创建一个channel实例，并把它register到eventLoopGroup中之后，这个channel然后处于inactive状态，仍然是不可用的。只有在bind或connect方法调用成功之后才能正常。因此bind或connect算是channel初始化的最后一步，本章这就重点分析这两个功能的实现。
接下来的代码分析如果没有特别说明，都是以NioSocketChannel为例。
bind实现
bind方法的调用栈如下:
io.netty.channel.AbstractChannel#bind(java.net.SocketAddress)
io.netty.channel.DefaultChannelPipeline#bind(java.net.SocketAddress)
io.netty.channel.AbstractChannelHandlerContext#bind(java.net.SocketAddress)
io.netty.channel.AbstractChannelHandlerContext#bind(java.net.SocketAddress, io.netty.channel.ChannelPromise)
io.netty.channel.AbstractChannelHandlerContext#invokeBind
io.netty.channel.DefaultChannelPipeline.HeadContext#bind
io.netty.channel.AbstractChannel.AbstractUnsafe#bind
io.netty.channel.socket.nio.NioSocketChannel#doBind
io.netty.channel.socket.nio.NioSocketChannel#doBind0
为了能简单明了地展示调用关系，这个调用栈忽略了一些调用。可能有多个AbstractChannelHandlerContext的方法在不同的线程中被调用。以后在描述调用栈时也会忽略这一点，不再赘述。
io.netty.channel.AbstractChannel.AbstractUnsafe#bind执行了主要的bind逻辑，它会调用doBind, 然后在channel的状态从inactive变成active，就调用pipline的fireChannelActive方法触发channelActives事件。doBind是io.netty.channel.AbstractChannel定义的抽象方法。NioSocketChannel只需要实现这个方法，整个bind功能就完整了。
1 @Override
2 protected void doBind(SocketAddress localAddress) throws Exception {
3 doBind0(localAddress);
4 }
5 private void doBind0(SocketAddress localAddress) throws Exception {
6 if (PlatformDependent.javaVersion() >= 7) {
7 SocketUtils.bind(javaChannel(), localAddress);
8 } else {
9 SocketUtils.bind(javaChannel().socket(), localAddress);
10 }
11 }
SocketUtils封装了通过AccessController调用JDK的socket API接口，事实上还是调用Socket或SocketChannel的bind方法。Nio的三个Channel类实现doBind的代码几乎一样。
connect实现
connect的调用栈如下:
io.netty.channel.AbstractChannel#connect(java.net.SocketAddress)
io.netty.channel.DefaultChannelPipeline#connect(java.net.SocketAddress)
io.netty.channel.AbstractChannelHandlerContext#connect(java.net.SocketAddress)
io.netty.channel.AbstractChannelHandlerContext#connect(java.net.SocketAddress, io.netty.channel.ChannelPromise)
io.netty.channel.AbstractChannelHandlerContext#connect(java.net.SocketAddress, java.net.SocketAddress, io.netty.channel.ChannelPromise)
io.netty.channel.AbstractChannelHandlerContext#invokeConnect
io.netty.channel.DefaultChannelPipeline.HeadContext#connect
io.netty.channel.nio.AbstractNioChannel.AbstractNioUnsafe#connect
io.netty.channel.socket.nio.NioSocketChannel#doConnect
connect的主要逻辑在io.netty.channel.nio.AbstractNioChannel.AbstractNioUnsafe#connect中实现，它的流程是:
1. 调用doConnect方法，这个方法是AbstractNioChanne定义的抽象方法。
2. 如果doConnect成功，且channel的状态从inactive变成active，则调用pipeline的fireChannelActive方法触发channelActive事件。
3. 如果doConnection失败，调用close关闭channel。
io.netty.channel.socket.nio.NioSocketChannel#doConnect中是socket connect API的调用。下面是connect的关键代码。
1 @Override
2 public final void connect(
3 final SocketAddress remoteAddress, final SocketAddress localAddress, final ChannelPromise promise) {
4 if (!promise.setUncancellable() || !ensureOpen(promise)) {
5 return;
6 }
7
8 try {
9 if (connectPromise != null) {
10 // Already a connect in process.
11 throw new ConnectionPendingException();
12 }
13
14 boolean wasActive = isActive();
15 if (doConnect(remoteAddress, localAddress)) {
16 fulfillConnectPromise(promise, wasActive);
17 } else {
18 connectPromise = promise;
19 requestedRemoteAddress = remoteAddress;
20
21 // Schedule connect timeout.
