Netty源码学习（三）NioEventLoop

0. NioEventLoop简介

NioEventLoop如同它的名字，它是一个无限循环（Loop），在循环中不断处理接收到的事件（Event）

在Reactor模型中，NioEventLoop就是Worker的角色，关联于多个Channel，监听这些Channel上的read/write事件，一旦有事件发生，就做出相应的处理

1. NioEventLoop类图

继承关系可以说是相当复杂了，我们慢慢分析

2.  NioEventLoop的构造方法

    NioEventLoop(NioEventLoopGroup parent, Executor executor, SelectorProvider selectorProvider,
                 SelectStrategy strategy, RejectedExecutionHandler rejectedExecutionHandler) {
        super(parent, executor, false, DEFAULT_MAX_PENDING_TASKS, rejectedExecutionHandler);
        if (selectorProvider == null) {
            throw new NullPointerException("selectorProvider");
        }
        if (strategy == null) {
            throw new NullPointerException("selectStrategy");
        }
        provider = selectorProvider;
        final SelectorTuple selectorTuple = openSelector();
        selector = selectorTuple.selector;
        unwrappedSelector = selectorTuple.unwrappedSelector;
        selectStrategy = strategy;
    }

    private SelectorTuple openSelector() {
        final Selector unwrappedSelector;
        try {
            unwrappedSelector = provider.openSelector();//利用JDK提供的SelectorProvider直接创建一个Selector
        } catch (IOException e) {
            throw new ChannelException("failed to open a new selector", e);
        }

        if (DISABLE_KEYSET_OPTIMIZATION) {//如果没有开启KEYSET优化，将上面的那个Selector直接返回
            return new SelectorTuple(unwrappedSelector);
        }

        final SelectedSelectionKeySet selectedKeySet = new SelectedSelectionKeySet();//创建一个专门存放SelectionKey的Set对象

        Object maybeSelectorImplClass = AccessController.doPrivileged(new PrivilegedAction<Object>() {
            @Override
            public Object run() {
                try {//用反射的方式创建一个SelectorImpl对象
                    return Class.forName(
                            "sun.nio.ch.SelectorImpl",
                            false,
                            PlatformDependent.getSystemClassLoader());
                } catch (Throwable cause) {
                    return cause;
                }
            }
        });

        if (!(maybeSelectorImplClass instanceof Class) ||
                // ensure the current selector implementation is what we can instrument.
                !((Class<?>) maybeSelectorImplClass).isAssignableFrom(unwrappedSelector.getClass())) {
            if (maybeSelectorImplClass instanceof Throwable) {
                Throwable t = (Throwable) maybeSelectorImplClass;
                logger.trace("failed to instrument a special java.util.Set into: {}", unwrappedSelector, t);
            }
            return new SelectorTuple(unwrappedSelector);
        }

        final Class<?> selectorImplClass = (Class<?>) maybeSelectorImplClass;

        Object maybeException = AccessController.doPrivileged(new PrivilegedAction<Object>() {
            @Override
            public Object run() {
                try {
                    Field selectedKeysField = selectorImplClass.getDeclaredField("selectedKeys");
                    Field publicSelectedKeysField = selectorImplClass.getDeclaredField("publicSelectedKeys");

                    Throwable cause = ReflectionUtil.trySetAccessible(selectedKeysField);
                    if (cause != null) {
                        return cause;
                    }
                    cause = ReflectionUtil.trySetAccessible(publicSelectedKeysField);
                    if (cause != null) {
                        return cause;
                    }

                    selectedKeysField.set(unwrappedSelector, selectedKeySet);//强制将SelectorImpl中的selectedKeys域替换为优化版的SelectedSelectionKeySet对象
                    publicSelectedKeysField.set(unwrappedSelector, selectedKeySet);//强制将SelectorImpl中的publicSelectedKeys域替换为优化版的SelectedSelectionKeySet对象
                    return null;
                } catch (NoSuchFieldException e) {
                    return e;
                } catch (IllegalAccessException e) {
                    return e;
                }
            }
        });

        if (maybeException instanceof Exception) {
            selectedKeys = null;
            Exception e = (Exception) maybeException;
            logger.trace("failed to instrument a special java.util.Set into: {}", unwrappedSelector, e);
            return new SelectorTuple(unwrappedSelector);
        }
        selectedKeys = selectedKeySet;
        logger.trace("instrumented a special java.util.Set into: {}", unwrappedSelector);
        return new SelectorTuple(unwrappedSelector,
                                 new SelectedSelectionKeySetSelector(unwrappedSelector, selectedKeySet));
    }

构造方法的主要作用是创建了一个SelectorImpl对象，如果没有设置DISABLE_KEYSET_OPTIMIZATION属性，SelectorImpl中类型为Set<SelectionKey>的selectedKeys与publicSelectedKeys域会被替换为一个SelectedSelectionKeySet对象。

