一. 代码下载

Netty代码下载和编译参考前一篇Netty文章
https://blog.51cto.com/483181/2112163

二. 服务器代码分析

2.1 服务器代码编写

一般Netty服务器端这样编写

    EventLoopGroup bossGroup = new NioEventLoopGroup();   //1. 实例化NioEventLoopGroup对象EventLoopGroup workerGroup = new NioEventLoopGroup();try {ServerBootstrap b = new ServerBootstrap(); //2. b.group(bossGroup, workerGroup) //3. .channel(NioServerSocketChannel.class).option(ChannelOption.SO_BACKLOG, 100).handler(new LoggingHandler(LogLevel.INFO)).childHandler(new ChannelInitializer<SocketChannel>() {@Overrideprotected void initChannel(SocketChannel ch) throws Exception {ch.pipeline().addLast(new FixedLengthFrameDecoder(20));}});ChannelFuture f = b.bind(port).sync(); //4.f.channel().closeFuture().sync();} catch (Exception e) {e.printStackTrace();} finally {bossGroup.shutdownGracefully();workerGroup.shutdownGracefully();}

2.2 NioEventLoopGroup

2.2.1 NioEventLoopGroup继承关系

一步步来看，首先看第一个注释，初始化NioEventLoopGroup对象

EventLoopGroup bossGroup = new NioEventLoopGroup();   //1. 实例化NioEventLoopGroup对象

下图是NioEventLoopGroup的类继承图，包含类成员和方法，比较详细。 这个功能是IntelliJ 自带的。
右击NioEventLoopGroup类名，选择Diagrams->Show Diagram->上面有f，m的按钮，分别对应field和method。

如下：

2.2.2 NioEventLoopGroup构造函数

public NioEventLoopGroup() {this(0);}public NioEventLoopGroup(int nThreads) {this(nThreads, (Executor) null);}public NioEventLoopGroup(int nThreads, Executor executor) {this(nThreads, executor, 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());}

我们可以看到几点

1. NioEventLoopGroup采用的是构造函数重载的方式，以适应不同的初始化场景
2. Executor传的是null
3. SelectorProvider用的是SelectorProvider.provider()
4. 然后把构造好的参数都传给父类MultithreadEventLoopGroup (继承关系可以看上图)

2.2.3 SelectorProvider.provider()

private static SelectorProvider provider = null;public static SelectorProvider provider() {synchronized (lock) {if (provider != null)return provider;return AccessController.doPrivileged(new PrivilegedAction<SelectorProvider>() {public SelectorProvider run() {if (loadProviderFromProperty())return provider;if (loadProviderAsService())return provider;provider = sun.nio.ch.DefaultSelectorProvider.create();return provider;}});}}public class DefaultSelectorProvider {private DefaultSelectorProvider() {}public static SelectorProvider create() {return new KQueueSelectorProvider();}
}public class KQueueSelectorProvider extends SelectorProviderImpl {public KQueueSelectorProvider() {}public AbstractSelector openSelector() throws IOException {return new KQueueSelectorImpl(this);}
}

这段代码我们也可以看到几点:

1. SelectorProvider provider是一个单例,static类型
2. SelectorProvider.provider的实现，产生了一个KQueueSelectorProvider
3. KQueueSelectorProvider的openSelector会生成一个KQueueSelectorImpl

这个先记下来，也许后面分析会有用，继续分析MultithreadEventLoopGroup的构造函数。

2.2.4 MultithreadEventLoopGroup

protected MultithreadEventLoopGroup(int nThreads, ThreadFactory threadFactory, Object... args) {super(nThreads == 0 ? DEFAULT_EVENT_LOOP_THREADS : nThreads, threadFactory, args);}private static final int DEFAULT_EVENT_LOOP_THREADS;static {DEFAULT_EVENT_LOOP_THREADS = Math.max(1, SystemPropertyUtil.getInt("io.netty.eventLoopThreads", NettyRuntime.availableProcessors() * 2));}       

