NioEventLoopGroup继承关系图

• AbstractEventExecutorGroup实现了Executor、ExecutorService、ScheduledExecutorService中的部分方法，都是委托给next()方法返回的EventExecutor
• MultithreadEventExecutorGroup主要是创建了EventExecutor[] children数组，并将多个方法委托给children
• MultithreadEventLoopGroup将EventExecutor类型的chlild转化成EventLoop（EventExecutor的子接口），并将register()方法委托给EventLoop
• NioEventLoopGroup实现newChild()方法，返回NioEventLoop（EventLoop的实现类），并将几个方法委托给NioEventLoop

Executor

??Executor接口比较简单，只有一个execute()方法，该方法在未来某个时间点执行参数Runnable实例，可以在当前线程、新建线程或线程池里面执行，取决于具体实现。

public interface Executor {
    void execute(Runnable command);
}

ExecutorService

??ExecutorService继承了Executor，是建立在Executor基础上的一个服务，它是Executor基础上增加了任务管理服务。
??除了执行任务的基础功能外，ExecutorService还提供submit类方法，用于提交任务，并返回futrue实例（可以用于取消执行任务，或者等待执行完成）用于和任务进行通信。
??同时，还支持对Executor进行shutdown()操作，这一步可以用于资源的释放。

public interface ExecutorService extends Executor {
    //拒绝新任务，但在terminate之前可以执行先前提交的任务
    void shutdown();

    //拒绝新任务，并删除之前提交的正在等待执行的任务，同时会尝试终止正在执行的任务
    List<Runnable> shutdownNow();

    boolean isShutdown();

    /**如果shutdown后所有任务都已完成，则返回true。 
    请注意，除非先调用shutdown或shutdownNow，否则isTerminated永远不会为真。**/
    boolean isTerminated();

    //阻塞，直到shutdown后剩下的所有任务被完成，或者等待超时，或者被中断（interrupted）
    boolean awaitTermination(long timeout, TimeUnit unit)
        throws InterruptedException;

    //提交任务，返回的Future实例可以用于取消执行任务，或者等待执行完成
    <T> Future<T> submit(Callable<T> task);

    <T> Future<T> submit(Runnable task, T result);

    Future<?> submit(Runnable task);

    <T> List<Future<T>> invokeAll(Collection<? extends Callable<T>> tasks)
        throws InterruptedException;

    <T> List<Future<T>> invokeAll(Collection<? extends Callable<T>> tasks,
                                  long timeout, TimeUnit unit)
        throws InterruptedException;

    <T> T invokeAny(Collection<? extends Callable<T>> tasks)
        throws InterruptedException, ExecutionException;

    <T> T invokeAny(Collection<? extends Callable<T>> tasks,
                    long timeout, TimeUnit unit)
        throws InterruptedException, ExecutionException, TimeoutException;
}

??Executors类提供ExecutorService类的多个工厂方法。

ScheduledExecutorService

??ScheduledExecutorService可以在给定延时后执行任务，或者定期执行任务。
??scheduleAtFixedRate和scheduleWithFixedDelay方法创建并执行定期运行的任务，直到被取消。
??scheduleAtFixedRate表示每隔多少时间，执行一次任务。
??scheduleWithFixedDelay等到前一个任务结束的时刻，才开始结算间隔时间，如0秒开始执行第一次任务，任务耗时5秒，任务间隔时间3秒，那么第二次任务执行的时间是在第8秒开始。
??通过 Executor.execute(Runnable)和ExecutorService的submit()方法提交的任务延迟时间为0。

public interface ScheduledExecutorService extends ExecutorService {

    public ScheduledFuture<?> schedule(Runnable command,
                                       long delay, TimeUnit unit);

    public <V> ScheduledFuture<V> schedule(Callable<V> callable,
                                           long delay, TimeUnit unit);

    public ScheduledFuture<?> scheduleAtFixedRate(Runnable command,
                                                  long initialDelay,
                                                  long period,
                                                  TimeUnit unit);

    public ScheduledFuture<?> scheduleWithFixedDelay(Runnable command,
                                                     long initialDelay,
                                                     long delay,
                                                     TimeUnit unit);

}

EventExecutorGroup

??EventExecutorGroup负责通过next()方法提供EventExecutor。 除此之外，它还负责处理它们的生命周期并允许以全局的方式关闭它们。

public interface EventExecutorGroup extends ScheduledExecutorService, Iterable<EventExecutor> {
    //当被此EventExecutorGroup管理的所有EventExecutor被shutdown或者被shutdownGracefully时，返回true
    boolean isShuttingDown();

