netty作为一个被广泛应用的通信框架,有必要我们多了解一点。

  实际上netty的几个重要的技术亮点:

    1. reactor的线程模型;
    2. 安全有效的nio非阻塞io模型应用;
    3. pipeline流水线式的灵活处理过程;
    4. channelHandler的灵活实现;
    5. 提供许多开箱即用的处理器和编解码器;

  我们可以从这些点去深入理解其过人之处。

1. 一个NettyServer的demo

  要想深入理解某个框架,一般还是要以demo作为一个抓手点的。以下,我们可以看到一个简单的nettyServer的创建过程,即netty的quick start样例吧。

@Slf4j
public class NettyServerHelloApplication { /**
* 一个server的样例
*/
public static void main(String[] args) throws Exception {
// 1. 创建对应的EventLoop线程池备用, 分bossGroup和workerGroup
EventLoopGroup bossGroup = new NioEventLoopGroup(1);
EventLoopGroup workerGroup = new NioEventLoopGroup(4);
try {
// 2. 创建netty对应的入口核心类 ServerBootstrap
ServerBootstrap b = new ServerBootstrap();
// 3. 设置server的各项参数,以及应用处理器
b.group(bossGroup, workerGroup)
.channel(NioServerSocketChannel.class)
.option(ChannelOption.SO_BACKLOG, 100) // 设置tcp协议的请求等待队列
.childHandler(new ChannelInitializer<SocketChannel>() {
@Override
public void initChannel(SocketChannel ch) throws Exception {
// 3.2. 最重要的,将各channelHandler绑定到netty的上下文中(暂且这么说吧)
ChannelPipeline p = ch.pipeline();
p.addLast(new LoggingHandler(LogLevel.INFO));
p.addLast("encoder", new MessageEncoder());
p.addLast("decoder", new MessageDecoder());
p.addLast(new EchoServerHandler());
}
}); // 4. 绑定tcp端口开启服务端监听, sync() 保证执行完成所有任务
ChannelFuture f = b.bind(ServerConstant.PORT).sync(); // 5. 等待关闭信号,让业务线程去服务业务了
f.channel().closeFuture().sync();
} finally {
// 6. 收到关闭信号后,优雅关闭server的线程池,保护应用
bossGroup.shutdownGracefully();
workerGroup.shutdownGracefully();
}
} }

  以上,就是一个简版的nettyServer的整个框架了,这也基本上整个nettyServer的编程范式了。主要即分为这么几步:

    1. 创建对应的EventLoop线程池备用, 分bossGroup和workerGroup;
    2. 创建netty对应的入口核心类 ServerBootstrap;
    3. 设置server的各项参数,以及应用处理器(必备的channelHandler业务接入过程);
    4. 绑定tcp端口开启服务端监听;
    5. 等待关闭信号,让业务线程去服务业务了;
    6. 收到关闭信号后,优雅关闭server的线程池,保护应用;

  事实上,如果我们直接基于jdk提供的ServerSocketChannel是否也差不了多少呢?是的,至少表面看起来是的,但我们要处理许多的异常情况,且可能面对变化繁多的业务类型。又该如何呢?

  毕竟一个框架的成功,绝非偶然。下面我们就这几个过程来看看netty都是如何处理的吧!

2. EventLoop 的创建

  EventLoop 直译为事件循环,但在这里我们也可以理解为一个线程池,因为所有的事件都是提交给其处理的。那么,它倒底是个什么样的循环呢?

  首先来看下其类继承情况:

  从类图可以看出,EventLoop也是一个executor或者说线程池的实现,它们也许有相通之处。

    // 调用方式如下
EventLoopGroup bossGroup = new NioEventLoopGroup(1);
EventLoopGroup workerGroup = new NioEventLoopGroup(4);
// io.netty.channel.nio.NioEventLoopGroup#NioEventLoopGroup(int, java.util.concurrent.ThreadFactory)
/**
* Create a new instance using the specified number of threads, the given {@link ThreadFactory} and the
* {@link SelectorProvider} which is returned by {@link SelectorProvider#provider()}.
*/
public NioEventLoopGroup(int nThreads, ThreadFactory threadFactory) {
this(nThreads, threadFactory, SelectorProvider.provider());
}
public NioEventLoopGroup(
int nThreads, Executor executor, final SelectorProvider selectorProvider) {
this(nThreads, executor, selectorProvider, DefaultSelectStrategyFactory.INSTANCE);
} public NioEventLoopGroup(int nThreads, Executor executor, final SelectorProvider selectorProvider,
final SelectStrategyFactory selectStrategyFactory) {
super(nThreads, executor, selectorProvider, selectStrategyFactory, RejectedExecutionHandlers.reject());
}
// io.netty.channel.MultithreadEventLoopGroup#MultithreadEventLoopGroup(int, java.util.concurrent.Executor, java.lang.Object...)
protected MultithreadEventLoopGroup(int nThreads, Executor executor, Object... args) {
// 默认线程是 cpu * 2
super(nThreads == 0 ? DEFAULT_EVENT_LOOP_THREADS : nThreads, executor, args);
}
// io.netty.util.concurrent.MultithreadEventExecutorGroup#MultithreadEventExecutorGroup(int, java.util.concurrent.Executor, java.lang.Object...)
/**
* Create a new instance.
*
* @param nThreads the number of threads that will be used by this instance.
* @param executor the Executor to use, or {@code null} if the default should be used.
* @param args arguments which will passed to each {@link #newChild(Executor, Object...)} call
*/
protected MultithreadEventExecutorGroup(int nThreads, Executor executor, Object... args) {
this(nThreads, executor, DefaultEventExecutorChooserFactory.INSTANCE, args);
} // io.netty.util.concurrent.MultithreadEventExecutorGroup#MultithreadEventExecutorGroup(int, java.util.concurrent.Executor, io.netty.util.concurrent.EventExecutorChooserFactory, java.lang.Object...)
/**
* Create a new instance.
*
* @param nThreads the number of threads that will be used by this instance.
* @param executor the Executor to use, or {@code null} if the default should be used.
* @param chooserFactory the {@link EventExecutorChooserFactory} to use.
* @param args arguments which will passed to each {@link #newChild(Executor, Object...)} call
*/
protected MultithreadEventExecutorGroup(int nThreads, Executor executor,
EventExecutorChooserFactory chooserFactory, Object... args) {
if (nThreads <= 0) {
throw new IllegalArgumentException(String.format("nThreads: %d (expected: > 0)", nThreads));
} // 创建一个执行器,该执行器每提交一个任务,就创建一个线程来运行,即并没有队列的概念
if (executor == null) {
executor = new ThreadPerTaskExecutor(newDefaultThreadFactory());
}
// 使用一个数组来保存整个可用的线程池
children = new EventExecutor[nThreads]; for (int i = 0; i < nThreads; i ++) {
boolean success = false;
try {
// 为每个child创建一个线程运行, 该方法由子类实现
children[i] = newChild(executor, args);
success = true;
} catch (Exception e) {
// TODO: Think about if this is a good exception type
throw new IllegalStateException("failed to create a child event loop", e);
} finally {
if (!success) {
// 如果创建失败,则把已经创建好的线程池关闭掉
// 不过值得注意的是,当某个线程池创建失败后,并没有立即停止后续创建工作,即无 return 操作,这是为啥?
// 实际上,发生异常时,Exeception 已经被抛出,此处无需关注
for (int j = 0; j < i; j ++) {
children[j].shutdownGracefully();
} for (int j = 0; j < i; j ++) {
EventExecutor e = children[j];
try {
while (!e.isTerminated()) {
e.awaitTermination(Integer.MAX_VALUE, TimeUnit.SECONDS);
}
} catch (InterruptedException interrupted) {
// Let the caller handle the interruption.
Thread.currentThread().interrupt();
break;
}
}
}
}
}
// 创建选择器,猜测是做负载均衡时使用
// 此处的chooser默认是 DefaultEventExecutorChooserFactory
chooser = chooserFactory.newChooser(children); final FutureListener<Object> terminationListener = new FutureListener<Object>() {
@Override
public void operationComplete(Future<Object> future) throws Exception {
if (terminatedChildren.incrementAndGet() == children.length) {
terminationFuture.setSuccess(null);
}
}
}; for (EventExecutor e: children) {
e.terminationFuture().addListener(terminationListener);
} Set<EventExecutor> childrenSet = new LinkedHashSet<EventExecutor>(children.length);
Collections.addAll(childrenSet, children);
readonlyChildren = Collections.unmodifiableSet(childrenSet);
} // io.netty.channel.nio.NioEventLoopGroup#newChild
@Override
protected EventLoop newChild(Executor executor, Object... args) throws Exception {
// 注意此处的参数类型是由外部进行保证的,在此直接做强转操作
return new NioEventLoop(this, executor, (SelectorProvider) args[0],
((SelectStrategyFactory) args[1]).newSelectStrategy(), (RejectedExecutionHandler) args[2]);
} // io.netty.channel.nio.NioEventLoop#NioEventLoop
NioEventLoop(NioEventLoopGroup parent, Executor executor, SelectorProvider selectorProvider,
SelectStrategy strategy, RejectedExecutionHandler rejectedExecutionHandler) {
// 此构造器会做很多事,比如创建队列,开启nio selector...
super(parent, executor, false, DEFAULT_MAX_PENDING_TASKS, rejectedExecutionHandler);
if (selectorProvider == null) {
throw new NullPointerException("selectorProvider");
}
if (strategy == null) {
throw new NullPointerException("selectStrategy");
}
provider = selectorProvider;
final SelectorTuple selectorTuple = openSelector();
selector = selectorTuple.selector;
unwrappedSelector = selectorTuple.unwrappedSelector;
selectStrategy = strategy;
} // io.netty.util.concurrent.DefaultEventExecutorChooserFactory#newChooser
@SuppressWarnings("unchecked")
@Override
public EventExecutorChooser newChooser(EventExecutor[] executors) {
// 如: 1,2,4,8... 都会创建 PowerOfTwoEventExecutorChooser
if (isPowerOfTwo(executors.length)) {
return new PowerOfTwoEventExecutorChooser(executors);
} else {
return new GenericEventExecutorChooser(executors);
}
} // io.netty.util.concurrent.DefaultPromise#addListener
@Override
public Promise<V> addListener(GenericFutureListener<? extends Future<? super V>> listener) {
checkNotNull(listener, "listener"); synchronized (this) {
addListener0(listener);
} if (isDone()) {
notifyListeners();
} return this;
}

