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netty4源码分析-accept
本文分析服务端如何accept客户端的connect请求，首先看下selector的I/O多路复用的分发逻辑：
//NioEventLoop
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();
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());
}
}
当有OP_ACCEPT事件到达时，分发给NioMessageUnsafe的read方法进行处理。
//NioMessageUnsafe
public void read() {
assert eventLoop().inEventLoop();
final SelectionKey key = selectionKey();
if (!config().isAutoRead()) {
int interestOps = key.interestOps();
if ((interestOps & readInterestOp) != 0) {
// only remove readInterestOp if needed
key.interestOps(interestOps & ~readInterestOp);
}
}
final ChannelConfig config = config();
final int maxMessagesPerRead = config.getMaxMessagesPerRead();
final boolean autoRead = config.isAutoRead();
final ChannelPipeline pipeline = pipeline();
boolean closed = false;
Throwable exception = null;
try {
for (;;) {
int localRead = doReadMessages(readBuf);
if (localRead == 0) {
break;
}
if (localRead < 0) {
closed = true;
break;
}
if (readBuf.size() >= maxMessagesPerRead | !autoRead) {
break;
}
}
} catch (Throwable t) {
exception = t;
}
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) {
// 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());
}
}
}
}
其中doReadMessages方法由NioServerSocketChannel实现：
// NioServerSocketChannel
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;
}
SocketChannel ch = javaChannel().accept()就为接受的客户端连接建立了一个已连接套接字socketChannel.
buf.add(new NioSocketChannel(this, ch))会构造一个NioSocketChannel，并将其缓存到buf中（buf是一个List<Object>）。该NioSocketChannel的模式为非阻塞，readInterestOp为SelectionKey.OP_READ，并创建对应的管道和NioByteUnsafe实例。
maxMessagesPerRead表示如果此时有多个connect，那么只有当SeverSocketChannel建立的已连接套接字个数超过maxMessagesPerRead后，才会对每个已连接套接字触发channelRead事件。maxMessagesPerRead的默认值是16.
接下来分析channelRead事件做了什么事情：
channelRead是Inbound事件，会调用ServerBootstrapAcceptor的channelRead方法：
// ServerBootstrapAcceptor
public void channelRead(ChannelHandlerContext ctx, Object msg) {
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);
} catch (Throwable t) {
child.unsafe().closeForcibly();
logger.warn("Failed to register an accepted channel: " + child, t);
}
}
首先child.pipeline().addLast(childHandler)将服务端main函数中实例化的ChannelInitializer加入到管道中，该处理器的initChannel方法会在channelRegistered事件触发时被调用
childHandler(new ChannelInitializer<SocketChannel>() {
@Override
public void initChannel(SocketChannel ch) throws Exception {
ch.pipeline().addLast(
//new LoggingHandler(LogLevel.INFO),
new EchoServerHandler());
}
});
然后设置NioSocketchannel的一些属性，最后进行注册：childGroup.register(child)。
这里采用的是childGroup，即worker线程池所在的Group，从Group中选择一个NioEventLoop，并启动其持有的worker线程，执行register0任务。
// AbstractUnsafe
public final void register(EventLoop eventLoop, final ChannelPromise promise) {
if (eventLoop == null) {
throw new NullPointerException("eventLoop");
}
if (isRegistered()) {
promise.setFailure(new IllegalStateException("registered to an event loop already"));
return;
}
if (!isCompatible(eventLoop)) {
promise.setFailure(
new IllegalStateException("incompatible event loop type: " + eventLoop.getClass().getName()));
return;
}
AbstractChannel.this.eventLoop = eventLoop;
if (eventLoop.inEventLoop()) {
register0(promise);
} else {
try {
eventLoop.execute(new Runnable() {
@Override
public void run() {
register0(promise);
}
});
} catch (Throwable t) {
logger.warn(
"Force-closing a channel whose registration task was not accepted by an event loop: {}",
AbstractChannel.this, t);
closeForcibly();
closeFuture.setClosed();
promise.setFailure(t);
}
}
}
private void register0(ChannelPromise promise) {
try {
// check if the channel is still open as it could be closed in the mean time when the register
// call was outside of the eventLoop
if (!ensureOpen(promise)) {
return;
}
doRegister();
registered = true;
promise.setSuccess();
pipeline.fireChannelRegistered();
if (isActive()) {
pipeline.fireChannelActive();
}
} catch (Throwable t) {
// Close the channel directly to avoid FD leak.
closeForcibly();
closeFuture.setClosed();
if (!promise.tryFailure(t)) {
logger.warn(
"Tried to fail the registration promise, but it is complete already. " +
"Swallowing the cause of the registration failure:", t);
}
}
}
此时worker线程就启动了。Register0任务在connect文章中已经描述，其主要功能就是将socketchannel注册到selector中；然后触发channelRegistered事件，调用ChannelInitializer的initChannel方法将服务端main函数中设置的处理器（本例为EchoServerHandler）加入到管道中，并将自己ChannelInitializer从管道中移除；最后触发channelActive事件，将ops设置为read。
// DefaultChannelPipeline
public ChannelPipeline fireChannelActive() {
head.fireChannelActive();
if (channel.config().isAutoRead()) {
channel.read();
}
return this;
}
到此，worker线程对应的selector就开始监听该socketChannel上的read事件了。
接下来继续分析boss线程的执行：
将本次readBuf中缓存的所有NioSocketChannel注册后，就将他们从readBuf中移除。然后触发ChannelReadComplete事件，
// DefaultChannelPipeline
public ChannelPipeline fireChannelReadComplete() {
head.fireChannelReadComplete();
if (channel.config().isAutoRead()) {
read();
}
return this;
}
head.fireChannelReadComplete()触发的是一个inbound事件，没有做任何事情。接着分析后续触发的read事件，这是一个outbound事件，也没有做任何事情（将ops重新设置为OP_ACCEPT，其实本来就是OP_ACCEPT）。
到此，一次accept的流程就执行完了。
总结：
一次accept的流程发生了以下事情：
- 为接受的客户端连接建立一个已连接套接字，设置为非阻塞。基于已连接套接字实例化一个NioSocketChannel，设置readInterestOp为SelectionKey.OP_READ，为其创建管道，并实例化内部的NioByteUnsafe。
- 在触发ServerSocketChannel的管道的channelRead方法之前，一个ServerSocketChannel一次可以最多缓存maxMessagesPerRead（默认为16）个NioSocketChannel。
- channelRead是一个Inbound事件，做了以下几件事：调用ServerBootstrapAcceptor处理器的channelRead方法为NioSocketChannel的管道加入ChannelInitializer处理器（该处理器的initChannel方法会在channalRegistered事件被触发时调用，将EchoServerHandler加入到管道中）；设置NioSocketChannel的属性;从worker线程池中启动一个worker线程，执行register0任务。
- register0任务做的事情是：将socketChannal注册到selector中，触发channelRegistered事件，调用ChannelInitializer的initChannel方法将main函数中设置的处理器（譬如：EchoServerHandler）加入到管道中，然后触发channelActive事件，最后里面触发read事件，将ops设置为read。到此，worker线程所属的NioEventLoop持有的selector就开始监听socketChannel的read事件了。
- 最后触发ChannelReadComplete（inbound）事件，里面又会触发read（outbound）事件，这两个事件均没有做任何实事。