【Netty源码系列】客户端启动流程

【相关源码都是出自4.1.55.Final-SNAPSHOT版本】

了解Netty服务端的启动过程后,现在换个角度,客户端是怎样启动呢?这一篇文章基于【Netty源码系列】服务端启动流程的解析,如果有相似的代码我会简单带过,不会再赘述。

现在咱们先看下官方example是怎样启动客户端的,为了使读者更加专注客户端启动的流程,所以我删去非启动不要的流程和加上一些注释,尽量精简代码,使我们能够快速的掌握Netty客户端启动的流程。

public final class EchoClient {
    // Netty服务端的主机名
    static final String HOST = System.getProperty("host", "127.0.0.1");
    // Netty服务端的端口
    static final int PORT = Integer.parseInt(System.getProperty("port", "8007"));

    public static void main(String[] args) throws Exception {
        // 配置线程池组
        EventLoopGroup group = new NioEventLoopGroup();
        try {
            Bootstrap b = new Bootstrap();
            b.group(group)
             .channel(NioSocketChannel.class) // SocketChannel类型为NioSocketChannel
             .option(ChannelOption.TCP_NODELAY, true) // SocketChannel相关TCP参数配置
             .handler(new ChannelInitializer() {
                 @Override
                 public void initChannel(SocketChannel ch) throws Exception { // 配置 SocketChannel 处理器
                     ChannelPipeline p = ch.pipeline();
                     p.addLast(new EchoClientHandler());
                 }
             });
            // 启动客户端
            ChannelFuture f = b.connect(HOST, PORT).sync();
            // 等待直到客户端channel关闭
            f.channel().closeFuture().sync();
        } finally {
            // 优雅关闭线程池组
            group.shutdownGracefully();
        }
    }
}

和Netty服务端启动的代码大部分都是相似的,无非都是要一开始启动线程池组,然后配置channel的类型,相关TCP参数和处理器,当配置完之后阻塞启动,最后优雅关闭线程池组。除了客户端启动时调用connect方法,其它基本与Netty服务端启动的流程基本一样,所以这篇文章主要就是分析Netty客户端调用connect的过程。

客户端启动——connect

    public ChannelFuture connect(String inetHost, int inetPort) {
        return connect(InetSocketAddress.createUnresolved(inetHost, inetPort));
    }
    
    // JDK原生方法,根据主机名和端口号创建未解析的套接字地址
    public static InetSocketAddress createUnresolved(String host, int port) {
        return new InetSocketAddress(checkPort(port), checkHost(host));
    }
    
    public ChannelFuture connect(SocketAddress remoteAddress) {
        ObjectUtil.checkNotNull(remoteAddress, "remoteAddress");
        validate();
        return doResolveAndConnect(remoteAddress, config.localAddress());
    }  
    
    private ChannelFuture doResolveAndConnect(final SocketAddress remoteAddress, final SocketAddress localAddress) {
        // 初始化channel并注册到SocketChannel上
        final ChannelFuture regFuture = initAndRegister();
        final Channel channel = regFuture.channel();

        // 因为initAndRegister方法是异步的,所以regFuture有可能是还未注册,因此程序需要作出判断
        if (regFuture.isDone()) {
            if (!regFuture.isSuccess()) {
                return regFuture;
            }
            return doResolveAndConnect0(channel, remoteAddress, localAddress, channel.newPromise());
        } else { // 如果regFuture还未完成注册,则加上监听器,当注册完成后回调doResolveAndConnect0方法
            final PendingRegistrationPromise promise = new PendingRegistrationPromise(channel);
            regFuture.addListener(new ChannelFutureListener() {
                @Override
                public void operationComplete(ChannelFuture future) throws Exception {
                    Throwable cause = future.cause();
                    if (cause != null) {
                        promise.setFailure(cause);
                    } else {
                        promise.registered();
                        doResolveAndConnect0(channel, remoteAddress, localAddress, promise);
                    }
                }
            });
            return promise;
        }
    }    

执行initAndRegister方法的流程,在Netty服务端启动流程系列已重点分析,不太清楚的读者可参考【Netty源码系列】服务端启动流程(三)绑定端口并启动

当NioSocketChannel初始化和注册完成后,下一步就会调用doResolveAndConnect0方法

    /**
     * 解析远程地址并进行连接
     */
    private ChannelFuture doResolveAndConnect0(final Channel channel, SocketAddress remoteAddress,
                                               final SocketAddress localAddress, final ChannelPromise promise) {
        try {
            // 获取channel绑定的eventLoop
            final EventLoop eventLoop = channel.eventLoop();
            AddressResolver resolver;
            try {
                resolver = this.resolver.getResolver(eventLoop);
            } catch (Throwable cause) {
                channel.close();
                return promise.setFailure(cause);
            }

            if (!resolver.isSupported(remoteAddress) || resolver.isResolved(remoteAddress)) {
                doConnect(remoteAddress, localAddress, promise);
                return promise;
            }

            // 解析远程地址
            final Future resolveFuture = resolver.resolve(remoteAddress);

            if (resolveFuture.isDone()) {
                final Throwable resolveFailureCause = resolveFuture.cause();
                if (resolveFailureCause != null) {
                    // 异常处理
                    channel.close();
                    promise.setFailure(resolveFailureCause);
                } else {
                    // 连接远程地址
                    doConnect(resolveFuture.getNow(), localAddress, promise);
                }
                return promise;
            }

