前面的一些章节，我们分析了 Netty 的三大组件 —— Channel 、EventLoop、Pipeline ，对 Netty 的工作原理有了深入的了解。在此基础上，我们来分析一下当 Netty 服务端启动后，Netty 是如何处理新连接接入的。

Netty 版本：4.1.30

本文内容主要分为以下四部分：

  • 新连接检测
  • NioSocketChannel 创建
  • NioSocketChannel 初始化与注册
  • NioSocketChannel 注册 READ 兴趣集

新连接检测

前面，我们在讲 EventLoop 的启动过程源码分析 时，解读过下面这段代码：

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public final class NioEventLoop extends SingleThreadEventLoop {
    
    ...
        
    private void processSelectedKey(SelectionKey k, AbstractNioChannel ch) {
    
    ...
    
        try {

            ...

            if ((readyOps & (SelectionKey.OP_READ | SelectionKey.OP_ACCEPT)) != 0 || readyOps == 0) {
                // 读取read事件
                unsafe.read();
            }
        } catch (CancelledKeyException ignored) {
            unsafe.close(unsafe.voidPromise());
        }
        
        ...
	}
    
    ...
    
}

我们还是以服务端 NioServerSocketChannel 为例，它绑定的 unsafe 实例为 NioMessageUnsafe 。上面的 unsafe.read() 接口，会向下调用到 NioMessageUnsafe.read () 接口，如下：

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public abstract class AbstractNioMessageChannel extends AbstractNioChannel {
	
    ...
   	
    private final class NioMessageUnsafe extends AbstractNioUnsafe {
		
        // 用于保存新建立的 NioSocketChannel 的集合
        private final List<Object> readBuf = new ArrayList<Object>();
		
        @Override
        public void read() {
            // 确保在当前线程与EventLoop中的一致
            assert eventLoop().inEventLoop();
            // 获取 NioServerSocketChannel config配置
            final ChannelConfig config = config();
            // 获取 NioServerSocketChannel 绑定的 pipeline
            final ChannelPipeline pipeline = pipeline();
            // 获取RecvByteBuf 分配器 Handle
            // 当channel在接收数据时，allocHandle 会用于分配ByteBuf来保存数据
            // 关于allocHandle后面再去做详细介绍
            final RecvByteBufAllocator.Handle allocHandle = unsafe().recvBufAllocHandle();
            // 重置已累积的所有计数器，并为下一个读取循环读取多少消息/字节数据提供建议
            allocHandle.reset(config);
			
            boolean closed = false;
            Throwable exception = null;
            try {
                try {
                    do {
                        // 调用后面的 doReadMessages 接口，读取到message则返回1
                        int localRead = doReadMessages(readBuf);
                        if (localRead == 0) {
                            break;
                        }
                        if (localRead < 0) {
                            closed = true;
                            break;
                        }
						// 对当前read循环所读取到的message数量计数+1
                        allocHandle.incMessagesRead(localRead);
                        // 判断是否继续读取message
                    } while (allocHandle.continueReading());
                } catch (Throwable t) {
                    exception = t;
                }
                
                int size = readBuf.size();
                for (int i = 0; i < size; i ++) {
                    readPending = false;
                    // 调用pipeline传播ChannelRead事件
                    pipeline.fireChannelRead(readBuf.get(i));
                }
                // 清空readBuf
                readBuf.clear();
                allocHandle.readComplete();
                // 调用pipeline传播 ChannelReadComplete 事件
                pipeline.fireChannelReadComplete();

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

                if (closed) {
                    inputShutdown = true;
                    if (isOpen()) {
                        close(voidPromise());
                    }
                }
            } finally {
                // Check if there is a readPending which was not processed yet.
                // This could be for two reasons:
                // * The user called Channel.read() or ChannelHandlerContext.read() in channelRead(...) method
                // * The user called Channel.read() or ChannelHandlerContext.read() in channelReadComplete(...) method
                //
                // See https://github.com/netty/netty/issues/2254
                if (!readPending && !config.isAutoRead()) {
                    removeReadOp();
                }
            }
        }
    }
    
    ...
    
}

对于 doReadMessages(...) 的分析：

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public class NioServerSocketChannel extends AbstractNioMessageChannel implements io.netty.channel.socket.ServerSocketChannel {
    
    ...
	