  以上,就是 NioEventLoopGroup 的创建过程了. 本质上其就是一个个的单独的线程组成的数组列表, 等待被调用.

3. ServerBootstrap 的创建

  ServerBootstrap是Netty的一个服务端核心入口类, 它可以很快速的创建一个稳定的netty服务.

  ServerBootstrap 的类图如下:

  还是非常纯粹的啊!其中有意思是的, ServerBootstrap继承自 AbstractBootstrap, 而这个 AbstractBootstrap 是一个自依赖的抽象类: AbstractBootstrap<B extends AbstractBootstrap<B, C>, C extends Channel> , 这样,即父类可以直接返回子类的信息了。

  其默认构造方法为空,所以所以参数都使用默认值, 因为还有后续的参数设置过程,接下来,我们看看其一些关键参数的设置:

    // 1. channel的设定
// io.netty.bootstrap.AbstractBootstrap#channel
/**
* The {@link Class} which is used to create {@link Channel} instances from.
* You either use this or {@link #channelFactory(io.netty.channel.ChannelFactory)} if your
* {@link Channel} implementation has no no-args constructor.
*/
public B channel(Class<? extends C> channelClass) {
if (channelClass == null) {
throw new NullPointerException("channelClass");
}
// 默认使用构造器反射的方式创建 channel
return channelFactory(new ReflectiveChannelFactory<C>(channelClass));
}
// io.netty.bootstrap.AbstractBootstrap#channelFactory(io.netty.channel.ChannelFactory<? extends C>)
/**
* {@link io.netty.channel.ChannelFactory} which is used to create {@link Channel} instances from
* when calling {@link #bind()}. This method is usually only used if {@link #channel(Class)}
* is not working for you because of some more complex needs. If your {@link Channel} implementation
* has a no-args constructor, its highly recommend to just use {@link #channel(Class)} for
* simplify your code.
*/
@SuppressWarnings({ "unchecked", "deprecation" })
public B channelFactory(io.netty.channel.ChannelFactory<? extends C> channelFactory) {
return channelFactory((ChannelFactory<C>) channelFactory);
}
// io.netty.bootstrap.AbstractBootstrap#channelFactory(io.netty.bootstrap.ChannelFactory<? extends C>)
/**
* @deprecated Use {@link #channelFactory(io.netty.channel.ChannelFactory)} instead.
*/
@Deprecated
public B channelFactory(ChannelFactory<? extends C> channelFactory) {
if (channelFactory == null) {
throw new NullPointerException("channelFactory");
}
if (this.channelFactory != null) {
throw new IllegalStateException("channelFactory set already");
} this.channelFactory = channelFactory;
return self();
}
@SuppressWarnings("unchecked")
private B self() {
return (B) this;
} // 2. option 参数选项设置, 它会承包各种特殊配置的设置, 是一个通用配置项设置的入口
/**
* Allow to specify a {@link ChannelOption} which is used for the {@link Channel} instances once they got
* created. Use a value of {@code null} to remove a previous set {@link ChannelOption}.
*/
public <T> B option(ChannelOption<T> option, T value) {
if (option == null) {
throw new NullPointerException("option");
}
// options 是一个 new LinkedHashMap<ChannelOption<?>, Object>(), 即非线程安全的容器, 所以设置值时要求使用 synchronized 保证线程安全
// value 为null时代表要将该选项设置删除, 如果key相同,后面的配置将会覆盖前面的配置
if (value == null) {
synchronized (options) {
options.remove(option);
}
} else {
synchronized (options) {
options.put(option, value);
}
}
return self();
} // 3. childHandler 添加channelHandler, 这是一个最重要的一个方法, 它会影响到后面的业务处理统筹
// 调用该方法仅将 channelHandler的上下文加入进来, 实际还未进行真正的添加操作 .childHandler(new ChannelInitializer<SocketChannel>() {
ServerBootstrap b = new ServerBootstrap();
b.group(bossGroup, workerGroup)
.channel(NioServerSocketChannel.class)
.option(ChannelOption.SO_BACKLOG, 100) // 设置tcp协议的请求等待队列
.childHandler(new ChannelInitializer<SocketChannel>() {
@Override
public void initChannel(SocketChannel ch) throws Exception {
ChannelPipeline p = ch.pipeline();
p.addLast(new LoggingHandler(LogLevel.INFO));
p.addLast("encoder", new MessageEncoder());
p.addLast("decoder", new MessageDecoder());
p.addLast(new EchoServerHandler());
}
});
/**
* Set the {@link ChannelHandler} which is used to serve the request for the {@link Channel}'s.
*/
public ServerBootstrap childHandler(ChannelHandler childHandler) {
if (childHandler == null) {
throw new NullPointerException("childHandler");
}
// 仅将 channelHandler 绑定到netty的上下文中
this.childHandler = childHandler;
return this;
} // 4. bossGroup, workGroup 如何被分配 ?
/**
* Set the {@link EventLoopGroup} for the parent (acceptor) and the child (client). These
* {@link EventLoopGroup}'s are used to handle all the events and IO for {@link ServerChannel} and
* {@link Channel}'s.
*/
public ServerBootstrap group(EventLoopGroup parentGroup, EventLoopGroup childGroup) {
// parentGroup 是给acceptor使用的, 主要用于对socket连接的接入,所以一般一个线程也够了
super.group(parentGroup);
if (childGroup == null) {
throw new NullPointerException("childGroup");
}
if (this.childGroup != null) {
throw new IllegalStateException("childGroup set already");
}
// childGroup 主要用于接入后的socket的事件的处理,一般要求数量较多,视业务属性决定
this.childGroup = childGroup;
return this;
}

