Netty4源码分析-Bootstrap

Bootstrap是Netty提供的一个便利的工具类, 通过Bootstrap类能够容易地启动一个客户端使之与服务器端进行通信。

下面的代码用于从服务器中获取时间：

public class TimeClient {

    public static void main(String[] args) throws Exception {
        int port = 8080;
        if (args != null && args.length > 0) {
            try {
                port = Integer.valueOf(args[0]);
            } catch (NumberFormatException e) {
                
            }
        }
        new TimeClient().connect(port, "127.0.0.1");
    }

    public void connect(int port, String host) throws Exception {
        // 配置客户端NIO线程组
        EventLoopGroup group = new NioEventLoopGroup();
        try {
            Bootstrap b = new Bootstrap();
            b.group(group).channel(NioSocketChannel.class)
                    .option(ChannelOption.TCP_NODELAY, true)
                    .handler(new ChannelInitializer<SocketChannel>() {
                        @Override
                        public void initChannel(SocketChannel ch)
                                throws Exception {
                            ch.pipeline().addLast(new TimeClientHandler());
                        }
                    });

            // 发起异步连接操作
            ChannelFuture f = b.connect(host, port).sync();

            // 当代客户端链路关闭
            f.channel().closeFuture().sync();
        } finally {
            // 优雅退出，释放NIO线程组
            group.shutdownGracefully();
        }
    }
}

TimeClientHandler代码如下：

public class TimeClientHandler extends ChannelInboundHandlerAdapter {

    private static final Logger logger = Logger
            .getLogger(TimeClientHandler.class.getName());

    private final ByteBuf firstMessage;

    /**
     * Creates a client-side handler.
     */
    public TimeClientHandler() {
        byte[] req = "QUERY TIME ORDER".getBytes();
        firstMessage = Unpooled.buffer(req.length);
        firstMessage.writeBytes(req);

    }

    @Override
    public void channelActive(ChannelHandlerContext ctx) {
        ctx.writeAndFlush(firstMessage);
    }

    @Override
    public void channelRead(ChannelHandlerContext ctx, Object msg)
            throws Exception {
        ByteBuf buf = (ByteBuf) msg;
        byte[] req = new byte[buf.readableBytes()];
        buf.readBytes(req);
        String body = new String(req, "UTF-8");
        System.out.println("Now is : " + body);
    }

    @Override
    public void exceptionCaught(ChannelHandlerContext ctx, Throwable cause) {
        // 释放资源
        logger.warning("Unexpected exception from downstream : "
                + cause.getMessage());
        ctx.close();
    }
}

在TimeClient中创建了一个NioEventLoopGroup对象，它用来处理IO事件。在group中添加了一个TimeClientHandler，在通道准备就绪时会调用channelActive方法向服务器发送QUERY TIME ORDER；在收到服务器的响应后，会调用channelRead方法，输出服务器返回的时间。

ChannelFuture f = b.connect(host, port).sync();用于异步连接服务端，我们进入到connect方法，该方法会调用doResolveAndConnect方法：

private ChannelFuture doResolveAndConnect(final SocketAddress remoteAddress, final SocketAddress localAddress) {
    
    // 初始化并注册
    final ChannelFuture regFuture = initAndRegister();
    final Channel channel = regFuture.channel();

    // 判断初始化和注册是否已经完成
    if (regFuture.isDone()) {
        // 如果注册失败，则返回ChannelFuture对象
        if (!regFuture.isSuccess()) {
            return regFuture;
        }
        // 如果注册成功，则创建一个该channel对应的Promise对象作为参数
        return doResolveAndConnect0(channel, remoteAddress, localAddress, channel.newPromise());
    } else {
        // Registration future is almost always fulfilled already, but just in case it's not.
        final PendingRegistrationPromise promise = new PendingRegistrationPromise(channel);
        regFuture.addListener(new ChannelFutureListener() {
            @Override
            public void operationComplete(ChannelFuture future) throws Exception {
                // Directly obtain the cause and do a null check so we only need one volatile read in case of a
                // failure.
                Throwable cause = future.cause();
                // 如果有异常，则将promise设置为失败
                if (cause != null) {
                    // Registration on the EventLoop failed so fail the ChannelPromise directly to not cause an
                    // IllegalStateException once we try to access the EventLoop of the Channel.
                    promise.setFailure(cause);
                } else {
                    // Registration was successful, so set the correct executor to use.
                    // See https://github.com/netty/netty/issues/2586
                    // 将promise标记为已注册
                    promise.registered();
                    doResolveAndConnect0(channel, remoteAddress, localAddress, promise);
                }
            }
        });
        return promise;
    }
}

如果注册未完成，则创建一个PendingRegistrationPromise类型的对象，并向regFuture对象中添加一个listener，在注册完成后会调用该listener的operationComplete方法。

