netty源码阅读之服务器启动之注册selector

淩亂°似流年 2022-05-17 02:26 233阅读 0赞

初始化完成之后，就是**把创建的channel注册到事件轮询器selector上面**去。

注册到selector上主要做以下两件事情：

1、绑定eventLoop，这个eventLoop就是服务端的event，不是childEventLoop\[绑定线程\]

2、registor0做实际的注册

2.1、doRegister(),调用jdk底层注册

2.2、invokeHandlerAddedIfNeeded()(主要做一些事件的回调)

2.3、fireChannelRegistered()，传播事件

回到《[netty源码阅读之服务器启动之服务端channel的创建][netty_channel]》这盘文章的AbstractBootstrap的initAndRegister方法，里面有一段代码，这是在创建和初始化完成channel之后的一段代码，register的

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

这个register方法层层追溯，可以到达AbstractChannel里面内部类的AbstractUnsafe的register方法（这个方法就是接下去我们要分析的方法）：

@Override
            public final void register(EventLoop eventLoop, final ChannelPromise promise) {
                if (eventLoop == null) {
                    throw new NullPointerException("eventLoop");
                }
                if (isRegistered()) {
                    promise.setFailure(new IllegalStateException("registered to an event loop already"));
                    return;
                }
                if (!isCompatible(eventLoop)) {
                    promise.setFailure(
                            new IllegalStateException("incompatible event loop type: " + eventLoop.getClass().getName()));
                    return;
                }
    
                AbstractChannel.this.eventLoop = eventLoop;
    
                if (eventLoop.inEventLoop()) {
                    register0(promise);
                } else {
                    try {
                        eventLoop.execute(new Runnable() {
                            @Override
                            public void run() {
                                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);
                    }
                }
            }

绑定线程用的就是AbstractChannel.this.eventLoop = eventLoop，也就是把nio线程和当前的channel做绑定。

后续所有的io操作，都是通过这个eventLoop来处理。

从用户代码的b.group(bossGroup, workerGroup)追溯，这个eventLoop就是和bossGroup有千丝万缕的关系。

然后我们看register0()，点进去，在AbstractChannl里面有个这样的函数：

private void register0(ChannelPromise promise) {
                try {
                    // check if the channel is still open as it could be closed in the mean time when the register
                    // call was outside of the eventLoop
                    if (!promise.setUncancellable() || !ensureOpen(promise)) {
                        return;
                    }
                    boolean firstRegistration = neverRegistered;
                    doRegister();
                    neverRegistered = false;
                    registered = true;
    
                    // Ensure we call handlerAdded(...) before we actually notify the promise. This is needed as the
                    // user may already fire events through the pipeline in the ChannelFutureListener.
                    pipeline.invokeHandlerAddedIfNeeded();
    
                    safeSetSuccess(promise);
                    pipeline.fireChannelRegistered();
                    // Only fire a channelActive if the channel has never been registered. This prevents firing
                    // multiple channel actives if the channel is deregistered and re-registered.
                    if (isActive()) {
                        if (firstRegistration) {
                            pipeline.fireChannelActive();
                        } else if (config().isAutoRead()) {
                            // This channel was registered before and autoRead() is set. This means we need to begin read
                            // again so that we process inbound data.
                            //
                            // See https://github.com/netty/netty/issues/4805
                            beginRead();
                        }
                    }
                } catch (Throwable t) {
                    // Close the channel directly to avoid FD leak.
                    closeForcibly();
                    closeFuture.setClosed();
                    safeSetFailure(promise, t);
                }
            }

在这里register0又做了三件事：

1、doRegister(),调用jdk底层注册

2、invokeHandlerAddedIfNeeded()

3、fireChannelRegistered()，传播事件

选择AbstractNioChannel的doregister()

@Override
        protected void doRegister() throws Exception {
            boolean selected = false;
            for (;;) {
                try {
                    selectionKey = javaChannel().register(eventLoop().selector, 0, this);
                    return;
                } catch (CancelledKeyException e) {
                    if (!selected) {
                        // Force the Selector to select now as the "canceled" SelectionKey may still be
                        // cached and not removed because no Select.select(..) operation was called yet.
                        eventLoop().selectNow();
                        selected = true;
                    } else {
                        // We forced a select operation on the selector before but the SelectionKey is still cached
                        // for whatever reason. JDK bug ?
                        throw e;
                    }
                }
            }
        }

这里就是调用jdk底层channel的register方法将jdk底层channel注册到selector轮询器上，并把NioServerSocketChannel自己作为attachment给添加进去。

javaChannel就是我们之前创建的jdk底层的channel；register就是调用jdk的api，参数第一个是selector，第二个是ops（需要关心的一些事件，0代表不关心任何事件，只是把channel绑定到selector上面去），第三个是attachment，也是把自己this也就是服务端channel作为attachment绑定到selector上面去（后面如果selector轮询到事件，可以直接把attachment也就是把netty的channel拿出来，然后针对这个nioChannel可以做一些事件的传播）。

在定义用户代码Handler，有这三个回调：

public void channelActive(ChannelHandlerContext ctx) {
            System.out.println("channelActive");
        }
    
        @Override
        public void channelRegistered(ChannelHandlerContext ctx) {
            System.out.println("channelRegistered");
        }
    
        @Override
        public void handlerAdded(ChannelHandlerContext ctx) {
            System.out.println("handlerAdded");
        }

有两个回调，就是在这里被调用了，先输出handleAdd然后输出handleRegistered。至于channelActive这个回调，是在端口绑定之后被回调的。此时的isActive()还是false。现在只有做了selector的注册，只有最终绑定端口之后，才会回调这个方法。下一篇文章我们会介绍。

[netty_channel]: https://blog.csdn.net/fst438060684/article/details/81427946