    // 读取消息
	@Override
    protected int doReadMessages(List<Object> buf) throws Exception {
        // 获取 SocketChannel 
        SocketChannel ch = SocketUtils.accept(javaChannel());
		
        try {
            if (ch != null) {
                // 使用SocketChannel创建NioSocketChannel，将其存入buf list中
                // 关于NioSocketChannel的创建请看后面的分析
                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;
    }
    
    ...
    
}

对于 continueReading() 接口的分析，至于结果为什么返回 false，后面会单独分析：

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public abstract class DefaultMaxMessagesRecvByteBufAllocator implements MaxMessagesRecvByteBufAllocator {
	
    private volatile int maxMessagesPerRead;
    private volatile boolean respectMaybeMoreData = true;
    
    ...
    
    public abstract class MaxMessageHandle implements ExtendedHandle {
        private ChannelConfig config;
        // 每次读取最大的消息数
        private int maxMessagePerRead;
        private int totalMessages;
        private int totalBytesRead;
        private int attemptedBytesRead;
        private int lastBytesRead;
        private final boolean respectMaybeMoreData = DefaultMaxMessagesRecvByteBufAllocator.this.respectMaybeMoreData;
        private final UncheckedBooleanSupplier defaultMaybeMoreSupplier = new UncheckedBooleanSupplier() {
            @Override
            public boolean get() {
                return attemptedBytesRead == lastBytesRead;
            }
        };
        
        ...
        
        // 判断是否继续读取message    
        @Override
        public boolean continueReading() {
            return continueReading(defaultMaybeMoreSupplier);
        }
        
        // 判断是否继续读取message
        @Override
        public boolean continueReading(UncheckedBooleanSupplier maybeMoreDataSupplier) {
            // 默认情况下 config.isAutoRead() 为true
            // respectMaybeMoreData 默认为 true
            // maybeMoreDataSupplier.get() 为false
            // totalMessages第一次循环则为1
            // maxMessagePerRead为16
			// 结果返回false
            return config.isAutoRead() &&
                (!respectMaybeMoreData || maybeMoreDataSupplier.get()) &&
                totalMessages < maxMessagePerRead &&
                totalBytesRead > 0;
        }
		
        ...
    
    }
    
    ...
        
}

NioSocketChannel 创建

上面分析新连接接入，提到了 NioSocketChannel 的创建，我们这里来详细分析一下，NioSocketChannel 的创建过程与此前我们分析 NioServerSocketChannel 创建 大体类似。

构造器

先来看看 NioSocketChannel 的构造函数：

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public class NioSocketChannel extends AbstractNioByteChannel implements io.netty.channel.socket.SocketChannel {

    ...
	
	public NioSocketChannel(Channel parent, SocketChannel socket) {
        // 调用父类构造器
        super(parent, socket);
        // 创建NioSocketChannelConfig
        config = new NioSocketChannelConfig(this, socket.socket());
    }
	
    ...
	
}

父类 AbstractNioByteChannel 构造器：

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public abstract class AbstractNioByteChannel extends AbstractNioChannel {

	...
    
    protected AbstractNioByteChannel(Channel parent, SelectableChannel ch) {
        // 调用父类构造器，并设置兴趣集为SelectionKey.OP_READ，对read事件感兴趣
        super(parent, ch, SelectionKey.OP_READ);
    }
    
    ...
	
}

父类 AbstractNioChannel 构造器：

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public abstract class AbstractNioChannel extends AbstractChannel {
    
    ...
    
    protected AbstractNioChannel(Channel parent, SelectableChannel ch, int readInterestOp) {
        // 调用父类构造器
        super(parent);
        // 设置channel
        this.ch = ch;
        // 设置兴趣集
        this.readInterestOp = readInterestOp;
        try {
            // 设置为非阻塞
            ch.configureBlocking(false);
        } catch (IOException e) {
            ...
        }
    }


}

父类 AbstractChannel 构造器：

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public abstract class AbstractChannel extends DefaultAttributeMap implements Channel {

    ...
    
	protected AbstractChannel(Channel parent) {
        // 设置parent
        this.parent = parent;
        // 创建channelId
        id = newId();
        // 创建unsafe
        unsafe = newUnsafe();
        // 创建pipeline
        pipeline = newChannelPipeline();
    }
	
    ...
}

ChannelConfig 创建

接着我们看看 NioSocketChannelConfig 的创建逻辑：

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public class NioSocketChannel extends AbstractNioByteChannel implements io.netty.channel.socket.SocketChannel {
	
    ...
       
    private NioSocketChannelConfig(NioSocketChannel channel, Socket javaSocket) {
        // 调用父类构造器
        super(channel, javaSocket);
        calculateMaxBytesPerGatheringWrite();
    }
    
    ...
	    
