引言
从本文开始，我们一起来阅读Netty的源码实现，主要针对Netty的核心实现进行进一步的梳理。但是话又说回来，如果我们直接看Netty源码的话，可能不知道该如何下手，大大小小那么多个源码包，就像一团乱麻，千头万绪。所以本文从Netty服务启动开始，根据启动流程来逐渐打开Netty的神秘面纱，理清Netty的技术脉络。
NioEventLoop创建- 总结
今天是周末，但是生物钟还是准时叫醒了疲惫的自己。不上班的日子，那就好好类写博客吧，哈哈。
下图是Netty的Reactor线程模型示意图，可以方便大家对于这部分内容的理解，后续的文章中这种图还会持续出现，本文主要涉及的内容包含了其中的几个步骤。
（图片来自于网络）
一、NioEventLoop创建
Netty服务启动，最重要的事情就是做好接受客户端连接的准备。我们分别看下自定义线程以及boss线程所要完成的任务以及流程，如下所示：
这里罗列了一些比较重要的类名称及其对应的主要功能说明，以便于大家在阅读源码时可以快速了解对应类的功能：
| 类 | 作用 |
|---|---|
| ServerBootstrap | 服务端启动辅助类 |
| NioEventLoop | Nio事件处理器（Reactor模型） |
| NioEventLoopGroup | 一组Nio事件处理器（Reactor模型） |
在Netty源码包中，我们首先看下 io.netty.example.echo包中的EchoServer 类，该类中包含了Netty服务启动相关代码，具体源码以及源码部分分析注释如下所示：
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;
}
//处理连接事件
EventLoopGroup bossGroup = new NioEventLoopGroup(1);
//处理读以及写的事件
EventLoopGroup workerGroup = new NioEventLoopGroup();
final EchoServerHandler serverHandler = new EchoServerHandler();
try {
//服务引导类
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 {
//初始化channel.pipeline
ChannelPipeline p = ch.pipeline();
if (sslCtx != null) {
p.addLast(sslCtx.newHandler(ch.alloc()));
}
//p.addLast(new LoggingHandler(LogLevel.INFO));
p.addLast(serverHandler);
}
});
// 启动服务，绑定端口
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();
}
}
}
在启动Netty服务过程中，创建了两个EventLoopGroup 对象，这两个对象是Netty的核心对象，其中bossGroup 用于接收请求，workerGroup用于处理请求。如下所示：
EventLoopGroup bossGroup = new NioEventLoopGroup(1);
EventLoopGroup workerGroup = new NioEventLoopGroup();
我们可以看下NioEventLoopGroup的类继承结构情况，如下所示：
从上图可知，NioEventLoopGroup实现了ScheduledExecutorService接口，因此它可以实现定时任务相关的功能。同时它还继承了SingleThreadEventExecutor类，从类名可以看出，这是一个单线程的线程执行器。在Netty中，通过NioEventLoopGroup的创建来达到创建NioEventLoop的目的。通过有参参数构建NioEventLoopGroup对象，实际调用的是MultithreadEventExecutorGroup的构造方法，如下所示：
protected MultithreadEventExecutorGroup(int nThreads, Executor executor,
EventExecutorChooserFactory chooserFactory, Object... args) {
if (nThreads <= 0) {
throw new IllegalArgumentException(String.format("nThreads: %d (expected: > 0)", nThreads));
}
if (executor == null) {
//ewDefaultThreadFactory()会创建一个线程工厂，该线程工厂的作用就是用来创建线程，同时给线程设置名称：nioEventLoop-1-XX
executor = new ThreadPerTaskExecutor(newDefaultThreadFactory());
}
// 根据传进来的线程数，来创建指定大小的数组大小，这个数组就是用来存放NioEventLoop对象实例
children = new EventExecutor[nThreads];
for (int i = 0; i < nThreads; i ++) {
//异常标志
boolean success = false;
try {
////创建nThreads个nioEventLoop保存到children数组中
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 {
//异常处理
if (!success) {
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;
}
}
}
}
}
// 通过线程执行器选择工厂来创建一个线程执行器
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);
}
对应的参数解析，如下表所示：
| 参数 | 说明 |
|---|---|
| nThreads | 创建的线程数量 |
| executor | 线程执行器（用户可自定义，没有则为null，后续进行初始化） |
| chooserFactory | 事件执行选择工厂 |
EventLoopGroup 在创建的时候会调用NioEventLoop 中的openSelector()方法。
EventLoopGroup创建本质就是创建多个NioEventLoop，这里创建NioEventLoop就是初始化一个Reactor，包括selector和taskQueue。
NioEventLoop 源码如下：
public final class NioEventLoop extends SingleThreadEventLoop {
...
