netty源码解析(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:

//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 }