上面这段代码我们可以看到这几点：

1. 如果我们实例化NioEventLoopGroup没有传入参数，也就是没有nThreads，那么就会采用默认的DEFAULT_EVENT_LOOP_THREADS
   DEFAULT_EVENT_LOOP_THREADS如果没有配置io.netty.eventLoopThreads的话，一般是cpu核数*2
2. MultithreadEventLoopGroup的实例化方法是继续调用父类的初始化方法。

继续父类MultithreadEventExecutorGroup

2.2.5 MultithreadEventExecutorGroup

protected MultithreadEventExecutorGroup(int nThreads, Executor executor,EventExecutorChooserFactory chooserFactory, Object... args) {...children = new EventExecutor[nThreads]; //1. 实例化children数组for (int i = 0; i < nThreads; i ++) { //2. 循环初始化childrenboolean success = false;try {children[i] = newChild(executor, args);success = true;} catch (Exception e) {throw new IllegalStateException("failed to create a child event loop", e);} finally {...}}chooser = chooserFactory.newChooser(children); //3. 实例化chooserfinal FutureListener<Object> terminationListener = new FutureListener<Object>() {@Overridepublic 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);}

上面这段代码可以从下面几个点分析：

private final EventExecutor[] children;

1. children - EventExecutor数组，大小是nThreads，线程数目。
2. newChild初始化
   实例类是NioEventLoopGroup.java，返回NioEventLoop对象

   protected EventLoop newChild(Executor executor, Object... args) throws Exception {return new NioEventLoop(this, executor, (SelectorProvider) args[0],((SelectStrategyFactory) args[1]).newSelectStrategy(), (RejectedExecutionHandler) args[2]);
   }

   NioEventLoop的继承关系是这样的，继承于SingleThreadEventLoop，别忘了上面我们看到NioEventLoopGroup继承自MultithreadEventLoopGroup.(看名字是单线程和多线程的区别？)

继续看NioEventLoop的构造函数

2.2.6 NioEventLoop

NioEventLoop(NioEventLoopGroup parent, Executor executor, SelectorProvider selectorProvider,SelectStrategy strategy, RejectedExecutionHandler rejectedExecutionHandler) {super(parent, executor, false, DEFAULT_MAX_PENDING_TASKS, rejectedExecutionHandler);provider = selectorProvider;final SelectorTuple selectorTuple = openSelector();selector = selectorTuple.selector;unwrappedSelector = selectorTuple.unwrappedSelector;selectStrategy = strategy;}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_KEYSET_OPTIMIZATION) {return new SelectorTuple(unwrappedSelector);}...
}

从上面这段代码我们可以看出这几点

1. NioEventLoop里面保存了SelectorProvider selectorProvider, Selector selector, unwrappedSelector(类型是KQueueSelectorImpl)

2. selector, unwrappedSelector是通过provider.openSelector()打开的.
   根据2.3段的介绍，provider之前介绍的类型是KQueueSelectorProvider，然后它的openSelector会生成一个KQueueSelectorImpl
   所以provider.openSelector()得到是KQueueSelectorImpl，KQueueSelectorImpl的继承关系如下：

   

继续往回看，看MultithreadEventExecutorGroup的构造函数。

2.2.7 newChooser

EventExecutorChooserFactory.EventExecutorChooser chooser;protected MultithreadEventExecutorGroup(int nThreads, Executor executor, Object... args) {this(nThreads, executor, DefaultEventExecutorChooserFactory.INSTANCE, args);}chooser = chooserFactory.newChooser(children);

上面代码我们可以看到：

1. chooserFactory的类型是DefaultEventExecutorChooserFactory，所以newChooser调用的是DefaultEventExecutorChooserFactory.newChooser方法。
   如下：

public EventExecutorChooser newChooser(EventExecutor[] executors) {if (isPowerOfTwo(executors.length)) {return new PowerOfTwoEventExecutorChooser(executors);} else {return new GenericEventExecutorChooser(executors);}}

1. 传入的参数是children，也就是NioEventLoop数组
2. DefaultEventExecutorChooserFactory INSTANCE是一个static final类型对象，也就是一种饿汉式的单例模式，如下：

public static final DefaultEventExecutorChooserFactory INSTANCE = new DefaultEventExecutorChooserFactory();

