netty分析(一) -- 服務啟動流程

Netty · 發表 2018-09-17 11:23:32

如果還不瞭解原生nio的socket程式設計，可以看前置博文

一個簡單的Demo程式

先貼一個簡單的netty的example中echo服務端程式碼

/*
 * Copyright 2012 The Netty Project
 *
 * The Netty Project licenses this file to you under the Apache License,
 * version 2.0 (the "License"); you may not use this file except in compliance
 * with the License. You may obtain a copy of the License at:
 *
 *http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
 * WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
 * License for the specific language governing permissions and limitations
 * under the License.
 */
package io.netty.example.echo;

import io.netty.bootstrap.ServerBootstrap;
import io.netty.channel.ChannelFuture;
import io.netty.channel.ChannelInitializer;
import io.netty.channel.ChannelOption;
import io.netty.channel.ChannelPipeline;
import io.netty.channel.EventLoopGroup;
import io.netty.channel.nio.NioEventLoopGroup;
import io.netty.channel.socket.SocketChannel;
import io.netty.channel.socket.nio.NioServerSocketChannel;
import io.netty.handler.logging.LogLevel;
import io.netty.handler.logging.LoggingHandler;
import io.netty.handler.ssl.SslContext;
import io.netty.handler.ssl.SslContextBuilder;
import io.netty.handler.ssl.util.SelfSignedCertificate;

/**
 * Echoes back any received data from a client.
 */
public final class EchoServer {

static final boolean SSL = System.getProperty("ssl") != null;
static final int PORT = Integer.parseInt(System.getProperty("port", "8007"));

public static void main(String[] args) throws Exception {
// Configure SSL.
final SslContext sslCtx;
if (SSL) {
SelfSignedCertificate ssc = new SelfSignedCertificate();
sslCtx = SslContextBuilder.forServer(ssc.certificate(), ssc.privateKey()).build();
} else {
sslCtx = null;
}

// Configure the server.
EventLoopGroup bossGroup = new NioEventLoopGroup(1);
EventLoopGroup workerGroup = new NioEventLoopGroup();
final EchoServerHandler serverHandler = new EchoServerHandler();
try {
ServerBootstrap b = new ServerBootstrap();
b.group(bossGroup, workerGroup)
.channel(NioServerSocketChannel.class)
.option(ChannelOption.SO_BACKLOG, 100)
.handler(new LoggingHandler(LogLevel.INFO))
.childHandler(new ChannelInitializer<SocketChannel>() {
@Override
public void initChannel(SocketChannel ch) throws Exception {
ChannelPipeline p = ch.pipeline();
if (sslCtx != null) {
p.addLast(sslCtx.newHandler(ch.alloc()));
}
//p.addLast(new LoggingHandler(LogLevel.INFO));
p.addLast(serverHandler);
}
});

// Start the server.
ChannelFuture f = b.bind(PORT).sync();

// Wait until the server socket is closed.
f.channel().closeFuture().sync();
} finally {
// Shut down all event loops to terminate all threads.
bossGroup.shutdownGracefully();
workerGroup.shutdownGracefully();
}
}
}

程式碼很簡潔，但是看不懂，因為使用的這些類均和Nio原生程式設計相差甚遠,下面先簡單分析一下。

ServerBootstrap b = new ServerBootstrap();
b.group(bossGroup, workerGroup);

此處首先是新建了一個ServerBootstrap 啟動類，分別設定好boss和worker工作執行緒。

b.channel(NioServerSocketChannel.class);

此處是設定channel的型別，內部會以建立一個ServerBootstrapChannelFactory工廠來儲存class，用於後續物件建立。

b.option(ChannelOption.SO_BACKLOG, 100);

此處設定了客戶端連線socket屬性。

b.childHandler(new ChannelInitializer<SocketChannel>() {
@Override
public void initChannel(SocketChannel ch) throws Exception {
ChannelPipeline p = ch.pipeline();
if (sslCtx != null) {
p.addLast(sslCtx.newHandler(ch.alloc()));
}
//p.addLast(new LoggingHandler(LogLevel.INFO));
p.addLast(serverHandler);
}
});

此處設定了客戶端連線建立以後對SocketChannel的初始化邏輯。

以上的程式碼均是給ServerBootstrap物件的各個引數賦值，真正讓netty跑起來的重點在下面程式碼。

ChannelFuture f = b.bind(port).sync();

閱讀這段程式碼之前，我們留個懸念，我們需要先了解另一個類:NioEventLoop。瞭解了NioEventLoop，netty中的執行緒模型就清晰起來了，後續分析將不會太費力。