为什么搞得这么麻烦呢？因为默认的域是Set类型，插入元素的开销是o(log n)，而优化版的SelectedSelectionKeySet继承了AbstractSet，具有Set的功能，但是内部是用数组实现，只具有add功能，而且其开销为o(1)。

3. NioEventLoop.run()

run方法是NioEventLoop的核心，此方法会在无限循环中监听关联的Channel上是否有新事件产生

    @Override
    protected void run() {
        for (;;) {//无限循环
            try {
                switch (selectStrategy.calculateStrategy(selectNowSupplier, hasTasks())) {//如果任务队列中有任务，调用selectNow()方法，如果没有，则直接返回SelectStrategy.SELECT
                    case SelectStrategy.CONTINUE://没搞懂这个分支的目的是什么，全局搜了一下SelectStrategy.CONTINUE，没发现有赋这个值的地方
                        continue;
                    case SelectStrategy.SELECT:
                        select(wakenUp.getAndSet(false));//调用select()方法，尝试从关联的channel里读取IO事件。需要注意的是这个select方法相当复杂，因为它悄悄的解决了老版本的JDK的select方法存在的bug，有兴趣的可以仔细分析一下相关源码
                        if (wakenUp.get()) {
                            selector.wakeup();
                        }
                    default:
                }

                cancelledKeys = 0;
                needsToSelectAgain = false;
                final int ioRatio = this.ioRatio;//ioRatio代表EventLoop会花多少时间在IO事件上
                if (ioRatio == 100) {
                    try {
                        processSelectedKeys();//处理IO事件
                    } finally {
                        // Ensure we always run tasks.
                        runAllTasks();//处理CPU事件
                    }
                } else {
                    final long ioStartTime = System.nanoTime();
                    try {
                        processSelectedKeys();//处理IO事件
                    } finally {
                        // Ensure we always run tasks.
                        final long ioTime = System.nanoTime() - ioStartTime;//本次循环处理IO事件的耗时
                        runAllTasks(ioTime * (100 - ioRatio) / ioRatio);//分给CPU事件的耗时
                    }
                }
            } catch (Throwable t) {
                handleLoopException(t);
            }
            // Always handle shutdown even if the loop processing threw an exception.
            try {
                if (isShuttingDown()) {
                    closeAll();
                    if (confirmShutdown()) {
                        return;
                    }
                }
            } catch (Throwable t) {
                handleLoopException(t);
            }
        }
    }

    private void processSelectedKeys() {
        if (selectedKeys != null) {
            processSelectedKeysOptimized();//处理IO事件。由于这里采用的是优化版的SelectorImpl，IO事件已经被写在selectedKeys属性里了，所以无需额外传参
        } else {
            processSelectedKeysPlain(selector.selectedKeys());//未优化版的SelectorImpl，需要调用selectedKeys()方法才能获取准备好的IO事件
        }
    }

    private void processSelectedKeysOptimized() {
        for (int i = 0; i < selectedKeys.size; ++i) {//遍历已经准备好的IO事件
            final SelectionKey k = selectedKeys.keys[i];
            // null out entry in the array to allow to have it GC'ed once the Channel close
            // See https://github.com/netty/netty/issues/2363
            selectedKeys.keys[i] = null;//手动将数组元素赋为null，以帮助gc（因为在系统压力大的时候，SelectionKey数组靠后的部分会被占用，如果不手动将用过的元素设置为null，那么在系统压力小的时候，这些元素是不会被释放的，也就是内存泄漏了）

            final Object a = k.attachment();//附件是这个事件所关联的Channel，后续的代码会直接从这个Channel上读取数据

            if (a instanceof AbstractNioChannel) {
                processSelectedKey(k, (AbstractNioChannel) a);//处理某个IO事件
            } else {
                @SuppressWarnings("unchecked")
                NioTask<SelectableChannel> task = (NioTask<SelectableChannel>) a;
                processSelectedKey(k, task);
            }

            if (needsToSelectAgain) {
                // null out entries in the array to allow to have it GC'ed once the Channel close
                // See https://github.com/netty/netty/issues/2363
                selectedKeys.reset(i + 1);

                selectAgain();
                i = -1;
            }
        }
    }

    private void processSelectedKey(SelectionKey k, AbstractNioChannel ch) {
        final AbstractNioChannel.NioUnsafe unsafe = ch.unsafe();
        if (!k.isValid()) {
            final EventLoop eventLoop;
            try {
                eventLoop = ch.eventLoop();
            } catch (Throwable ignored) {
                // If the channel implementation throws an exception because there is no event loop, we ignore this
                // because we are only trying to determine if ch is registered to this event loop and thus has authority
                // to close ch.
                return;
            }
            // Only close ch if ch is still registered to this EventLoop. ch could have deregistered from the event loop
            // and thus the SelectionKey could be cancelled as part of the deregistration process, but the channel is
            // still healthy and should not be closed.
            // See https://github.com/netty/netty/issues/5125
            if (eventLoop != this || eventLoop == null) {
                return;
            }
            // close the channel if the key is not valid anymore
            unsafe.close(unsafe.voidPromise());
            return;
        }