}

父类 DefaultSocketChannelConfig 构造器：

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public class DefaultSocketChannelConfig extends DefaultChannelConfig implements SocketChannelConfig {
   
   ...
      
   public DefaultSocketChannelConfig(SocketChannel channel, Socket javaSocket) {
       // 调用父类构造器，绑定socketchannel 
       super(channel);
        if (javaSocket == null) {
            throw new NullPointerException("javaSocket");
        }
        // 绑定java socket
        this.javaSocket = javaSocket;
		
        // Enable TCP_NODELAY by default if possible.
        // netty一般运行在服务器上，不在Android上，canEnableTcpNoDelayByDefault返回true
        if (PlatformDependent.canEnableTcpNoDelayByDefault()) {
            try {
            	// 开启 TCP_NODELAY ，开启TCP的nagle算法
                // 尽量不要等待，只要发送缓冲区中有数据，并且发送窗口是打开的，就尽量把数据发送到网络上去。
                setTcpNoDelay(true);
            } catch (Exception e) {
                // Ignore.
            }
        }
    }                                  
     
    ... 
                                        
}

NioSocketChannel 初始化与注册

上面小节分析了 NioSocketChannel 的创建逻辑，创建完成之后，我们来分析一下 NioSocketChannel 是如何注册到 NioEventLoop 上去的。

在前面小节分析新连接检测的有如下小段代码：

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private final class NioMessageUnsafe extends AbstractNioUnsafe {
    
    ...

    int size = readBuf.size();
    for (int i = 0; i < size; i ++) {
        readPending = false;
        // 调用pipeline传播ChannelRead事件
        pipeline.fireChannelRead(readBuf.get(i));
    }

    ...

}

调用 pipeline 传播 ChannelRead 事件，这里的 Pipeline 是服务端 Channel，也就是 NioServerSocketChannel 所绑定的 Pipeline，此时的 Pipeline 的内部结构是怎么样子的呢？

Pipeline-ServerBootstrapAcceptor

那这个 ServerBootstrapAcceptor 是从哪里来的呢？

在此前，我们分析 NioServerSocketChannel 初始化 时，有过下面这段代码：

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public class ServerBootstrap extends AbstractBootstrap<ServerBootstrap, ServerChannel> {
 	
    ...
    
    // NioServerSocketChannel初始化    
    void init(Channel channel) throws Exception {
        // 获取启动器 启动时配置的option参数，主要是TCP的一些属性
        final Map<ChannelOption<?>, Object> options = options0();
        // 将获得到 options 配置到 ChannelConfig 中去
        synchronized (options) {
            setChannelOptions(channel, options, logger);
        }

        // 获取 ServerBootstrap 启动时配置的 attr 参数
        final Map<AttributeKey<?>, Object> attrs = attrs0();
        // 配置 Channel attr，主要是设置用户自定义的一些参数
        synchronized (attrs) {
            for (Entry<AttributeKey<?>, Object> e: attrs.entrySet()) {
                @SuppressWarnings("unchecked")
                AttributeKey<Object> key = (AttributeKey<Object>) e.getKey();
                channel.attr(key).set(e.getValue());
            }
        }

        // 获取channel中的 pipeline，这个pipeline使我们前面在channel创建过程中设置的 pipeline
        ChannelPipeline p = channel.pipeline();

        // 将启动器中配置的 childGroup 保存到局部变量 currentChildGroup
        final EventLoopGroup currentChildGroup = childGroup;
        // 将启动器中配置的 childHandler 保存到局部变量 currentChildHandler
        final ChannelHandler currentChildHandler = childHandler;
        final Entry<ChannelOption<?>, Object>[] currentChildOptions;
        final Entry<AttributeKey<?>, Object>[] currentChildAttrs;
        // 保存用户设置的 childOptions 到局部变量 currentChildOptions
        synchronized (childOptions) {
            currentChildOptions = childOptions.entrySet().toArray(newOptionArray(childOptions.size()));
        }
        // 保存用户设置的 childAttrs 到局部变量 currentChildAttrs
        synchronized (childAttrs) {
            currentChildAttrs = childAttrs.entrySet().toArray(newAttrArray(childAttrs.size()));
        }