    //向executor发出关闭信号，此方法一旦被调用，isShuttingDown()就返回true，然后executor准备关闭自己
    Future<?> shutdownGracefully();

    Future<?> shutdownGracefully(long quietPeriod, long timeout, TimeUnit unit);

    //返回一个future，当所有被当前这个EventExecutorGroup管理的EventExecutor被终止时，会通知这个future
    Future<?> terminationFuture();

    //返回一个EventExecutor
    EventExecutor next();
   
    //被shutdownGracefully方法替代
    @Override
    @Deprecated
    void shutdown();

    //被shutdownGracefully方法替代
    @Override
    @Deprecated
    List<Runnable> shutdownNow();

    @Override
    Iterator<EventExecutor> iterator();

    @Override
    Future<?> submit(Runnable task);

    @Override
    <T> Future<T> submit(Runnable task, T result);

    @Override
    <T> Future<T> submit(Callable<T> task);

    @Override
    ScheduledFuture<?> schedule(Runnable command, long delay, TimeUnit unit);

    @Override
    <V> ScheduledFuture<V> schedule(Callable<V> callable, long delay, TimeUnit unit);

    @Override
    ScheduledFuture<?> scheduleAtFixedRate(Runnable command, long initialDelay, long period, TimeUnit unit);

    @Override
    ScheduledFuture<?> scheduleWithFixedDelay(Runnable command, long initialDelay, long delay, TimeUnit unit);
}

EventLoopGroup

??EventLoopGroup是特殊的EventExecutorGroup，它可以注册channel，被注册的channel会在事件循环（event loop）中被select和处理。

public interface EventLoopGroup extends EventExecutorGroup {
    //返回下一个EventLoop
    @Override
    EventLoop next();

    //注册Channel，返回的ChannelFuture会在注册完成时得到通知
    ChannelFuture register(Channel channel);

    ChannelFuture register(ChannelPromise promise);

    @Deprecated
    ChannelFuture register(Channel channel, ChannelPromise promise);
}

AbstractEventExecutorGroup

??AbstractEventExecutorGroup只是将部分方法的实现的委托给next()方法返回的EventExecutor，但是没具体实现next()方法。

public abstract class AbstractEventExecutorGroup implements EventExecutorGroup {
    @Override
    public Future<?> submit(Runnable task) {
        return next().submit(task);
    }

    @Override
    public <T> Future<T> submit(Runnable task, T result) {
        return next().submit(task, result);
    }

    @Override
    public <T> Future<T> submit(Callable<T> task) {
        return next().submit(task);
    }

    @Override
    public ScheduledFuture<?> schedule(Runnable command, long delay, TimeUnit unit) {
        return next().schedule(command, delay, unit);
    }

    @Override
    public <V> ScheduledFuture<V> schedule(Callable<V> callable, long delay, TimeUnit unit) {
        return next().schedule(callable, delay, unit);
    }

    @Override
    public ScheduledFuture<?> scheduleAtFixedRate(Runnable command, long initialDelay, long period, TimeUnit unit) {
        return next().scheduleAtFixedRate(command, initialDelay, period, unit);
    }

    @Override
    public ScheduledFuture<?> scheduleWithFixedDelay(Runnable command, long initialDelay, long delay, TimeUnit unit) {
        return next().scheduleWithFixedDelay(command, initialDelay, delay, unit);
    }

    @Override
    public Future<?> shutdownGracefully() {
        return shutdownGracefully(DEFAULT_SHUTDOWN_QUIET_PERIOD, DEFAULT_SHUTDOWN_TIMEOUT, TimeUnit.SECONDS);
    }

    /**
     * @deprecated {@link #shutdownGracefully(long, long, TimeUnit)} or {@link #shutdownGracefully()} instead.
     */
    @Override
    @Deprecated
    public abstract void shutdown();

    /**
     * @deprecated {@link #shutdownGracefully(long, long, TimeUnit)} or {@link #shutdownGracefully()} instead.
     */
    @Override
    @Deprecated
    public List<Runnable> shutdownNow() {
        shutdown();
        return Collections.emptyList();
    }

