netty 原理

netty 实现

1. 各组件之间的关系

每个ServerBootstrap与一个事件循环对象（一个线程）都会与一个Channel绑定，如NioServerSocketChannel

2. 如何绑定

在做bind操作时，会执行方法，register进行注册

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

关键接口及类之间的关系：

EventLoopGroup与EventLoop及其类关系图之间形成组合模式。

@Override
public ChannelFuture register(Channel channel) {
    return next().register(channel);//选择一个线程执行器，调用register方法，绑定channel
}

@Override
public ChannelFuture register(Channel channel) {
    return register(new DefaultChannelPromise(channel, this));
}

3. 代表客户端的channel创建

注意到ServerBootstrap有两个EventLoopGroup,parent 负责代表客户端channel的分发，child负责代码客户端channel的处理。

• Accept事件，创建代表客户端的channel

if ((readyOps & (SelectionKey.OP_READ | SelectionKey.OP_ACCEPT)) != 0 || readyOps == 0) {
    unsafe.read();
}

try {
    do {
        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;
    pipeline.fireChannelRead(readBuf.get(i));
}
readBuf.clear();
allocHandle.readComplete();
pipeline.fireChannelReadComplete();

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

    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;
}

注意到doReadMessages会创建代表客户端的channel,创建完成会触发，fireChannelRead 事件。

4. 代表客户端的channel分发

上一节知道，channel创建完成会触发fireChannelRead 事件。在ServerSocketChannel初始化时，会注册ServerBootstrapAcceptor 用户接收代表客户端channel并分发到child EventLoopGroup中执行。

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() {
                pipeline.addLast(new ServerBootstrapAcceptor(
                        ch, currentChildGroup, currentChildHandler, currentChildOptions, currentChildAttrs));
            }
        });
    }
});

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

    child.pipeline().addLast(childHandler);

    setChannelOptions(child, childOptions, logger);

    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);
    }
}

5. netty线程模型

不管是客户端还是服务端可以通过一个Channel同时读或者写，不需要阻塞，读写多路复用。

6. handler链模型