Netty 源码解析系列-客户端连接接入及读I/O解析

上一章节《Netty 源码解析系列-服务端启动流程解析》 我们完成了服务端启动，那么服务端启动完成后，客户端接入以及读 I/O 事件是怎么哪里开始的？以及 netty 的 boss 线程接收到客户端 TCP 连接请求后如何将链路注册到 worker 线程池？带着这些疑问，我们开始客户端连接接入及读写 I/O 解析。

1.NioEventLoop run()开始

processSelectedKeys();
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private void processSelectedKeys() {
    if (selectedKeys != null) {
        processSelectedKeysOptimized(selectedKeys.flip());
    } else {
        processSelectedKeysPlain(selector.selectedKeys());
    }
}
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根据 selectedKeys 是否为空，判断是否采用优化后的 selectedKeys ，进到 processSelectedKeysOptimized 。

private void processSelectedKeysOptimized(SelectionKey[] selectedKeys) {
    for (int i = 0;; i ++) {
        final SelectionKey k = selectedKeys[i];
        if (k == null) {
            break;
        }
        selectedKeys[i] = null;

        final Object a = k.attachment();

        if (a instanceof AbstractNioChannel) {
               processSelectedKey(k, (AbstractNioChannel) a);
        } else {
               @SuppressWarnings("unchecked")
               NioTask<SelectableChannel> task = (NioTask<SelectableChannel>) a;
               processSelectedKey(k, task);
        }
            ...
}
}
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k.attachment()获取附加的对象，那我们是在哪里附加上去的呢？上一篇 《Netty 源码解析-服务端启动流程解析》 注册时 attach 上去的对象,其实就是 NioServerSocketChannel 自身。

@Override
protected void doRegister() throws Exception {
    boolean selected = false;
    for (;;) {
	...
	selectionKey = javaChannel().register(eventLoop().selector, 0, this);
	...        
    }
}
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我们再回到 k.attachment() ，在取出附加对象后，判断类型是否为 AbstractNioChannel ，从这里我们可以看到，不是附加 AbstractNioChannel 类型，那么就是附加的 NioTask 对象，在这里我们只看关于 AbstractNioChannel 的，进到 processSelectedKey() 方法。

private static void processSelectedKey(SelectionKey k, AbstractNioChannel ch) {
    final NioUnsafe unsafe = ch.unsafe();
    ...
    int readyOps = k.readyOps();
    if ((readyOps & (SelectionKey.OP_READ | SelectionKey.OP_ACCEPT)) != 0 || readyOps == 0) {
         unsafe.read();
    if (!ch.isOpen()) {
         return;
    }
    if ((readyOps & SelectionKey.OP_WRITE) != 0) {
          ch.unsafe().forceFlush();
    }
    if ((readyOps & SelectionKey.OP_CONNECT) != 0) {
          int ops = k.interestOps();
          ops &= ~SelectionKey.OP_CONNECT;
          k.interestOps(ops);
          unsafe.finishConnect();
    }
    ...
}
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当操作类型是读操作或者连接操作，进入 unsafe.read() ,有两个类实现了这个方法，一个是 AbstractNioByteChannel 的内部类 NioByteUnsafe ，一个是 AbstractNioMessageChannel 的内部类 NioMessageUnsafe ，这两个类都是 NioUnsafe 实现类 AbstractNioChannel 的子类，那到底是哪一个子类？我们看看 NioServerSocketChannel 创建时是创建的 NioByteUnsafe 还是 NioMessageUnsafe 。

public class NioServerSocketChannel extends AbstractNioMessageChannel
                             implements io.netty.channel.socket.ServerSocketChannel {
        public NioServerSocketChannel() {
                this(newSocket(DEFAULT_SELECTOR_PROVIDER));
        }
}
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public NioServerSocketChannel(ServerSocketChannel channel) {
        super(null, channel, SelectionKey.OP_ACCEPT);
        config = new NioServerSocketChannelConfig(this, javaChannel().socket());
}
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public abstract class AbstractNioMessageChannel extends AbstractNioChannel {
    protected AbstractNioMessageChannel(Channel parent, SelectableChannel ch, int readInterestOp) {
          super(parent, ch, readInterestOp);
      }
}
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public abstract class AbstractNioChannel extends AbstractChannel {
	protected AbstractNioChannel(Channel parent, SelectableChannel ch, int readInterestOp) {
    		super(parent);
	}
}

