Netty心跳基本檢測機制

首先赡勘，了解下為什么需要心跳从橘？
假設(shè)客戶端（如手機矩距，PAD）與服務(wù)器端已經(jīng)建立了長連接拗盒，客戶端開啟飛行模式或者強制關(guān)機了，服務(wù)器端的handlerRemoved方法不會被調(diào)用锥债，因為感知不到锣咒。此時，客戶端往服務(wù)器端發(fā)送的消息無法到達赞弥。如何處理這種情況呢毅整？那么就是通過心跳來做的，每隔一段時間客戶端向服務(wù)器端發(fā)送心跳包绽左，客戶端經(jīng)過一段時間收到后悼嫉，客戶端認為與服務(wù)器端連接是正常的。如果客戶端經(jīng)過一段時間沒有收到心跳包拼窥，客戶端則認為與服務(wù)端無法通信戏蔑，由客戶端發(fā)起斷開連接，然后重新向服務(wù)器端發(fā)起新的連接鲁纠。

Netty提供了空閑狀態(tài)監(jiān)測的處理器总棵。IdleStateHandler
我們來看下Javadoc說明

/**
 * Triggers an {@link IdleStateEvent} when a {@link Channel} has not performed
 * read, write, or both operation for a while.
 *
 * <h3>Supported idle states</h3>
 * <table border="1">
 * <tr>
 * <th>Property</th><th>Meaning</th>
 * </tr>
 * <tr>
 * <td>{@code readerIdleTime}</td>
 * <td>an {@link IdleStateEvent} whose state is {@link IdleState#READER_IDLE}
 *     will be triggered when no read was performed for the specified period of
 *     time.  Specify {@code 0} to disable.</td>
 * </tr>
 * <tr>
 * <td>{@code writerIdleTime}</td>
 * <td>an {@link IdleStateEvent} whose state is {@link IdleState#WRITER_IDLE}
 *     will be triggered when no write was performed for the specified period of
 *     time.  Specify {@code 0} to disable.</td>
 * </tr>
 * <tr>
 * <td>{@code allIdleTime}</td>
 * <td>an {@link IdleStateEvent} whose state is {@link IdleState#ALL_IDLE}
 *     will be triggered when neither read nor write was performed for the
 *     specified period of time.  Specify {@code 0} to disable.</td>
 * </tr>
 * </table>
 *
 * <pre>
 * // An example that sends a ping message when there is no outbound traffic
 * // for 30 seconds.  The connection is closed when there is no inbound traffic
 * // for 60 seconds.
 *
 * public class MyChannelInitializer extends {@link ChannelInitializer}&lt;{@link Channel}&gt; {
 *     {@code @Override}
 *     public void initChannel({@link Channel} channel) {
 *         channel.pipeline().addLast("idleStateHandler", new {@link IdleStateHandler}(60, 30, 0));
 *         channel.pipeline().addLast("myHandler", new MyHandler());
 *     }
 * }
 *
 * // Handler should handle the {@link IdleStateEvent} triggered by {@link IdleStateHandler}.
 * public class MyHandler extends {@link ChannelDuplexHandler} {
 *     {@code @Override}
 *     public void userEventTriggered({@link ChannelHandlerContext} ctx, {@link Object} evt) throws {@link Exception} {
 *         if (evt instanceof {@link IdleStateEvent}) {
 *             {@link IdleStateEvent} e = ({@link IdleStateEvent}) evt;
 *             if (e.state() == {@link IdleState}.READER_IDLE) {
 *                 ctx.close();
 *             } else if (e.state() == {@link IdleState}.WRITER_IDLE) {
 *                 ctx.writeAndFlush(new PingMessage());
 *             }
 *         }
 *     }
 * }
 *
 * {@link ServerBootstrap} bootstrap = ...;
 * ...
 * bootstrap.childHandler(new MyChannelInitializer());
 * ...
 * </pre>
 *
 * @see ReadTimeoutHandler
 * @see WriteTimeoutHandler
 */
public class IdleStateHandler extends ChannelDuplexHandler {

}

在一定時間內(nèi)當(dāng)有讀、寫或者讀寫沒有被執(zhí)行的時候改含，會觸發(fā)一個空閑狀態(tài)監(jiān)測事件情龄。
從Javadoc中我們也看到了基本的示例代碼，理論+實踐模式 :）
進一步看下構(gòu)造方法：

/**
     * Creates a new instance firing {@link IdleStateEvent}s.
     *
     * @param readerIdleTimeSeconds
     *        an {@link IdleStateEvent} whose state is {@link IdleState#READER_IDLE}
     *        will be triggered when no read was performed for the specified
     *        period of time.  Specify {@code 0} to disable.
     * @param writerIdleTimeSeconds
     *        an {@link IdleStateEvent} whose state is {@link IdleState#WRITER_IDLE}
     *        will be triggered when no write was performed for the specified
     *        period of time.  Specify {@code 0} to disable.
     * @param allIdleTimeSeconds
     *        an {@link IdleStateEvent} whose state is {@link IdleState#ALL_IDLE}
     *        will be triggered when neither read nor write was performed for
     *        the specified period of time.  Specify {@code 0} to disable.
     */
    public IdleStateHandler(
            int readerIdleTimeSeconds,
            int writerIdleTimeSeconds,
            int allIdleTimeSeconds) {

        this(readerIdleTimeSeconds, writerIdleTimeSeconds, allIdleTimeSeconds,
             TimeUnit.SECONDS);
    }

