A nexus for I/O operations.上文的Channel的Java doc的说明翻译过来就是，Channel代表一个到某个实体的打开的连接，这个实体可以是设备、文件、网络套接字或者是具备IO操纵的程序组件。与Java IO流不同的是，Channel的界说显得更加抽象，只要满足上面的界说都可以称作一个Channel，从这个意义上来说，Channel更的含义更像“信道”这个词。
A channel represents an open connection to an entity such as a hardware device, a file, a network socket, or a program component that is capable of performing one or more distinct I/O operations, for example reading or writing.
A channel that can be asynchronously closed and interrupted.实现InterruptibleChannel接口的Channel支持异步关闭和中断关闭
A channel that implements this interface is asynchronously closeable: If a thread is blocked in an I/O operation on an interruptible channel then another thread may invoke the channel's close method. This will cause the blocked thread to receive an AsynchronousCloseException.
A channel that implements this interface is also interruptible: If a thread is blocked in an I/O operation on an interruptible channel then another thread may invoke the blocked thread's interrupt method. This will cause the channel to be closed, the blocked thread to receive a ClosedByInterruptException, and the blocked thread's interrupt status to be set.
If a thread's interrupt status is already set and it invokes a blocking I/O operation upon a channel then the channel will be closed and the thread will immediately receive a ClosedByInterruptException; its interrupt status will remain set.
A channel supports asynchronous closing and interruption if, and only if, it implements this interface. This can be tested at runtime, if necessary, via the instanceof operator.
A channel that can be multiplexed via a Selector.上面的注释要注意的一点就是：selectable channel可以在阻塞模式下工作也可以在非阻塞模式下工作。
In order to be used with a selector, an instance of this class must first be registered via the register method. This method returns a new SelectionKey object that represents the channel's registration with the selector.
Once registered with a selector, a channel remains registered until it is deregistered. This involves deallocating whatever resources were allocated to the channel by the selector.
A channel cannot be deregistered directly; instead, the key representing its registration must be cancelled. Cancelling a key requests that the channel be deregistered during the selector's next selection operation. A key may be cancelled explicitly by invoking its cancel method. All of a channel's keys are cancelled implicitly when the channel is closed, whether by invoking its close method or by interrupting a thread blocked in an I/O operation upon the channel.
If the selector itself is closed then the channel will be deregistered, and the key representing its registration will be invalidated, without further delay.
A channel may be registered at most once with any particular selector.
Whether or not a channel is registered with one or more selectors may be determined by invoking the isRegistered method.
Selectable channels are safe for use by multiple concurrent threads.
Blocking mode
A selectable channel is either in blocking mode or in non-blocking mode. In blocking mode, every I/O operation invoked upon the channel will block until it completes. In non-blocking mode an I/O operation will never block and may transfer fewer bytes than were requested or possibly no bytes at all. The blocking mode of a selectable channel may be determined by invoking its isBlocking method.
Newly-created selectable channels are always in blocking mode. Non-blocking mode is most useful in conjunction with selector-based multiplexing. A channel must be placed into non-blocking mode before being registered with a selector, and may not be returned to blocking mode until it has been deregistered
A multiplexor of SelectableChannel objects.图9 Selector
A selector may be created by invoking the open method of this class, which will use the system's default selector provider to create a new selector. A selector may also be created by invoking the openSelector method of a custom selector provider. A selector remains open until it is closed via its close method.
A selectable channel's registration with a selector is represented by a SelectionKey object. A selector maintains three sets of selection keys:
The key set contains the keys representing the current channel registrations of this selector. This set is returned by the keys method.
The selected-key set is the set of keys such that each key's channel was detected to be ready for at least one of the operations identified in the key's interest set during a prior selection operation. This set is returned by the selectedKeys method. The selected-key set is always a subset of the key set.
The cancelled-key set is the set of keys that have been cancelled but whose channels have not yet been deregistered. This set is not directly accessible. The cancelled-key set is always a subset of the key set.
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