【Netty源码学习】BootStrap

BootStrap是客户端的启动类，其主要功能就是设置必要的参数然后启动客户端。

实现如下：

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 {    
                     ChannelPipeline p = ch.pipeline();    
                     p.addLast("decoder", new StringDecoder());    
                     p.addLast("encoder", new StringEncoder());    
                     p.addLast(new ClientHandler());    
                 }    
             });    
    
ChannelFuture future = b.connect(HOST, PORT).sync();

首先是创建实例，然后设置group、channel、option和handler。

接下来就是连接服务器，主要的操作都在这部分进行操作。

public ChannelFuture connect(String inetHost, int inetPort) {
        return connect(InetSocketAddress.createUnresolved(inetHost, inetPort));
    }
    public ChannelFuture connect(InetAddress inetHost, int inetPort) {
        return connect(new InetSocketAddress(inetHost, inetPort));
    }
    public ChannelFuture connect(SocketAddress remoteAddress) {
        if (remoteAddress == null) {
            throw new NullPointerException("remoteAddress");
        }
        validate();
        return doResolveAndConnect(remoteAddress, config.localAddress());
    }

接下来的操作是在doResolveAndConnect中，主要有两个重要的操作：一是调用父类的initAndregister函数将NioServerSocketChannel注册到Selector中，具体实现是在AbstractNioChannel的doRegister函数中，二是调用自身函数doConnect连接到Server，并在线程池中执行。具体操作如下：

 private ChannelFuture doResolveAndConnect(final SocketAddress remoteAddress, final SocketAddress localAddress) {
        final ChannelFuture regFuture = initAndRegister();
        final Channel channel = regFuture.channel();

        if (regFuture.isDone()) {
            if (!regFuture.isSuccess()) {
                return regFuture;
            }
            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 {
                    // Direclty obtain the cause and do a null check so we only need one volatile read in case of a
                    // failure.
                    Throwable cause = future.cause();
                    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
                        promise.registered();
                        doResolveAndConnect0(channel, remoteAddress, localAddress, promise);
                    }
                }
            });
            return promise;
        }
    }

首先是执行final ChannelFuture regFuture = initAndRegister();调用父类的initAndRegister()函数，实现将Channel注册到Selector中，实现如下，调用config().group().register(channel)，实现将Channel注册到Selector中。

   final ChannelFuture initAndRegister() {
        Channel channel = null;
        try {
            channel = channelFactory.newChannel();
            init(channel);
        } catch (Throwable t) {
            if (channel != null) {
                channel.unsafe().closeForcibly();
            }
            return new DefaultChannelPromise(channel, GlobalEventExecutor.INSTANCE).setFailure(t);
        }

        ChannelFuture regFuture = config().group().register(channel);
        if (regFuture.cause() != null) {
            if (channel.isRegistered()) {
                channel.close();
            } else {
                channel.unsafe().closeForcibly();
            }
        }
        return regFuture;
    }

接下的操作是将NioSocketChannel连接到Server端，实现的函数是doResolveAndConnect0，看如下代码，连接的具体实现在doContect中。

 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;
    }

客户端连接到服务端的操作在doConnect中，发现在EventLoop线程池中执行线程，Channel去连接服务器。

 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);
            }
        });
    }

BootStrap总结起来有两个主要的功能，

（1）将调用父类AbstractBootStrap中的initAndRegister函数将NioSocketChannel注册到Selector，具体实现是在AbstractNioChannel的doRegister函数中。

（2）调用自身函数doConnect连接到Server，并在线程池中执行。