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 _channelMap and channelNumberAllocator */
private final Object monitor = new Object();/** Mapping from 1.._channelMax to {@link ChannelN} instance */private final Map _channelMap = new HashMap();private final IntAllocator channelNumberAllocator;private final ConsumerWorkService workService;private final Set shutdownSet = new HashSet();/** 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 numberchannelMax = (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 addedreturn 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 addedreturn 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 itexnWrappingRpc(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。