    @Override
    public <T> List<java.util.concurrent.Future<T>> invokeAll(Collection<? extends Callable<T>> tasks)
            throws InterruptedException {
        return next().invokeAll(tasks);
    }

    @Override
    public <T> List<java.util.concurrent.Future<T>> invokeAll(
            Collection<? extends Callable<T>> tasks, long timeout, TimeUnit unit) throws InterruptedException {
        return next().invokeAll(tasks, timeout, unit);
    }

    @Override
    public <T> T invokeAny(Collection<? extends Callable<T>> tasks) throws InterruptedException, ExecutionException {
        return next().invokeAny(tasks);
    }

    @Override
    public <T> T invokeAny(Collection<? extends Callable<T>> tasks, long timeout, TimeUnit unit)
            throws InterruptedException, ExecutionException, TimeoutException {
        return next().invokeAny(tasks, timeout, unit);
    }

    @Override
    public void execute(Runnable command) {
        next().execute(command);
    }
}

MultithreadEventExecutorGroup

??MultithreadEventExecutorGroup主要是创建了EventExecutor[] children
数组，并将多个方法委托给children。

public abstract class MultithreadEventExecutorGroup extends AbstractEventExecutorGroup {

    private final EventExecutor[] children;
    private final Set<EventExecutor> readonlyChildren;
    private final AtomicInteger terminatedChildren = new AtomicInteger();
    private final Promise<?> terminationFuture = new DefaultPromise(GlobalEventExecutor.INSTANCE);
    private final EventExecutorChooserFactory.EventExecutorChooser chooser;

    protected MultithreadEventExecutorGroup(int nThreads, ThreadFactory threadFactory, Object... args) {
        this(nThreads, threadFactory == null ? null : new ThreadPerTaskExecutor(threadFactory), args);
    }

    protected MultithreadEventExecutorGroup(int nThreads, Executor executor, Object... args) {
        this(nThreads, executor, DefaultEventExecutorChooserFactory.INSTANCE, args);
    }

    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) {
            executor = new ThreadPerTaskExecutor(newDefaultThreadFactory());
        }

        children = new EventExecutor[nThreads];

        for (int i = 0; i < nThreads; i ++) {
            boolean success = false;
            try {
                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) {
                    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;
                        }
                    }
                }
            }
        }

        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);
    }

    protected ThreadFactory newDefaultThreadFactory() {
        return new DefaultThreadFactory(getClass());
    }

    @Override
    public EventExecutor next() {
        return chooser.next();
    }

    @Override
    public Iterator<EventExecutor> iterator() {
        return readonlyChildren.iterator();
    }

    public final int executorCount() {
        return children.length;
    }

    protected abstract EventExecutor newChild(Executor executor, Object... args) throws Exception;

    @Override
    public Future<?> shutdownGracefully(long quietPeriod, long timeout, TimeUnit unit) {
        for (EventExecutor l: children) {
            l.shutdownGracefully(quietPeriod, timeout, unit);
        }
        return terminationFuture();
    }

    @Override
    public Future<?> terminationFuture() {
        return terminationFuture;
    }

    @Override
    @Deprecated
    public void shutdown() {
        for (EventExecutor l: children) {
            l.shutdown();
        }
    }

    @Override
    public boolean isShuttingDown() {
        for (EventExecutor l: children) {
            if (!l.isShuttingDown()) {
                return false;
            }
        }
        return true;
    }

    @Override
    public boolean isShutdown() {
        for (EventExecutor l: children) {
            if (!l.isShutdown()) {
                return false;
            }
        }
        return true;
    }

    @Override
    public boolean isTerminated() {
        for (EventExecutor l: children) {
            if (!l.isTerminated()) {
                return false;
            }
        }
        return true;
    }

    @Override
    public boolean awaitTermination(long timeout, TimeUnit unit)
            throws InterruptedException {
        long deadline = System.nanoTime() + unit.toNanos(timeout);
        loop: for (EventExecutor l: children) {
            for (;;) {
                long timeLeft = deadline - System.nanoTime();
                if (timeLeft <= 0) {
                    break loop;
                }
                if (l.awaitTermination(timeLeft, TimeUnit.NANOSECONDS)) {
                    break;
                }
            }
        }
        return isTerminated();
    }
}

