1.什么是SelectionKey
A token representing the registration of a SelectableChannel with a Selector.
一個(gè)表示 SelectableChannel 向 Selector 注冊的令牌。
2.什么時(shí)候被創(chuàng)建
A selection key is created each time a channel is registered with a selector.
注冊到selector的時(shí)候被創(chuàng)建.
3.SelectionKey的生命周期
A key remains valid until it is cancelled by invoking its cancel method, by closing its channel, or by closing its selector.
4.取消
Cancelling a key does not immediately remove it from its selector; it is instead added to the selector's cancelled-key set for removal during the next selection operation. (
取消一個(gè)鍵并不會立即將其從其選擇器中移除；而是將其添加到選擇器的取消鍵集中，以便在下一次選擇操作期間刪除。)
5.兩個(gè)集合
A selection key contains two operation sets represented as integer values. Each bit of an operation set denotes a category of selectable operations that are supported by the key's channel.

• The interest set
• The ready set
  6.幾個(gè)枚舉值

public static final int OP_READ = 1 << 0;
public static final int OP_WRITE = 1 << 2;
public static final int OP_CONNECT = 1 << 3;
public static final int OP_ACCEPT = 1 << 4;

7.netty中的使用.
NioSocketChannel里

   //連接.
    @Override
    protected boolean doConnect(SocketAddress remoteAddress, SocketAddress localAddress) throws Exception {
        if (localAddress != null) {
            doBind0(localAddress);
        }
        boolean success = false;
        try {
            boolean connected = SocketUtils.connect(javaChannel(), remoteAddress);
            //沒有連接的時(shí)候
            if (!connected) {
                //注冊connect事件.
                selectionKey().interestOps(SelectionKey.OP_CONNECT);
            }
            success = true;
            return connected;
        } finally {
            if (!success) {
                doClose();
            }
        }
    }

  selectionKey = javaChannel().register(eventLoop().unwrappedSelector(), 0, this);