NioEventLoop

NioEventLoop是與jdk層nio互動的最重要的物件，是在NioEventLoopGroup物件中創建出來的。

NioEventGroup內部有個名為children的陣列，我們把它理解成一個頭尾相連的環，每次我們呼叫NioEventLoopGroup.next()方法時，會返回這個環的下一個元素。這個元素就是一個NioEventLoop。

這個children的大小由什麼決定呢？答案就是NioEventLoopGroup物件構造時傳入的執行緒數量。

接下來我們來看看NioEventLoop的具體實現,建構函式

NioEventLoop(NioEventLoopGroup parent, Executor executor, SelectorProvider selectorProvider) {
super(parent, executor, false);
if (selectorProvider == null) {
throw new NullPointerException("selectorProvider");
}
provider = selectorProvider;
selector = openSelector();
}

看一下openSelector()的實現。

private Selector openSelector() {
final Selector selector;
try {
selector = provider.openSelector();
} catch (IOException e) {
throw new ChannelException("failed to open a new selector", e);
}

if (DISABLE_KEYSET_OPTIMIZATION) {
return selector;
}

try {
SelectedSelectionKeySet selectedKeySet = new SelectedSelectionKeySet();

Class<?> selectorImplClass =
Class.forName("sun.nio.ch.SelectorImpl", false, ClassLoader.getSystemClassLoader());

// Ensure the current selector implementation is what we can instrument.
if (!selectorImplClass.isAssignableFrom(selector.getClass())) {
return selector;
}

Field selectedKeysField = selectorImplClass.getDeclaredField("selectedKeys");
Field publicSelectedKeysField = selectorImplClass.getDeclaredField("publicSelectedKeys");

selectedKeysField.setAccessible(true);
publicSelectedKeysField.setAccessible(true);

selectedKeysField.set(selector, selectedKeySet);
publicSelectedKeysField.set(selector, selectedKeySet);

selectedKeys = selectedKeySet;
logger.trace("Instrumented an optimized java.util.Set into: {}", selector);
} catch (Throwable t) {
selectedKeys = null;
logger.trace("Failed to instrument an optimized java.util.Set into: {}", selector, t);
}

return selector;
}

建構函式呼叫了provider.openSelector()來產生一個多路複用選擇器物件。

jdk原生Nio實現中，selector內部有一個HashSet物件selectedKeys，用來儲存呼叫select函式之後的結果集。如果未禁用優化，此處還利用反射將selector內部的selectedKeys值設定成本地物件。這麼做有一個好處，每次呼叫Selector的select函式以後，能很方便的檢視selectedKeys的值以確認是否產生了發生了新的事件。

外界可以呼叫NioEventLoop的execute方法來放入任務，檢視其實現。

public void execute(Runnable task) {
if (task == null) {
throw new NullPointerException("task");
}

boolean inEventLoop = inEventLoop();
if (inEventLoop) {
addTask(task);
} else {
startThread();
addTask(task);
if (isShutdown() && removeTask(task)) {
reject();
}
}

if (!addTaskWakesUp) {
wakeup(inEventLoop);
}
}

private void startThread() {
synchronized (stateLock) {
if (state == ST_NOT_STARTED) {
state = ST_STARTED;
delayedTaskQueue.add(new ScheduledFutureTask<Void>(
this, delayedTaskQueue, Executors.<Void>callable(new PurgeTask(), null),
ScheduledFutureTask.deadlineNanos(SCHEDULE_PURGE_INTERVAL), -SCHEDULE_PURGE_INTERVAL));
doStartThread();
}
}
}

NioEventLoop內部有一個狀態變數state，這保證了在呼叫startThread方法時，只會呼叫一次doStartThread。而doStartThread，在首次呼叫的時候，會建立新的執行緒,檢視doStartThread

private void doStartThread() {
assert thread == null;
executor.execute(new Runnable() {
@Override
public void run() {
thread = Thread.currentThread();
if (interrupted) {
thread.interrupt();
}

boolean success = false;
updateLastExecutionTime();
try {
SingleThreadEventExecutor.this.run();
success = true;
} catch (Throwable t) {
logger.warn("Unexpected exception from an event executor: ", t);
} finally {
if (state < ST_SHUTTING_DOWN) {
state = ST_SHUTTING_DOWN;
}

