netty源码解解析(4.0)-12 Channel NIO实现:channel初始化

　　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:

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的实现如下:

 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 }