_channelManager中对其初始化：

protected ChannelManager instantiateChannelManager(int channelMax, ThreadFactory threadFactory) {
    return new ChannelManager(this._workService, channelMax, threadFactory);
}

再调用其构造函数：

public ChannelManager(ConsumerWorkService workService, int channelMax, ThreadFactory threadFactory) {
    if (channelMax == 0) {
        // The framing encoding only allows for unsigned 16-bit integers
        // for the channel number
        channelMax = (1 << 16) - 1;
    }
    _channelMax = channelMax;
    channelNumberAllocator = new IntAllocator(1, channelMax);

    this.workService = workService;
    this.threadFactory = threadFactory;
}

这里的ConsumerWorkService也在AMQConnection的start()方法中初始化——initializeConsumerWorkService()：

private void initializeConsumerWorkService() {
    this._workService  = new ConsumerWorkService(executor, threadFactory, shutdownTimeout);
}

再回到构造函数。

channelMax参数是在client接收到broker的Connection.Tune帧中的“Channel-Max”参数之后设置的，如果为0则表示没有限制，这里就会设置为默认的最大值：2的16次方-1。
threadFactory参数是指：Executors.defaultThreadFactory();

关于ConsumerWorkService请参考文章末尾处。

使用过RabbitMQ的同学知道要生产或者消费消息之前必须要初始化Channel,如下：

Channel channel = connection.createChannel();

这个createChannel()是AMQConnection中的方法：

public Channel createChannel(int channelNumber) throws IOException {
    ensureIsOpen();
    ChannelManager cm = _channelManager;
    if (cm == null) return null;
    return cm.createChannel(this, channelNumber);
}
public Channel createChannel() throws IOException {
    ensureIsOpen();
    ChannelManager cm = _channelManager;
    if (cm == null) return null;
    return cm.createChannel(this);
}

这里就是调用了ChannelManager的createChannel方法。

下面是ChannelManager中关于创建Channel的代码：

public ChannelN createChannel(AMQConnection connection) throws IOException {
    ChannelN ch;
    synchronized (this.monitor) {
        int channelNumber = channelNumberAllocator.allocate();
        if (channelNumber == -1) {
            return null;
        } else {
            ch = addNewChannel(connection, channelNumber);
        }
    }
    ch.open(); // now that it's been safely added
    return ch;
}

public ChannelN createChannel(AMQConnection connection, int channelNumber) throws IOException {
    ChannelN ch;
    synchronized (this.monitor) {
        if (channelNumberAllocator.reserve(channelNumber)) {
            ch = addNewChannel(connection, channelNumber);
        } else {
            return null;
        }
    }
    ch.open(); // now that it's been safely added
    return ch;
}

private ChannelN addNewChannel(AMQConnection connection, int channelNumber) throws IOException {
    if (_channelMap.containsKey(channelNumber)) {
        // That number's already allocated! Can't do it
        // This should never happen unless something has gone
        // badly wrong with our implementation.
        throw new IllegalStateException("We have attempted to "
                + "create a channel with a number that is already in "
                + "use. This should never happen. "
                + "Please report this as a bug.");
    }
    ChannelN ch = instantiateChannel(connection, channelNumber, this.workService);
    _channelMap.put(ch.getChannelNumber(), ch);
    return ch;
}

protected ChannelN instantiateChannel(AMQConnection connection, int channelNumber, ConsumerWorkService workService) {
    return new ChannelN(connection, channelNumber, workService);
}

上面有两个createChannel方法，一个是带了channelNumber的，一个是自动分片channelNumber的，分别对应AMQConnection中的两个方法。最后都调用addNewChannel方法。

注意两个createChannel方法中都有这样一句代码：

ch.open();

这个是什么呢？其实是调用ChannelN的open方法：

/**
 * Package method: open the channel.
 * This is only called from {@link ChannelManager}.
 * @throws IOException if any problem is encountered
 */
public void open() throws IOException {
    // wait for the Channel.OpenOk response, and ignore it
    exnWrappingRpc(new Channel.Open(UNSPECIFIED_OUT_OF_BAND));
}

这样就调用了AMQChannel的rpc方法，向broker发送了一个Channel.Open帧。

addNewChannel方法实际上是创建了一个ChannelN对象，然后置其于ChannelManager中的_channelMap中，方便管理。

channelNumberAllocator是channelNumber的分配器，其原理是采用BitSet来实现channelNumber的分配，有兴趣的同学可以深究进去看看。

关于ChannelN类会有专门一篇博文来讲述，其实整个RabbitMQ-client的代码最关键的就是ChannelN这个类，需要着重讲述。

细心的朋友可能会发现关于ConsumerWorkService这个，我并没有做什么阐述。这个主要牵涉到Channel层面的处理，涉及到的类有AMQConnection, ChannelN, ConsumerDispatcher等。ConsumerWorkService是在AMQConnection中初始化，在ChannelManager中引用。至于这里怎么理解，在ChannelN中这么解释：
service for managing this channel’s consumer callbacks。意思是管理消费回调的服务。
综述，ChannelManager主要用来管理Channel, 包括channelNumber与Channel之间的映射关系。

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