        try {
            int readyOps = k.readyOps();//获取事件的类型
            // We first need to call finishConnect() before try to trigger a read(...) or write(...) as otherwise
            // the NIO JDK channel implementation may throw a NotYetConnectedException.
            if ((readyOps & SelectionKey.OP_CONNECT) != 0) {//不太理解这段代码的原因
                // remove OP_CONNECT as otherwise Selector.select(..) will always return without blocking
                // See https://github.com/netty/netty/issues/924
                int ops = k.interestOps();
                ops &= ~SelectionKey.OP_CONNECT;
                k.interestOps(ops);

                unsafe.finishConnect();
            }

            // Process OP_WRITE first as we may be able to write some queued buffers and so free memory.
            if ((readyOps & SelectionKey.OP_WRITE) != 0) {//如果是可写事件，则flush缓存
                // Call forceFlush which will also take care of clear the OP_WRITE once there is nothing left to write
                ch.unsafe().forceFlush();
            }

            // Also check for readOps of 0 to workaround possible JDK bug which may otherwise lead
            // to a spin loop
            if ((readyOps & (SelectionKey.OP_READ | SelectionKey.OP_ACCEPT)) != 0 || readyOps == 0) {//如果是可写事件，则调用unsafe.read()方法
                unsafe.read();//此处的unsafe为NioSocketChannel.NioSocketChannelUnsafe
            }
        } catch (CancelledKeyException ignored) {
            unsafe.close(unsafe.voidPromise());
        }
    }

代码很长，大概逻辑如下：

a. 在无限循环中调用Selector.select方法

b. 使用ioRatio控制IO事件与CPU事件的耗时比例（ioRatio的默认值为50）

c. 如果有IO事件发生，遍历所有IO事件并调用processSelectedKey方法

d. 在processSelectedKey中，如果发现事件是READ/ACCEPT类型，则调用NioSocketChannel.NioSocketChannelUnsafe.read()方法对事件进行处理。其实际实现位于AbstractNioByteChannel中：

        @Override
        public final void read() {
            final ChannelConfig config = config();
            final ChannelPipeline pipeline = pipeline();//获取用户注册的pipeline
            final ByteBufAllocator allocator = config.getAllocator();//默认值为PooledByteBufAllocator，也就是申请direct memory
            final RecvByteBufAllocator.Handle allocHandle = recvBufAllocHandle();
            allocHandle.reset(config);

            ByteBuf byteBuf = null;
            boolean close = false;
            try {
                do {
                    byteBuf = allocHandle.allocate(allocator);//开辟一块内存作为buffer
                    allocHandle.lastBytesRead(doReadBytes(byteBuf));//doReadBytes方法会从绑定的Channel里读取数据到buffer里。顺便记录一下本次读了多少字节的数据出来
                    if (allocHandle.lastBytesRead() <= 0) {
                        // nothing was read. release the buffer.
                        byteBuf.release();
                        byteBuf = null;
                        close = allocHandle.lastBytesRead() < 0;
                        break;
                    }

                    allocHandle.incMessagesRead(1);//计数
                    readPending = false;
                    pipeline.fireChannelRead(byteBuf);//触发pipeline的channelRead事件
                    byteBuf = null;
                } while (allocHandle.continueReading());

                allocHandle.readComplete();
                pipeline.fireChannelReadComplete();//触发pipeline的channelReadComplete事件

                if (close) {
                    closeOnRead(pipeline);
                }
            } catch (Throwable t) {
                handleReadException(pipeline, byteBuf, t, close, allocHandle);
            } finally {
                // Check if there is a readPending which was not processed yet.
                // This could be for two reasons:
                // * The user called Channel.read() or ChannelHandlerContext.read() in channelRead(...) method
                // * The user called Channel.read() or ChannelHandlerContext.read() in channelReadComplete(...) method
                //
                // See https://github.com/netty/netty/issues/2254
                if (!readPending && !config.isAutoRead()) {
                    removeReadOp();
                }
            }
        }
    }

这段代码主要做了两件事情：

a. 在循环中将Channel可读的数据读到一个临时的ByteBuf中

b. 调用pipeline.fireChannelRead方法，处理读取到数据。（具体的处理逻辑在后续文章中阐述）

现在我们已经基本搞清楚NioEventLoop的工作逻辑了：

在无限循环中监听绑定的Channel上的事件，如果有ACCEPT/READ事件发生，则从Channel里读取数据，并调用关联的pipeline的fireChannelRead方法进行处理。

目前仍不清楚的地方是：NioEventLoop是如何与Channel/Pipeline绑定的？

且听后文分解。