  bind 绑定tcp端口,这个是真正触发server初始化的一步,工作量比较大,我们另开一段讲解。

4. nettyServer 的初始化

  前面所有工作都是在准备, 都并未体现在外部, 而 bind 则会是开启一个对外服务, 对外可见, 真正启动server.

    // io.netty.bootstrap.AbstractBootstrap#bind(int)
/**
* Create a new {@link Channel} and bind it.
*/
public ChannelFuture bind(int inetPort) {
return bind(new InetSocketAddress(inetPort));
}
// io.netty.bootstrap.AbstractBootstrap#bind(java.net.SocketAddress)
/**
* Create a new {@link Channel} and bind it.
*/
public ChannelFuture bind(SocketAddress localAddress) {
// 先验证各种参数是否设置完整, 如线程池是否设置, channelHandler 是否设置...
validate();
if (localAddress == null) {
throw new NullPointerException("localAddress");
}
// 绑定tcp端口
return doBind(localAddress);
}
private ChannelFuture doBind(final SocketAddress localAddress) {
// 1. 创建一些channel使用, 与eventloop绑定, 统一管理嘛
final ChannelFuture regFuture = initAndRegister();
final Channel channel = regFuture.channel();
if (regFuture.cause() != null) {
return regFuture;
} if (regFuture.isDone()) {
// At this point we know that the registration was complete and successful.
ChannelPromise promise = channel.newPromise();
// 2. 注册成功之后, 开始实际的 bind() 操作, 实际就是调用 channel.bind()
// doBind0() 是一个异步的操作,所以使用的一个 promise 作为结果驱动
doBind0(regFuture, channel, localAddress, promise);
return promise;
} else {
// Registration future is almost always fulfilled already, but just in case it's not.
final PendingRegistrationPromise promise = new PendingRegistrationPromise(channel);
regFuture.addListener(new ChannelFutureListener() {
@Override
public void operationComplete(ChannelFuture future) throws Exception {
Throwable cause = future.cause();
if (cause != null) {
// Registration on the EventLoop failed so fail the ChannelPromise directly to not cause an
// IllegalStateException once we try to access the EventLoop of the Channel.
promise.setFailure(cause);
} else {
// Registration was successful, so set the correct executor to use.
// See https://github.com/netty/netty/issues/2586
promise.registered(); doBind0(regFuture, channel, localAddress, promise);
}
}
});
return promise;
}
}

  所以,从整体来说,bind()过程分两大步走:1. 初始化channel,与nio关联; 2. 落实channel和本地端口的绑定工作; 我们来细看下:

