Netty源码解析 -- 客户端启动过程

上一篇文章分享了Netty服务端启动过程，本文继续分享Netty客户端启动过程。
源码分析基于Netty 4.1

Connect

客户端启动过程比较简单，主要是Connect操作。
Netty客户端启动引导类是Bootstrap，同样继承了AbstractBootstrap，它只有一个EventLoopGroup，下文称为ConnectGroup。

Bootstrap#connect -> doResolveAndConnect -> doResolveAndConnect0

private ChannelFuture doResolveAndConnect0(final Channel channel, SocketAddress remoteAddress,
										   final SocketAddress localAddress, final ChannelPromise promise) {
	try {
		final EventLoop eventLoop = channel.eventLoop();
		// #1
		final AddressResolver<SocketAddress> resolver = this.resolver.getResolver(eventLoop);
		
		...
		
		final Future<SocketAddress> resolveFuture = resolver.resolve(remoteAddress);

		if (resolveFuture.isDone()) {
			final Throwable resolveFailureCause = resolveFuture.cause();

			if (resolveFailureCause != null) {
				channel.close();
				promise.setFailure(resolveFailureCause);
			} else {
				// #2
				doConnect(resolveFuture.getNow(), localAddress, promise);
			}
			return promise;
		}

		...
	} catch (Throwable cause) {
		promise.tryFailure(cause);
	}
	return promise;
}

#1
AddressResolver负责解析SocketAddress。它可以做一些地址转换工作。如Netty提供了RoundRobinInetAddressResolver，可以对下游服务集群进行轮询调用。
Bootstrap#resolver是一个AddressResolverGroup，它负责构造AddressResolver，默认使用DefaultAddressResolverGroup。
#2 调用doConnect，执行Connect操作。

doConnect -> AbstractChannel#connect -> DefaultChannelPipeline#connect -> HeadContext#connect -> AbstractNioUnsafe#connect
（这里涉及DefaultChannelPipeline的内容后续有文章解析）

public final void connect(
		final SocketAddress remoteAddress, final SocketAddress localAddress, final ChannelPromise promise) {
	...

	try {
		...

		boolean wasActive = isActive();
		// #1
		if (doConnect(remoteAddress, localAddress)) {
			fulfillConnectPromise(promise, wasActive);
		} else {
			connectPromise = promise;
			requestedRemoteAddress = remoteAddress;

			// #2
			int connectTimeoutMillis = config().getConnectTimeoutMillis();
			if (connectTimeoutMillis > 0) {
				connectTimeoutFuture = eventLoop().schedule(new Runnable() {
					public void run() {
						ChannelPromise connectPromise = AbstractNioChannel.this.connectPromise;
						ConnectTimeoutException cause =
								new ConnectTimeoutException("connection timed out: " + remoteAddress);
						if (connectPromise != null && connectPromise.tryFailure(cause)) {
							close(voidPromise());
						}
					}
				}, connectTimeoutMillis, TimeUnit.MILLISECONDS);
			}
			// #3
			promise.addListener(new ChannelFutureListener() {
				public void operationComplete(ChannelFuture future) throws Exception {
					if (future.isCancelled()) {
						if (connectTimeoutFuture != null) {
							connectTimeoutFuture.cancel(false);
						}
						connectPromise = null;
						close(voidPromise());
					}
				}
			});
		}
	} catch (Throwable t) {
		promise.tryFailure(annotateConnectException(t, remoteAddress));
		closeIfClosed();
	}
}

#1 调用SocketChannel#connect，如果是非阻塞Socket调用，该方法返回false。
#2 给EventLoop添加一个定时任务，如果连接超时则关闭Channel。
Netty中也提供了ReadTimeoutHandler处理读超时的场景。
#3 给promise添加一个回调方法，connect操作完成时，如果connect操作被取消了，则关闭Channel。

NioSocketChannel#doConnect

protected boolean doConnect(SocketAddress remoteAddress, SocketAddress localAddress) throws Exception {
    ...

    boolean success = false;
    try {
    	// #1
        boolean connected = SocketUtils.connect(javaChannel(), remoteAddress);
        // #2
        if (!connected) {
            selectionKey().interestOps(SelectionKey.OP_CONNECT);
        }
        success = true;
        return connected;
    } finally {
        if (!success) {
            doClose();
        }
    }
}

#1 调用(jvm)SocketChannel#connect方法，同样，非阻塞SocketChannel调用该方法，返回false。
#2 关注OP_CONNECT事件。

EventLoop中负责处理OP_CONNECT事件(EventLoop后面有文章解析)，调用AbstractNioUnsafe#finishConnect完成连接操作。

public final void finishConnect() {
	...
	try {
		boolean wasActive = isActive();
		// #1
		doFinishConnect();
		// #2
		fulfillConnectPromise(connectPromise, wasActive);
	} catch (Throwable t) {
		fulfillConnectPromise(connectPromise, annotateConnectException(t, requestedRemoteAddress));
	} finally {
		// #3
		if (connectTimeoutFuture != null) {
			connectTimeoutFuture.cancel(false);
		}
		connectPromise = null;
	}
}

#1 doFinishConnect方法由子类NioSocketChannel实现，就是调用(jvm)SocketChannel#finishConnect()方法
#2 设置connectPromise处理成功
#3 取消connectTimeoutFuture延迟任务

注册关注Read事件
AbstractNioUnsafe#fulfillConnectPromise -> DefaultChannelPipeline#fireChannelActive -> HeadContext#channelActive
前面解析服务端启动过程时说过，HeadContext#channelActive会调用readIfIsAutoRead方法，判断是否开启autoRead，开启则自动触发read事件处理方法。
HeadContext#readIfIsAutoRead -> AbstractChannel#read -> HeadContext#read -> AbstractUnsafe#beginRead -> AbstractNioChannel#doBeginRead
AbstractNioChannel#doBeginRead在解析服务端启动过程时也说过，这里会注册关注Read事件。

客户端启动完成后，客户端和服务端就可以开始进行Read/Write操作了。

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