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).registerchannel);

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 registerChannel channel) {
    return next).registerchannel);//选择一个线程执行器，调用register方法，绑定channel
}

@Override
public ChannelFuture registerChannel channel) {
    return registernew DefaultChannelPromisechannel, 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 = doReadMessagesreadBuf);
        if localRead == 0) {
            break;
        }
        if localRead < 0) {
            closed = true;
            break;
        }

        allocHandle.incMessagesReadlocalRead);
    } while allocHandle.continueReading));
} catch Throwable t) {
    exception = t;
}

int size = readBuf.size);
for int i = 0; i < size; i ++) {
    readPending = false;
    pipeline.fireChannelReadreadBuf.geti));
}
readBuf.clear);
allocHandle.readComplete);
pipeline.fireChannelReadComplete);

protected int doReadMessagesList<Object> buf) throws Exception {
    SocketChannel ch = SocketUtils.acceptjavaChannel));

    try {
        if ch != null) {
            buf.addnew NioSocketChannelthis, 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.addLastnew ChannelInitializer<Channel>) {
    @Override
    public void initChannelfinal Channel ch) throws Exception {
        final ChannelPipeline pipeline = ch.pipeline);
        ChannelHandler handler = config.handler);
        if handler != null) {
            pipeline.addLasthandler);
        }

        ch.eventLoop).executenew Runnable) {
            @Override
            public void run) {
                pipeline.addLastnew ServerBootstrapAcceptor
                        ch, currentChildGroup, currentChildHandler, currentChildOptions, currentChildAttrs));
            }
        });
    }
});

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

    child.pipeline).addLastchildHandler);

    setChannelOptionschild, childOptions, logger);

    for Entry<AttributeKey<?>, Object> e: childAttrs) {
        child.attrAttributeKey<Object>) e.getKey)).sete.getValue));
    }

    try {
        childGroup.registerchild).addListenernew ChannelFutureListener) {
            @Override
            public void operationCompleteChannelFuture future) throws Exception {
                if !future.isSuccess)) {
                    forceClosechild, future.cause));
                }
            }
        });
    } catch Throwable t) {
        forceClosechild, 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