4.1 初始化channel

  初始化channel, 并注册到 selector上, 这个操作实际上非常重要。

    // 以下我们先看下执行框架
// io.netty.bootstrap.AbstractBootstrap#initAndRegister
final ChannelFuture initAndRegister() {
Channel channel = null;
try {
// 即根据前面设置的channel 使用反射创建一个实例出来
// 即此处将会实例化出一个 ServerSocketChannel 出来
// 所以如果你想用jdk的nio实现,则设置channel时使用 NioServerSocketChannel.class即可, 而你想使用其他更优化的实现时比如EpollServerSocketChannel时,改变一下即可
// 而此处的 channelFactory 就是一个反射的实现 ReflectiveChannelFactory, 它会调用如上channel的无参构造方法实例化
// 重点工作就需要在这个无参构造器中完成,我们接下来看看
channel = channelFactory.newChannel();
// 初始化channel的一些公共参数, 相当于做一些属性的继承, 因为后续它将不再依赖 ServerBootstrap, 它需要有独立自主能力
init(channel);
} catch (Throwable t) {
if (channel != null) {
// channel can be null if newChannel crashed (eg SocketException("too many open files"))
channel.unsafe().closeForcibly();
// as the Channel is not registered yet we need to force the usage of the GlobalEventExecutor
return new DefaultChannelPromise(channel, GlobalEventExecutor.INSTANCE).setFailure(t);
}
// as the Channel is not registered yet we need to force the usage of the GlobalEventExecutor
return new DefaultChannelPromise(new FailedChannel(), GlobalEventExecutor.INSTANCE).setFailure(t);
}
// 注册创建好的 channel 到eventLoop中
ChannelFuture regFuture = config().group().register(channel);
if (regFuture.cause() != null) {
if (channel.isRegistered()) {
channel.close();
} else {
channel.unsafe().closeForcibly();
}
} // If we are here and the promise is not failed, it's one of the following cases:
// 1) If we attempted registration from the event loop, the registration has been completed at this point.
// i.e. It's safe to attempt bind() or connect() now because the channel has been registered.
// 2) If we attempted registration from the other thread, the registration request has been successfully
// added to the event loop's task queue for later execution.
// i.e. It's safe to attempt bind() or connect() now:
// because bind() or connect() will be executed *after* the scheduled registration task is executed
// because register(), bind(), and connect() are all bound to the same thread. return regFuture;
} // 1. 先看看 NioServerSocketChannel 的构造过程
// io.netty.channel.socket.nio.NioServerSocketChannel#NioServerSocketChannel()
/**
* Create a new instance
*/
public NioServerSocketChannel() {
// newSocket 简单说就是创建一个本地socket, api调用: SelectorProvider.provider().openServerSocketChannel()
// 但此时本 socket 并未和任何端口绑定
this(newSocket(DEFAULT_SELECTOR_PROVIDER));
}
/**
* Create a new instance using the given {@link ServerSocketChannel}.
*/
public NioServerSocketChannel(ServerSocketChannel channel) {
// 注册 OP_ACCEPT 事件
super(null, channel, SelectionKey.OP_ACCEPT);
// 此处的 javaChannel() 实际就是 channel, 这样调用只是为统一吧
// 创建一个新的 socket 传入 NioServerSocketChannelConfig 中
// 主要用于一些 RecvByteBufAllocator 的设置,及channel的保存
config = new NioServerSocketChannelConfig(this, javaChannel().socket());
}
// io.netty.channel.nio.AbstractNioChannel#AbstractNioChannel
/**
* Create a new instance
*
* @param parent the parent {@link Channel} by which this instance was created. May be {@code null}
* @param ch the underlying {@link SelectableChannel} on which it operates
* @param readInterestOp the ops to set to receive data from the {@link SelectableChannel}
*/
protected AbstractNioChannel(Channel parent, SelectableChannel ch, int readInterestOp) {
// 先让父类初始化必要的上下文
super(parent);
// 保留 channel 信息,并设置非阻塞标识
this.ch = ch;
this.readInterestOp = readInterestOp;
try {
ch.configureBlocking(false);
} catch (IOException e) {
try {
ch.close();
} catch (IOException e2) {
if (logger.isWarnEnabled()) {
logger.warn(
"Failed to close a partially initialized socket.", e2);
}
} throw new ChannelException("Failed to enter non-blocking mode.", e);
}
}
// io.netty.channel.AbstractChannel#AbstractChannel(io.netty.channel.Channel)
/**
* Creates a new instance.
*
* @param parent
* the parent of this channel. {@code null} if there's no parent.
*/
protected AbstractChannel(Channel parent) {
// 初始化上下文
this.parent = parent;
// DefaultChannelId
id = newId();
// NioMessageUnsafe
unsafe = newUnsafe();
// new DefaultChannelPipeline(this);
// 比较重要,将会初始化 head, tail 节点
pipeline = newChannelPipeline();
}
// io.netty.channel.DefaultChannelPipeline#DefaultChannelPipeline
protected DefaultChannelPipeline(Channel channel) {
this.channel = ObjectUtil.checkNotNull(channel, "channel");
succeededFuture = new SucceededChannelFuture(channel, null);
voidPromise = new VoidChannelPromise(channel, true);
// 初始化 head, tail
tail = new TailContext(this);
head = new HeadContext(this);
// 构成双向链表
head.next = tail;
tail.prev = head;
} // 2. 初始化channel, 有个最重要的动作是将 Acceptor 接入到 pipeline 中
// io.netty.bootstrap.ServerBootstrap#init
@Override
void init(Channel channel) throws Exception {
final Map<ChannelOption<?>, Object> options = options0();
// 根据前面的设置, 将各种属性copy过来, 放到 config 字段中
// 同样, 因为 options 和 attrs 都不是线程安全的, 所以都要上锁操作
synchronized (options) {
setChannelOptions(channel, options, logger);
} final Map<AttributeKey<?>, Object> attrs = attrs0();
synchronized (attrs) {
for (Entry<AttributeKey<?>, Object> e: attrs.entrySet()) {
@SuppressWarnings("unchecked")
AttributeKey<Object> key = (AttributeKey<Object>) e.getKey();
channel.attr(key).set(e.getValue());
}
}
// 此处的pipeline, 就是在 NioServerSocketChannel 中初始化好head,tail的pipeline
ChannelPipeline p = channel.pipeline();
// childGroup 实际就是外部的 workGroup
final EventLoopGroup currentChildGroup = childGroup;
final ChannelHandler currentChildHandler = childHandler;
final Entry<ChannelOption<?>, Object>[] currentChildOptions;
final Entry<AttributeKey<?>, Object>[] currentChildAttrs;
synchronized (childOptions) {
currentChildOptions = childOptions.entrySet().toArray(newOptionArray(childOptions.size()));
}
synchronized (childAttrs) {
currentChildAttrs = childAttrs.entrySet().toArray(newAttrArray(childAttrs.size()));
}
// 这个就比较重要了, 关联 ServerBootstrapAcceptor
// 主动添加一个 initializer, 它将作为第一个被调用的 channelInitializer 存在
// 而 channelInitializer 只会被调用一次
p.addLast(new ChannelInitializer<Channel>() {
@Override
public void initChannel(final Channel ch) throws Exception {
final ChannelPipeline pipeline = ch.pipeline();
ChannelHandler handler = config.handler();
if (handler != null) {
pipeline.addLast(handler);
} ch.eventLoop().execute(new Runnable() {
@Override
public void run() {
// 添加 Acceptor 到 pipeline 中, 形成一个 head -> ServerBootstrapAcceptor -> tail 的pipeline
pipeline.addLast(new ServerBootstrapAcceptor(
ch, currentChildGroup, currentChildHandler, currentChildOptions, currentChildAttrs));
}
});
}
});
// 此操作过后,当前pipeline中,就只有此一handler
}

  。。。

4.2 handler的添加过程

  addLast() 看起来只是一个添加元素的过程, 总体来说就是一个双向链表的添加, 但也蛮有意思的, 有兴趣可以戳开详情看看.

