Java NIO根据操作系统不同，比如 macosx 是KQueueSelectorProvider、windows有WindowsSelectorProvider、Linux有EPollSelectorProvider （Linux kernels >= 2.6，是epoll模式）或PollSelectorProvider（selector模式）， 足以可见不同的系统对nio中的Selector有不同的实现，自4.0.16起，Netty为Linux通过JNI的方式提供了native socket transport。

Oracle jdk会自动选择合适的Selector，Oracle JDK在Linux已经默认使用epoll方式， 为什么netty还要提供一个基于epoll的实现呢？

stackoverflow也解释过，具体可参阅官方native-transports，Tomcat Native

If you are running on linux you can use EpollEventLoopGroup and so get better performance, less GC and have more advanced features that are only available on linux.

• Netty的 epoll transport使用 epoll edge-triggered 而 java的 nio 使用 level-triggered
• Netty的 epoll transport 暴露了更多的nio没有的配置参数， 如 TCP_CORK, SO_REUSEADDR等。
• C代码，更少GC，更少synchronized

总之，linux上使用EpollEventLoopGroup会有较少的gc有更高级的特性，性能更好~！

那该如何使用native socket transport（epoll）呢？

其实只需将相应的类替换即可

很多优秀的源码中对此都进行了兼容性处理，比如rocketmq

//源于org.apache.rocketmq.remoting.netty
public class NettyRemotingServer extends NettyRemotingAbstract implements RemotingServer {
    public NettyRemotingServer(final NettyServerConfig nettyServerConfig,
        final ChannelEventListener channelEventListener) {
        super(nettyServerConfig.getServerOnewaySemaphoreValue(), nettyServerConfig.getServerAsyncSemaphoreValue());
        this.serverBootstrap = new ServerBootstrap();
        this.nettyServerConfig = nettyServerConfig;
        this.channelEventListener = channelEventListener;

        int publicThreadNums = nettyServerConfig.getServerCallbackExecutorThreads();
        if (publicThreadNums <= 0) {
            publicThreadNums = 4;
        }

        this.publicExecutor = Executors.newFixedThreadPool(publicThreadNums, new ThreadFactory() {
            private AtomicInteger threadIndex = new AtomicInteger(0);

            @Override
            public Thread newThread(Runnable r) {
                return new Thread(r, "NettyServerPublicExecutor_" + this.threadIndex.incrementAndGet());
            }
        });

        if (useEpoll()) {
            this.eventLoopGroupBoss = new EpollEventLoopGroup(1, new ThreadFactory() {
                private AtomicInteger threadIndex = new AtomicInteger(0);

                @Override
                public Thread newThread(Runnable r) {
                    return new Thread(r, String.format("NettyEPOLLBoss_%d", this.threadIndex.incrementAndGet()));
                }
            });

            this.eventLoopGroupSelector = new EpollEventLoopGroup(nettyServerConfig.getServerSelectorThreads(), new ThreadFactory() {
                private AtomicInteger threadIndex = new AtomicInteger(0);
                private int threadTotal = nettyServerConfig.getServerSelectorThreads();

                @Override
                public Thread newThread(Runnable r) {
                    return new Thread(r, String.format("NettyServerEPOLLSelector_%d_%d", threadTotal, this.threadIndex.incrementAndGet()));
                }
            });
        } else {
            this.eventLoopGroupBoss = new NioEventLoopGroup(1, new ThreadFactory() {
                private AtomicInteger threadIndex = new AtomicInteger(0);

                @Override
                public Thread newThread(Runnable r) {
                    return new Thread(r, String.format("NettyNIOBoss_%d", this.threadIndex.incrementAndGet()));
                }
            });

            this.eventLoopGroupSelector = new NioEventLoopGroup(nettyServerConfig.getServerSelectorThreads(), new ThreadFactory() {
                private AtomicInteger threadIndex = new AtomicInteger(0);
                private int threadTotal = nettyServerConfig.getServerSelectorThreads();