// Check if confirmShutdown() was called at the end of the loop.
if (success && gracefulShutdownStartTime == 0) {
logger.error("Buggy " + EventExecutor.class.getSimpleName() + " implementation; " +
SingleThreadEventExecutor.class.getSimpleName() + ".confirmShutdown() must be called " +
"before run() implementation terminates.");
}

try {
// Run all remaining tasks and shutdown hooks.
for (;;) {
if (confirmShutdown()) {
break;
}
}
} finally {
try {
cleanup();
} finally {
synchronized (stateLock) {
state = ST_TERMINATED;
}
threadLock.release();
if (!taskQueue.isEmpty()) {
logger.warn(
"An event executor terminated with " +
"non-empty task queue (" + taskQueue.size() + ')');
}

terminationFuture.setSuccess(null);
}
}
}
}
});
}

主角是executor，看下其預設實現。

public final class ThreadPerTaskExecutor implements Executor {
private final ThreadFactory threadFactory;

public ThreadPerTaskExecutor(ThreadFactory threadFactory) {
if (threadFactory == null) {
throw new NullPointerException("threadFactory");
}
this.threadFactory = threadFactory;
}

@Override
public void execute(Runnable command) {
threadFactory.newThread(command).start();
}
}

可以看到，每次呼叫executor的execute方法將會產生一個新的執行緒，實際上只調用了一次doStartThread，所以只會建立一個執行緒。

新執行緒最後呼叫到了"SingleThreadEventExecutor.this.run();"。好了，我們離真相已經很近了。貼一下run的實現。

protected void run() {
for (;;) {
oldWakenUp = wakenUp.getAndSet(false);
try {
if (hasTasks()) {
selectNow();
} else {
select();

// '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();
}
}

cancelledKeys = 0;

final long ioStartTime = System.nanoTime();
needsToSelectAgain = false;
if (selectedKeys != null) {
processSelectedKeysOptimized(selectedKeys.flip());
} else {
processSelectedKeysPlain(selector.selectedKeys());
}
final long ioTime = System.nanoTime() - ioStartTime;

final int ioRatio = this.ioRatio;
runAllTasks(ioTime * (100 - ioRatio) / ioRatio);

if (isShuttingDown()) {
closeAll();
if (confirmShutdown()) {
break;
}
}
} catch (Throwable t) {
logger.warn("Unexpected exception in the selector loop.", t);

// Prevent possible consecutive immediate failures that lead to
// excessive CPU consumption.
try {
Thread.sleep(1000);
} catch (InterruptedException e) {
// Ignore.
}
}
}
}

run方法,是一個死迴圈，做的事情就是週期執行Selector的select函式獲取事件並處理，以及執行一些拋進佇列的任務。

1. select()/selectNow(),檢視函式內部， 執行了原生Selector的select方法 ,第一步已經浮出水面了，根據nio的呼叫流程（詳細程式碼在這篇博文中有），下一步應該就是ServerSocketChannel呼叫accept函式來接受客戶端連結了，讓我們找一下。
2.如果select呼叫之後有事件發生。那麼selectedKeys將發生改變(注意selectedKeys變數實際是指向底層Selector的觸發事件集合的引用),此時進入processSelectedKeysOptimized函式處理：

private void processSelectedKeysOptimized(SelectionKey[] selectedKeys) {
for (int i = 0;; i ++) {
final SelectionKey k = selectedKeys[i];
if (k == null) {
break;
}

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) {
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;
}
}
}

進一步看processSelectedKey

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 e) {
unsafe.close(unsafe.voidPromise());
}
}

當客戶端觸發連線的時候，readyOps應該是16 ，對應著SelectionKey.OP_ACCEPT（如果觸發了OP_READ，那麼將觸發讀取客戶端資料操作，這個在下篇博文中再詳盡分析，地址），進一步檢視unsafe.read()中呼叫的doReadMessages方法。

public void read() {
assert eventLoop().inEventLoop();
if (!config().isAutoRead()) {
removeReadOp();
}

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());
}
}
}

@Override
protected int doReadMessages(List<Object> buf) throws Exception {
SocketChannel ch = javaChannel().accept();

try {
if (ch != null) {
buf.add(new NioSocketChannel(this, childEventLoopGroup().next(), 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;
}

由此我們也找到了accept，藏的還挺深,呼叫accept之後我們拿到具體對接客戶端連線的socket繫結到一個work執行緒，放入list buf中。接著我們回到上層的read方法。

一步步呼叫到了這裡。

設定SocketChannel的pipline屬性堆疊

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());
}

child.unsafe().register(child.newPromise());
}

首先是這句 "child.pipeline().addLast(childHandler);" 很熟悉不是嗎，childHandler是開頭我們呼叫ServerBootstrap的childHandler方法傳入的處理物件,接下來設定好socket屬性

檢視register實現。

public final void register(final ChannelPromise promise) {
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);
}
}
}

向eventLoop投遞了一個register事件，在eventLoop（NioEventLoop）執行緒(此時的eventLoop是workerGroup中的執行緒)中，將會把這個SocketChannel也註冊到eventLoop中的selector，注意到這裡實現和我們原生的nio呼叫有區別，每個執行緒都啟用了一個Selector物件來輪詢事件。

