Netty怎么切换三种I/O模式和源码解释

参考文献：极客时间傅健老师的《Netty源码剖析与实战》Talk is cheap.show me the code!

三种I/O模式

　　BIO:Block I/O，即同步并阻塞的IO；BIO就是传统的java.io包下的代码实现

　　NIO:New IO（non-blocking IO）:同步非阻塞的IO,jdk1.4及以上版本提供

　　AIO:Async IO: 异步非阻塞IO,jdk1.7

阻塞和非阻塞

　　　阻塞：没有数据传输过来时，读会阻塞直到有数据；缓冲区满时，写操作也会阻塞。

　　非阻塞：非阻塞遇到这些情况都是直接返回。

同步和异步

　　同步：数据就绪后需要自己去读是同步。

　　异步：数据就绪后直接读好再回调给程序是异步。

Netty对三种IO的支持

首先Netty是都支持三种IO模式的，准确的来说是曾经都支持过，因为BIO的被Netty给过期了，AIO被Netty给删除了，具体原因这就不多赘述；知道BIO在Netty被称为OIO,NIO在多平台下都有对应的支持，有人会问为啥有common的支持了还有Linux等其他的意义吗，这好比全栈和后端前端之分一样，一个通用一个专用的区别。

Netty切换IO模式

　　如上图所示，对应的实现类都差不多，甚至可以看出都是头不一样，如果NIO的通用是NioEventLoopGroup,而OIO的实现则是OioEventLoopGroup,先看之前的一个demo

public class MyServer {
    public static void main(String[] args) throws Exception {
        EventLoopGroup bossGroup = new NioEventLoopGroup();
        EventLoopGroup workerGroup = new NioEventLoopGroup();
        try {
            ServerBootstrap sb = new ServerBootstrap();
            sb.group(bossGroup, workerGroup).channel(NioServerSocketChannel.class)
                    .handler(new LoggingHandler(LogLevel.INFO)).childHandler(new ChannelInitializer<SocketChannel>() {
                @Override
                public void initChannel(SocketChannel ch) throws Exception {
                    ChannelPipeline p = ch.pipeline();
                    p.addLast(new LoggingHandler(LogLevel.INFO));
                    p.addLast(new MyServerHandler());
                }
            });
            ChannelFuture f = sb.bind(8090).sync();
            f.channel().closeFuture().sync();
        } finally {
            bossGroup.shutdownGracefully();
            workerGroup.shutdownGracefully();
        }
    }
}

　　图上标粗的就是切换的模式的关键点。现在看看切换成OIO的代码

public class MyServer {
    public static void main(String[] args) throws Exception {
        EventLoopGroup bossGroup = new OioEventLoopGroup();
        EventLoopGroup workerGroup = new OioEventLoopGroup();
        try {
            ServerBootstrap sb = new ServerBootstrap();
            sb.group(bossGroup, workerGroup).channel(OioServerSocketChannel.class)
                    .handler(new LoggingHandler(LogLevel.INFO)).childHandler(new ChannelInitializer<SocketChannel>() {
                @Override
                public void initChannel(SocketChannel ch) throws Exception {
                    ChannelPipeline p = ch.pipeline();
                    p.addLast(new LoggingHandler(LogLevel.INFO));
                    p.addLast(new MyServerHandler());
                }
            });
            ChannelFuture f = sb.bind(8090).sync();
            f.channel().closeFuture().sync();
        } finally {
            bossGroup.shutdownGracefully();
            workerGroup.shutdownGracefully();
        }
    }
}

　　上面代码改动也就是标粗的那些。运行起来是完全没问题的。

　　那么具体是怎么做的呢，我们看看源码就知道了；

　　点进channel()方法里，核心步骤在此：

　　不难发现传入的是OioServerSocketChannel.class,由channelFactory工厂创建返回，进入ReflectiveChannelFactory();

上图的代码中有“this.constructor = clazz.getConstructor();”获取无参构造；

可以看出“return constructor.newInstance();”返回泛型“T” 就是要使用的IO模式。

总结来说：Netty实现IO模式的切换就是泛型+反射+工厂实现的。

除此之外，还有一点，"EventLoopGroup bossGroup = new NioEventLoopGroup();";实际上，它就相当于一个死循环，在“NioEventLoop.java”中，有个run(),如下图源码，可以看出它是个死循环（for (;;) {}），现在可以简单的理解它就是循环监听、处理事件的。

@Override
    protected void run() {
        for (;;) {
            try {
                try {
                    switch (selectStrategy.calculateStrategy(selectNowSupplier, hasTasks())) {
                    case SelectStrategy.CONTINUE:
                        continue;

                    case SelectStrategy.BUSY_WAIT:
                        // fall-through to SELECT since the busy-wait is not supported with NIO

                    case SelectStrategy.SELECT:
                        select(wakenUp.getAndSet(false));

                        // 'wakenUp.compareAndSet(false, true)' is always evaluated
                        // before calling 'selector.wakeup()' to reduce the wake-up
                        // overhead. (Selector.wakeup() is an expensive operation.)
                        //
                        // However, there is a race condition in this approach.
                        // The race condition is triggered when 'wakenUp' is set to
                        // true too early.
                        //
                        // 'wakenUp' is set to true too early if:
                        // 1) Selector is waken up between 'wakenUp.set(false)' and
                        //    'selector.select(...)'. (BAD)
                        // 2) Selector is waken up between 'selector.select(...)' and
                        //    'if (wakenUp.get()) { ... }'. (OK)
                        //
                        // In the first case, 'wakenUp' is set to true and the
                        // following 'selector.select(...)' will wake up immediately.
                        // Until 'wakenUp' is set to false again in the next round,
                        // 'wakenUp.compareAndSet(false, true)' will fail, and therefore
                        // any attempt to wake up the Selector will fail, too, causing
                        // the following 'selector.select(...)' call to block
                        // unnecessarily.
                        //
                        // To fix this problem, we wake up the selector again if wakenUp
                        // is true immediately after selector.select(...).
                        // It is inefficient in that it wakes up the selector for both
                        // the first case (BAD - wake-up required) and the second case
                        // (OK - no wake-up required).

                        if (wakenUp.get()) {
                            selector.wakeup();
                        }
                        // fall through
                    default:
                    }
                } catch (IOException e) {
                    // If we receive an IOException here its because the Selector is messed up. Let's rebuild
                    // the selector and retry. https://github.com/netty/netty/issues/8566
                    rebuildSelector0();
                    handleLoopException(e);
                    continue;
                }

                cancelledKeys = 0;
                needsToSelectAgain = false;
                final int ioRatio = this.ioRatio;
                if (ioRatio == 100) {
                    try {
                        processSelectedKeys();
                    } finally {
                        // Ensure we always run tasks.
                        runAllTasks();
                    }
                } else {
                    final long ioStartTime = System.nanoTime();
                    try {
                        processSelectedKeys();
                    } finally {
                        // Ensure we always run tasks.
                        final long ioTime = System.nanoTime() - ioStartTime;
                        runAllTasks(ioTime * (100 - ioRatio) / ioRatio);
                    }
                }
            } catch (Throwable t) {
                handleLoopException(t);
            }
            // Always handle shutdown even if the loop processing threw an exception.
            try {
                if (isShuttingDown()) {
                    closeAll();
                    if (confirmShutdown()) {
                        return;
                    }
                }
            } catch (Throwable t) {
                handleLoopException(t);
            }
        }
    }

我只想做的更好，仅此而已