下面看下doResolveAndConnect0方法：

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

        if (!resolver.isSupported(remoteAddress) || resolver.isResolved(remoteAddress)) {
            // Resolver has no idea about what to do with the specified remote address or it's resolved already.
            doConnect(remoteAddress, localAddress, promise);
            return promise;
        }

        final Future<SocketAddress> resolveFuture = resolver.resolve(remoteAddress);

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

            if (resolveFailureCause != null) {
                // Failed to resolve immediately
                channel.close();
                promise.setFailure(resolveFailureCause);
            } else {
                // Succeeded to resolve immediately; cached? (or did a blocking lookup)
                doConnect(resolveFuture.getNow(), localAddress, promise);
            }
            return promise;
        }

        // Wait until the name resolution is finished.
        resolveFuture.addListener(new FutureListener<SocketAddress>() {
            @Override
            public void operationComplete(Future<SocketAddress> future) throws Exception {
                if (future.cause() != null) {
                    channel.close();
                    promise.setFailure(future.cause());
                } else {
                    doConnect(future.getNow(), localAddress, promise);
                }
            }
        });
    } catch (Throwable cause) {
        promise.tryFailure(cause);
    }
    return promise;
}

该方法主要对地址进行一些检查和解析，然后会调用doConnect方法：

private static void doConnect(
            final SocketAddress remoteAddress, final SocketAddress localAddress, final ChannelPromise connectPromise) {

    // This method is invoked before channelRegistered() is triggered.  Give user handlers a chance to set up
    // the pipeline in its channelRegistered() implementation.
    final Channel channel = connectPromise.channel();
    channel.eventLoop().execute(new Runnable() {
        @Override
        public void run() {
            if (localAddress == null) {
                channel.connect(remoteAddress, connectPromise);
            } else {
                channel.connect(remoteAddress, localAddress, connectPromise);
            }
            connectPromise.addListener(ChannelFutureListener.CLOSE_ON_FAILURE);
        }
    });
}

这里把具体的连接工作放到了channel对应的eventLoop来执行，这方面的内容以后会讲到，现在可以理解为放入一个线程池中执行。

具体的连接操作其实是通过pipeline调用了AbstractNioUnsafe类中的connect方法来执行，这些内容将在以后分析。

我们回过头来看下doResolveAndConnect方法中调用的initAndRegister方法，该方法在AbstractBootstrap中实现：

final ChannelFuture initAndRegister() {
    Channel channel = null;
    try {
        // 创建一个Channel对象
        channel = channelFactory.newChannel();
        init(channel);
    } catch (Throwable t) {
        if (channel != null) {
            // channel can be null if newChannel crashed (eg SocketException("too many open files"))
            channel.unsafe().closeForcibly();
        }
        // as the Channel is not registered yet we need to force the usage of the GlobalEventExecutor
        return new DefaultChannelPromise(channel, GlobalEventExecutor.INSTANCE).setFailure(t);
    }

    // 通过group执行注册
    ChannelFuture regFuture = config().group().register(channel);
    if (regFuture.cause() != null) {
        if (channel.isRegistered()) {
            channel.close();
        } else {
            channel.unsafe().closeForcibly();
        }
    }

    // If we are here and the promise is not failed, it's one of the following cases:
    // 1) If we attempted registration from the event loop, the registration has been completed at this point.
    //    i.e. It's safe to attempt bind() or connect() now because the channel has been registered.
    // 2) If we attempted registration from the other thread, the registration request has been successfully
    //    added to the event loop's task queue for later execution.
    //    i.e. It's safe to attempt bind() or connect() now:
    //         because bind() or connect() will be executed *after* the scheduled registration task is executed
    //         because register(), bind(), and connect() are all bound to the same thread.

    return regFuture;
}

该方法很简单，创建了一个Channel对象，然后调用init方法对该对象进行初始化：

@Override
@SuppressWarnings("unchecked")
void init(Channel channel) throws Exception {
    ChannelPipeline p = channel.pipeline();
    p.addLast(config.handler());

    final Map<ChannelOption<?>, Object> options = options0();
    synchronized (options) {
        setChannelOptions(channel, options, logger);
    }

    final Map<AttributeKey<?>, Object> attrs = attrs0();
    synchronized (attrs) {
        for (Entry<AttributeKey<?>, Object> e: attrs.entrySet()) {
            channel.attr((AttributeKey<Object>) e.getKey()).set(e.getValue());
        }
    }
}

回顾一下在TimeClient中的如下代码：

b.group(group).channel(NioSocketChannel.class)
                .option(ChannelOption.TCP_NODELAY, true)
                .handler(new ChannelInitializer<SocketChannel>() {
                    @Override
                    public void initChannel(SocketChannel ch)
                            throws Exception {
                        ch.pipeline().addLast(new TimeClientHandler());
                    }
                });

init方法就是把上面的option和handler添加到channel中。

至此，整个连接操作算是完成了，以后的通信工作就会交给EventLoop来执行，Bootstrap只是提供了一个启动的工作，主要是进行初始化和连接的操作。