Netty源码解析---服务端启动

一个简单的服务端代码:

public class SimpleServer {

    public static void main(String[] args) {
        NioEventLoopGroup boss = new NioEventLoopGroup(1);
        NioEventLoopGroup worker = new NioEventLoopGroup();

        try {
            ServerBootstrap b = new ServerBootstrap();
            b.group(boss,worker)
                    .channel(NioServerSocketChannel.class)
                    .handler(new ServerHandler())
                    .childHandler(new ChannelInitializer<SocketChannel>() {
                        @Override
                        protected void initChannel(SocketChannel ch) throws Exception {

                        }
                    });

            ChannelFuture f = b.bind(8000).sync();
            f.channel().closeFuture().sync();
        } catch (InterruptedException e) {
            e.printStackTrace();
        }finally {
            boss.shutdownGracefully();
            worker.shutdownGracefully();
        }
    }

    private static class ServerHandler extends ChannelInboundHandlerAdapter{

        @Override
        public void channelActive(ChannelHandlerContext ctx) throws Exception {
            System.out.println("channelActive");
        }

        @Override
        public void channelRegistered(ChannelHandlerContext ctx) throws Exception {
            System.out.println("channelRegistered");
        }

        @Override
        public void handlerAdded(ChannelHandlerContext ctx) throws Exception {
            System.out.println("handlerAdded");
        }
    }
}

我们从bind(...)方法开始

AbstractBootstrap.java

    /**
     * Create a new {@link Channel} and bind it.
     */
    public ChannelFuture bind(int inetPort) {
        return bind(new InetSocketAddress(inetPort));
    }

    /**
     * Create a new {@link Channel} and bind it.
     */
    public ChannelFuture bind(SocketAddress localAddress) {
        validate();
        if (localAddress == null) {
            throw new NullPointerException("localAddress");
        }
        return doBind(localAddress);
    }

然后我们调了doBind()方法:

AbstractBootstrap.java
  
    private ChannelFuture doBind(final SocketAddress localAddress) {
        final ChannelFuture regFuture = initAndRegister();//初始化及注册
        final Channel channel = regFuture.channel();
        if (regFuture.cause() != null) {
            return regFuture;
        }

        if (regFuture.isDone()) {
            // At this point we know that the registration was complete and successful.
            ChannelPromise promise = channel.newPromise();
            doBind0(regFuture, channel, localAddress, promise);//绑定端口
            return promise;
        } 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 {
                    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();

                        doBind0(regFuture, channel, localAddress, promise);
                    }
                }
            });
            return promise;
        }
    }

我们继续断点进initAndRegister();这个方法内部

  AbstractBootstrap.java
  final ChannelFuture initAndRegister() {
        Channel channel = null;
        try {
          //这里的channelFactory是ReflectiveChannelFactory
            channel = channelFactory.newChannel();
          //初始化参数
            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);
        }

        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;
    }

initAndRegister()做了几件事情

1. new一个channel
2. init这个channel
3. 将这个channel register到某个对象

newChannel();我们先来分析一个

   ReflectiveChannelFactory.java
   @Override
    public T newChannel() {
        try {
            return clazz.newInstance();
        } catch (Throwable t) {
            throw new ChannelException("Unable to create Channel from class " + clazz, t);
        }
    }

可以看到里面是通过反射创建了一个NioServerSocketChannel 这个对象

我们再来看看NioServerSocketChannel

NioServerSocketChannel.java
    public NioServerSocketChannel() {
        this(newSocket(DEFAULT_SELECTOR_PROVIDER));
    }
    
    /**
     * Create a new instance using the given {@link ServerSocketChannel}.
     */
    public NioServerSocketChannel(ServerSocketChannel channel) {
        super(null, channel, SelectionKey.OP_ACCEPT);
        config = new NioServerSocketChannelConfig(this, javaChannel().socket());
    }

