本章节分析服务端如何accept客户端的connect请求。

在Netty源码分析之NioEventLoop章节中，已经分析了NioEventLoop的工作机制，当有客户端connect请求，selector可以返回其对应的SelectionKey，方法processSelectedKeys进行后续的处理。

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

默认采用优化过的SelectedSelectionKeySet保存有事件发生的selectedKey。

1、SelectedSelectionKeySet内部使用两个大小为1024的SelectionKey数组keysA和keysB保存selectedKey。

2、把SelectedSelectionKeySet实例映射到selector的原生selectedKeys和publicSelectedKeys。

private void processSelectedKeysOptimized(SelectionKey[] selectedKeys) { for (int i = 0;; i ++) { final SelectionKey k = selectedKeys[i]; if (k == null) { break; } // 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[i] = null; final Object a = k.attachment(); if (a instanceof AbstractNioChannel) { processSelectedKey(k, (AbstractNioChannel) a); } else { @SuppressWarnings("unchecked") NioTask task = (NioTask) 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 for (;;) { i++; if (selectedKeys[i] == null) { break; } selectedKeys[i] = null; } selectAgain(); // Need to flip the optimized selectedKeys to get the right reference to the array // and reset the index to -1 which will then set to 0 on the for loop // to start over again. // // See https://github.com/netty/netty/issues/1523 selectedKeys = this.selectedKeys.flip(); i = -1; } }}

因为selector的I/O多路复用机制，一次可以返回多个selectedKey，所以要用for循环处理全部selectionKey。

假设这时有请求进来，selectedKeys中就存在一个selectionKey，这块逻辑不清楚的可以回头看看深入浅出Nio Socket。

1、通过k.attachment()可以获取ServerSocketChannel注册时绑定上去的附件，其实这个附件就是ServerSocketChannel自身。

2、如果selectedKey的附件是AbstractNioChannel类型的，执行processSelectedKey(k, (AbstractNioChannel) a)方法进行下一步操作。

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 ignored) { unsafe.close(unsafe.voidPromise()); }}

1、获取ServerSocketChannel的unsafe对象。

2、当前selectionKey发生的事件是SelectionKey.OP_ACCEPT，执行unsafe的read方法。

该read方法定义在NioMessageUnsafe类中：

private final List readBuf = new ArrayList();@Overridepublic void read() { assert eventLoop().inEventLoop(); final ChannelConfig config = config(); if (!config.isAutoRead() && !isReadPending()) { // ChannelConfig.setAutoRead(false) was called in the meantime removeReadOp(); return; } final int maxMessagesPerRead = config.getMaxMessagesPerRead(); final ChannelPipeline pipeline = pipeline(); boolean closed = false; Throwable exception = null; try { try { for (;;) { int localRead = doReadMessages(readBuf); if (localRead == 0) { break; } if (localRead < 0) { closed = true; break; } // stop reading and remove op if (!config.isAutoRead()) { break; } if (readBuf.size() >= maxMessagesPerRead) { break; } } } catch (Throwable t) { exception = t; } setReadPending(false); int size = readBuf.size(); for (int i = 0; i < size; i ++) { pipeline.fireChannelRead(readBuf.get(i)); } readBuf.clear(); pipeline.fireChannelReadComplete(); if (exception != null) { if (exception instanceof IOException && !(exception instanceof PortUnreachableException)) { // ServerChannel should not be closed even on IOException because it can often continue // accepting incoming connections. (e.g. too many open files) closed = !(AbstractNioMessageChannel.this instanceof ServerChannel); } pipeline.fireExceptionCaught(exception); } if (closed) { if (isOpen()) { close(voidPromise()); } } } 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(); } }}

1、readBuf 用来保存客户端NioSocketChannel，默认一次不超过16个。

2、方法doReadMessages进行处理ServerSocketChannel的accept操作。

protected int doReadMessages(List buf) throws Exception { SocketChannel ch = javaChannel().accept(); try { if (ch != null) { buf.add(new NioSocketChannel(this, ch)); return 1; } } catch (Throwable t) { logger.warn("Failed to create a new channel from an accepted socket.