    // io.netty.channel.ChannelHandler
@Override
public final ChannelPipeline addLast(ChannelHandler... handlers) {
return addLast(null, handlers);
}
// io.netty.channel.DefaultChannelPipeline#addLast(io.netty.util.concurrent.EventExecutorGroup, io.netty.channel.ChannelHandler...)
@Override
public final ChannelPipeline addLast(EventExecutorGroup executor, ChannelHandler... handlers) {
if (handlers == null) {
throw new NullPointerException("handlers");
}
// 支持同时添加多个 handler
for (ChannelHandler h: handlers) {
if (h == null) {
break;
}
addLast(executor, null, h);
} return this;
}
// io.netty.channel.DefaultChannelPipeline#addLast(io.netty.util.concurrent.EventExecutorGroup, java.lang.String, io.netty.channel.ChannelHandler)
@Override
public final ChannelPipeline addLast(EventExecutorGroup group, String name, ChannelHandler handler) {
final AbstractChannelHandlerContext newCtx;
synchronized (this) {
// 重复性检查 @Shareable 参数使用
checkMultiplicity(handler);
// 生成一个新的上下文, filterName()将会生成一个唯一的名称, 如 ServerBootstrap$1#0
newCtx = newContext(group, filterName(name, handler), handler);
// 将当前ctx添加到链表中
addLast0(newCtx); // If the registered is false it means that the channel was not registered on an eventloop yet.
// In this case we add the context to the pipeline and add a task that will call
// ChannelHandler.handlerAdded(...) once the channel is registered.
if (!registered) {
newCtx.setAddPending();
// 未注册情况下, 不会进行下一步了
callHandlerCallbackLater(newCtx, true);
return this;
}
// 而已注册情况下, 则会使用 executor 提交callHandlerAdded0, 即调用 pipeline 的头节点
EventExecutor executor = newCtx.executor();
if (!executor.inEventLoop()) {
newCtx.setAddPending();
executor.execute(new Runnable() {
@Override
public void run() {
callHandlerAdded0(newCtx);
}
});
return this;
}
}
callHandlerAdded0(newCtx);
return this;
}
private AbstractChannelHandlerContext newContext(EventExecutorGroup group, String name, ChannelHandler handler) {
return new DefaultChannelHandlerContext(this, childExecutor(group), name, handler);
}
private void addLast0(AbstractChannelHandlerContext newCtx) {
// 一个双向链表保存上下文
AbstractChannelHandlerContext prev = tail.prev;
newCtx.prev = prev;
newCtx.next = tail;
prev.next = newCtx;
tail.prev = newCtx;
}
// 添加ctx到队列尾部
private void callHandlerCallbackLater(AbstractChannelHandlerContext ctx, boolean added) {
assert !registered; PendingHandlerCallback task = added ? new PendingHandlerAddedTask(ctx) : new PendingHandlerRemovedTask(ctx);
PendingHandlerCallback pending = pendingHandlerCallbackHead;
if (pending == null) {
pendingHandlerCallbackHead = task;
} else {
// Find the tail of the linked-list.
while (pending.next != null) {
pending = pending.next;
}
pending.next = task;
}
}
// 对每一次添加 handler, 则都会产生一个事件, 通知现有的handler, handlerAdded()
private void callHandlerAdded0(final AbstractChannelHandlerContext ctx) {
try {
// We must call setAddComplete before calling handlerAdded. Otherwise if the handlerAdded method generates
// any pipeline events ctx.handler() will miss them because the state will not allow it.
ctx.setAddComplete();
ctx.handler().handlerAdded(ctx);
} catch (Throwable t) {
boolean removed = false;
try {
remove0(ctx);
try {
ctx.handler().handlerRemoved(ctx);
} finally {
ctx.setRemoved();
}
removed = true;
} catch (Throwable t2) {
if (logger.isWarnEnabled()) {
logger.warn("Failed to remove a handler: " + ctx.name(), t2);
}
} if (removed) {
fireExceptionCaught(new ChannelPipelineException(
ctx.handler().getClass().getName() +
".handlerAdded() has thrown an exception; removed.", t));
} else {
fireExceptionCaught(new ChannelPipelineException(
ctx.handler().getClass().getName() +
".handlerAdded() has thrown an exception; also failed to remove.", t));
}
}
}

查看 handler 的添加过程

4.3 注册channel,绑定eventloop线程

  经过前面两步, channel已经创建好和初始化好了, 但还没有看到 eventLoop 的影子. 实际上eventloop和channel间就差一个注册了.

  也就是前面看到的 ChannelFuture regFuture = config().group().register(channel); 此处的group 即是 bossGroup.

    // io.netty.channel.MultithreadEventLoopGroup#register(io.netty.channel.Channel)
@Override
public ChannelFuture register(Channel channel) {
// next() 相当于是一个负载均衡器, 会选择出一个合适的 eventloop 出来, 默认是round-robin
return next().register(channel);
}
// io.netty.channel.MultithreadEventLoopGroup#next
@Override
public EventLoop next() {
return (EventLoop) super.next();
}
// io.netty.util.concurrent.MultithreadEventExecutorGroup#next
@Override
public EventExecutor next() {
// 使用前面创建的 PowerOfTwoEventExecutorChooser 进行调用
// 默认实现为轮询
return chooser.next();
}
// io.netty.util.concurrent.DefaultEventExecutorChooserFactory.PowerOfTwoEventExecutorChooser#next
@Override
public EventExecutor next() {
return executors[idx.getAndIncrement() & executors.length - 1];
} // io.netty.channel.SingleThreadEventLoop#register(io.netty.channel.Channel)
@Override
public ChannelFuture register(Channel channel) {
// 使用 DefaultChannelPromise 封装channel, 再注册到 eventloop 中
return register(new DefaultChannelPromise(channel, this));
}
@Override
public ChannelFuture register(final ChannelPromise promise) {
ObjectUtil.checkNotNull(promise, "promise");
// NioMessageUnsafe
promise.channel().unsafe().register(this, promise);
return promise;
} // io.netty.channel.AbstractChannel.AbstractUnsafe#register
@Override
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;
// inEventLoop() 判断当前线程是否在 eventLoop 中
// 判断方式为直接比较 eventloop 线程也当前线程是否是同一个即可 Thread.currentThread() == this.thread;
// 核心注册方法 register0()
if (eventLoop.inEventLoop()) {
register0(promise);
} else {
// 不在 eventLoop 中, 则异步提交任务给 eventloop 处理
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();
safeSetFailure(promise, t);
}
}
} // register0() 做真正的注册
// io.netty.channel.AbstractChannel.AbstractUnsafe#register0
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 (!promise.setUncancellable() || !ensureOpen(promise)) {
return;
}
boolean firstRegistration = neverRegistered;
// 具体的注册逻辑由子类实现, NioServerSocketChannel
doRegister();
neverRegistered = false;
registered = true; // Ensure we call handlerAdded(...) before we actually notify the promise. This is needed as the
// user may already fire events through the pipeline in the ChannelFutureListener.
// 几个扩展点: fireHandlerAdded() -> fireChannelRegistered() -> fireChannelActive()
// part1: fireChannelAdded(), 它将会回调上面的 ServerBootstrapAcceptor 的添加 channelInitializer
pipeline.invokeHandlerAddedIfNeeded(); safeSetSuccess(promise);
// part2: fireChannelRegistered()
pipeline.fireChannelRegistered();
// Only fire a channelActive if the channel has never been registered. This prevents firing
// multiple channel actives if the channel is deregistered and re-registered.
if (isActive()) {
if (firstRegistration) {
pipeline.fireChannelActive();
} else if (config().isAutoRead()) {
// This channel was registered before and autoRead() is set. This means we need to begin read
// again so that we process inbound data.
//
// See https://github.com/netty/netty/issues/4805
beginRead();
}
}
} catch (Throwable t) {
// Close the channel directly to avoid FD leak.
closeForcibly();
closeFuture.setClosed();
safeSetFailure(promise, t);
}
}
// io.netty.channel.nio.AbstractNioChannel#doRegister
@Override
protected void doRegister() throws Exception {
boolean selected = false;
// 进行注册即是 JDK 的 ServerSocketChannel.register() 过程
// 即 netty 与 socket 建立了关系连接, ops=0, 代表监听所有读事件
for (;;) {
try {
// 一直注册直到成功
// 此处 ops=0, 即不关注任何事件哦, 那么前面的 OP_ACCEPT 和这里又是什么关系呢?
selectionKey = javaChannel().register(eventLoop().unwrappedSelector(), 0, this);
return;
} catch (CancelledKeyException e) {
if (!selected) {
// Force the Selector to select now as the "canceled" SelectionKey may still be
// cached and not removed because no Select.select(..) operation was called yet.
eventLoop().selectNow();
selected = true;
} else {
// We forced a select operation on the selector before but the SelectionKey is still cached
// for whatever reason. JDK bug ?
throw e;
}
}
}
}

