Netty 4源码解析：请求处理

Netty 4源码解析：请求处理

通过之前《Netty 4源码解析：服务端启动》的分析，我们知道在最前端“扛压力”的是NioEventLoop.run()方法。我们指定创建出的NioServerSocketChannel就是注册到了NioEventLoop中的Selector上。所以我们继续顺藤摸瓜，看看服务端启动完成后，Netty是如何处理每个请求的。

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1.MainReactor

1.1 事件轮询

之前我们曾分析过到NioEventLoop.run()方法，但因为之前只关注启动流程所以“浅尝辄止”了，这次我们就以它为起点开始深入分析。NioEventLoop于Selector绑定，它是真正轮询Selector的地方。至于有哪一个或哪些Channel的事件绑定到Selector了，则是注册阶段决定的。对于MainReactor来说，只有一个NioEventLoop负责处理一个ServerSocketChannel的事件。

// NioEventLoop
    @Override
    protected void run() {
        for (;;) {
            boolean oldWakenUp = wakenUp.getAndSet(false);
            try {
                if (hasTasks()) {
                    selectNow();
                } else {
                    select(oldWakenUp);

                    if (wakenUp.get()) {
                        selector.wakeup();
                    }
                }

                if (ioRatio == 100) {
                    processSelectedKeys();
                    runAllTasks();
                }
            } catch (Throwable t) {
                logger.warn("Unexpected exception in the selector loop.", t);
            }
        }
    }

    private void processSelectedKeys() {
        if (selectedKeys != null) {
            processSelectedKeysOptimized(selectedKeys.flip());
        } else {
            processSelectedKeysPlain(selector.selectedKeys());
        }
    }

    private void processSelectedKeysPlain(Set<SelectionKey> selectedKeys) {
        if (selectedKeys.isEmpty()) {
            return;
        }

        Iterator<SelectionKey> i = selectedKeys.iterator();
        for (;;) {
            final SelectionKey k = i.next();
            final Object a = k.attachment();
            i.remove();

            if (a instanceof AbstractNioChannel) {
                processSelectedKey(k, (AbstractNioChannel) a);
            }

            if (!i.hasNext()) {
                break;
            }
        }
    }

1.2 事件触发

不知道大家是否还记得，NioServerSocketChannel注册时有一个小细节，就是它将自己作为了attachment。所以上面处理SelectionKey时就能通过attachement取到注册时的Channel。为什么一定要拿到当时的Channel呢？继续往下看。

    @Override
    protected void doRegister() throws Exception {
        for (;;) {
            try {
                selectionKey = javaChannel().register(eventLoop().selector, 0, this);
                return;
            } catch (CancelledKeyException e) {
                // ...
            }
        }
    }

原来跟之前介绍过的注册和绑定一样，最终都是通过Channel的unsafe工具类来完成的。

// NioEventLoop
private static void processSelectedKey(SelectionKey k, AbstractNioChannel ch) {
        final NioUnsafe unsafe = ch.unsafe();
        try {
            int readyOps = k.readyOps();
            // Also check for readOps of 0 to workaround possible JDK bug which may otherwise lead to a spin loop
            if ((readyOps & (SelectionKey.OP_READ | SelectionKey.OP_ACCEPT)) != 0 || readyOps == 0) {
                unsafe.read();
                if (!ch.isOpen()) {
                    // Connection already closed - no need to handle write.
                    return;
                }
            }
            if ((readyOps & SelectionKey.OP_WRITE) != 0) {
                // Call forceFlush which will also take care of clear the OP_WRITE once there is nothing left to write
                ch.unsafe().forceFlush();
            }
            if ((readyOps & SelectionKey.OP_CONNECT) != 0) {
                // remove OP_CONNECT as otherwise Selector.select(..) will always return without blocking
                int ops = k.interestOps();
                ops &= ~SelectionKey.OP_CONNECT;
                k.interestOps(ops);

                unsafe.finishConnect();
            }
        } catch (CancelledKeyException ignored) {
            unsafe.close(unsafe.voidPromise());
        }
    }

1.3 感兴趣的事件

还有一个小细节，就是doRegister()注册时实际传给register()的事件是0，也就是对任何事件都不感兴趣，这又是怎么回事呢？原来具体对什么事件感兴趣是在子类的构造方法里传入的。如果是isAutoRead()，那么一旦Channel连接成功就会自动触发一次读操作。真正注册感兴趣事件的地方就是第一次读操作的时候。

// NioServerSocketChannel
    public NioServerSocketChannel(ServerSocketChannel channel) {
        super(null, channel, SelectionKey.OP_ACCEPT);
    }

// AbstractNioChannel
    protected AbstractNioChannel(Channel parent, SelectableChannel ch, int readInterestOp) {
        super(parent);
        this.ch = ch;
        this.readInterestOp = readInterestOp;
        try {
            ch.configureBlocking(false);
        } catch (IOException e) {
            //...
        }
    }

