Netty服务端处理新连接的流程：
1.检测新连接
2.基于NioServerSocketChannel创建客户端的NioSocketChannel
3.分配客户端channel的线程，注册线程所对应的selector
4.向selector注册读事件
新连接检测
服务端在创建完服务端的NioServerSocketChannel之后，绑定完端口号之后，会注册accept事件。当有新连接进入的时候，会触发accpet事件。之前博客有分析过EventLoop的thread的run方法会循环select检测是否有新的IO事件，如果检测到有IO事件，就通过processSelectedKey来处理对应的IO事件，这里的IO事件是accept,就会调用channel内部聚合的UnSafe类的read()方法。
这里循环调用doReadMessage()方法的条件是是否自动读，读取的连接数是否小于最大连接数，服务端channel默认一次最多读取16个新连接。
当没有超过最大连接数，并且是自动读的状态时候，就会循环调用doReadMessage，直到没有读到新连接，跳出while循环，
public void read() {
assert AbstractNioMessageChannel.this.eventLoop().inEventLoop();
ChannelConfig config = AbstractNioMessageChannel.this.config();
if(!config.isAutoRead() && !AbstractNioMessageChannel.this.isReadPending()) {
this.removeReadOp();
} else {
int maxMessagesPerRead = config.getMaxMessagesPerRead();
ChannelPipeline pipeline = AbstractNioMessageChannel.this.pipeline();
boolean closed = false;
Throwable exception = null;
try {
int size;
try {
do {
size = AbstractNioMessageChannel.this.doReadMessages(this.readBuf);
if(size == 0) {
break;
}
if(size < 0) {
closed = true;
break;
}
} while(config.isAutoRead() && this.readBuf.size() < maxMessagesPerRead);
} catch (Throwable var11) {
exception = var11;
}
AbstractNioMessageChannel.this.setReadPending(false);
size = this.readBuf.size();
int i = 0;
while(true) {
if(i >= size) {
this.readBuf.clear();
pipeline.fireChannelReadComplete();
if(exception != null) {
if(exception instanceof IOException && !(exception instanceof PortUnreachableException)) {
closed = !(AbstractNioMessageChannel.this instanceof ServerChannel);
}
pipeline.fireExceptionCaught(exception);
}
if(closed && AbstractNioMessageChannel.this.isOpen()) {
this.close(this.voidPromise());
}
break;
}
pipeline.fireChannelRead(this.readBuf.get(i));
++i;
}
} finally {
if(!config.isAutoRead() && !AbstractNioMessageChannel.this.isReadPending()) {
this.removeReadOp();
}
}
}
}
创建NioSocketChannel
这里read()方法是通过循环调用NioServerSocket的doReadMessage(byteBuf)方法进行实现channel的读取新连接。而doReadMessage是通过java nio的channel的accept获取当前新连接的channel，这里获取的channel也是java nio中的channel，然后将这个channel封装成NioSocketChannel，将NioServerSocketChannel和javaChannel都作为参数构造NioSocketChannel，放到buf中去，返回1，表示已经读取一条连接。
protected int doReadMessages(List<Object> buf) throws Exception {
SocketChannel ch = this.javaChannel().accept();
try {
if(ch != null) {
buf.add(new NioSocketChannel(this, ch));
return 1;
}
} catch (Throwable var6) {
logger.warn("Failed to create a new channel from an accepted socket.", var6);
try {
ch.close();
} catch (Throwable var5) {
logger.warn("Failed to close a socket.", var5);
}
}
return 0;
}
NioSocketChannel的构造函数。
//配置Config类public NioSocketChannel(Channel parent, java.nio.channels.SocketChannel socket) {
super(parent, socket);
this.config = new NioSocketChannel.NioSocketChannelConfig(this, socket.socket(), null);
}
protected AbstractNioByteChannel(Channel parent, SelectableChannel ch) { super(parent, ch, 1);}
//保存channel感兴趣的读事件，并将channel设置为非阻塞的protected AbstractNioChannel(Channel parent, SelectableChannel ch, int readInterestOp) { super(parent); this.ch = ch; this.readInterestOp = readInterestOp;