                @Override
                public Thread newThread(Runnable r) {
                    return new Thread(r, String.format("NettyServerNIOSelector_%d_%d", threadTotal, this.threadIndex.incrementAndGet()));
                }
            });
        }

        loadSslContext();
    }
    private boolean useEpoll() {
        return RemotingUtil.isLinuxPlatform()
            && nettyServerConfig.isUseEpollNativeSelector()
            && Epoll.isAvailable();
    }
}

怎么读懂这块的源码？

首先从EventExecutorGroup开始，EventExecutorGroup是NioEventLoopGroup最上层的接口，它继承了java.util.concurrent.ScheduledExecutorService接口，因此它可以调度执行task。

• EventExecutorGroup内部管理了n个EventExecutor，next()方法返回其中的一个
• EventExecutor也是EventExecutorGroup（的子类）

EventExecutorGroup就像一个BOSS，每当有活儿的时候，就派一个小弟（EventExecutor）去干。

MultithreadEventExecutorGroup的每一个小弟都是一个SingleThreadEventExecutor，而且小弟的数量在构造的时候就确定了，这个BOSS的小弟分配逻辑相当简单，无非就是轮流使唤（next方法），这个BOSS的每一个小弟都是一个NioEventLoop。

public interface EventExecutorGroup extends ScheduledExecutorService, Iterable<EventExecutor> {
}
//实现了ScheduledExecutorService接口
public abstract class AbstractEventExecutorGroup implements EventExecutorGroup {
    public Future<?> submit(Runnable task) {
        return this.next().submit(task);
    }
    public <T> Future<T> submit(Callable<T> task) {
        return this.next().submit(task);
    }
}
public abstract class MultithreadEventExecutorGroup extends AbstractEventExecutorGroup {
    private final EventExecutor[] children;
    private final Set<EventExecutor> readonlyChildren;
    private final AtomicInteger terminatedChildren;
    private final Promise<?> terminationFuture;
    private final EventExecutorChooser chooser;

    public EventExecutor next() {
        return this.chooser.next();
    }
}
public final class DefaultEventExecutorChooserFactory implements EventExecutorChooserFactory {
    public static final DefaultEventExecutorChooserFactory INSTANCE = new DefaultEventExecutorChooserFactory();
    private static final class GenericEventExecutorChooser implements EventExecutorChooser {
        //调度逻辑，这里调用了Math.abs()方法以防止executors溢出
        public EventExecutor next() {
            return this.executors[Math.abs(this.idx.getAndIncrement() % this.executors.length)];
        }
    }
}
public abstract class MultithreadEventLoopGroup extends MultithreadEventExecutorGroup implements EventLoopGroup {
    public EventLoop next() {
        return (EventLoop)super.next();
    }

    protected abstract EventLoop newChild(Executor var1, Object... var2) throws Exception;

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

    public ChannelFuture register(ChannelPromise promise) {
        return this.next().register(promise);
    }
}
public class NioEventLoopGroup extends MultithreadEventLoopGroup {
    public void rebuildSelectors() {
        Iterator var1 = this.iterator();

        while(var1.hasNext()) {
            EventExecutor e = (EventExecutor)var1.next();
            ((NioEventLoop)e).rebuildSelector();
        }

    }
}

• NioEventLoopGroup实际上就是个线程池
• NioEventLoopGroup在后台启动了n个NioEventLoop来处理Channel事件
• 每一个NioEventLoop负责处理m个Channel，可以理解为nio中Selector，它有两个职责：一是作为IO线程处理IO事件，二是作为普通的线程处理通过execute等方法提交上来的任务
• NioEventLoopGroup从NioEventLoop数组里挨个取出NioEventLoop来处理Channel

NioEventLoop在单线程里同时处理IO事件和其他任务（单线程模式下的Reactor），NioEventLoop尽量（但不能保证）按照给定的比率（默认为50%）来分配花在这两种事情上的时间。换句话说，我们不应该在NioEventLoop里执行耗时的操作（比如数据库操作），这样会卡死NioEventLoop，降低程序的响应性。