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public abstract class AbstractChannel extends DefaultAttributeMap implements Channel {
        protected AbstractChannel(Channel parent) {
                this.parent = parent;
                unsafe = newUnsafe();
                pipeline = new DefaultChannelPipeline(this);
        }
}
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NioServerSocketChannel是 AbstractNioMessageChannel 的子类， AbstractNioMessageChannel 是 AbstractNioChannel 的子类， newUnsafe() 是 AbstractChannel 的抽象方法，那么我们从这里就知道， AbstractNioMessageChannel 实现了 AbstractChannel的newUnsafe() 抽象方法，由此判断，我们选择 AbstractNioMessageChannel 的内部类 NioMessageUnsafe 的 read() 。

private final class NioMessageUnsafe extends AbstractNioUnsafe {
    private final List<Object> readBuf = new ArrayList<Object>();
    @Override
    public void read() {
        ...
        for (;;) {
           int localRead = doReadMessages(readBuf);
           ...
    }
    setReadPending(false);
    int size = readBuf.size();
    for (int i = 0; i < size; i ++) {
            pipeline.fireChannelRead(readBuf.get(i));
    }
    readBuf.clear();
    pipeline.fireChannelReadComplete();
    ...
}
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这里分两部分，一个是处理消息，一个是处理事件。

1.处理消息

@Override
protected int doReadMessages(List<Object> buf) throws Exception {
    SocketChannel ch = javaChannel().accept();
    ...
    buf.add(new NioSocketChannel(this, ch));
    return 1;
    ...
}
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接受了一个客户端 SocketChannel ，封装到 NioSocketChannel ，添加到 list 集合中，我们看看 new NioSocketChannel() 。

public class NioSocketChannel extends AbstractNioByteChannel implements io.netty.channel.socket.SocketChannel {
	public NioSocketChannel(Channel parent, SocketChannel socket) {
    		super(parent, socket);
    		config = new NioSocketChannelConfig(this, socket.socket());
	}
}
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public abstract class AbstractNioByteChannel extends AbstractNioChannel {
	protected AbstractNioByteChannel(Channel parent, SelectableChannel ch) {
    		super(parent, ch, SelectionKey.OP_READ);
	}

    @Override
    protected AbstractNioUnsafe newUnsafe() {
    	return new NioByteUnsafe();
    }

    protected class NioByteUnsafe extends AbstractNioUnsafe {
	    @Override
	    public final void read() {
		    ...
	    }
    }
}
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AbstractNioByteChannel也继承了 AbstractNioChannel ，并实现了 newUnsafe() 方法,由此我们可以推断出当客户端第一次连接时，走的是 AbstractNioMessageChannel 的子类 NioMessageUnsafe的read() ，当客户端发送数据时，走的是 AbstractNioByteChannel 的内部类 AbstractNioUnsafe 的 read() 方法。

2.处理事件

for (int i = 0; i < size; i ++) {
    	   pipeline.fireChannelRead(readBuf.get(i));
     }

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@Override
public ChannelPipeline fireChannelRead(Object msg) {
    head.fireChannelRead(msg);
    return this;
}
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@Override
public ChannelHandlerContext fireChannelRead(final Object msg) {
    final AbstractChannelHandlerContext next = findContextInbound();
    EventExecutor executor = next.executor();
    if (executor.inEventLoop()) {
        next.invokeChannelRead(msg);
    } else {
        executor.execute(new OneTimeTask() {
            @Override
            public void run() {
                next.invokeChannelRead(msg);
            }
        });
    }
    return this;
}
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处理器。

public final void channelRegistered(ChannelHandlerContext ctx) throws Exception {
    initChannel((C) ctx.channel());
    ctx.pipeline().remove(this);
    ctx.fireChannelRegistered();
}
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我们继续看 ServerBootstrapAcceptor 的 ChannelRead() 方法。