// DefaultChannelPipeline
    @Override
    public ChannelPipeline fireChannelActive() {
        head.fireChannelActive();

        if (channel.config().isAutoRead()) {
            channel.read();
        }
        return this;
    }

// AbstractNioChannel
    @Override
    protected void doBeginRead() throws Exception {
        final SelectionKey selectionKey = this.selectionKey;
        final int interestOps = selectionKey.interestOps();
        if ((interestOps & readInterestOp) == 0) {
            selectionKey.interestOps(interestOps | readInterestOp);
        }
    }

1.4 事件处理

紧接着刚才的processSelectedKey()说，既然OP_ACCEPT都已经注册上了，当接收到新用户连接时就会触发unsafe.read()方法。read()会不断调用doReadMessages()，将产生的readBuf逐一发送给Pipeline.fireChannelRead()去处理。

    private final class NioMessageUnsafe extends AbstractNioUnsafe {

        private final List<Object> readBuf = new ArrayList<Object>();

        @Override
        public void read() {
            final ChannelConfig config = config();
            final int maxMessagesPerRead = config.getMaxMessagesPerRead();
            final ChannelPipeline pipeline = pipeline();
            boolean closed = false;
            Throwable exception = null;
            try {
                try {
                    for (;;) {
                        int localRead = doReadMessages(readBuf);
                        if (localRead == 0) {
                            break;
                        }
                        if (localRead < 0) {
                            closed = true;
                            break;
                        }
                        if (readBuf.size() >= maxMessagesPerRead) {
                            break;
                        }
                    }
                } catch (Throwable t) {
                    exception = t;
                }
                setReadPending(false);
                int size = readBuf.size();
                for (int i = 0; i < size; i ++) {
                    pipeline.fireChannelRead(readBuf.get(i));
                }

                readBuf.clear();
                pipeline.fireChannelReadComplete();

                if (exception != null) {
                    pipeline.fireExceptionCaught(exception);
                }
            }
        }
    }

来看看NioServerSocketChannel对doReadMessages()的覆写吧，原来接收并创建Channel的工作就是在这完成的。JDK Channel被保存到Netty包装后的NioSocketChannel中，然后传给Pipeline处理。

// NioServerSocketChannel
    @Override
    protected int doReadMessages(List<Object> buf) throws Exception {
        SocketChannel ch = javaChannel().accept();

        try {
            if (ch != null) {
                buf.add(new NioSocketChannel(this, ch));
                return 1;
            }
        } catch (Throwable t) {
            logger.warn("Failed to create a new channel from an accepted socket.", t);

            try {
                ch.close();
            } catch (Throwable t2) {
                logger.warn("Failed to close a socket.", t2);
            }
        }
        return 0;
    }

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2.主Pipeline

2.1 “主从”的桥梁

又到了“温故时间”，还记得初始化Channel时Netty是怎么做的吗？我们创建了一个叫做ServerBootstrapAcceptor的Handler，它持有的childGroup和childHandler就是SubReactor的NioEventLoopGroup和Handler。

// ServerBootstrap
    @Override
    void init(Channel channel) throws Exception {
        // ...
        final EventLoopGroup currentChildGroup = childGroup;
        final ChannelHandler currentChildHandler = childHandler;
        final Entry<ChannelOption<?>, Object>[] currentChildOptions;
        final Entry<AttributeKey<?>, Object>[] currentChildAttrs;
        // ...

        p.addLast(new ChannelInitializer<Channel>() {
            @Override
            public void initChannel(Channel ch) throws Exception {
                ch.pipeline().addLast(new ServerBootstrapAcceptor(
                        currentChildGroup, currentChildHandler, currentChildOptions, currentChildAttrs));
            }
        });
    }

紧接着前面unsafe.read()方法中的fireChannelRead()，会触发ServerBootstrapAcceptor的channelRead()。在这里，msg就是新创建的SocketChannel，将我们的定义的Handler都加入到子Pipeline中。所以说，ServerBootstrapAcceptor就是主从Reactor间的桥梁，它不断将从主Reactor接收到的Channel绑定到从Reactor的一个EventLoop上。

    private static class ServerBootstrapAcceptor extends ChannelInboundHandlerAdapter {

        @Override
        public void channelRead(ChannelHandlerContext ctx, Object msg) {
            final Channel child = (Channel) msg;

            child.pipeline().addLast(childHandler);

            try {
                childGroup.register(child);
            } catch (Throwable t) {
                forceClose(child, t);
            }
        }
    }

2.2 注册读事件

因为Netty 4中已经完全统一了EventLoopGroup的代码，已经不区分主从Reactor的逻辑了。所以实际上，这里的注册过程我们已经分析过了。子EventLoopGroup会选择出一个EventLoop负责轮询绑定上的Channel的事件，而Channel感兴趣的事件前面也提到了，就是Channel构造方法中传入的。