try { ch.configureBlocking(false); } catch (IOException var7) { try { ch.close(); } catch (IOException var6) { if(logger.isWarnEnabled()) { logger.warn("Failed to close a partially initialized socket.", var6); } }
throw new ChannelException("Failed to enter non-blocking mode.", var7); }}
这里配置channel的Config类使用了setTcpNoDelay(true)，这里禁止了Nagle算法，Nagle算法的目的是让小的数据包尽量集合成大的数据包发送出去，Netty为了使数据能够及时发出去，禁止了Nagle算法。
public DefaultSocketChannelConfig(io.netty.channel.socket.SocketChannel channel, Socket javaSocket) {
super(channel);
if(javaSocket == null) {
throw new NullPointerException("javaSocket");
} else {
this.javaSocket = javaSocket;
if(PlatformDependent.canEnableTcpNoDelayByDefault()) {
try {
this.setTcpNoDelay(true);
} catch (Exception var4) {
;
}
}
}
}
public SocketChannelConfig setTcpNoDelay(boolean tcpNoDelay) { try { this.javaSocket.setTcpNoDelay(tcpNoDelay); return this; } catch (SocketException var3) { throw new ChannelException(var3); }}
新连接NioEventLoop的分配和selector的注册
在读取完新连接之后，会调用fireChannelRead方法，而服务端的NioServerSocketChannel在初始化阶段，在上面的pipeline添加了连接处理器ServerBootstrap.ServerBootstrapAcceptor，read事件会从head传送到serverBootstrapAcceptor，serverBootstrapAcceptor也是一个ChannelHandler，它会对新连接进行处理。
处理流程：
1.设置客户端channel的childHandler
添加channelHandler，这里的channelHandler一般是一个ChannelInitializer，他可以获取channel的pipeline，并且在上面添加一系列的Handler，最后再将ChannelInitializer这个Handler删除。
2.设置options和attrs
options是底层tcp读写的相关参数，attrs可以在客户端channel上面绑定一些属性。这里的options和attrs都是用户通过代码设置的。比如
bootstrap.childOption(ChannelOption.SO_KEEPALIVE, true)
设置的这些都会保存到ServerBootstrap这个类，然后在initChannel的时候会将这些参数都传入，构造一个ServerBootstrapAcceptor，这样当连接器接受到新的连接之后，新建子channel，就会带有这些属性。
3.选择NioEventLoop，并且注册selector
public void channelRead(ChannelHandlerContext ctx, Object msg) {
final Channel child = (Channel)msg;
//添加ChannelHandler child.pipeline().addLast(new ChannelHandler[]{this.childHandler});
Map.Entry[] t = this.childOptions;
int len$ = t.length;
int i$;
Map.Entry e;
for(i$ = 0; i$ < len$; ++i$) {
e = t[i$];
try {
if(!child.config().setOption((ChannelOption)e.getKey(), e.getValue())) {
ServerBootstrap.logger.warn("Unknown channel option: " + e);
}
} catch (Throwable var10) {
ServerBootstrap.logger.warn("Failed to set a channel option: " + child, var10);
}
}
t = this.childAttrs;
len$ = t.length;
for(i$ = 0; i$ < len$; ++i$) {
e = t[i$];
child.attr((AttributeKey)e.getKey()).set(e.getValue());
}
try {
this.childGroup.register(child).addListener(new ChannelFutureListener() {
public void operationComplete(ChannelFuture future) throws Exception {
if(!future.isSuccess()) {
ServerBootstrap.ServerBootstrapAcceptor.forceClose(child, future.cause());
}
}
});
} catch (Throwable var9) {
forceClose(child, var9);
}
}
这里注册是使用用户传进来的workerGroup线程池，使用register方法完成注册。
public ChannelFuture register(Channel channel) {
return this.next().register(channel);
}
这里的next()函数返回一个NioEventLoop,相当于从线程池里面挑选一个线程与这个channel进行绑定。最后通过层层调用，还是调用了java nio中channel的register方法，这时注册的时候，不关心任何事件。
public ChannelFuture register(Channel channel, ChannelPromise promise) {
if(channel == null) {
throw new NullPointerException("channel");
