关于 Netty Channel 的 Autoread

Netty 4 的 Channel 多了一个 autoread 参数, 它的用处是在让 channel 在触发某些事件以后(例如 channelActive, channelReadComplete)以后还会自动调用一次 read(), 代码:

DefaultChannelPipeline.java

    @Override
    public ChannelPipeline fireChannelActive() {
        head.fireChannelActive();

        if (channel.config().isAutoRead()) {
            channel.read();
        }

        return this;
    }

-----------------------------------

    @Override
    public ChannelPipeline fireChannelReadComplete() {
        head.fireChannelReadComplete();
        if (channel.config().isAutoRead()) {
            read();
        }
        return this;
    }

另外一个很重要的作用是在触发某些事件(例如 socket 关闭)时, 在 NioEventloop 的 OP_READ 位为 1 时, 是否真正的去 channel 读取数据, 去 channel 读取数据的意义还包括可以更新 channel 的状态, 例如改变 active 状态等, 代码:

Eventloop.java

private static void processSelectedKey(SelectionKey k, AbstractNioChannel ch) {
        final NioUnsafe unsafe = ch.unsafe();
        if (!k.isValid()) {
            // close the channel if the key is not valid anymore
            unsafe.close(unsafe.voidPromise());
            return;
        }

        try {
            int readyOps = k.readyOps();
            // 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) {
                unsafe.read();
                if (!ch.isOpen()) {
                    // Connection already closed - no need to handle write.
                    return;
                }
            }
            if ((readyOps & SelectionKey.OP_WRITE) != 0) {
                // Call forceFlush which will also take care of clear the OP_WRITE once there is nothing left to write
                ch.unsafe().forceFlush();
            }
            if ((readyOps & SelectionKey.OP_CONNECT) != 0) {
                // 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();
            }
        } catch (CancelledKeyException e) {
            unsafe.close(unsafe.voidPromise());
        }
    }

AbstractNioByteChannel.java (unsafe.read())

public void read() {
            final ChannelConfig config = config();
            // 判断是否是 autoread, 如果不是直接移除 opRead 标志位返回
            if (!config.isAutoRead() && !isReadPending()) {
                // ChannelConfig.setAutoRead(false) was called in the meantime
                removeReadOp();
                return;
            }

            final ChannelPipeline pipeline = pipeline();
            final ByteBufAllocator allocator = config.getAllocator();
            final int maxMessagesPerRead = config.getMaxMessagesPerRead();
            RecvByteBufAllocator.Handle allocHandle = this.allocHandle;
            if (allocHandle == null) {
                this.allocHandle = allocHandle = config.getRecvByteBufAllocator().newHandle();
            }

            ByteBuf byteBuf = null;
            int messages = 0;
            boolean close = false;
            try {
                int totalReadAmount = 0;
                boolean readPendingReset = false;
                do {
                    byteBuf = allocHandle.allocate(allocator);
                    int writable = byteBuf.writableBytes();
                    // 这里会到 channel 中 read 一次, 如果返回 -1, 则表示 channel 关闭
                    // 假如说 autoread 为 false, 则在 socket close 时永远不会进入这里, 从而改变 channel 的 active 状态 
                    int localReadAmount = doReadBytes(byteBuf);
                    if (localReadAmount <= 0) {
                        // not was read release the buffer
                        byteBuf.release();
                        close = localReadAmount < 0;
                        break;
                    }
                    if (!readPendingReset) {
                        readPendingReset = true;
                        setReadPending(false);
                    }
                    pipeline.fireChannelRead(byteBuf);
                    byteBuf = null;

                    if (totalReadAmount >= Integer.MAX_VALUE - localReadAmount) {
                        // Avoid overflow.
                        totalReadAmount = Integer.MAX_VALUE;
                        break;
                    }

                    totalReadAmount += localReadAmount;

                    // stop reading
                    if (!config.isAutoRead()) {
                        break;
                    }

                    if (localReadAmount < writable) {
                        // Read less than what the buffer can hold,
                        // which might mean we drained the recv buffer completely.
                        break;
                    }
                } while (++ messages < maxMessagesPerRead);

                pipeline.fireChannelReadComplete();
                allocHandle.record(totalReadAmount);
                // 关闭 channel, 并改变 channel active 状态
                if (close) {
                    closeOnRead(pipeline);
                    close = false;
                }
            } catch (Throwable t) {
                handleReadException(pipeline, byteBuf, t, close);
            } 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 (!config.isAutoRead() && !isReadPending()) {
                    removeReadOp();
                }
            }
        }
    }