// NioSocketChannel
    public NioSocketChannel(Channel parent, SocketChannel socket) {
        super(parent, socket);
        config = new NioSocketChannelConfig(this, socket.socket());
    }
// AbstractNioByteChannel
    protected AbstractNioByteChannel(Channel parent, SelectableChannel ch) {
        super(parent, ch, SelectionKey.OP_READ);
    }

2.3 创建Handler

使用Netty时我们通常会在ChannelInitializer中初始化Handler，但Netty是什么时候调用它的呢？答案就在Channel注册到子EventLoop之后。之前看到的fireChannelRegistered()会触发ChannelInitializer。所以说：每个客户端Channel建立成功后会创建Handler，并且后续请求处理都由这一组Handler完成。

public abstract class ChannelInitializer<C extends Channel> extends ChannelInboundHandlerAdapter {
    /**
     * This method will be called once the {@link Channel} was registered.
     */
    protected abstract void initChannel(C ch) throws Exception;

    @Override
    public final void channelRegistered(ChannelHandlerContext ctx) throws Exception {
        ChannelPipeline pipeline = ctx.pipeline();
        try {
            initChannel((C) ctx.channel());
            pipeline.remove(this);
            ctx.fireChannelRegistered();
        } catch (Throwable t) {
            logger.warn("Failed to initialize a channel. Closing: " + ctx.channel(), t);
        }
    }
}

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3.SubReactor

通过前面的分析能够看到，EventLoop轮询到的事件最终会交给unsafe.read()去处理。NioSocketChannel与NioServerSocketChannel的一个重要区别就是：NioSocketChannel继承AbstractNioByteChannel，而后者继承AbstractNioMessageChannel，两者的unsafe工具类实现是不同的。

public abstract class AbstractNioByteChannel extends AbstractNioChannel {

        @Override
        public void read() {
            final ChannelConfig config = config();
            final ChannelPipeline pipeline = pipeline();
            final ByteBufAllocator allocator = config.getAllocator();
            final int maxMessagesPerRead = config.getMaxMessagesPerRead();
            RecvByteBufAllocator.Handle allocHandle = this.allocHandle;
            if (allocHandle == null) {
                this.allocHandle = allocHandle = config.getRecvByteBufAllocator().newHandle();
            }

            ByteBuf byteBuf = null;
            int messages = 0;
            boolean close = false;
            try {
                int totalReadAmount = 0;
                boolean readPendingReset = false;
                do {
                    byteBuf = allocHandle.allocate(allocator);
                    int writable = byteBuf.writableBytes();
                    int localReadAmount = doReadBytes(byteBuf);
                    if (localReadAmount <= 0) {
                        // not was read release the buffer
                        byteBuf.release();
                        close = localReadAmount < 0;
                        break;
                    }

                    pipeline.fireChannelRead(byteBuf);
                    byteBuf = null;

                    if (totalReadAmount >= Integer.MAX_VALUE - localReadAmount) {
                        // Avoid overflow.
                        totalReadAmount = Integer.MAX_VALUE;
                        break;
                    }

                    totalReadAmount += localReadAmount;

                    if (localReadAmount < writable) {
                        // Read less than what the buffer can hold,
                        // which might mean we drained the recv buffer completely.
                        break;
                    }
                } while (++ messages < maxMessagesPerRead);

                pipeline.fireChannelReadComplete();
                allocHandle.record(totalReadAmount);
            } catch (Throwable t) {
                handleReadException(pipeline, byteBuf, t, close);
            }
        }
    }
}

在AbstractNioMessageChannel中接收到的是SocketChannel，所以并没有发生真正的读操作。而AbstractNioByteChannel是真正地从SocketChannel中读，所以这也是申请缓冲区的地方。每次发生读事件时，都会分配一块ByteBuf，然后尝试从Channel中读出数据写到ByteBuf中。之后触发fireChannelRead()，由Pipeline中的Handler继续处理，最终Tail处理器负责释放掉ByteBuf。

    static final class TailContext extends AbstractChannelHandlerContext implements ChannelInboundHandler {
        @Override
        public void channelRead(ChannelHandlerContext ctx, Object msg) throws Exception {
            try {
                logger.debug(
                        "Discarded inbound message {} that reached at the tail of the pipeline. " +
                                "Please check your pipeline configuration.", msg);
            } finally {
                ReferenceCountUtil.release(msg);
            }
        }
    }

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4.总结

这次代码分析一个很直接的目的就是想了解Netty的线程和对象模型，至此已经差不多摸清了。首先，ServerSocketChannel会由一个EventLoop负责轮询接收事件，得到的SocketChannel是交给子Reactor中的一个EventLoop负责轮询读事件。也就是说多个客户端可能会对应一个EventLoop线程。每个SocketChannel注册完毕就会创建Handler，所以说每个客户端都对应自己的Handler实例，并且一直使用到连接断开。