@Override
@SuppressWarnings("unchecked")
public void channelRead(ChannelHandlerContext ctx, Object msg) {
    final Channel child = (Channel) msg;
    child.pipeline().addLast(childHandler);
    for (Entry<ChannelOption<?>, Object> e: childOptions) {
       try {
          if (!child.config().setOption((ChannelOption<Object>) e.getKey(), e.getValue())) {
              logger.warn("Unknown channel option: " + e);
          }
        } catch (Throwable t) {
              logger.warn("Failed to set a channel option: " + child, t);
        }
    }
    for (Entry<AttributeKey<?>, Object> e: childAttrs) {
         child.attr((AttributeKey<Object>) e.getKey()).set(e.getValue());
    }
    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);
     }
}
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这里分三个步骤

(1)将 childHandler 添加到处理器上，这个从哪里来？就是从最开始设置 serverBootstrap.childHandler(new IOChannelInitialize()) 。

(2)设置一些参数。

(3) work线程池 register 客户端的 channel 。

@Override
public ChannelFuture register(Channel channel) {
    return next().register(channel);
}

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@Override
public EventLoop next() {
    return (EventLoop) super.next();
}
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@Override
public EventExecutor next() {
    return chooser.next();
}
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private final class GenericEventExecutorChooser implements EventExecutorChooser {
    @Override
    public EventExecutor next() {
        return children[Math.abs(childIndex.getAndIncrement() % children.length)];
    }
}
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从 work 线程池选一个线程来执行 register 。

@Override
public ChannelFuture register(Channel channel) {
    return register(channel, new DefaultChannelPromise(channel, this));
}
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@Override
public ChannelFuture register(final Channel channel, final ChannelPromise promise) {
	 ...
        channel.unsafe().register(this, promise);
        return promise;
}
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@Override
public final void register(EventLoop eventLoop, final ChannelPromise promise) {
	 ...
     AbstractChannel.this.eventLoop = eventLoop;
     if (eventLoop.inEventLoop()) {
     register0(promise);
     } else {
          try {
              eventLoop.execute(new OneTimeTask() {
              @Override
              public void run() {
                 register0(promise);
              }
              });
           } catch (Throwable t) {
	            ...
           }
     }
}
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@Override
protected void doRegister() throws Exception {
	...
	selectionKey = javaChannel().register(eventLoop().selector, 0, this);
	...
}
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后面的流程和 上一篇《Netty 源码解析-服务端启动流程解析》 的注册流程是一样的，区别在于服务启动时注册是在 boss 线程池任务队列中执行注册，客户端新接入注册是在 work 线程池任务队列中执行 register0() 方法，并将 work 线程池的 selector 注册到 Java NIO 到这里，我们就可以回答开篇的的几个问题：客户端是如何接入？ netty 的 boss 线程接收到客户端 TCP 连接请求后如何将链路注册到 worker 线程池？ 现在我们还剩下一个问题：读写 I/O 事件是怎么哪里开始的？

我们回到文章开头

private void processSelectedKeysOptimized(SelectionKey[] selectedKeys) {
    for (int i = 0;; i ++) {
        final SelectionKey k = selectedKeys[i];
        if (k == null) {
            break;
        }
        selectedKeys[i] = null;

        final Object a = k.attachment();

        if (a instanceof AbstractNioChannel) {
               processSelectedKey(k, (AbstractNioChannel) a);
        } else {
               @SuppressWarnings("unchecked")
               NioTask<SelectableChannel> task = (NioTask<SelectableChannel>) a;
               processSelectedKey(k, task);
        }
             ...
    } 
}
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前面 boss 线程池在这里完成了客户端连接接入，并将链路注册到 worker 线程池任务队列，添加了 read 事件的监听，那么现在 work 线程不停循环 selectedKeys 中有没有待处理的事件，当有待处理事件，那么会执行 processSelectedKey() 方法。

private static void processSelectedKey(SelectionKey k, AbstractNioChannel ch) {
	...
	int readyOps = k.readyOps();
	if ((readyOps & (SelectionKey.OP_READ | SelectionKey.OP_ACCEPT)) != 0 || readyOps == 0) {
    		unsafe.read();
    		...
	}
	...
}
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在这里 unsafe.read() 选择 AbstractNioByteChannel 的 read() 。