        p.addLast(new ChannelInitializer<Channel>() {
            @Override
            public void initChannel(final Channel ch) throws Exception {
                final ChannelPipeline pipeline = ch.pipeline();
                // 获取启动器上配置的handler
                ChannelHandler handler = config.handler();
                if (handler != null) {
                    // 添加 handler 到 pipeline 中
                    pipeline.addLast(handler);
                }
                ch.eventLoop().execute(new Runnable() {
                    @Override
                    public void run() {
                        // 用child相关的参数创建出一个新连接接入器ServerBootstrapAcceptor
                        // 通过 ServerBootstrapAcceptor 可以将一个新连接绑定到一个线程上去
                        // 每次有新的连接进来 ServerBootstrapAcceptor 都会用child相关的属性对它们进行配置，并注册到ChaildGroup上去
                        pipeline.addLast(new ServerBootstrapAcceptor(
                                ch, currentChildGroup, currentChildHandler, currentChildOptions, currentChildAttrs));
                    }
                });
            }
        });
    }
	    
    ...    
 	   
}

ServerBootstrapAcceptor

NioServerSocketChannel 初始化时，向 NioServerSocketChannel 所绑定的 Pipeline 添加了一个 InboundHandler 节点 —— ServerBootstrapAcceptor ，其代码如下：

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public class ServerBootstrap extends AbstractBootstrap<ServerBootstrap, ServerChannel> {
	
    ...
    
    private static class ServerBootstrapAcceptor extends ChannelInboundHandlerAdapter {
		
        // 子EventLoopGroup，即为workGroup
        private final EventLoopGroup childGroup;
        // ServerBootstrap启动时配置的 childHandler
        private final ChannelHandler childHandler;
        // ServerBootstrap启动时配置的 childOptions
        private final Entry<ChannelOption<?>, Object>[] childOptions;
        // ServerBootstrap启动时配置的 childAttrs
        private final Entry<AttributeKey<?>, Object>[] childAttrs;
        private final Runnable enableAutoReadTask;
		
        // 构造函数
        ServerBootstrapAcceptor(
                final Channel channel, EventLoopGroup childGroup, ChannelHandler childHandler,
                Entry<ChannelOption<?>, Object>[] childOptions, Entry<AttributeKey<?>, Object>[] childAttrs) {
            this.childGroup = childGroup;
            this.childHandler = childHandler;
            this.childOptions = childOptions;
            this.childAttrs = childAttrs;

            // Task which is scheduled to re-enable auto-read.
            // It's important to create this Runnable before we try to submit it as otherwise the URLClassLoader may
            // not be able to load the class because of the file limit it already reached.
            //
            // See https://github.com/netty/netty/issues/1328
            enableAutoReadTask = new Runnable() {
                @Override
                public void run() {
                    channel.config().setAutoRead(true);
                }
            };
        }

        // 处理Pipeline所传播的channelRead事件
        // 也就是前面新连接检测时看到的那段代码
        // pipeline.fireChannelRead(readBuf.get(i));
        // ServerBootstrapAcceptor的channelRead接口将会被调用，用于处理channelRead事件
        @Override
        @SuppressWarnings("unchecked")
        public void channelRead(ChannelHandlerContext ctx, Object msg) {
            // 获取传播事件的对象数据，即为前面的readBuf.get(i)
            // readBuf.get(i)取出的对象为 NioSocketChannel
            final Channel child = (Channel) msg;
			// 向 NioSocketChannel 添加childHandler，也就是我们常看到的
            // ServerBootstrap在启动时配置的代码：
            // ServerBootstrap.childHandler(new ChannelInitializer<SocketChannel>() {...} ）
            // 最终的结果就是向NioSocketChannel的Pipeline添加用户自定义的ChannelHandler
            // 用于处理客户端的channel连接
            child.pipeline().addLast(childHandler);
			// 配置 NioSocketChannel的TCP属性
            setChannelOptions(child, childOptions, logger);
			// 配置 NioSocketChannel 一些用户自定义数据
            for (Entry<AttributeKey<?>, Object> e: childAttrs) {
                child.attr((AttributeKey<Object>) e.getKey()).set(e.getValue());
            }
			// 将NioSocketChannel注册到childGroup，也就是Netty的WorkerGroup当中去
            try {
                childGroup.register(child).addListener(new ChannelFutureListener() {
                    @Override
                    public void operationComplete(ChannelFuture future) throws Exception {
                        if (!future.isSuccess()) {
                            forceClose(child, future.cause());
                        }
                    }
                });
            } catch (Throwable t) {
                forceClose(child, t);
            }
        }
		
        ...
    }
	
    ...
	
