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
- [Conclusion]RabbitMQ-客户端源码之总结
- [一]RabbitMQ-客户端源码之ConnectionFactory
- [二]RabbitMQ-客户端源码之AMQConnection
- [三]RabbitMQ-客户端源码之ChannelManager
- [四]RabbitMQ-客户端源码之Frame
- [五]RabbitMQ-客户端源码之AMQChannel
- [六]RabbitMQ-客户端源码之AMQCommand
- [七]RabbitMQ-客户端源码之AMQPImpl+Method
- [八]RabbitMQ-客户端源码之ChannelN
- [九]RabbitMQ-客户端源码之Consumer