MultithreadEventLoopGroup

??MultithreadEventLoopGroup将EventExecutor类型的chlild转化成EventLoop（EventExecutor的子接口），并将register()方法委托给EventLoop。

public abstract class MultithreadEventLoopGroup extends MultithreadEventExecutorGroup implements EventLoopGroup {

    private static final InternalLogger logger = InternalLoggerFactory.getInstance(MultithreadEventLoopGroup.class);

    private static final int DEFAULT_EVENT_LOOP_THREADS;

    static {
        DEFAULT_EVENT_LOOP_THREADS = Math.max(1, SystemPropertyUtil.getInt(
                "io.netty.eventLoopThreads", NettyRuntime.availableProcessors() * 2));

        if (logger.isDebugEnabled()) {
            logger.debug("-Dio.netty.eventLoopThreads: {}", DEFAULT_EVENT_LOOP_THREADS);
        }
    }

    protected MultithreadEventLoopGroup(int nThreads, Executor executor, Object... args) {
        super(nThreads == 0 ? DEFAULT_EVENT_LOOP_THREADS : nThreads, executor, args);
    }

    protected MultithreadEventLoopGroup(int nThreads, ThreadFactory threadFactory, Object... args) {
        super(nThreads == 0 ? DEFAULT_EVENT_LOOP_THREADS : nThreads, threadFactory, args);
    }

    protected MultithreadEventLoopGroup(int nThreads, Executor executor, EventExecutorChooserFactory chooserFactory,
                                     Object... args) {
        super(nThreads == 0 ? DEFAULT_EVENT_LOOP_THREADS : nThreads, executor, chooserFactory, args);
    }

    @Override
    protected ThreadFactory newDefaultThreadFactory() {
        return new DefaultThreadFactory(getClass(), Thread.MAX_PRIORITY);
    }

    @Override
    public EventLoop next() {
        return (EventLoop) super.next();
    }

    @Override
    protected abstract EventLoop newChild(Executor executor, Object... args) throws Exception;

    @Override
    public ChannelFuture register(Channel channel) {
        return next().register(channel);
    }

    @Override
    public ChannelFuture register(ChannelPromise promise) {
        return next().register(promise);
    }

    @Deprecated
    @Override
    public ChannelFuture register(Channel channel, ChannelPromise promise) {
        return next().register(channel, promise);
    }
}

NioEventLoopGroup

??NioEventLoopGroup实现newChild()方法，返回NioEventLoop（EventLoop的实现类），并将几个方法委托给NioEventLoop。

public class NioEventLoopGroup extends MultithreadEventLoopGroup {

    public NioEventLoopGroup() {
        this(0);
    }

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

    public NioEventLoopGroup(int nThreads, ThreadFactory threadFactory) {
        this(nThreads, threadFactory, SelectorProvider.provider());
    }

    public NioEventLoopGroup(int nThreads, Executor executor) {
        this(nThreads, executor, SelectorProvider.provider());
    }

    public NioEventLoopGroup(
            int nThreads, ThreadFactory threadFactory, final SelectorProvider selectorProvider) {
        this(nThreads, threadFactory, selectorProvider, DefaultSelectStrategyFactory.INSTANCE);
    }

    public NioEventLoopGroup(int nThreads, ThreadFactory threadFactory,
        final SelectorProvider selectorProvider, final SelectStrategyFactory selectStrategyFactory) {
        super(nThreads, threadFactory, selectorProvider, selectStrategyFactory, RejectedExecutionHandlers.reject());
    }

    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());
    }

    public NioEventLoopGroup(int nThreads, Executor executor, EventExecutorChooserFactory chooserFactory,
                             final SelectorProvider selectorProvider,
                             final SelectStrategyFactory selectStrategyFactory) {
        super(nThreads, executor, chooserFactory, selectorProvider, selectStrategyFactory,
                RejectedExecutionHandlers.reject());
    }

    public NioEventLoopGroup(int nThreads, Executor executor, EventExecutorChooserFactory chooserFactory,
                             final SelectorProvider selectorProvider,
                             final SelectStrategyFactory selectStrategyFactory,
                             final RejectedExecutionHandler rejectedExecutionHandler) {
        super(nThreads, executor, chooserFactory, selectorProvider, selectStrategyFactory, rejectedExecutionHandler);
    }

    public void setIoRatio(int ioRatio) {
        for (EventExecutor e: this) {
            ((NioEventLoop) e).setIoRatio(ioRatio);
        }
    }

    public void rebuildSelectors() {
        for (EventExecutor e: this) {
            ((NioEventLoop) e).rebuildSelector();
        }
    }

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