JAVA NIO SELECTOR

JAVA SELECTOR

Java NIO根据操作系统不同， 针对nio中的Selector有不同的实现
java.nio.channels.spi.SelectorProvider

/**
     * Returns the system-wide default selector provider for this invocation of
     * the Java virtual machine.
     *
     * <p> The first invocation of this method locates the default provider
     * object as follows: </p>
     *
     * <ol>
     *
     *   <li><p> If the system property
     *   <tt>java.nio.channels.spi.SelectorProvider</tt> is defined then it is
     *   taken to be the fully-qualified name of a concrete provider class.
     *   The class is loaded and instantiated; if this process fails then an
     *   unspecified error is thrown.  </p></li>
     *
     *   <li><p> If a provider class has been installed in a jar file that is
     *   visible to the system class loader, and that jar file contains a
     *   provider-configuration file named
     *   <tt>java.nio.channels.spi.SelectorProvider</tt> in the resource
     *   directory <tt>META-INF/services</tt>, then the first class name
     *   specified in that file is taken.  The class is loaded and
     *   instantiated; if this process fails then an unspecified error is
     *   thrown.  </p></li>
     *
     *   <li><p> Finally, if no provider has been specified by any of the above
     *   means then the system-default provider class is instantiated and the
     *   result is returned.  </p></li>
     *
     * </ol>
     *
     * <p> Subsequent invocations of this method return the provider that was
     * returned by the first invocation.  </p>
     *
     * @return  The system-wide default selector provider
     */
    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;
                        }
                    });
        }
    }

DefaultSelectorProvider

根据系统不同编译的二进制JDK不同

• mac

public class DefaultSelectorProvider {

    /**
     * Prevent instantiation.
     */
    private DefaultSelectorProvider() { }

    /**
     * Returns the default SelectorProvider.
     */
    public static SelectorProvider create() {
        return new sun.nio.ch.KQueueSelectorProvider();
    }

}

• solaris

 /**
     * Returns the default SelectorProvider.
     */
    public static SelectorProvider create() {
        String osname = AccessController
            .doPrivileged(new GetPropertyAction("os.name"));
        if (osname.equals("SunOS"))
            return createProvider("sun.nio.ch.DevPollSelectorProvider");
        if (osname.equals("Linux"))
            return createProvider("sun.nio.ch.EPollSelectorProvider");
        return new sun.nio.ch.PollSelectorProvider();
    }

• windows

 /**
     * Returns the default SelectorProvider.
     */
    public static SelectorProvider create() {
        return new sun.nio.ch.WindowsSelectorProvider();
    }

以上是不同操作系统的默认实现,用户可以指定不同的selector实现

• 指定系统变量

 private static boolean loadProviderFromProperty() {
        String cn = System.getProperty("java.nio.channels.spi.SelectorProvider");
        if (cn == null)
            return false;
        try {
            Class<?> c = Class.forName(cn, true,
                                       ClassLoader.getSystemClassLoader());
            provider = (SelectorProvider)c.newInstance();
            return true;
        } catch (ClassNotFoundException x) {
            throw new ServiceConfigurationError(null, x);
        } catch (IllegalAccessException x) {
            throw new ServiceConfigurationError(null, x);
        } catch (InstantiationException x) {
            throw new ServiceConfigurationError(null, x);
        } catch (SecurityException x) {
            throw new ServiceConfigurationError(null, x);
        }
    }

Djava.nio.channels.spi.SelectorProvider=sun.nio.ch.EPollSelectorProvider

• SPI

private static boolean loadProviderAsService() {

        ServiceLoader<SelectorProvider> sl =
            ServiceLoader.load(SelectorProvider.class,
                               ClassLoader.getSystemClassLoader());
        Iterator<SelectorProvider> i = sl.iterator();
        for (;;) {
            try {
                if (!i.hasNext())
                    return false;
                provider = i.next();
                return true;
            } catch (ServiceConfigurationError sce) {
                if (sce.getCause() instanceof SecurityException) {
                    // Ignore the security exception, try the next provider
                    continue;
                }
                throw sce;
            }
        }
    }

JDK NIO 在linux 平台默认使用epoll方式， 为什么netty还要提供一个基于epoll的实现?

NETTY作者回答

https://stackoverflow.com/questions/23465401/why-native-epoll-support-is-introduced-in-netty

• Netty的 epoll transport使用 epoll edge-triggered 而 java的 nio 使用 level-triggered.
• 另外netty epoll transport 暴露了更多的nio没有的配置参数， 如 TCP_CORK, SO_REUSEADDR等等

netty 作者博客

http://normanmaurer.me/