java NIO --selector

源码：

* <p> A selector may be created by invoking the {@link #open open} method of
* this class, which will use the system‘s default {@link
* java.nio.channels.spi.SelectorProvider selector provider} to
* create a new selector. A selector may also be created by invoking the
* {@link java.nio.channels.spi.SelectorProvider#openSelector openSelector}
* method of a custom selector provider. A selector remains open until it is
* closed via its {@link #close close} method.

　翻译:

　　Selector 是通过调用这个类的open()方法进行创建的，它使用的是系统默认的 选择器提供者去创建一个新的Selector；

　　也可以调用spi 里的 openSelector 方法进行自定义Selector 的创建；

　　在调用close 方法之前，Selector 是保持打开的；

结果：

Selector selector = Selector.open();  //Selector 构造方式

源码：
 * <p> A key is added to a selector‘s key set as a side effect of registering a
 * channel via the channel‘s {@link SelectableChannel#register(Selector,int)
 * register} method.  Cancelled keys are removed from the key set during
 * selection operations.  The key set itself is not directly modifiable.

翻译解析：

　　我们可以通提供过继承了 SelectableChannel 的类的 register 方法进行通道的注册，返回SelectionKey 在注册的同时，一个key 也会加入到选择器的键集里；

　　需要注意的是 只有继承了 SelectableChannel 的类 才能够进行注册；

　　在注册方法中有两个参数  SelectionKey register(Selector sel, int ops) ；Selector  就是我们的选择器， ops 是我们需要监听的事件，

　
以socketChannel 为例：
　　

//创建Selector 选择器对象
Selector selector = Selector.open();

//创建ServerSocketChannel

ServerSocketChannel socketChannel = ServerSocketChannel.open();
//设置非阻塞模式
socketChannel.configureBlocking(false);
//创建ServerSocket 绑定端口
ServerSocket socket=socketChannel.socket();
socket.bind(new InetSocketAddress(9999));

//进行通道的注册

socketChannel.register(selector, SelectionKey.OP_ACCEPT);

源码：
* <p> A selectable channel‘s registration with a selector is represented by a
 * {@link SelectionKey} object.  A selector maintains three sets of selection
 * keys:

翻译：
selector 选择器中可选择的通道注册是由 SelectionKey 这个对象表示的，也就是说，在Selector 注册的不同通道，都会有一个 SelectionKey 对应；，这个

SelectionKey 是可以追溯的，也可以通过 SelectionKey 来获取当前 SelectionKey 的通道channel;

一个选择器里维护了三种选择集 分别是如下：


源码：

* <li><p> The <i>key set</i> contains the keys representing the current
* channel registrations of this selector. This set is returned by the
* {@link #keys() keys} method. </p></li>
*
* <li><p> The <i>selected-key set</i> 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 {@link #selectedKeys() selectedKeys} method.
* The selected-key set is always a subset of the key set. </p></li>
*

* <li><p> The <i>cancelled-key</i> 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. </p></li>

翻译理解：

1. 键集：包含了选择器里当前通道注册的键，可以通过keys 方法进行返回这个键集；它是最全所有的键集

2.selected 键集 ： 一句话就是当前selector 监听的事件触发返回的一个键集合， 可以通过selectKeys()方法进行获取；它是键集（1）的子类

3. cancelled 键集：是一个被取消的键集，但是通道还没有被撤销登记，这几集合不能直接被操作，也是键集（1）的子类