这里调用了父类的构造器

我们接下来跟踪一下父类

 AbstractNioMessageChannel.java
   /**
     * @see {@link AbstractNioChannel#AbstractNioChannel(Channel, SelectableChannel, int)}
     */
    protected AbstractNioMessageChannel(Channel parent, SelectableChannel ch, int readInterestOp) {
        super(parent, ch, readInterestOp);
    }
 AbstractNioChannel.java
    /**
     * Create a new instance
     *
     * @param parent            the parent {@link Channel} by which this instance was created. May be {@code null}
     * @param ch                the underlying {@link SelectableChannel} on which it operates
     * @param readInterestOp    the ops to set to receive data from the {@link SelectableChannel}
     */
    protected AbstractNioChannel(Channel parent, SelectableChannel ch, int readInterestOp) {
        super(parent);
        this.ch = ch;
        this.readInterestOp = readInterestOp;
        try {
            ch.configureBlocking(false);
        } catch (IOException e) {
            try {
                ch.close();
            } catch (IOException e2) {
                if (logger.isWarnEnabled()) {
                    logger.warn(
                            "Failed to close a partially initialized socket.", e2);
                }
            }

            throw new ChannelException("Failed to enter non-blocking mode.", e);
        }
    }

AbstractChannel.java
    /**
     * Creates a new instance.
     *
     * @param parent
     *        the parent of this channel. {@code null} if there's no parent.
     */
    protected AbstractChannel(Channel parent) {
        this.parent = parent;
 // 创建 ChannelId 对象
        id = newId();
   // 创建 Unsafe 对象
        unsafe = newUnsafe();
  // 创建 DefaultChannelPipeline 对象
        pipeline = newChannelPipeline();
    }

通过不断跟踪我们把Channel对象创建完毕 , 它总共包含一下几个核心组件:

• ChannelId
• Unsafe
• Pipeline
  • ChannelHandler
• ChannelConfig
• Java 原生 NIO Channel

接下来我们回到initAndRegister()这个方法,继续往下分析int(channel)这个方法

  ServerBootstrap.java  
    @Override
    void init(Channel channel) throws Exception {
        final Map<ChannelOption<?>, Object> options = options0();
        synchronized (options) {
            channel.config().setOptions(options);
        }

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

        ChannelPipeline p = channel.pipeline();

        final EventLoopGroup currentChildGroup = childGroup;
        final ChannelHandler currentChildHandler = childHandler;
        final Entry<ChannelOption<?>, Object>[] currentChildOptions;
        final Entry<AttributeKey<?>, Object>[] currentChildAttrs;
        synchronized (childOptions) {
            currentChildOptions = childOptions.entrySet().toArray(newOptionArray(childOptions.size()));
        }
        synchronized (childAttrs) {
            currentChildAttrs = childAttrs.entrySet().toArray(newAttrArray(childAttrs.size()));
        }

        p.addLast(new ChannelInitializer<Channel>() {
            @Override
            public void initChannel(Channel ch) throws Exception {
                final ChannelPipeline pipeline = ch.pipeline();
                ChannelHandler handler = config.handler();
                if (handler != null) {
                    pipeline.addLast(handler);
                }

                // We add this handler via the EventLoop as the user may have used a ChannelInitializer as handler.
                // In this case the initChannel(...) method will only be called after this method returns. Because
                // of this we need to ensure we add our handler in a delayed fashion so all the users handler are
                // placed in front of the ServerBootstrapAcceptor.
                ch.eventLoop().execute(new Runnable() {
                    @Override
                    public void run() {
                        pipeline.addLast(new ServerBootstrapAcceptor(
                                currentChildGroup, currentChildHandler, currentChildOptions, currentChildAttrs));
                    }
                });
            }
        });
    }

看到这个方法, 我们大概知道以下几件事:

1. 设置option和attr
2. 设置新接入channel的option和attr
3. 加入新连接处理器

接下来我们再分析

        ChannelFuture regFuture = config().group().register(channel);

通过断点可以看到调用到:

    SingleThreadEventLoop.java
    @Override
    public ChannelFuture register(Channel channel) {
        return register(new DefaultChannelPromise(channel, this));
    }
    
    @Override
    public ChannelFuture register(final ChannelPromise promise) {
        ObjectUtil.checkNotNull(promise, "promise");
        promise.channel().unsafe().register(this, promise);
        return promise;
    }

在调用到:

AbstractChannel.java

        @Override
        public final void register(EventLoop eventLoop, final ChannelPromise promise) {
            if (eventLoop == null) {
                throw new NullPointerException("eventLoop");
            }
            if (isRegistered()) {
                promise.setFailure(new IllegalStateException("registered to an event loop already"));
                return;
            }
            if (!isCompatible(eventLoop)) {
                promise.setFailure(
                        new IllegalStateException("incompatible event loop type: " + eventLoop.getClass().getName()));
                return;
            }