看看bind做了什麼

public ChannelFuture bind(SocketAddress localAddress) {
validate();//判斷引數合法性
if (localAddress == null) {
throw new NullPointerException("localAddress");
}
return doBind(localAddress);
}

看doBind

private ChannelFuture doBind(final SocketAddress localAddress) {
final ChannelFuture regFuture = initAndRegister();
final Channel channel = regFuture.channel();
if (regFuture.cause() != null) {
return regFuture;
}

final ChannelPromise promise;
if (regFuture.isDone()) {
promise = channel.newPromise();
doBind0(regFuture, channel, localAddress, promise);
} else {
// Registration future is almost always fulfilled already, but just in case it's not.
promise = new DefaultChannelPromise(channel, GlobalEventExecutor.INSTANCE);
regFuture.addListener(new ChannelFutureListener() {
@Override
public void operationComplete(ChannelFuture future) throws Exception {
doBind0(regFuture, channel, localAddress, promise);
}
});
}

return promise;
}

初始化了一個Channel，並將其繫結到boss執行緒。我們進一步看下initAndRegister

final ChannelFuture initAndRegister() {
Channel channel;
try {
channel = createChannel();
} catch (Throwable t) {
return VoidChannel.INSTANCE.newFailedFuture(t);
}

try {
init(channel);
} catch (Throwable t) {
channel.unsafe().closeForcibly();
return channel.newFailedFuture(t);
}

ChannelPromise regFuture = channel.newPromise();
channel.unsafe().register(regFuture);
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 beause 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;
}

進一步分為三個步驟，createChannel，init和register。

Channel createChannel() {
EventLoop eventLoop = group().next();
return channelFactory().newChannel(eventLoop, childGroup);
}

void init(Channel channel) throws Exception {
final Map<ChannelOption<?>, Object> options = options();
synchronized (options) {
channel.config().setOptions(options);
}

final Map<AttributeKey<?>, Object> attrs = attrs();
synchronized (attrs) {
for (Entry<AttributeKey<?>, Object> e: attrs.entrySet()) {
@SuppressWarnings("unchecked")
AttributeKey<Object> key = (AttributeKey<Object>) e.getKey();
channel.attr(key).set(e.getValue());
}
}

ChannelPipeline p = channel.pipeline();
if (handler() != null) {
p.addLast(handler());
}

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()));
}

p.addLast(new ChannelInitializer<Channel>() {
@Override
public void initChannel(Channel ch) throws Exception {
ch.pipeline().addLast(new ServerBootstrapAcceptor(currentChildHandler, currentChildOptions,
currentChildAttrs));
}
});
}

根據createChannel的實現所示，ServerBootstrap.channel設定進來的Channel型別派上用場了。這裡將bossGroup中的NioeventLoop繫結到

創建出來的channel中，為什麼也同時綁了workerGroup呢，因為這個ServerChannel接收到的客戶端連線要拋給指定的worker處理呀。

init函式完成了setoption，及給ServerChannel的pipline綁定了對於的處理ChannelHandler。

接下來我們著重看下register的實現。

public final void register(final ChannelPromise promise) {
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);
}
}
}

終於看到了呼叫了eventLoop.execute方法。這裡由於不是Eventloop的內部執行緒因此會走到execute的邏輯。結合我們之前對NioEventLoop的分析，首次呼叫會建立一個新的執行緒來執行投遞進去Runnable物件的run方法，最後執行了ServerChannel的註冊邏輯。注意到傳進去的promise是一個future物件，在註冊成功以後，可以由其他執行緒通過promise看到是否執行完成

至此，我們總結一下。

ServerBootstrap設定了兩個執行緒組，bossGroup和workerGroup，每個執行緒內部均有一個selector迴圈地執行select函式來查詢監聽的事件。正常場景下，我們應該只有一個監聽埠，此時bossGroup僅有一個執行緒在工作。
boss執行緒的selector只綁定了一個ServerSocketChannel，當其accept到一個客戶端連線以後，會呼叫執行緒組的next()函式獲取一個NioEventLoop來將SocketChannel放入worker中執行邏輯。
同時NioEventLoop還有一個execute方法，支援了其他執行緒往內部執行緒拋入Runnable任務。這個主要場景是boss執行緒檢測到有新連線到來時，將channel註冊到worker執行緒組。以及使用者執行緒函式在呼叫ServerBootstrap的bind
時註冊serverChannel到boss執行緒。

還需要擴充套件認識的部分

還是有許多疑惑，資料的拆分包的實現原理是怎樣的,ChannelHandler處理資料的流程，新增多個ChannelHandler時如何工作。下回合分析。

netty分析(一) -- 服務啟動流程

一個簡單的Demo程式

NioEventLoop

還需要擴充套件認識的部分

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