創建一個channel實例，並把它register到eventLoopGroup中之後，這個channel然後處於inactive狀態，仍然是不可用的。只有在bind或connect方法調用成功之後才能正常。因此bind或connect算是channel初始化的最後一步，本章這就重點分析這兩個功能的實現。
接下來的代碼分析如果沒有特別說明，都是以NioSocketChannel爲例。
bind實現
bind方法的調用棧如下:
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
爲了能簡單明瞭地展示調用關係，這個調用棧忽略了一些調用。可能有多個AbstractChannelHandlerContext的方法在不同的線程中被調用。以後在描述調用棧時也會忽略這一點，不再贅述。
io.netty.channel.AbstractChannel.AbstractUnsafe#bind執行了主要的bind邏輯，它會調用doBind, 然後在channel的狀態從inactive變成active，就調用pipline的fireChannelActive方法觸發channelActives事件。doBind是io.netty.channel.AbstractChannel定義的抽象方法。NioSocketChannel只需要實現這個方法，整個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封裝了通過AccessController調用JDK的socket API接口，事實上還是調用Socket或SocketChannel的bind方法。Nio的三個Channel類實現doBind的代碼幾乎一樣。
connect實現
connect的調用棧如下:
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的主要邏輯在io.netty.channel.nio.AbstractNioChannel.AbstractNioUnsafe#connect中實現，它的流程是:
1. 調用doConnect方法，這個方法是AbstractNioChanne定義的抽象方法。
2. 如果doConnect成功，且channel的狀態從inactive變成active，則調用pipeline的fireChannelActive方法觸發channelActive事件。
3. 如果doConnection失敗，調用close關閉channel。
io.netty.channel.socket.nio.NioSocketChannel#doConnect中是socket connect API的調用。下面是connect的關鍵代碼。
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行和整個fulfillConnectPromise方法處理正常流程。
第18-52行處理異常流程。代碼雖然多，但總結起來就一句話: 設置promis返回錯誤，確保能夠調用close方法
io.netty.channel.socket.nio.NioSocketChannel#doConnect實現和doBind實現類似:
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，註冊OP_CONNECT事件。由於channel在初始化是被設置成非阻塞模式，connect方法可能返回false, 如果返回false表示connect操作沒有完成，需要通過selector關注OP_CONNECT事件，把connect變成一個異步過程。只有異步調用io.netty.channel.nio.AbstractNioChannel.AbstractNioUnsafe#finishConnect之後，connect纔算完成。finishConnect在eventLoop中被調用:
1 //io.netty.channel.nio.NioEventLoop#processSelectedKey(java.nio.channels.SelectionKey, io.netty.channel.nio.AbstractNioChannel)
2 if ((readyOps & SelectionKey.OP_CONNECT) != 0) {
3 int ops = k.interestOps();
4 ops &= ~SelectionKey.OP_CONNECT;
5 k.interestOps(ops);
6 unsafe.finishConnect();
7 }
finishConnection的實現如下:
1 //io.netty.channel.nio.AbstractNioChannel.AbstractNioUnsafe#finishConnect
2 @Override
3 public final void finishConnect() {
4 // Note this method is invoked by the event loop only if the connection attempt was
5 // neither cancelled nor timed out.
6
7 assert eventLoop().inEventLoop();
8 try {
9 boolean wasActive = isActive();
10 doFinishConnect();
11 fulfillConnectPromise(connectPromise, wasActive);
12 } catch (Throwable t) {
13 fulfillConnectPromise(connectPromise, annotateConnectException(t, requestedRemoteAddress));
14 } finally {
15 // Check for null as the connectTimeoutFuture is only created if a connectTimeoutMillis > 0 is used
16 // See https://github.com/netty/netty/issues/1770
17 if (connectTimeoutFuture != null) {
18 connectTimeoutFuture.cancel(false);
19 }
20 connectPromise = null;
21 }
22 }
23
24 //io.netty.channel.socket.nio.NioSocketChannel#doFinishConnect
25 @Override
26 protected void doFinishConnect() throws Exception {
27 if (!javaChannel().finishConnect()) {
28 throw new Error();
29 }
30 }
9-11行是finishConnection的關鍵代碼, 先調用doFinishConnect執行完成連接之後的操作，NioSocketChannel實現是檢查連接是否真的已經完成(27-29行)，然後調用fulfillConnectPromise觸發事件,設置promise返回值。在前面分析netty.channel.nio.AbstractNioChannel.AbstractNioUnsafe#connect代碼時，可以看到在doConnect調用成功以後會立即調用這個方法。這個方法被調用兩次是爲了確保channelActive事件一定會被觸發一次。
localAddress,remoteAddress實現：得到channel的本地和遠程地址
這個兩個方法的實現幾乎一樣，這裏只分析localAddress，它的調用棧如下:
1 io.netty.channel.AbstractChannel#localAddress
2 io.netty.channel.AbstractChannel.AbstractUnsafe#localAddress
3 io.netty.channel.socket.nio.NioSocketChannel#localAddress0
這個方法不會觸發任何事件，因此沒有通過pipline調用unsafe，它直接調用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行直接調用unsafe的locallAddress方法，這個方法在AbstractUnsafe中實現，它調用了localAddress0，這一個protected的抽象方法，在NioSocketChannel中的實現是:
1 @Override
2 protected SocketAddress localAddress0() {
3 return javaChannel().socket().getLocalSocketAddress();
4 }