構(gòu)造方法中提供了三個參數(shù)捍壤，第一個參數(shù)readerIdleTimeSeconds是指給定時間內(nèi)沒有讀動作骤视，則觸發(fā)READER_IDLE事件；第二個參數(shù)writerIdleTimeSeconds是指給定時間內(nèi)服務(wù)端沒有寫入動作則觸發(fā)WRITER_IDLE事件鹃觉，第三個參數(shù)allIdleTimeSeconds表示給定實踐內(nèi)沒有發(fā)生讀或者寫動作专酗，則觸發(fā)ALL_IDLE事件。

我們可以編寫自定義處理器盗扇，監(jiān)測空閑狀態(tài)祷肯，繼承SimpleChannelInboundHandler沉填，同時，重寫
userEeventTriggered(ChannelHandlerContext ctx佑笋，Object evt)方法拜轨，兩個參數(shù)分別是上下文對象，事件對象允青。userEeventTriggered方法是SimpleChannelInboundHandler父類ChannelInboundHandlerAdapter抽象適配類的方法。
當(dāng)有操作操作超出指定空閑秒數(shù)時卵沉，便會觸發(fā)UserEventTriggered事件颠锉。

IdleState代表了Channel的空心閑狀態(tài)枚舉，三種狀態(tài)史汗，包括READ_IDLE琼掠、WRITE_IDLE、ALL_IDLE

下面附上示例代碼：

1. Main方法啟動類停撞，這里我們增加了LoggerHandler日志處理器

public class MyIdleServer {

    public static void main(String[] args) {
        EventLoopGroup bossGroup = new NioEventLoopGroup();
        EventLoopGroup workerGroup = new NioEventLoopGroup();

        try {
            ServerBootstrap bootstrap = new ServerBootstrap();
            bootstrap.group(bossGroup, workerGroup).channel(NioServerSocketChannel.class).handler(new LoggingHandler(LogLevel.INFO))
                    .childHandler(new MyIdleServerInitializer());

            ChannelFuture channelFuture = bootstrap.bind(8899).sync();
            channelFuture.channel().closeFuture().sync();

        } catch (InterruptedException e) {
            e.printStackTrace();
        } finally {
            bossGroup.shutdownGracefully();
            workerGroup.shutdownGracefully();
        }
    }
}

2. 編寫MyIdleServerInitializer瓷蛙，初始化處理器鏈，增加了IdleStateHandler空閑事件處理器.

public class MyIdleServerInitializer extends ChannelInitializer<SocketChannel>{

    @Override protected void initChannel(SocketChannel ch) throws Exception {
        ChannelPipeline channelPipeline = ch.pipeline();

        channelPipeline.addLast(new IdleStateHandler(6, 4, 10));
        channelPipeline.addLast(new MyIdleServerHandler());
    }
}

3. 編寫自定義處理器MyIdleServerHandler戈毒，重寫了channelRead0艰猬，目的主要為了測試寫超時觸發(fā)事件.

public class MyIdleServerHandler extends SimpleChannelInboundHandler<Object> {

    @Override public void channelRead0(ChannelHandlerContext ctx, Object msg) throws Exception {
        int i=0;
        while (i< 3) {
            sleep(3000);
            System.out.println("服務(wù)端執(zhí)行寫入操作3s");
            ctx.channel().writeAndFlush("服務(wù)端寫個客戶端數(shù)據(jù)");
            i++;
        }
    }

    @Override public void userEventTriggered(ChannelHandlerContext ctx, Object evt) throws Exception {
        if (evt instanceof IdleStateEvent) {
            IdleStateEvent idleStateEvt = (IdleStateEvent) evt;

            String eventType = null; // 空閑事件類型

            switch (idleStateEvt.state()) {
            case READER_IDLE:
                eventType = "讀空閑";
                break;
            case WRITER_IDLE:
                eventType = "寫空閑";
                break;
            case ALL_IDLE:
                eventType = "讀寫空閑";
                break;
            }

            System.out.println("客戶端地址：" + ctx.channel().remoteAddress() + "超時事件被觸發(fā)，eventType:" + eventType);
            ctx.channel().close();
        }
    }
}

4. 啟動MyIdleServer后埋市，使用MyChatClient請參見Netty多客戶端通信一章
   因設(shè)置寫超時時間設(shè)置4秒冠桃，MyChatClient不輸入任何內(nèi)容，結(jié)果如下：

MyChatClient不輸入任何內(nèi)容

當(dāng)MyChatClient輸入內(nèi)容后道宅，執(zhí)行結(jié)果會看到等到channelRead0執(zhí)行完寫入操作后食听，超過寫超時時間才會觸發(fā)WRITE_IDLE狀態(tài).

寫入操作完成后觸發(fā)事件