22 int connectTimeoutMillis = config().getConnectTimeoutMillis();
23 if (connectTimeoutMillis > 0) {
24 connectTimeoutFuture = eventLoop().schedule(new Runnable() {
25 @Override
26 public void run() {
27 ChannelPromise connectPromise = AbstractNioChannel.this.connectPromise;
28 ConnectTimeoutException cause =
29 new ConnectTimeoutException("connection timed out: " + remoteAddress);
30 if (connectPromise != null && connectPromise.tryFailure(cause)) {
31 close(voidPromise());
32 }
33 }
34 }, connectTimeoutMillis, TimeUnit.MILLISECONDS);
35 }
36
37 promise.addListener(new ChannelFutureListener() {
38 @Override
39 public void operationComplete(ChannelFuture future) throws Exception {
40 if (future.isCancelled()) {
41 if (connectTimeoutFuture != null) {
42 connectTimeoutFuture.cancel(false);
43 }
44 connectPromise = null;
45 close(voidPromise());
46 }
47 }
48 });
49 }
50 } catch (Throwable t) {
51 promise.tryFailure(annotateConnectException(t, remoteAddress));
52 closeIfClosed();
53 }
54 }
55
56 private void fulfillConnectPromise(ChannelPromise promise, boolean wasActive) {
57 if (promise == null) {
58 return;
59 }
60 boolean active = isActive();
61 boolean promiseSet = promise.trySuccess();
62
63 if (!wasActive && active) {
64 pipeline().fireChannelActive();
65 }
66 if (!promiseSet) {
67 close(voidPromise());
68 }
69 }
第14,15行和整个fulfillConnectPromise方法处理正常流程。
第18-52行处理异常流程。代码虽然多，但总结起来就一句话: 设置promis返回错误，确保能够调用close方法
io.netty.channel.socket.nio.NioSocketChannel#doConnect实现和doBind实现类似:
1 @Override
2 protected boolean doConnect(SocketAddress remoteAddress, SocketAddress localAddress) throws Exception {
3 if (localAddress != null) {
4 doBind0(localAddress);
5 }
6
7 boolean success = false;
8 try {
9 boolean connected = SocketUtils.connect(javaChannel(), remoteAddress);
10 if (!connected) {
11 selectionKey().interestOps(SelectionKey.OP_CONNECT);
12 }
13 success = true;
14 return connected;
15 } finally {
16 if (!success) {
17 doClose();
18 }
19 }
20 }
在第11行，注册OP_CONNECT事件。由于channel在初始化是被设置成非阻塞模式，connect方法可能返回false, 如果返回false表示connect操作没有完成，需要通过selector关注OP_CONNECT事件，把connect变成一个异步过程。只有异步调用io.netty.channel.nio.AbstractNioChannel.AbstractNioUnsafe#finishConnect之后，connect才算完成。finishConnect在eventLoop中被调用:
//io.netty.channel.nio.NioEventLoop#processSelectedKey(java.nio.channels.SelectionKey, io.netty.channel.nio.AbstractNioChannel)
if ((readyOps & SelectionKey.OP_CONNECT) != 0) {
int ops = k.interestOps();
ops &= ~SelectionKey.OP_CONNECT;
k.interestOps(ops);
unsafe.finishConnect();
}
finishConnection的实现如下:
//io.netty.channel.nio.AbstractNioChannel.AbstractNioUnsafe#finishConnect
@Override
public final void finishConnect() {
// Note this method is invoked by the event loop only if the connection attempt was
// neither cancelled nor timed out. assert eventLoop().inEventLoop();
try {
9 boolean wasActive = isActive();
10 doFinishConnect();
11 fulfillConnectPromise(connectPromise, wasActive);
} catch (Throwable t) {
fulfillConnectPromise(connectPromise, annotateConnectException(t, requestedRemoteAddress));
} finally {
// Check for null as the connectTimeoutFuture is only created if a connectTimeoutMillis > 0 is used
// See https://github.com/netty/netty/issues/1770
if (connectTimeoutFuture != null) {
connectTimeoutFuture.cancel(false);
}
connectPromise = null;
}
} //io.netty.channel.socket.nio.NioSocketChannel#doFinishConnect
@Override
protected void doFinishConnect() throws Exception {
if (!javaChannel().finishConnect()) {
throw new Error();
}
}
9-11行是finishConnection的关键代码, 先调用doFinishConnect执行完成连接之后的操作，NioSocketChannel实现是检查连接是否真的已经完成(27-29行)，然后调用fulfillConnectPromise触发事件,设置promise返回值。在前面分析netty.channel.nio.AbstractNioChannel.AbstractNioUnsafe#connect代码时，可以看到在doConnect调用成功以后会立即调用这个方法。这个方法被调用两次是为了确保channelActive事件一定会被触发一次。
localAddress,remoteAddress实现：得到channel的本地和远程地址
这个两个方法的实现几乎一样，这里只分析localAddress，它的调用栈如下:
1 io.netty.channel.AbstractChannel#localAddress
2 io.netty.channel.AbstractChannel.AbstractUnsafe#localAddress
3 io.netty.channel.socket.nio.NioSocketChannel#localAddress0
这个方法不会触发任何事件，因此没有通过pipline调用unsafe，它直接调用unsafe的方法:
1 //io.netty.channel.AbstractChannel#localAddress
2 @Override
3 public SocketAddress localAddress() {
4 SocketAddress localAddress = this.localAddress;
5 if (localAddress == null) {
6 try {
7 this.localAddress = localAddress = unsafe().localAddress();
8 } catch (Throwable t) {
9 // Sometimes fails on a closed socket in Windows.
10 return null;
11 }
12 }
13 return localAddress;
14 }
在第7行直接调用unsafe的locallAddress方法，这个方法在AbstractUnsafe中实现，它调用了localAddress0，这一个protected的抽象方法，在NioSocketChannel中的实现是:
1 @Override
2 protected SocketAddress localAddress0() {
3 return javaChannel().socket().getLocalSocketAddress();
4 }