本系列：
本章節分析服務端如何accept客戶端的connect請求。
在《深入淺出Netty：NioEventLoop》章節中，已經分析了NioEventLoop的工作機制，當有客戶端connect請求，selector可以返回其對應的SelectionKey，方法processSelectedKeys進行後續的處理。
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private void processSelectedKeys() { if (selectedKeys != null) { processSelectedKeysOptimized(selectedKeys.flip()); } else { processSelectedKeysPlain(selector.selectedKeys()); } } |
預設採用優化過的SelectedSelectionKeySet儲存有事件發生的selectedKey。
1、SelectedSelectionKeySet內部使用兩個大小為1024的SelectionKey陣列keysA和keysB儲存selectedKey。
2、把SelectedSelectionKeySet例項對映到selector的原生selectedKeys和publicSelectedKeys。
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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<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 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; } } } |
因為selector的I/O多路複用機制，一次可以返回多個selectedKey，所以要用for迴圈處理全部selectionKey。
假設這時有請求進來，selectedKeys中就存在一個selectionKey，這塊邏輯不清楚的可以回頭看看深入淺出Nio Socket。
1、通過k.attachment()可以獲取ServerSocketChannel註冊時繫結上去的附件，其實這個附件就是ServerSocketChannel自身。
2、如果selectedKey的附件是AbstractNioChannel型別的，執行processSelectedKey(k, (AbstractNioChannel) a)方法進行下一步操作。
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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、獲取ServerSocketChannel的unsafe物件。
2、當前selectionKey發生的事件是SelectionKey.OP_ACCEPT，執行unsafe的read方法。
該read方法定義在NioMessageUnsafe類中：
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private final List<Object> readBuf = new ArrayList<Object>(); @Override public 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 用來儲存客戶端NioSocketChannel，預設一次不超過16個。
2、方法doReadMessages進行處理ServerSocketChannel的accept操作。
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protected int doReadMessages(List<Object> 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.", t); try { ch.close(); } catch (Throwable t2) { logger.warn("Failed to close a socket.", t2); } } return 0; } |
1、javaChannel()返回NioServerSocketChannel對應的ServerSocketChannel。
2、ServerSocketChannel.accept返回客戶端的socketChannel 。
3、把 NioServerSocketChannel 和 socketChannel 封裝成 NioSocketChannel，並快取到readBuf。
4、遍歷redBuf中的NioSocketChannel，觸發各自pipeline的ChannelRead事件，從pipeline的head開始遍歷，最終執行ServerBootstrapAcceptor的channelRead方法。
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public void channelRead(ChannelHandlerContext ctx, Object msg) { final Channel child = (Channel) msg; child.pipeline().addLast(childHandler); for (Entry<ChannelOption<?>, Object> e: childOptions) { try { if (!child.config().setOption((ChannelOption<Object>) e.getKey(), e.getValue())) { logger.warn("Unknown channel option: " + e); } } catch (Throwable t) { logger.warn("Failed to set a channel option: " + child, t); } } for (Entry<AttributeKey<?>, Object> e: childAttrs) { child.attr((AttributeKey<Object>) e.getKey()).set(e.getValue()); } try { childGroup.register(child).addListener(new ChannelFutureListener() { @Override public void operationComplete(ChannelFuture future) throws Exception { if (!future.isSuccess()) { forceClose(child, future.cause()); } } }); } catch (Throwable t) { forceClose(child, t); } } |
1、child.pipeline().addLast(childHandler)新增childHandler到NioSocketChannel的pipeline。
其中childHandler是通過ServerBootstrap的childHandler方法進行配置的，和NioServerSocketChannel類似，NioSocketChannel在註冊到selector後會觸發其pipeline的fireChannelRegistered方法，並執行initChannel方法，為NioSocketChannel的pipeline新增更多自定義的handler，進行業務處理。
2、childGroup.register(child)將NioSocketChannel註冊到work的eventLoop中，這個過程和NioServerSocketChannel註冊到boss的eventLoop的過程一樣，最終由work執行緒對應的selector進行read事件的監聽。
當readBuf中快取的NioSocketChannel都處理完成後，清空readBuf，並觸發ChannelReadComplete。
到此為止，一次accept流程已經執行完。
END。
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