netty源码分析(十七)Netty线程模型深度解读与架构设计原则

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 结构，其实是一个异步返回结果的封装，它持有channel和当前的SingleThreadEventLoop :

进入unsafe实现类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;
            }
             //上边的逻辑主要是一些非空判断之类的东西
            AbstractChannel.this.eventLoop = eventLoop;
            //主要的注册逻辑分支，if和else分支可以看到最后调用的都是register0，但是else里边加了一个外壳---线程
            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()是分支逻辑的主要判断依据(eventLoop实体是SingleThreadEventExecutor):

public interface EventExecutor extends EventExecutorGroup {
...略
    /**
     * Calls {@link #inEventLoop(Thread)} with {@link Thread#currentThread()} as argument
     * 将当前线程作为参数。
     */
    boolean inEventLoop();
    ...略
    }

找到他的实现类:

public abstract class AbstractEventExecutor extends AbstractExecutorService implements EventExecutor {
    ...略
    public boolean inEventLoop() {
        return inEventLoop(Thread.currentThread());//参数为当前线程。
    }
    ...略
}

在SingleThreadEventExecutor:

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

很简单，即，判断当前线程是不是SingleThreadEventExecutor里边维护的线程。所以else里边的逻辑是SingleThreadEventExecutor里边的线程不是当前线程的时候，新建一个Thread去执行register0，下边看一下register0的逻辑：

        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();//核心注册方法
                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 实现类的doRegister逻辑：

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是将channel注册到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
}

到这里我们看到了最终netty的JavaNio的注册实现。最后说一下比较重要的一点，上边提到的主要的注册逻辑分支，if和else分支可以看到最后调用的都是register0，但是else里边加了一个外壳—线程，这个地方有这么五点需要注意：

1. 一个EventLoopGroup当中会包含一个或多个EventLoop。
2. 一个EventLoop在它的整个生命周期当中都只会与唯一一个Thread进行绑定。
3. 所有由EventLoop所处理的各种I/O事件都将在它所关联的那个Thread上进行处理。
4. 一个Channel在它的整个生命周期中只会注册在一个EventLoop上。
5. 一个EventLoop在运行过程中，会被分配给一个或多个Channel。

这是netty的核心的架构理念，非常重要！！！