} else if(promise == null) {
throw new NullPointerException("promise");
} else {
channel.unsafe().register(this, promise);
return promise;
}
}
//AbstractChannelpublic final void register(EventLoop eventLoop, final ChannelPromise promise) { if(eventLoop == null) { throw new NullPointerException("eventLoop"); } else if(AbstractChannel.this.isRegistered()) { promise.setFailure(new IllegalStateException("registered to an event loop already")); } else if(!AbstractChannel.this.isCompatible(eventLoop)) { promise.setFailure(new IllegalStateException("incompatible event loop type: " + eventLoop.getClass().getName())); } else { AbstractChannel.this.eventLoop = eventLoop; if(eventLoop.inEventLoop()) { this.register0(promise); } else { try { eventLoop.execute(new OneTimeTask() { public void run() { AbstractUnsafe.this.register0(promise); } }); } catch (Throwable var4) { AbstractChannel.logger.warn("Force-closing a channel whose registration task was not accepted by an event loop: {}", AbstractChannel.this, var4); this.closeForcibly(); AbstractChannel.this.closeFuture.setClosed(); this.safeSetFailure(promise, var4); } }
}}
private void register0(ChannelPromise promise) { try { if(!promise.setUncancellable() || !this.ensureOpen(promise)) { return; }
AbstractChannel.this.doRegister(); AbstractChannel.this.registered = true; this.safeSetSuccess(promise); AbstractChannel.this.pipeline.fireChannelRegistered(); if(AbstractChannel.this.isActive()) { AbstractChannel.this.pipeline.fireChannelActive(); } } catch (Throwable var3) { this.closeForcibly(); AbstractChannel.this.closeFuture.setClosed(); this.safeSetFailure(promise, var3); }
}
protected void doRegister() throws Exception { boolean selected = false;
while(true) { try { this.selectionKey = this.javaChannel().register(this.eventLoop().selector, 0, this); return; } catch (CancelledKeyException var3) { if(selected) { throw var3; }
this.eventLoop().selectNow(); selected = true; } }}
NioSocketChannel读事件的注册
通过传播channelActive方法，最终会调用channel的read()方法，channel在创建的时候都是默认自动读的。
public ChannelPipeline fireChannelActive() {
this.head.fireChannelActive();
if(this.channel.config().isAutoRead()) {
this.channel.read();
}
return this;
}
会将channel的Active状态在pipeline上面传播，调用read方法，最后会调用doBeginRead,去注册感兴趣的事件，NioSocketChannel感兴趣的事件是读事件，而NioServerSocketChannel感兴趣的事件则是Accept事件。
public ChannelHandlerContext read() {
final AbstractChannelHandlerContext next = this.findContextOutbound();
EventExecutor executor = next.executor();
if(executor.inEventLoop()) {
next.invokeRead();
} else {
Runnable task = next.invokeReadTask;
if(task == null) {
next.invokeReadTask = task = new Runnable() {
public void run() {
next.invokeRead();
}
};
}
executor.execute(task);
}
return this;
}
protected void doBeginRead() throws Exception { if(!this.inputShutdown) { SelectionKey selectionKey = this.selectionKey; if(selectionKey.isValid()) { this.readPending = true; int interestOps = selectionKey.interestOps(); if((interestOps & this.readInterestOp) == 0) { selectionKey.interestOps(interestOps | this.readInterestOp); }
} }}
原文地址：https://www.cnblogs.com/xiaobaituyun/p/10801495.html