            // 解析没完成时等待解析完成
            resolveFuture.addListener(new FutureListener() {
                @Override
                public void operationComplete(Future future) throws Exception {
                    if (future.cause() != null) { // 异常处理
                        channel.close();
                        promise.setFailure(future.cause());
                    } else {
                        // 解析成功开始连接
                        doConnect(future.getNow(), localAddress, promise);
                    }
                }
            });
        } catch (Throwable cause) {
            promise.tryFailure(cause);
        }
        return promise;
    }

调用doResolveAndConnect0是为了连接服务器,该方法中对远程地址的解析,最终都会调用doConnect方法

    private static void doConnect(
            final SocketAddress remoteAddress, final SocketAddress localAddress, final ChannelPromise connectPromise) {
        final Channel channel = connectPromise.channel();
        // 根据channel绑定的eventLoop线程执行connect方法
        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);
            }
        });
    }

channel.connect方法实际上是在AbstractChannel类声明的

    @Override
    public ChannelFuture connect(SocketAddress remoteAddress, ChannelPromise promise) {
        return pipeline.connect(remoteAddress, promise);
    }
    
    /**
     * DefaultChannelPipeline类
     */
    @Override
    public final ChannelFuture connect(SocketAddress remoteAddress, ChannelPromise promise) {
        return tail.connect(remoteAddress, promise);
    }
    
    /**
     * AbstractChannelHandlerContext类
     */
    @Override
    public ChannelFuture connect(SocketAddress remoteAddress, ChannelPromise promise) {
        return connect(remoteAddress, null, promise);
    } 
    
    /**
     * AbstractChannelHandlerContext类
     */    
    @Override
    public ChannelFuture connect(
            final SocketAddress remoteAddress, final SocketAddress localAddress, final ChannelPromise promise) {
        ObjectUtil.checkNotNull(remoteAddress, "remoteAddress");

        if (isNotValidPromise(promise, false)) {
            return promise;
        }
        final AbstractChannelHandlerContext next = findContextOutbound(MASK_CONNECT);
        EventExecutor executor = next.executor();
        if (executor.inEventLoop()) {
            next.invokeConnect(remoteAddress, localAddress, promise);
        } else {
            safeExecute(executor, new Runnable() {
                @Override
                public void run() {
                    next.invokeConnect(remoteAddress, localAddress, promise);
                }
            }, promise, null, false);
        }
        return promise;
    }

通过findContextOutbound方法,从TailContext向前找到第一个OutBound类型为true,实际上也就是HeadContext,然后调用当前类的invokeConnect方法,调用connect方法

    private void invokeConnect(SocketAddress remoteAddress, SocketAddress localAddress, ChannelPromise promise) {
        if (invokeHandler()) {
            try {
                ((ChannelOutboundHandler) handler()).connect(this, remoteAddress, localAddress, promise);
            } catch (Throwable t) {
                notifyOutboundHandlerException(t, promise);
            }
        } else {
            connect(remoteAddress, localAddress, promise);
        }
    }
    
    /**
     * DefaultChannelPipeline类
     */
    final class HeadContext extends AbstractChannelHandlerContext implements ChannelOutboundHandler, ChannelInboundHandler {
        @Override
        public void connect(
                ChannelHandlerContext ctx,
                SocketAddress remoteAddress, SocketAddress localAddress,
                ChannelPromise promise) {
            unsafe.connect(remoteAddress, localAddress, promise);
        }    
    }
    
    /**
     * AbstractNioChannel类
     */
    protected abstract class AbstractNioUnsafe extends AbstractUnsafe implements NioUnsafe {
        @Override
        public final void connect(
                final SocketAddress remoteAddress, final SocketAddress localAddress, final ChannelPromise promise) {
            if (!promise.setUncancellable() || !ensureOpen(promise)) {
                return;
            }

            try {
                if (connectPromise != null) {
                    throw new ConnectionPendingException();
                }

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

                    int connectTimeoutMillis = config().getConnectTimeoutMillis();
                    if (connectTimeoutMillis > 0) {
                        connectTimeoutFuture = eventLoop().schedule(new Runnable() {
                            @Override
                            public void run() {
                                ChannelPromise connectPromise = AbstractNioChannel.this.connectPromise;
                                if (connectPromise != null && !connectPromise.isDone()
                                        && connectPromise.tryFailure(new ConnectTimeoutException(
                                                "connection timed out: " + remoteAddress))) {
                                    close(voidPromise());
                                }
                            }
                        }, connectTimeoutMillis, TimeUnit.MILLISECONDS);
                    }

                    promise.addListener(new ChannelFutureListener() {
                        @Override
                        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();
            }
        }
    }
    
    /**
     * NioSocketChannel类
     */
    @Override
    protected boolean doConnect(SocketAddress remoteAddress, SocketAddress localAddress) throws Exception {
        if (localAddress != null) {
            doBind0(localAddress);
        }

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

经过漫长的debug,终于到达客户端启动连接服务端的最底层,实际上就是通过JDK原生类SocketUtils调用connect方法完成服务端的连接。为了梳理整个客户端启动连接服务端的流程,特意画出时序图帮助理解。

以上就是Netty客户端的启动连接服务端过程,那么当Netty客户端发起请求的时候,Netty服务端会有哪些操作和改变呢?接下来请看【Netty源码系列】服务端接收请求流程

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