private SelectorTuple openSelector() {
final Selector unwrappedSelector;
try {
unwrappedSelector = provider.openSelector();
} catch (IOException e) {
throw new ChannelException("failed to open a new selector", e);
}
if (DISABLE_KEY_SET_OPTIMIZATION) {
return new SelectorTuple(unwrappedSelector);
}
Object maybeSelectorImplClass = AccessController.doPrivileged(new PrivilegedAction<Object>() {
@Override
public Object run() {
try {
return Class.forName(
"sun.nio.ch.SelectorImpl",
false,
PlatformDependent.getSystemClassLoader());
} catch (Throwable cause) {
return cause;
}
}
});
if (!(maybeSelectorImplClass instanceof Class) ||
// ensure the current selector implementation is what we can instrument.
!((Class<?>) maybeSelectorImplClass).isAssignableFrom(unwrappedSelector.getClass())) {
if (maybeSelectorImplClass instanceof Throwable) {
Throwable t = (Throwable) maybeSelectorImplClass;
logger.trace("failed to instrument a special java.util.Set into: {}", unwrappedSelector, t);
}
return new SelectorTuple(unwrappedSelector);
}
final Class<?> selectorImplClass = (Class<?>) maybeSelectorImplClass;
final SelectedSelectionKeySet selectedKeySet = new SelectedSelectionKeySet();
Object maybeException = AccessController.doPrivileged(new PrivilegedAction<Object>() {
@Override
public Object run() {
try {
Field selectedKeysField = selectorImplClass.getDeclaredField("selectedKeys");
Field publicSelectedKeysField = selectorImplClass.getDeclaredField("publicSelectedKeys");
if (PlatformDependent.javaVersion() >= 9 && PlatformDependent.hasUnsafe()) {
// Let us try to use sun.misc.Unsafe to replace the SelectionKeySet.
// This allows us to also do this in Java9+ without any extra flags.
long selectedKeysFieldOffset = PlatformDependent.objectFieldOffset(selectedKeysField);
long publicSelectedKeysFieldOffset =
PlatformDependent.objectFieldOffset(publicSelectedKeysField);
if (selectedKeysFieldOffset != -1 && publicSelectedKeysFieldOffset != -1) {
PlatformDependent.putObject(
unwrappedSelector, selectedKeysFieldOffset, selectedKeySet);
PlatformDependent.putObject(
unwrappedSelector, publicSelectedKeysFieldOffset, selectedKeySet);
return null;
}
// We could not retrieve the offset, lets try reflection as last-resort.
}
Throwable cause = ReflectionUtil.trySetAccessible(selectedKeysField, true);
if (cause != null) {
return cause;
}
cause = ReflectionUtil.trySetAccessible(publicSelectedKeysField, true);
if (cause != null) {
return cause;
}
selectedKeysField.set(unwrappedSelector, selectedKeySet);
publicSelectedKeysField.set(unwrappedSelector, selectedKeySet);
return null;
} catch (NoSuchFieldException e) {
return e;
} catch (IllegalAccessException e) {
return e;
}
}
});
if (maybeException instanceof Exception) {
selectedKeys = null;
Exception e = (Exception) maybeException;
logger.trace("failed to instrument a special java.util.Set into: {}", unwrappedSelector, e);
return new SelectorTuple(unwrappedSelector);
}
selectedKeys = selectedKeySet;
logger.trace("instrumented a special java.util.Set into: {}", unwrappedSelector);
return new SelectorTuple(unwrappedSelector,
new SelectedSelectionKeySetSelector(unwrappedSelector, selectedKeySet));
}
...
}
二、总结
本文主要通过Netty的服务的启动代码，初步了解了Netty在启动过程中做了哪些事情。重点分析了NioEventLoop的创建过程，之后的文章再介绍启动过程中，Netty的其他操作。
