首先客戶端連線到服務端時服務端會開啟一個執行緒，不斷的監聽客戶端的操作。

這個執行緒的執行操作在NioEventLoop的run方法中，其實操作是在processSelectedKeys中，監聽是否進行讀操作

protected void run() {
        for (;;) {
            try {
                switch (selectStrategy.calculateStrategy(selectNowSupplier, hasTasks())) {
                    case SelectStrategy.CONTINUE:
                        continue;
                    case SelectStrategy.SELECT:
                        select(wakenUp.getAndSet(false));
                        if (wakenUp.get()) {
                            selector.wakeup();
                        }
                    default:
                        // fallthrough
                }

                cancelledKeys = 0;
                needsToSelectAgain = false;
                final int ioRatio = this.ioRatio;
                if (ioRatio == 100) {
                    processSelectedKeys();
                    runAllTasks();
                } else {
                    final long ioStartTime = System.nanoTime();
                    processSelectedKeys();
                    final long ioTime = System.nanoTime() - ioStartTime;
                    runAllTasks(ioTime * (100 - ioRatio) / ioRatio);
                }

                if (isShuttingDown()) {
                    closeAll();
                    if (confirmShutdown()) {
                        break;
                    }
                }
            } 
		}
    }
 

private void processSelectedKeys() {
        if (selectedKeys != null) {
            processSelectedKeysOptimized(selectedKeys.flip());
        } else {
            processSelectedKeysPlain(selector.selectedKeys());
        }
    }

private void processSelectedKeysOptimized(SelectionKey[] selectedKeys) {
        for (int i = 0;; i ++) {
            final SelectionKey k = selectedKeys[i];
            if (k == null) {
                break;
            }
            selectedKeys[i] = null;

            final Object a = k.attachment();

            if (a instanceof AbstractNioChannel) {
                processSelectedKey(k, (AbstractNioChannel) a);
            } else {
                @SuppressWarnings("unchecked")
                NioTask<SelectableChannel> task = (NioTask<SelectableChannel>) a;
                processSelectedKey(k, task);
            }
        }
    }
 

主要判斷int readyOps = k.readyOps();拿到的值是否是SelectionKey.OP_READ | SelectionKey.OP_ACCEPT

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 (eventLoop != this || eventLoop == null) {
                return;
            }
          
            unsafe.close(unsafe.voidPromise());
            return;
        }

        try {
            int readyOps = k.readyOps();
        
            if ((readyOps & (SelectionKey.OP_READ | SelectionKey.OP_ACCEPT)) != 0 || readyOps == 0) {
                unsafe.read();
                if (!ch.isOpen()) {
                    return;
                }
            }
            if ((readyOps & SelectionKey.OP_WRITE) != 0) {
              
                ch.unsafe().forceFlush();
            }
            if ((readyOps & SelectionKey.OP_CONNECT) != 0) {
                int ops = k.interestOps();
                ops &= ~SelectionKey.OP_CONNECT;
                k.interestOps(ops);

                unsafe.finishConnect();
            }
        } catch (CancelledKeyException ignored) {
            unsafe.close(unsafe.voidPromise());
        }
    }
 

如果int readyOps = k.readyOps();拿到的值是SelectionKey.OP_READ | SelectionKey.OP_ACCEPT就執行unsafe.read();

具體實現在AbstractNioByteChannel執行read操作

@Override
public final void read() {
	final ChannelConfig config = config();
	final ChannelPipeline pipeline = pipeline();
	final ByteBufAllocator allocator = config.getAllocator();
	final RecvByteBufAllocator.Handle allocHandle = recvBufAllocHandle();
	allocHandle.reset(config);

	ByteBuf byteBuf = null;
	boolean close = false;
	try {
		do {
			byteBuf = allocHandle.allocate(allocator);
			allocHandle.lastBytesRead(doReadBytes(byteBuf));
			if (allocHandle.lastBytesRead() <= 0) {
				// nothing was read. release the buffer.
				byteBuf.release();
				byteBuf = null;
				close = allocHandle.lastBytesRead() < 0;
				break;
			}
			allocHandle.incMessagesRead(1);
			readPending = false;
			pipeline.fireChannelRead(byteBuf);
			byteBuf = null;
            } while (allocHandle.continueReading());

		allocHandle.readComplete();
		pipeline.fireChannelReadComplete();

		if (close) {
			closeOnRead(pipeline);
		}
	} catch (Throwable t) {
		handleReadException(pipeline, byteBuf, t, close, allocHandle);
	} 
}

讀取操作是在doReadBytes(byteBuf)中，具體實現是在NioSocketChannel中

@Override
    protected int doReadBytes(ByteBuf byteBuf) throws Exception {
        final RecvByteBufAllocator.Handle allocHandle = unsafe().recvBufAllocHandle();
        allocHandle.attemptedBytesRead(byteBuf.writableBytes());
        return byteBuf.writeBytes(javaChannel(), allocHandle.attemptedBytesRead());
    }

@Override
    public int writeBytes(ScatteringByteChannel in, int length) throws IOException {
        ensureAccessible();
        ensureWritable(length);
        int writtenBytes = setBytes(writerIndex, in, length);
        if (writtenBytes > 0) {
            writerIndex += writtenBytes;
        }
        return writtenBytes;
    }

 @Override
    public int setBytes(int index, ScatteringByteChannel in, int length) throws IOException {
        checkIndex(index, length);
        ByteBuffer tmpBuf = internalNioBuffer();
        index = idx(index);
        tmpBuf.clear().position(index).limit(index + length);
        try {
            return in.read(tmpBuf);
        } catch (ClosedChannelException ignored) {
            return -1;
        }
    }