@Override
public final void read() {
    final ChannelConfig config = config();
    if (!config.isAutoRead() && !isReadPending()) {
        // ChannelConfig.setAutoRead(false) was called in the meantime
        removeReadOp();
        return;
    }
    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();
              byteBuf = null;
              close = localReadAmount < 0;
              break;
           }
          if (!readPendingReset) {
               readPendingReset = true;
               setReadPending(false);
          }
          pipeline.fireChannelRead(byteBuf);
          byteBuf = null;

          if (totalReadAmount >= Integer.MAX_VALUE - localReadAmount) {
               totalReadAmount = Integer.MAX_VALUE;
               break;
          }
          totalReadAmount += localReadAmount;

          if (!config.isAutoRead()) {
               break;
          }

          if (localReadAmount < writable) {
              break;
          }
       } while (++ messages < maxMessagesPerRead);
         pipeline.fireChannelReadComplete();
         allocHandle.record(totalReadAmount);

        if (close) {
            closeOnRead(pipeline);
            close = false;
        }
     } catch (Throwable t) {
          handleReadException(pipeline, byteBuf, t, close);
     } finally {
         if (!config.isAutoRead() && !isReadPending()) {
                removeReadOp();
          }
     }
    }
}
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把这一大段代码分解成几部分

1.设置循环读,16次，未读完则会等到下一轮 select 继续读取， maxMessagesPerRead 默认等于16。

2.获取缓存操作 handler ， config.getRecvByteBufAllocator().newHandle() 。

3.申请缓存空间， allocHandle.allocate(allocator) 。

4.从 socket 中读取数据到 byteBuf 中。

5.传递读事件到下一个 handler 处理器。

6.读完之后发送读完时间到下一个 handler 处理器 我们只看读事件，其他细节后面的文章再详细解析。

@Override
public ChannelPipeline fireChannelRead(Object msg) {
    head.fireChannelRead(msg);
    return this;
}
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@Override
public ChannelHandlerContext fireChannelRead(final Object msg) {
    if (msg == null) {
        throw new NullPointerException("msg");
    }

    final AbstractChannelHandlerContext next = findContextInbound();
    EventExecutor executor = next.executor();
    if (executor.inEventLoop()) {
        next.invokeChannelRead(msg);
 } else {
        executor.execute(new OneTimeTask() {
            @Override
            public void run() {
                next.invokeChannelRead(msg);
            }
        });
    }
    return this;
}
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private void invokeChannelRead(Object msg) {
    try {
        ((ChannelInboundHandler) handler()).channelRead(this, msg);
    } catch (Throwable t) {
        notifyHandlerException(t);
    }
}
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进到 IdleStateHandler

@Override
public void channelRead(ChannelHandlerContext ctx, Object msg) throws Exception {
    if (readerIdleTimeNanos > 0 || allIdleTimeNanos > 0) {
        reading = true;
        firstReaderIdleEvent = firstAllIdleEvent = true;
    }
    ctx.fireChannelRead(msg);
}
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设置读事件为true，为后面状态检测做准备，继续向下传递读事件，这次是 IOHandler 的读事件。

public class IOHandler extends ChannelInboundHandlerAdapter {
    @Override
    public void channelRead(ChannelHandlerContext ctx, Object msg) throws Exception {
        super.channelRead(ctx, msg);
        System.out.println(msg.toString());
    }
	...
}
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交给用户自定义 handler 处理读事件，自此读 I/O 事件是怎么哪里开始，如何交给用户 handler 处理已解析完毕。

总结:

1.boss线程处理 NioServerSocketChannel 的 accept 事件，并将客户端添加到 work 任务队列，任务队列执行 redister0() 方法， 将 read 事件注册到 work 线程的 selector 。

2.work线程轮询 selectkeys ，当有事件上来时，将缓存数据发送到用户 handler 。