            AbstractChannel.this.eventLoop = eventLoop;

            if (eventLoop.inEventLoop()) {
                register0(promise);
            } else {
                try {
                    eventLoop.execute(new Runnable() {
                        @Override
                        public void run() {
                            register0(promise);
                        }
                    });
                } catch (Throwable t) {
                    logger.warn(
                            "Force-closing a channel whose registration task was not accepted by an event loop: {}",
                            AbstractChannel.this, t);
                    closeForcibly();
                    closeFuture.setClosed();
                    safeSetFailure(promise, t);
                }
            }
        }

这段这么长的代码,其实重点就两行代码:

        @Override
        public final void register(EventLoop eventLoop, final ChannelPromise promise) {
            //...
            AbstractChannel.this.eventLoop = eventLoop;
            //...
            register0(promise);
            //...
        }

我们断点进入到register0

        private void register0(ChannelPromise promise) {
            try {
                // check if the channel is still open as it could be closed in the mean time when the register
                // call was outside of the eventLoop
                if (!promise.setUncancellable() || !ensureOpen(promise)) {// 确保 Channel 是打开的
                    return;
                }
               // 记录是否为首次注册
                boolean firstRegistration = neverRegistered;
              // 执行注册逻辑
                doRegister();
              // 标记首次注册为 false
                neverRegistered = false;
               // 标记 Channel 为已注册
                registered = true;

                // Ensure we call handlerAdded(...) before we actually notify the promise. This is needed as the
                // user may already fire events through the pipeline in the ChannelFutureListener.
                pipeline.invokeHandlerAddedIfNeeded();
                // 回调通知 `promise` 执行成功
                safeSetSuccess(promise);
              // 触发通知已注册事件
                pipeline.fireChannelRegistered();
                // Only fire a channelActive if the channel has never been registered. This prevents firing
                // multiple channel actives if the channel is deregistered and re-registered.
                if (isActive()) {
                    if (firstRegistration) {
                        pipeline.fireChannelActive();
                    } else if (config().isAutoRead()) {
                        // This channel was registered before and autoRead() is set. This means we need to begin read
                        // again so that we process inbound data.
                        //
                        // See https://github.com/netty/netty/issues/4805
                        beginRead();
                    }
                }
            } catch (Throwable t) {
                // Close the channel directly to avoid FD leak.
                closeForcibly();
                closeFuture.setClosed();
                safeSetFailure(promise, t);
            }
        }

最终我们在doRegister()里面看到了java NIO的注册方法

    AbstractNioChannel.java
    @Override
    protected void doRegister() throws Exception {
        boolean selected = false;
        for (;;) {
            try {
                selectionKey = javaChannel().register(eventLoop().selector, 0, this);
                return;
            } catch (CancelledKeyException e) {
                if (!selected) {
                    // Force the Selector to select now as the "canceled" SelectionKey may still be
                    // cached and not removed because no Select.select(..) operation was called yet.
                    eventLoop().selectNow();
                    selected = true;
                } else {
                    // We forced a select operation on the selector before but the SelectionKey is still cached
                    // for whatever reason. JDK bug ?
                    throw e;
                }
            }
        }
    }

至此我们分析完了doBind()里面的initAndRegister(),

接下来我们分析doBind0(regFuture, channel, localAddress, promise);

    private static void doBind0(
            final ChannelFuture regFuture, final Channel channel,
            final SocketAddress localAddress, final ChannelPromise promise) {

        // This method is invoked before channelRegistered() is triggered.  Give user handlers a chance to set up
        // the pipeline in its channelRegistered() implementation.
        channel.eventLoop().execute(new Runnable() {
            @Override
            public void run() {
              //注册成功,绑定端口
                if (regFuture.isSuccess()) {
                    channel.bind(localAddress, promise).addListener(ChannelFutureListener.CLOSE_ON_FAILURE);
                  //注册失败,回调通知promise异常
                } else {
                    promise.setFailure(regFuture.cause());
                }
            }
        });
    }

我们继续断点跟踪bind()方法:

最后我们调用到AbstractUnsafe 的bind()方法

        @Override
        public final void bind(final SocketAddress localAddress, final ChannelPromise promise) {
            // 判断是否在 EventLoop 的线程中。
            assertEventLoop();

            if (!promise.setUncancellable() || !ensureOpen(promise)) {
                return;
            }

            // See: https://github.com/netty/netty/issues/576
            if (Boolean.TRUE.equals(config().getOption(ChannelOption.SO_BROADCAST)) &&
                localAddress instanceof InetSocketAddress &&
                !((InetSocketAddress) localAddress).getAddress().isAnyLocalAddress() &&
                !PlatformDependent.isWindows() && !PlatformDependent.isRoot()) {
                // Warn a user about the fact that a non-root user can't receive a
                // broadcast packet on *nix if the socket is bound on non-wildcard address.
                logger.warn(
                        "A non-root user can't receive a broadcast packet if the socket " +
                        "is not bound to a wildcard address; binding to a non-wildcard " +
                        "address (" + localAddress + ") anyway as requested.");
            }
            // 记录 Channel 是否激活
            boolean wasActive = isActive();
            try {
                // 绑定 Channel 的端口
                doBind(localAddress);
            } catch (Throwable t) {
                safeSetFailure(promise, t);
                closeIfClosed();
                return;
            }
            // 若 Channel 是新激活的，触发通知 Channel 已激活的事件。
            if (!wasActive && isActive()) {
                invokeLater(new Runnable() {
                    @Override
                    public void run() {
                        pipeline.fireChannelActive();
                    }
                });
            }
            // 回调通知 promise 执行成功
            safeSetSuccess(promise);
        }

在dobind()方法里面终于用java NIO绑定了端口

NioServerSocketChannel.java    
    @Override
    protected void doBind(SocketAddress localAddress) throws Exception {
        if (PlatformDependent.javaVersion() >= 7) {
            javaChannel().bind(localAddress, config.getBacklog());
        } else {
            javaChannel().socket().bind(localAddress, config.getBacklog());
        }
    }

在invokeLater里面加入了异步执行过程,通过提交一个新任务到EventLoop的线程中

        private void invokeLater(Runnable task) {
            try {
                // This method is used by outbound operation implementations to trigger an inbound event later.
                // They do not trigger an inbound event immediately because an outbound operation might have been
                // triggered by another inbound event handler method.  If fired immediately, the call stack
                // will look like this for example:
                //
                //   handlerA.inboundBufferUpdated() - (1) an inbound handler method closes a connection.
                //   -> handlerA.ctx.close()
                //      -> channel.unsafe.close()
                //         -> handlerA.channelInactive() - (2) another inbound handler method called while in (1) yet
                //
                // which means the execution of two inbound handler methods of the same handler overlap undesirably.
                eventLoop().execute(task);
            } catch (RejectedExecutionException e) {
                logger.warn("Can't invoke task later as EventLoop rejected it", e);
            }
        }

通过断点跟踪,可以到AbstractUnsafe#beginRead()方法

        @Override
        public final void beginRead() {
          // 判断是否在 EventLoop 的线程中。
            assertEventLoop();
            // Channel 必须激活
            if (!isActive()) {
                return;
            }
            // 执行开始读取
            try {
                doBeginRead();
            } catch (final Exception e) {
                invokeLater(new Runnable() {
                    @Override
                    public void run() {
                        pipeline.fireExceptionCaught(e);
                    }
                });
                close(voidPromise());
            }
        }

doBeginRead()方法,执行开始读取,对于NioServerSocketChannel 来说,实现如下:

    AbstractNioMessageChannel.java
    @Override
    protected void doBeginRead() throws Exception {
        if (inputShutdown) {
            return;
        }
        super.doBeginRead();
    }
    AbstractNioChannel.java
    @Override
    protected void doBeginRead() throws Exception {
        // Channel.read() or ChannelHandlerContext.read() was called
        final SelectionKey selectionKey = this.selectionKey;
        if (!selectionKey.isValid()) {
            return;
        }

        readPending = true;

        final int interestOps = selectionKey.interestOps();
        if ((interestOps & readInterestOp) == 0) {
            selectionKey.interestOps(interestOps | readInterestOp);
        }
    }

selectionKey.interestOps()方法调用后,服务端可以开始处理客户端的连接事件.