  。。。

4.4 ServerBootstrapAcceptor 速览

  前面我们看到, 在做 register() 完了之后, netty 会触发一个invokeHandlerAddedIfNeeded, 从而调用fireHandlerAdded. 此时将会触发 handlerAdded() 从而首次调用 ChannelInitializer.initChannel(), 从而将 ServerBootstrapAcceptor 添加到pipeline进来. ServerBootstrapAcceptor 独立做的事情不多,更多是交给父类处理。

        ServerBootstrapAcceptor(
final Channel channel, EventLoopGroup childGroup, ChannelHandler childHandler,
Entry<ChannelOption<?>, Object>[] childOptions, Entry<AttributeKey<?>, Object>[] childAttrs) {
this.childGroup = childGroup;
this.childHandler = childHandler;
this.childOptions = childOptions;
this.childAttrs = childAttrs; // Task which is scheduled to re-enable auto-read.
// It's important to create this Runnable before we try to submit it as otherwise the URLClassLoader may
// not be able to load the class because of the file limit it already reached.
//
// See https://github.com/netty/netty/issues/1328
//
enableAutoReadTask = new Runnable() {
@Override
public void run() {
channel.config().setAutoRead(true);
}
};
} // ServerBootstrapAcceptor 大部分情况下都是普通的 InboundHandler, 除了 channelRead() 时
// io.netty.bootstrap.ServerBootstrap.ServerBootstrapAcceptor#channelRead
@Override
@SuppressWarnings("unchecked")
public void channelRead(ChannelHandlerContext ctx, Object msg) {
final Channel child = (Channel) msg; child.pipeline().addLast(childHandler); setChannelOptions(child, childOptions, logger); for (Entry<AttributeKey<?>, Object> e: childAttrs) {
child.attr((AttributeKey<Object>) e.getKey()).set(e.getValue());
} try {
// 它会向 childGroup 中提交channel过去, 从而使用 childGroup 产生作用
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);
}
}

  。。。

4.6 端口的绑定 doBind0

  经过前面的channel的创建,初始化, Acceptor 的添加到handlerAdded(), 整个pipeline已经work起来了. 然后netty会回调之前添加好的 listeners, 其中一个便是 doBind0();

    // 回顾下:
...
// Registration future is almost always fulfilled already, but just in case it's not.
final PendingRegistrationPromise promise = new PendingRegistrationPromise(channel);
regFuture.addListener(new ChannelFutureListener() {
@Override
public void operationComplete(ChannelFuture future) throws Exception {
Throwable cause = future.cause();
if (cause != null) {
// Registration on the EventLoop failed so fail the ChannelPromise directly to not cause an
// IllegalStateException once we try to access the EventLoop of the Channel.
promise.setFailure(cause);
} else {
// Registration was successful, so set the correct executor to use.
// See https://github.com/netty/netty/issues/2586
promise.registered(); doBind0(regFuture, channel, localAddress, promise);
}
}
});
...
// io.netty.bootstrap.AbstractBootstrap#doBind0
private static void doBind0(
final ChannelFuture regFuture, final Channel channel,
final SocketAddress localAddress, final ChannelPromise promise) { // This method is invoked before channelRegistered() is triggered. Give user handlers a chance to set up
// the pipeline in its channelRegistered() implementation.
// 这还是一个异步过程
channel.eventLoop().execute(new Runnable() {
@Override
public void run() {
// channel.bind(), channel 与 端口绑定
if (regFuture.isSuccess()) {
channel.bind(localAddress, promise).addListener(ChannelFutureListener.CLOSE_ON_FAILURE);
} else {
promise.setFailure(regFuture.cause());
}
}
});
}
// io.netty.channel.AbstractChannel#bind(java.net.SocketAddress, io.netty.channel.ChannelPromise)
@Override
public ChannelFuture bind(SocketAddress localAddress, ChannelPromise promise) {
// bind() 被当作一个普通的出站事件, 在pipeline中被传递
return pipeline.bind(localAddress, promise);
} // io.netty.channel.DefaultChannelPipeline#bind(java.net.SocketAddress, io.netty.channel.ChannelPromise)
@Override
public final ChannelFuture bind(SocketAddress localAddress, ChannelPromise promise) {
// 从tail开始传递
return tail.bind(localAddress, promise);
}
// io.netty.channel.AbstractChannelHandlerContext#bind(java.net.SocketAddress, io.netty.channel.ChannelPromise)
@Override
public ChannelFuture bind(final SocketAddress localAddress, final ChannelPromise promise) {
if (localAddress == null) {
throw new NullPointerException("localAddress");
}
if (isNotValidPromise(promise, false)) {
// cancelled
return promise;
}
// 同样是一个pipeline式调用, bind() 是一个出站事件, 所以查找 outbound
// 最终会调到 DefaultChannelPipeline 中
// netty的pipeline机制就体现在这里, 它会一直查找可用的handler, 然后执行它, 直到结束
final AbstractChannelHandlerContext next = findContextOutbound();
// 获取其绑定的 executor
EventExecutor executor = next.executor();
if (executor.inEventLoop()) {
next.invokeBind(localAddress, promise);
} else {
safeExecute(executor, new Runnable() {
@Override
public void run() {
next.invokeBind(localAddress, promise);
}
}, promise, null);
}
return promise;
}
// -------------------------------------------------------------------------
// 出入站handler的查找实现, 非常简单, 却很有效 (该方法在 AbstractChannelHandlerContext 中实现,被所有handler通用)
// io.netty.channel.AbstractChannelHandlerContext#findContextInbound
private AbstractChannelHandlerContext findContextInbound() {
// 以当前节点作为起点开始查找, 取第一个入站handler返回, 没有则说明 pipeline 已结束
AbstractChannelHandlerContext ctx = this;
do {
ctx = ctx.next;
} while (!ctx.inbound);
return ctx;
}
// io.netty.channel.AbstractChannelHandlerContext#findContextOutbound
private AbstractChannelHandlerContext findContextOutbound() {
// 以当前节点作为起点开始查找, 取第一个出站handler返回, 没有则说明 pipeline 已结束
AbstractChannelHandlerContext ctx = this;
do {
ctx = ctx.prev;
} while (!ctx.outbound);
return ctx;
}
// ------------------------------------------------------------------------- // io.netty.channel.AbstractChannelHandlerContext#invokeBind
private void invokeBind(SocketAddress localAddress, ChannelPromise promise) {
if (invokeHandler()) {
try {
((ChannelOutboundHandler) handler()).bind(this, localAddress, promise);
} catch (Throwable t) {
notifyOutboundHandlerException(t, promise);
}
} else {
bind(localAddress, promise);
}
}
// 最终传递到 HeadContext 中进行处理
// io.netty.channel.DefaultChannelPipeline.HeadContext#bind
@Override
public void bind(
ChannelHandlerContext ctx, SocketAddress localAddress, ChannelPromise promise)
throws Exception {
// unsafe 处理bind() 操作
unsafe.bind(localAddress, promise);
}
// io.netty.channel.AbstractChannel.AbstractUnsafe#bind
@Override
public final void bind(final SocketAddress localAddress, final ChannelPromise promise) {
assertEventLoop(); if (!promise.setUncancellable() || !ensureOpen(promise)) {
return;
} // See: https://github.com/netty/netty/issues/576
if (Boolean.TRUE.equals(config().getOption(ChannelOption.SO_BROADCAST)) &&
localAddress instanceof InetSocketAddress &&
!((InetSocketAddress) localAddress).getAddress().isAnyLocalAddress() &&
!PlatformDependent.isWindows() && !PlatformDependent.maybeSuperUser()) {
// Warn a user about the fact that a non-root user can't receive a
// broadcast packet on *nix if the socket is bound on non-wildcard address.
logger.warn(
"A non-root user can't receive a broadcast packet if the socket " +
"is not bound to a wildcard address; binding to a non-wildcard " +
"address (" + localAddress + ") anyway as requested.");
} boolean wasActive = isActive();
try {
// 这里会调用 jdk 的ServerSocketChannel接口, 实现真正的端口绑定
// 至此, 服务对外可见
doBind(localAddress);
} catch (Throwable t) {
safeSetFailure(promise, t);
closeIfClosed();
return;
}
// 判断是否是首次创建 channel, 如果是, 则调用 fireChannelActive() 传播channelActive事件
if (!wasActive && isActive()) {
// 这将会被稍后执行
invokeLater(new Runnable() {
@Override
public void run() {
pipeline.fireChannelActive();
}
});
}
// 触发一些通知什么的, 结束了
safeSetSuccess(promise);
}
// 最终的bind(), 是通过 jdk 底层的 serverSocketChannel 开启socket监听
// io.netty.channel.socket.nio.NioServerSocketChannel#doBind
@Override
protected void doBind(SocketAddress localAddress) throws Exception {
if (PlatformDependent.javaVersion() >= 7) {
// 调用 serverSocketChannel bind() 方法,开启socket监听
javaChannel().bind(localAddress, config.getBacklog());
} else {
javaChannel().socket().bind(localAddress, config.getBacklog());
}
}

