本人花费半年的时间总结的《Java面试指南》已拿腾讯等大厂offer，已开源在github ，欢迎star！
点击上方“芋道源码”，选择“置顶公众号”
技术文章第一时间送达！
摘要: 原创出处 https://www.jianshu.com/p/ffc6fd82e32b 「占小狼」欢迎转载，保留摘要，谢谢！
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")
NioTaskSelectableChannel task = (NioTaskSelectableChannel) 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 ListObject readBuf = new ArrayListObject();
@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(ListObject 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对应的ServerSocketChannel。
2、ServerSocketChannel.accept返回客户端的socketChannel 。
3、把 NioServerSocketChannel 和 socketChannel 封装成 NioSocketChannel，并缓存到readBuf。
4、遍历redBuf中的NioSocketChannel，触发各自pipeline的ChannelRead事件，从pipeline的head开始遍历，最终执行ServerBootstrapAcceptor的channelRead方法。
public void channelRead(ChannelHandlerContext ctx, Object msg) {
final Channel child = (Channel) msg;
child.pipeline().addLast(childHandler);
for (EntryChannelOption?, Object e: childOptions) {
try {
if (!child.config().setOption((ChannelOptionObject) e.getKey(), e.getValue())) {
logger.warn("Unknown channel option: " + e);
}
} catch (Throwable t) {
logger.warn("Failed to set a channel option: " + child, t);
}
}
for (EntryAttributeKey?, Object e: childAttrs) {
child.attr((AttributeKeyObject) 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)添加childHandler到NioSocketChannel的pipeline。
其中childHandler是通过ServerBootstrap的childHandler方法进行配置的，和NioServerSocketChannel类似，NioSocketChannel在注册到selector后会触发其pipeline的fireChannelRegistered方法，并执行initChannel方法，为NioSocketChannel的pipeline添加更多自定义的handler，进行业务处理。
2、childGroup.register(child)将NioSocketChannel注册到work的eventLoop中，这个过程和NioServerSocketChannel注册到boss的eventLoop的过程一样，最终由work线程对应的selector进行read事件的监听。
当readBuf中缓存的NioSocketChannel都处理完成后，清空readBuf，并触发ChannelReadComplete。
到此为止，一次accept流程已经执行完。
666. 彩蛋
如果你对 Netty 并发感兴趣，欢迎加入我的知识星球一起交流。
目前在知识星球（https://t.zsxq.com/2VbiaEu）更新了如下 Dubbo 源码解析如下：
01. 调试环境搭建
02. 项目结构一览
03. API 配置（一）之应用
04. API 配置（二）之服务提供者
05. API 配置（三）之服务消费者
06. 属性配置
07. XML 配置
08. 核心流程一览
09. 拓展机制 SPI
- 线程池
…
一共 60 篇++
