NIO 源码分析(04) 从 SelectorProvider 看 JDK SPI 机制

NIO 源码分析(04) 从 SelectorProvider 看 JDK SPI 机制

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Netty 系列目录(https://www.cnblogs.com/binarylei/p/10117436.html)

SelectorProvider 定义了建立 Selector、ServerSocketChannel、SocketChannel 等方法，采用 JDK 的 Service Provider Interface (SPI) 方式实现。

public static ServerSocketChannel open() throws IOException {
    return SelectorProvider.provider().openServerSocketChannel();
}

1、SelectorProvider SPI

SelectorProvider 是一个抽象类，须要子类实现。主要方法以下：

public abstract DatagramChannel openDatagramChannel() throws IOException;
public abstract DatagramChannel openDatagramChannel(ProtocolFamily family) throws IOException;
public abstract ServerSocketChannel openServerSocketChannel() throws IOException;
public abstract SocketChannel openSocketChannel() throws IOException;

public abstract AbstractSelector openSelector() throws IOException;
public abstract Pipe openPipe() throws IOException;

<b>总结：</b> SelectorProvider 至关于一个工厂类，提供了对 DatagramChannel、ServerSocketChannel、SocketChannel、Selector 了建立方法。

java.nio.channels.spi 中提供了一系列的抽象类，由具体的厂商实现，固然咱们通常使用的都是 JDK 本身的实现。相关的 SPI 接口以下：

AbstractInterruptibleChannel    -> SocketChannelImpl/ServerSocketChannelImpl     
AbstractSelectableChannel   
AbstractSelectionKey            -> SelectionKeyImpl
AbstractSelector                -> WindowsSelectorImpl/PollSelectorImpl/EpollSelectorImpl
SelectorProvider                -> DefaultSelectorProvider

2、SelectorProvider 加载过程

2.1 SelectorProvider 加载

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() {
                        // 1. java.nio.channels.spi.SelectorProvider 属性指定实现类
                        if (loadProviderFromProperty())
                            return provider;
                        // 2. SPI 指定实现类
                        if (loadProviderAsService())
                            return provider;
                        // 3. 默认实现，Windows 和 Linux 下不一样
                        provider = sun.nio.ch.DefaultSelectorProvider.create();
                        return provider;
                    }
                });
    }
}

<b>总结：</b> SelectorProvider 提供了三种方式来自定义 SelectorProvider 的实现类。

1. java.nio.channels.spi.SelectorProvider 属性指定实现类
2. 采用 SPI 方法建立 SelectorProvider
3. 默认实现 DefaultSelectorProvider，Windows 和 Linux 下具体的实现不一样。

public abstract class SelectorProviderImpl extends SelectorProvider {
    public DatagramChannel openDatagramChannel() throws IOException {
        return new DatagramChannelImpl(this);
    }
    public DatagramChannel openDatagramChannel(ProtocolFamily family) throws IOException {
        return new DatagramChannelImpl(this, family);
    }

    public Pipe openPipe() throws IOException {
        return new PipeImpl(this);
    }

    public abstract AbstractSelector openSelector() throws IOException;

    public ServerSocketChannel openServerSocketChannel() throws IOException {
        return new ServerSocketChannelImpl(this);
    }
    public SocketChannel openSocketChannel() throws IOException {
        return new SocketChannelImpl(this);
    }
}

<b>总结：</b> SelectorProviderImpl 提供了 ServerSocketChannel、SocketChanne 的建立，至于 Selector 在不一样的平台下有不一样的实现。

2.2 Windows 下 DefaultSelectorProvider

public class DefaultSelectorProvider {
    public static SelectorProvider create() {
        return new sun.nio.ch.WindowsSelectorProvider();
    }
}

public class WindowsSelectorProvider extends SelectorProviderImpl {
    public AbstractSelector openSelector() throws IOException {
        return new WindowsSelectorImpl(this);
    }
}

2.3 Unix 下 DefaultSelectorProvider

public class DefaultSelectorProvider {
    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();
    }
}

<b>总结：</b> Unix 平台下须要根据不一样的操做系统选择不一样的 Selector，例如 Linux 下是 EPollSelectorProvider。

public class EPollSelectorProvider extends SelectorProviderImpl {
    public AbstractSelector openSelector() throws IOException {
        return new EPollSelectorImpl(this);
    }

    public Channel inheritedChannel() throws IOException {
        return InheritedChannel.getChannel();
    }
}

<b>总结：</b> 不管是 WindowsSelectorProvider 仍是 EPollSelectorImpl，它们都继承 SelectorProviderImpl，关于 ServerSocketChannel、SocketChanne 的建立都是同样的，区别是 Selector 有兼容性问题。难道 Socket 在 Windows 和 Linux 下就没有区别吗，确定也是有兼容性问题的。

ServerSocketChannelImpl(SelectorProvider sp) throws IOException {
    super(sp);
    this.fd =  Net.serverSocket(true);  // 建立 socket，这个 Net 工具自己是跨平台的
    this.fdVal = IOUtil.fdVal(fd);
    this.state = ST_INUSE;
}

Socket 的建立是在 sun.nio.ch.Net 工具类的 socket0 完成的，这个类不少方法都是 native 方法，在不一样的平台有不一样的实现。

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