  至此, bind 工作总算是完成了.我们来总结下它的主要工作:

    1. 初始化一个channel, 根据设置里来, 我们使用 NioServerSocketChannel;
    2. 过继现有的配置项给到channel;
    3. 将channel与eventloop绑定做注册, 添加 ServerBootstrapAcceptor 到 pipeline 中;
    4. 绑定完成后, 通知现有的handler, 触发系列事件: fireHandlerAdded() -> fireChannelRegistered() -> fireChannelActive();
    5. 而bind()则作为一个出站事件, 被处理, 最终调用 jdk的ServerSocketChannel.register() 完成端口的开启;

  不过有一点需要注意, 在这个过程中, 只有 bossGroup 起作用, 所有的 workGroup 都还在待命中. 我们目前看到的 pipeline 是这样的: head -> Acceptor -> tail;

  讲了这么多, 有一种绕了一大圈的感觉有木有, 如果你自己直接使用nio写, 估计10行代码都不要就搞定了. 尴尬!

5. netty eventloop 主循环

  evenloop是netty的重要概念, 但在前面我们并未细讲这玩意如何起作用(仅看过其创建过程而已), 不过这并不意味着它还没起作用, 而我们暂时忽略了它. 每次要执行任务时, 总是会调用 eventloop().execute(...), 实际上这就是 eventloop的入口:

    // io.netty.util.concurrent.SingleThreadEventExecutor#execute
@Override
public void execute(Runnable task) {
// execute 在线程池中, 是一个异步任务的提交方法, eventloop中同样也一样
// 但是大部分情况下只是添加队列, 因为 eventloop 是单线程的
if (task == null) {
throw new NullPointerException("task");
}
// 向eventLoop队列中添加task
boolean inEventLoop = inEventLoop();
addTask(task);
// 如果自身就是运行在 eventloop 环境中, 添加完task后则不再做更多的事
if (!inEventLoop) {
// 如果不是在eventLoop线程中,则都会尝试创建新线程运行, 但实际会重新检测线程是否创建
startThread();
if (isShutdown() && removeTask(task)) {
reject();
}
} if (!addTaskWakesUp && wakesUpForTask(task)) {
wakeup(inEventLoop);
}
}
// io.netty.util.concurrent.SingleThreadEventExecutor#addTask
/**
* Add a task to the task queue, or throws a {@link RejectedExecutionException} if this instance was shutdown
* before.
*/
protected void addTask(Runnable task) {
if (task == null) {
throw new NullPointerException("task");
}
// taskQueue = MpscUnsafeUnboundedArrayQueue, 基于Unsafe 和 cas 实现的线程安全的队列
if (!offerTask(task)) {
// 添加失败,则走拒绝策略
reject(task);
}
}
// startThread, 看起来是开启线程的意思, 却又不太一样
private void startThread() {
// 所以实际上只会创建一次线程
if (state == ST_NOT_STARTED) {
// 抢到锁的线程才能调用start()方法
if (STATE_UPDATER.compareAndSet(this, ST_NOT_STARTED, ST_STARTED)) {
try {
doStartThread();
} catch (Throwable cause) {
STATE_UPDATER.set(this, ST_NOT_STARTED);
PlatformDependent.throwException(cause);
}
}
}
}
// 开启eventLoop的线程
// io.netty.util.concurrent.SingleThreadEventExecutor#doStartThread
private void doStartThread() {
assert thread == null;
// 它并不是简单的thread.start()
executor.execute(new Runnable() {
@Override
public void run() {
thread = Thread.currentThread();
if (interrupted) {
thread.interrupt();
} boolean success = false;
updateLastExecutionTime();
try {
// 核心方法,由 SingleThreadEventExecutor.run() 实现
// 当然是由具体的executor具体实现了, 此文为 NioEventLoop.run()
SingleThreadEventExecutor.this.run();
success = true;
} catch (Throwable t) {
logger.warn("Unexpected exception from an event executor: ", t);
} finally {
// 线程池关闭,优雅停机
...
}
}
});
}

