Netty源码—四、事件处理

前面经过channel初始化、注册,所需要的数据结构(epoll_event)基本上准备好了,serverSocket也处于监听状态,可以接收来自客户端的请求了。NioServerSocketChannel注册在了NioEventLoop#selector,在注册过程中启动了NioEventLoop,run方法会循环执行,每次循环都会执行select和执行所有的task。如果select有事件,则会处理收到的事件。

private void processSelectedKeys() {
    if (selectedKeys != null) {
        // 是否使用优化过的selectionKey
        processSelectedKeysOptimized();
    } else {
        processSelectedKeysPlain(selector.selectedKeys());
    }
}

前面在NioEventLoop初始化的时候说过关于selectionKey优化的问题,这里不再赘述。两种方式主要是遍历selectionKey的方式不同,具体处理事件的调用是一样的。这里以processSelectedKeysOptimized为例。

accept

private void processSelectedKey(SelectionKey k, AbstractNioChannel ch) {
    // channel是NioServerSocketChannel
    // unsafe是NioMessageUnsafe
    final AbstractNioChannel.NioUnsafe unsafe = ch.unsafe();
    // 省略中间代码...
    if ((readyOps & (SelectionKey.OP_READ | SelectionKey.OP_ACCEPT)) != 0 || readyOps == 0) {
        // 调用NioMessageUnsafe.read
        unsafe.read();
    }
} catch (CancelledKeyException ignored) {
    unsafe.close(unsafe.voidPromise());
}

public void read() {
    // 省略中间代码...
                // 由于是ServerSocket,只负责accept,如果有IO事件说明就是有新的客户端连接,所以这里就是创建NioSocketChannel
                int localRead = doReadMessages(readBuf);
                if (localRead == 0) {
                    break;
                }
                if (localRead < 0) {
                    closed = true;
                    break;
                }

                allocHandle.incMessagesRead(localRead);
            } while (allocHandle.continueReading());
        } catch (Throwable t) {
            exception = t;
        }

        int size = readBuf.size();
        for (int i = 0; i < size; i ++) {
            readPending = false;
            // 注册刚刚创建的NioSocketChannel
            pipeline.fireChannelRead(readBuf.get(i));
        }
        readBuf.clear();
        allocHandle.readComplete();
        pipeline.fireChannelReadComplete();
        // 省略中间代码...
    }
}

protected int doReadMessages(List<Object> buf) throws Exception {
    // 调用java.nio.channels.ServerSocketChannel#accept来创建SocketChannel
    SocketChannel ch = SocketUtils.accept(javaChannel());

    try {
        if (ch != null) {
            // 创建NioSocketChannel
            buf.add(new NioSocketChannel(this, ch));
            return 1;
        }
    } catch (Throwable t) {
        // 省略中间代码...
    }

    return 0;
}

上面创建了NioSocketChannel之后,接下来注册所有客户端连接的NioSocketChannel,调用的是DefaultChannelPipeline#fireChannelRead方法,接下来是执行pipeline中的handler,在初始化的时候添加了LoggingHandler (如果启动的时候配置了的话),那么目前pipeline中的handler有

  • io.netty.channel.DefaultChannelPipeline$HeadContext:pipeline创建的时候默认的第一个handler
  • io.netty.handler.logging.LoggingHandler:启动的时候用户配置的handler
  • io.netty.bootstrap.ServerBootstrap$ServerBootstrapAcceptor
  • io.netty.channel.DefaultChannelPipeline$TailContext:pipeline创建的时候默认的最后一个handler

下面看下ServerBootstrap$ServerBootstrapAcceptor是什么时候添加到handler的

// io.netty.bootstrap.ServerBootstrap#init
// 这个方法是NioServerSocketChannel初始化的时候调用的
void init(Channel channel) throws Exception {
    // 省略中间代码...
    p.addLast(new ChannelInitializer<Channel>() {
        @Override
        public void initChannel(final Channel ch) throws Exception {
            final ChannelPipeline pipeline = ch.pipeline();
            ChannelHandler handler = config.handler();
            if (handler != null) {
                pipeline.addLast(handler);
            }

            ch.eventLoop().execute(new Runnable() {
                @Override
                public void run() {
                    // 在pipeline中添加ServerBootstrapAcceptor
                    pipeline.addLast(new ServerBootstrapAcceptor(
                        ch, currentChildGroup, currentChildHandler, currentChildOptions, currentChildAttrs));
                }
            });
        }
    });
}