继续看newChooser的实现

2.2.8 newChooser

newChooser的代码就不贴了，上面就有，从上面代码可以看到：

1. isPowerOfTwo是用来判断一个整数是否是2的幂，比如(2,4, 8,16,32等等)，它的实现方式如下：

private static boolean isPowerOfTwo(int val) {return (val & -val) == val;}private static final class PowerOfTwoEventExecutorChooser implements EventExecutorChooser {private final AtomicInteger idx = new AtomicInteger();private final EventExecutor[] executors;PowerOfTwoEventExecutorChooser(EventExecutor[] executors) {this.executors = executors;}@Overridepublic EventExecutor next() {return executors[idx.getAndIncrement() & executors.length - 1];}}private static final class GenericEventExecutorChooser implements EventExecutorChooser {private final AtomicInteger idx = new AtomicInteger();private final EventExecutor[] executors;GenericEventExecutorChooser(EventExecutor[] executors) {this.executors = executors;}@Overridepublic EventExecutor next() {return executors[Math.abs(idx.getAndIncrement() % executors.length)];}}       

这种实现方法感觉比较优雅和高效，首先拿到-val，也就是val的二进制倒转，然后+1。再做&运算。
大家自己可以拿到数字举个例子，比较巧妙。后续自己写代码可以借鉴，这是读源码的一个好处，可以学习到别人很多优秀的写法。

1. PowerOfTwoEventExecutorChooser和GenericEventExecutorChooser的不同之处在于next方法的算法不一样，作用都是从NioEventLoop数组里面选出一个NioEventLoop对象来。

但是说实话，我没有想到这两种算法有什么区别，如果谁知道，请告诉我，谢谢。

return executors[idx.getAndIncrement() & executors.length - 1];return executors[Math.abs(idx.getAndIncrement() % executors.length)];

继续往回走，MultithreadEventExecutorGroup的构造函数就基本看完了。

三. 总结

我们来总结下NioEventLoopGroup的实例化过程，可以得到以下几点。

1. NioEventLoopGroup的父类MultithreadEventExecutorGroup包含一个NioEventLoop数组children，数组的大小等于nThreads线程数目。如果没有指定，默认一般是cpu核数 x 2

2. NioEventLoopGroup和NioEventLoop一样都是继承自Executor，但是NioEventLoopGroup又包含多个NioEventLoop(children数组)，这种关系有点像android里面ViewGroup和View的关系。或者装饰者模式？

3. NioEventLoopGroup继承自MultithreadEventLoopGroup，而NioEventLoop继承自SingleThreadEventLoop，从名字看，不知道和多线程，单线程有没有关系。

4. MultithreadEventLoopGroup有个chooser，执行next方法的时候，会选择下一个NioEventLoop对象，虽然并不知道两个chooser算法有何区别。

5. NioEventLoopGroup里面重写了newChild方法，里面实例化NioEventLoop。

6. NioEventLoop里面包含了Selector，类型是KQueueSelectorImpl

SelectorProvider provider
SelectStrategy selectStrategy

SelectStrategy这个我们上面我们没有关注，其实它是NioEventLoopGroup构造函数传进去的，如下：

public NioEventLoopGroup(int nThreads, Executor executor, final SelectorProvider selectorProvider) {this(nThreads, executor, selectorProvider, DefaultSelectStrategyFactory.INSTANCE);}public final class DefaultSelectStrategyFactory implements SelectStrategyFactory {public static final SelectStrategyFactory INSTANCE = new DefaultSelectStrategyFactory();private DefaultSelectStrategyFactory() { }@Overridepublic SelectStrategy newSelectStrategy() {return DefaultSelectStrategy.INSTANCE;}
}final class DefaultSelectStrategy implements SelectStrategy {static final SelectStrategy INSTANCE = new DefaultSelectStrategy();private DefaultSelectStrategy() { }@Overridepublic int calculateStrategy(IntSupplier selectSupplier, boolean hasTasks) throws Exception {return hasTasks ? selectSupplier.get() : SelectStrategy.SELECT;}
}

所以SelectStrategy的实现类是DefaultSelectStrategy.

在理清楚NioEventLoopGroup实例化的过程之后，我们下一篇继续按照源代码分析Netty服务器端的源代码。