  核心: 事件循环主框架, 既然是事件循环,则其必然是不会退出的。

    // io.netty.channel.nio.NioEventLoop#run
@Override
protected void run() {
// 一个死循环检测任务, 这就 eventloop 的大杀器哦
for (;;) {
try {
switch (selectStrategy.calculateStrategy(selectNowSupplier, hasTasks())) {
case SelectStrategy.CONTINUE:
continue;
// 有任务时执行任务, 否则阻塞等待网络事件, 或被唤醒
case SelectStrategy.SELECT:
// select.select(), 带超时限制
select(wakenUp.getAndSet(false)); // 'wakenUp.compareAndSet(false, true)' is always evaluated
// before calling 'selector.wakeup()' to reduce the wake-up
// overhead. (Selector.wakeup() is an expensive operation.)
//
// However, there is a race condition in this approach.
// The race condition is triggered when 'wakenUp' is set to
// true too early.
//
// 'wakenUp' is set to true too early if:
// 1) Selector is waken up between 'wakenUp.set(false)' and
// 'selector.select(...)'. (BAD)
// 2) Selector is waken up between 'selector.select(...)' and
// 'if (wakenUp.get()) { ... }'. (OK)
//
// In the first case, 'wakenUp' is set to true and the
// following 'selector.select(...)' will wake up immediately.
// Until 'wakenUp' is set to false again in the next round,
// 'wakenUp.compareAndSet(false, true)' will fail, and therefore
// any attempt to wake up the Selector will fail, too, causing
// the following 'selector.select(...)' call to block
// unnecessarily.
//
// To fix this problem, we wake up the selector again if wakenUp
// is true immediately after selector.select(...).
// It is inefficient in that it wakes up the selector for both
// the first case (BAD - wake-up required) and the second case
// (OK - no wake-up required). if (wakenUp.get()) {
selector.wakeup();
}
// fall through
default:
} cancelledKeys = 0;
needsToSelectAgain = false;
// ioRatio 为io操作的占比, 和运行任务相比, 默认为 50:50
final int ioRatio = this.ioRatio;
if (ioRatio == 100) {
try {
// step1. 运行io操作
processSelectedKeys();
} finally {
// Ensure we always run tasks.
// step2. 运行task任务
runAllTasks();
}
} else {
final long ioStartTime = System.nanoTime();
try {
processSelectedKeys();
} finally {
// Ensure we always run tasks.
final long ioTime = System.nanoTime() - ioStartTime;
// 运行任务的最长时间
runAllTasks(ioTime * (100 - ioRatio) / ioRatio);
}
}
} catch (Throwable t) {
handleLoopException(t);
}
// Always handle shutdown even if the loop processing threw an exception.
try {
if (isShuttingDown()) {
closeAll();
if (confirmShutdown()) {
return;
}
}
} catch (Throwable t) {
handleLoopException(t);
}
}
}
// select, 事件循环的依据
private void select(boolean oldWakenUp) throws IOException {
Selector selector = this.selector;
try {
int selectCnt = 0;
long currentTimeNanos = System.nanoTime();
// 带超时限制, 默认最大超时1s, 但当有延时任务处理时, 以它为标准
long selectDeadLineNanos = currentTimeNanos + delayNanos(currentTimeNanos); for (;;) {
long timeoutMillis = (selectDeadLineNanos - currentTimeNanos + 500000L) / 1000000L;
if (timeoutMillis <= 0) {
// 超时则立即返回
if (selectCnt == 0) {
selector.selectNow();
selectCnt = 1;
}
break;
} // If a task was submitted when wakenUp value was true, the task didn't get a chance to call
// Selector#wakeup. So we need to check task queue again before executing select operation.
// If we don't, the task might be pended until select operation was timed out.
// It might be pended until idle timeout if IdleStateHandler existed in pipeline.
if (hasTasks() && wakenUp.compareAndSet(false, true)) {
selector.selectNow();
selectCnt = 1;
break;
} int selectedKeys = selector.select(timeoutMillis);
selectCnt ++; if (selectedKeys != 0 || oldWakenUp || wakenUp.get() || hasTasks() || hasScheduledTasks()) {
// - Selected something,
// - waken up by user, or
// - the task queue has a pending task.
// - a scheduled task is ready for processing
break;
}
if (Thread.interrupted()) {
// Thread was interrupted so reset selected keys and break so we not run into a busy loop.
// As this is most likely a bug in the handler of the user or it's client library we will
// also log it.
//
// See https://github.com/netty/netty/issues/2426
if (logger.isDebugEnabled()) {
logger.debug("Selector.select() returned prematurely because " +
"Thread.currentThread().interrupt() was called. Use " +
"NioEventLoop.shutdownGracefully() to shutdown the NioEventLoop.");
}
selectCnt = 1;
break;
} long time = System.nanoTime();
if (time - TimeUnit.MILLISECONDS.toNanos(timeoutMillis) >= currentTimeNanos) {
// timeoutMillis elapsed without anything selected.
selectCnt = 1;
} else if (SELECTOR_AUTO_REBUILD_THRESHOLD > 0 &&
selectCnt >= SELECTOR_AUTO_REBUILD_THRESHOLD) {
// The selector returned prematurely many times in a row.
// Rebuild the selector to work around the problem.
logger.warn(
"Selector.select() returned prematurely {} times in a row; rebuilding Selector {}.",
selectCnt, selector); rebuildSelector();
selector = this.selector; // Select again to populate selectedKeys.
selector.selectNow();
selectCnt = 1;
break;
} currentTimeNanos = time;
} if (selectCnt > MIN_PREMATURE_SELECTOR_RETURNS) {
if (logger.isDebugEnabled()) {
logger.debug("Selector.select() returned prematurely {} times in a row for Selector {}.",
selectCnt - 1, selector);
}
}
} catch (CancelledKeyException e) {
if (logger.isDebugEnabled()) {
logger.debug(CancelledKeyException.class.getSimpleName() + " raised by a Selector {} - JDK bug?",
selector, e);
}
// Harmless exception - log anyway
}
}

  反正整体就是这样了, 循环检测select, 运行io事件及execute task.

  有了这个 eventloop, 整体server就可以run起来了, 不管是有外部请求进来, 还是有内部任务提交, 都将被eventloop执行.

  不过还有一点未澄清的: 前面在做channel.register()时传递了一个 ops=0, 那它是如何监听新连接事件的呢?

  实际上它是在注册激活完成之后, 再进行了一个read()的操作, 重新将 OP_ACCEPT 添加到 selectionKey 中了.(没错,底层永远没那么多花招)

        // io.netty.channel.DefaultChannelPipeline.HeadContext#channelActive
@Override
public void channelActive(ChannelHandlerContext ctx) throws Exception {
ctx.fireChannelActive();
// 会触发 read() 流程, 修改 selectionKey 的 ops 标志位
readIfIsAutoRead();
}
...
// io.netty.channel.AbstractChannel.AbstractUnsafe#beginRead
@Override
public final void beginRead() {
assertEventLoop(); if (!isActive()) {
return;
} try {
doBeginRead();
} catch (final Exception e) {
invokeLater(new Runnable() {
@Override
public void run() {
pipeline.fireExceptionCaught(e);
}
});
close(voidPromise());
}
}
// io.netty.channel.nio.AbstractNioMessageChannel#doBeginRead
@Override
protected void doBeginRead() throws Exception {
if (inputShutdown) {
return;
}
super.doBeginRead();
}
// io.netty.channel.nio.AbstractNioChannel#doBeginRead
@Override
protected void doBeginRead() throws Exception {
// Channel.read() or ChannelHandlerContext.read() was called
final SelectionKey selectionKey = this.selectionKey;
if (!selectionKey.isValid()) {
return;
} readPending = true; final int interestOps = selectionKey.interestOps();
if ((interestOps & readInterestOp) == 0) {
// readInterestOp, 即是前面设置的 OP_ACCEPT
selectionKey.interestOps(interestOps | readInterestOp);
}
}

  本文有点长了, 留点东西下篇继续: io事件如何处理? 任务如何执行?

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