之所以说ServerBootstrapAcceptor,是因为NioSocketChannel的register过程是这个handler的channelRead方法开始的

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

    child.pipeline().addLast(childHandler);

    setChannelOptions(child, childOptions, logger);

    // 配置NioSocketChannel
    for (Entry<AttributeKey<?>, Object> e: childAttrs) {
        child.attr((AttributeKey<Object>) e.getKey()).set(e.getValue());
    }

    try {
        // 这里childGroup就是一开始我们配置的workerGroup
        // 所以调用的是io.netty.channel.MultithreadEventLoopGroup#register(io.netty.channel.Channel)
        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);
    }
}

接下来的注册过程和NioServerSocketChannel的注册过程是类似的,创建socket,创建SelectionKeyImpl等。只不过NioSocketChannel不监听accept事件。

read

上面在接收到来自客户端的连接请求后,将NioSocketChannel注册到selector上,这个selector也是在NioEventLoop里面的,后面和这个客户端的通信都会通过这个channel进行,如果客户端发送来数据,也是selector收到读事件通知,然后调用processSelectedKey来处理read事件。

private void processSelectedKey(SelectionKey k, AbstractNioChannel ch) {
    // channel是NioSocketChannel
    // unsafe是NioSocketChannelUnsafe
    final AbstractNioChannel.NioUnsafe unsafe = ch.unsafe();
    // 省略中间代码...
    if ((readyOps & (SelectionKey.OP_READ | SelectionKey.OP_ACCEPT)) != 0 || readyOps == 0) {
        // 调用NioByteUnsafe.read
        unsafe.read();
    }
} catch (CancelledKeyException ignored) {
    unsafe.close(unsafe.voidPromise());
}

public final void read() {
    final ChannelConfig config = config();
    if (shouldBreakReadReady(config)) {
        clearReadPending();
        return;
    }
    final ChannelPipeline pipeline = pipeline();
    // PooledByteBufAllocator,默认的内存申请管理器
    final ByteBufAllocator allocator = config.getAllocator();
    // AdaptiveRecvByteBufAllocator$HandleImpl
    final RecvByteBufAllocator.Handle allocHandle = recvBufAllocHandle();
    allocHandle.reset(config);

    ByteBuf byteBuf = null;
    boolean close = false;
    try {
        do {
            // 申请内存
            byteBuf = allocHandle.allocate(allocator);
            // 读取数据
            allocHandle.lastBytesRead(doReadBytes(byteBuf));
            if (allocHandle.lastBytesRead() <= 0) {
                // nothing was read. release the buffer.
                byteBuf.release();
                byteBuf = null;
                close = allocHandle.lastBytesRead() < 0;
                if (close) {
                    // There is nothing left to read as we received an EOF.
                    readPending = false;
                }
                break;
            }

            allocHandle.incMessagesRead(1);
            readPending = false;
            // 执行pipeline中的handler
            pipeline.fireChannelRead(byteBuf);
            byteBuf = null;
        } while (allocHandle.continueReading());
        // 省略中间代码
    }
}

一般来说NioSocketChannel中的handler包括

  • io.netty.channel.DefaultChannelPipeline$HeadContext
  • org.lep.test.netty.protocol.custom.codec.NettyMessageDecoder:自定义的解码器
  • org.lep.test.netty.protocol.custom.codec.NettyMessageEncoder:自定义的编码器
  • org.lep.test.netty.protocol.custom.server.LoginAuthRespHandler:自定义的handler
  • org.lep.test.netty.protocol.custom.server.HeartBeatRespHandler:自定义的handler
  • io.netty.channel.DefaultChannelPipeline$TailContext

netty提供了一些基本的编解码功能,自己可以根据实际需要扩展使用,然后自定义自己的逻辑处理handler。

上面还涉及到内存的分配部分留在下一节介绍。

总结

read事件处理过程:

  1. 接收到read事件
  2. 分配内存,初始化buffer
  3. 调用channel.read将数据读取到buffer中
  4. 执行pipeline中的handler,包括了编解码的handler,自定义的handler来处理数据