上次分析到：

public abstract class SingleThreadEventLoop extends SingleThreadEventExecutor implements EventLoop
    public ChannelFuture register(final ChannelPromise promise) {
        ObjectUtil.checkNotNull(promise, "promise");
        promise.channel().unsafe().register(this, promise);
        return
 
 promise;
    }
}

之前提到的ChannelPromise 結構，其實是一個非同步返回結果的封裝，它持有channel和當前的SingleThreadEventLoop :

進入unsafe實現類AbstractUnsafe的register方法：

 protected abstract class AbstractUnsafe implements Unsafe {

        private volatile ChannelOutboundBuffer outboundBuffer = new ChannelOutboundBuffer(AbstractChannel.this
 
);
        private RecvByteBufAllocator.Handle recvHandle;
        private boolean inFlush0;
        /** true if the channel has never been registered, false otherwise */
        private boolean neverRegistered = true;
  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;
            }
             //上邊的邏輯主要是一些非空判斷之類的東西
            AbstractChannel.this.eventLoop = eventLoop;
            //主要的註冊邏輯分支，if和else分支可以看到最後呼叫的都是register0，但是else裡邊加了一個外殼---執行緒
            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);
                }
            }
        }
}

eventLoop.inEventLoop()是分支邏輯的主要判斷依據(eventLoop實體是SingleThreadEventExecutor):

public interface EventExecutor extends EventExecutorGroup {
...略
    /**
     * Calls {@link #inEventLoop(Thread)} with {@link Thread#currentThread()} as argument
     * 將當前執行緒作為引數。
     */
    boolean inEventLoop();
    ...略
    }

找到他的實現類:

public abstract class AbstractEventExecutor extends AbstractExecutorService implements EventExecutor {
    ...略
    public boolean inEventLoop() {
        return inEventLoop(Thread.currentThread());//引數為當前執行緒。
    }
    ...略
}

在SingleThreadEventExecutor:

    public boolean inEventLoop(Thread thread) {
        return thread == this.thread;
    }

很簡單，即，判斷當前執行緒是不是SingleThreadEventExecutor裡邊維護的執行緒。所以else裡邊的邏輯是SingleThreadEventExecutor裡邊的執行緒不是當前執行緒的時候，新建一個Thread去執行register0，下邊看一下register0的邏輯：

        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();//核心註冊方法
                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);
            }
        }

看一下doRegister()：
看一下AbstractNioChannel 實現類的doRegister邏輯：

public abstract class AbstractNioChannel extends AbstractChannel {
...略
    protected void doRegister() throws Exception {
        boolean selected = false;
        for (;;) {
            try {
                selectionKey = javaChannel().register(eventLoop().unwrappedSelector(), 0, this);
                //javaChannel()返回的是SelectableChannel(jdk的java.nio.channels.SelectableChannel)
                //javaChannel().register是將channel註冊到Selector上去，所以eventLoop().unwrappedSelector()返回的是Selector
                //（jdk的java.nio.channels.Selector）
                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;
                }
            }
        }
    }
    ...略
}

到這裡我們看到了最終netty的JavaNio的註冊實現。最後說一下比較重要的一點，上邊提到的主要的註冊邏輯分支，if和else分支可以看到最後呼叫的都是register0，但是else裡邊加了一個外殼—執行緒，這個地方有這麼五點需要注意：

1. 一個EventLoopGroup當中會包含一個或多個EventLoop。
2. 一個EventLoop在它的整個生命週期當中都只會與唯一一個Thread進行繫結。
3. 所有由EventLoop所處理的各種I/O事件都將在它所關聯的那個Thread上進行處理。
4. 一個Channel在它的整個生命週期中只會註冊在一個EventLoop上。
5. 一個EventLoop在執行過程中，會被分配給一個或多個Channel。

這是netty的核心的架構理念，非常重要！！！