}

关于 ChannelInitializer 的讲解，可以看此前 Pipeline 源码分析 文章。

后面的 register 逻辑，就与我们前面讲解 NioServerSocketChannel 注册 大体类似了，这里简单介绍一下。

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public abstract class AbstractChannel extends DefaultAttributeMap implements Channel {

    ...
	
    // 注册NioSocketChannel
    // eventLoop为childGroup    
    @Override
    public final void register(EventLoop eventLoop, final ChannelPromise promise) {
        
        ...
		// 绑定eventLoop到NioSocketChannel上
        AbstractChannel.this.eventLoop = eventLoop;
		// 现在分析的逻辑是在服务端的线程上，eventLoop与主线程不同，返回false
        if (eventLoop.inEventLoop()) {
            register0(promise);
        } else {
            try {
                eventLoop.execute(new Runnable() {
                    @Override
                    public void run() {
                        // 这里来调用register0方法
                        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);
            }
        }
    }
	
    // 注册
    private void register0(ChannelPromise promise) {
        try {
            
            ...
               
            boolean firstRegistration = neverRegistered;
            // 调用 doRegister()
            doRegister();
            neverRegistered = false;
            registered = true;
            
            pipeline.invokeHandlerAddedIfNeeded();

            safeSetSuccess(promise);
            pipeline.fireChannelRegistered();
            
            // 服务端的NioServerSocketChannel已经与客户端的NioSocketChannel建立了连接
            // 所以，NioSocketChannel是处于激活状态，isActive()返回ture
            if (isActive()) {
                // 对于新连接，是第一次注册
                if (firstRegistration) {
                    // 传播ChannelActive事件
                    pipeline.fireChannelActive();
                } else if (config().isAutoRead()) {
                    beginRead();
                }
            }
            ...
                
        } catch (Throwable t) {
            // Close the channel directly to avoid FD leak.
            closeForcibly();
            closeFuture.setClosed();
            safeSetFailure(promise, t);
        }
    }
    
    ...
	        
}

调用到 NioSocketChannel 中的 doRegister () 方法：

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public abstract class AbstractNioChannel extends AbstractChannel {

    ...
        
    @Override
    protected void doRegister() throws Exception {
        boolean selected = false;
        for (;;) {
            try {
                // 将selector注册到底层JDK channel上，并附加了NioSocketChannel对象
                // 兴趣集设置为0，表示不关心任何事件
                selectionKey = javaChannel().register(eventLoop().unwrappedSelector(), 0, this);
                return;
            } catch (CancelledKeyException e) {
                ...
            }
        }
    }
 	
    ...
    
}

NioSocketChannel 注册 OP_READ 兴趣集

紧接着上面的分析，传播 ChannelActive 事件之后的逻辑，主要就是向客户端的 NioSocketChannel 注册一个 Read 兴趣集

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if (isActive()) {
    // 对于新连接，是第一次注册
    if (firstRegistration) {
        // 传播ChannelActive事件
        pipeline.fireChannelActive();
    } else if (config().isAutoRead()) {
        beginRead();
    }
}

通过 Pipeline 的传播机制 ，最终会调用到 doBeginRead () 接口，如下：

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public abstract class AbstractNioChannel extends AbstractChannel {
	
    ...

    protected abstract class AbstractNioUnsafe extends AbstractUnsafe implements NioUnsafe {

        ...
        
        @Override
        protected void doBeginRead() throws Exception {
            // Channel.read() or ChannelHandlerContext.read() was called
            // 保存selectionKey到局部变量
            final SelectionKey selectionKey = this.selectionKey;
            // 判断有效性
            if (!selectionKey.isValid()) {
                return;
            }
		
            readPending = true;
			
            // 获取selectionKey的兴趣集
            // 前面小结分析doRegister()接口提到，selectionKey的兴趣集设置为0
            final int interestOps = selectionKey.interestOps();
            // 这里的 readInterestOp 是前面讲NioSocketChannel创建时设置的值
            // 为 SelectionKey.OP_READ，也就是1
            if ((interestOps & readInterestOp) == 0) {
                // 这样，selectionKey最终设置的兴趣集为SelectionKey.OP_READ
                // 表示对读事件感兴趣
                selectionKey.interestOps(interestOps | readInterestOp);
            }
        }    
            
        ...    
        
    }    
	
    ...
        
}

小结

  • Netty 是在哪里检测有新连接接入的？
  • 新连接是怎样注册到 NioEventLoop 线程上的？
  • NioSocketChannel 是怎样初始化及注册的？

参考资料