Java NIO 根据操作系统不同，对 Selector 有不同的实现。比如：

    • macosx 是 KQueueSelectorProvider
    • windows 有 WindowsSelectorProvider
    • Linux 有 EPollSelectorProvider （Linux kernels >= 2.6，是 epoll 模式）或 PollSelectorProvider（selector模式）

    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
    image.png
    很多优秀的源码中对此都进行了兼容性处理，比如 rocketmq

    1. //源于org.apache.rocketmq.remoting.netty
    2. public class NettyRemotingServer extends NettyRemotingAbstract implements RemotingServer {
    3.     public NettyRemotingServer(final NettyServerConfig nettyServerConfig,
    4.         final ChannelEventListener channelEventListener) {
    5.         super(nettyServerConfig.getServerOnewaySemaphoreValue(), nettyServerConfig.getServerAsyncSemaphoreValue());
    6.         this.serverBootstrap = new ServerBootstrap();
    7.         this.nettyServerConfig = nettyServerConfig;
    8.         this.channelEventListener = channelEventListener;
    10.         int publicThreadNums = nettyServerConfig.getServerCallbackExecutorThreads();
    11.         if (publicThreadNums <= 0) {
    12.             publicThreadNums = 4;
    13.         }
    15.         this.publicExecutor = Executors.newFixedThreadPool(publicThreadNums, new ThreadFactory() {
    16.             private AtomicInteger threadIndex = new AtomicInteger(0);
    18.             @Override
    19.             public Thread newThread(Runnable r) {
    20.                 return new Thread(r, "NettyServerPublicExecutor_" + this.threadIndex.incrementAndGet());
    21.             }
    22.         });
    24.         if (useEpoll()) {
    25.             this.eventLoopGroupBoss = new EpollEventLoopGroup(1, new ThreadFactory() {
    26.                 private AtomicInteger threadIndex = new AtomicInteger(0);
    28.                 @Override
    29.                 public Thread newThread(Runnable r) {
    30.                     return new Thread(r, String.format("NettyEPOLLBoss_%d", this.threadIndex.incrementAndGet()));
    31.                 }
    32.             });
    34.             this.eventLoopGroupSelector = new EpollEventLoopGroup(nettyServerConfig.getServerSelectorThreads(), new ThreadFactory() {
    35.                 private AtomicInteger threadIndex = new AtomicInteger(0);
    36.                 private int threadTotal = nettyServerConfig.getServerSelectorThreads();
    38.                 @Override
    39.                 public Thread newThread(Runnable r) {
    40.                     return new Thread(r, String.format("NettyServerEPOLLSelector_%d_%d", threadTotal, this.threadIndex.incrementAndGet()));
    41.                 }
    42.             });
    43.         } else {
    44.             this.eventLoopGroupBoss = new NioEventLoopGroup(1, new ThreadFactory() {
    45.                 private AtomicInteger threadIndex = new AtomicInteger(0);
    47.                 @Override
    48.                 public Thread newThread(Runnable r) {
    49.                     return new Thread(r, String.format("NettyNIOBoss_%d", this.threadIndex.incrementAndGet()));
    50.                 }
    51.             });
    53.             this.eventLoopGroupSelector = new NioEventLoopGroup(nettyServerConfig.getServerSelectorThreads(), new ThreadFactory() {
    54.                 private AtomicInteger threadIndex = new AtomicInteger(0);
    55.                 private int threadTotal = nettyServerConfig.getServerSelectorThreads();
    57.                 @Override
    58.                 public Thread newThread(Runnable r) {
    59.                     return new Thread(r, String.format("NettyServerNIOSelector_%d_%d", threadTotal, this.threadIndex.incrementAndGet()));
    60.                 }
    61.             });
    62.         }
    64.         loadSslContext();
    65.     }
    66.     private boolean useEpoll() {
    67.         return RemotingUtil.isLinuxPlatform()
    68.             && nettyServerConfig.isUseEpollNativeSelector()
    69.             && Epoll.isAvailable();
    70.     }
    71. }

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    ](https://blog.csdn.net/alex_xfboy/article/details/89643638)