Netty Pipeline源碼分析(2)

原文：https://wangwei.one/posts/netty-pipeline-source-analyse-2.html

前面 ，我們分析了Netty Pipeline的初始化及節(jié)點(diǎn)添加與刪除邏輯。接下來(lái)，我們將來(lái)分析Pipeline的事件傳播機(jī)制。

Netty版本：4.1.30

inBound事件傳播

示例

我們通過(guò)下面這個(gè)例子來(lái)演示Netty Pipeline的事件傳播機(jī)制。

public class NettyPipelineInboundExample {

    public static void main(String[] args) {
        EventLoopGroup group = new NioEventLoopGroup(1);
        ServerBootstrap strap = new ServerBootstrap();
        strap.group(group)
                .channel(NioServerSocketChannel.class)
                .localAddress(new InetSocketAddress(8888))
                .childOption(ChannelOption.TCP_NODELAY, true)
                .childHandler(new ChannelInitializer<SocketChannel>() {
                    @Override
                    protected void initChannel(SocketChannel ch) throws Exception {
                        ch.pipeline().addLast(new InboundHandlerA());
                        ch.pipeline().addLast(new InboundHandlerB());
                        ch.pipeline().addLast(new InboundHandlerC());
                    }
                });
        try {
            ChannelFuture future = strap.bind().sync();
            future.channel().closeFuture().sync();
        } catch (InterruptedException e) {
            e.printStackTrace();
        } finally {
            group.shutdownGracefully();
        }
    }
}

class InboundHandlerA extends ChannelInboundHandlerAdapter {

    @Override
    public void channelRead(ChannelHandlerContext ctx, Object msg) throws Exception {
        System.out.println("InboundHandler A : " + msg);
        // 傳播read事件到下一個(gè)channelhandler
        ctx.fireChannelRead(msg);
    }

}

class InboundHandlerB extends ChannelInboundHandlerAdapter {

    @Override
    public void channelRead(ChannelHandlerContext ctx, Object msg) throws Exception {
        System.out.println("InboundHandler B : " + msg);
        // 傳播read事件到下一個(gè)channelhandler
        ctx.fireChannelRead(msg);
    }

    @Override
    public void channelActive(ChannelHandlerContext ctx) throws Exception {
        // channel激活，觸發(fā)channelRead事件，從pipeline的heandContext節(jié)點(diǎn)開(kāi)始往下傳播
        ctx.channel().pipeline().fireChannelRead("Hello world");
    }
}

class InboundHandlerC extends ChannelInboundHandlerAdapter {

    @Override
    public void channelRead(ChannelHandlerContext ctx, Object msg) throws Exception {
        System.out.println("InboundHandler C : " + msg);
        // 傳播read事件到下一個(gè)channelhandler
        ctx.fireChannelRead(msg);
    }
}

源碼

通過(guò) telnet 來(lái)連接上面啟動(dòng)好的netty服務(wù)，觸發(fā)channel active事件：

$ telnet 127.0.0.1 8888

按照InboundHandlerA、InboundHandlerB、InboundHandlerC的添加順序，控制臺(tái)輸出如下信息：

InboundHandler A : Hello world
InboundHandler B : Hello world
InboundHandler C : Hello world

若是調(diào)用它們的添加順序，則會(huì)輸出對(duì)應(yīng)順序的信息，e.g:

...

ch.pipeline().addLast(new InboundHandlerB());
ch.pipeline().addLast(new InboundHandlerA());
ch.pipeline().addLast(new InboundHandlerC());

...

輸出如下信息：

InboundHandler B : Hello world
InboundHandler A : Hello world
InboundHandler C : Hello world

源碼分析

強(qiáng)烈建議 下面的流程，自己通過(guò)IDE的Debug模式來(lái)分析

待netty啟動(dòng)成功，通過(guò)telnet連接到netty，然后通過(guò)telnet終端輸入任意字符（這一步才開(kāi)啟Debug模式），進(jìn)入Debug模式。

觸發(fā)channel read事件，從下面的入口開(kāi)始調(diào)用

public class DefaultChannelPipeline implements ChannelPipeline {
    
    ...
    
    // 出發(fā)channel read事件
    @Override
    public final ChannelPipeline fireChannelRead(Object msg) {
        // 從head節(jié)點(diǎn)開(kāi)始往下傳播read事件
        AbstractChannelHandlerContext.invokeChannelRead(head, msg);
        return this;
    }
    
    ...

}

調(diào)用 AbstractChannelHandlerContext 中的 invokeChannelRead(head, msg) 接口：

abstract class AbstractChannelHandlerContext extends DefaultAttributeMap
        implements ChannelHandlerContext, ResourceLeakHint {

    ...

    // 調(diào)用channel read
    static void invokeChannelRead(final AbstractChannelHandlerContext next, Object msg) {
        // 獲取消息
        final Object m = next.pipeline.touch(ObjectUtil.checkNotNull(msg, "msg"), next);
        // 獲取 EventExecutor
        EventExecutor executor = next.executor();
        // true
        if (executor.inEventLoop()) {
            // 調(diào)用下面的invokeChannelRead接口：invokeChannelRead(Object msg)
            next.invokeChannelRead(m);
        } else {
            executor.execute(new Runnable() {
                @Override
                public void run() {
                    next.invokeChannelRead(m);
                }
            });
        }
    }
    
    private void invokeChannelRead(Object msg) {
        if (invokeHandler()) {
            try {
                // handler():獲取當(dāng)前遍歷到的channelHandler，第一個(gè)為HeandContext，最后為TailContext
                // 調(diào)用channel handler的channelRead接口
                ((ChannelInboundHandler) handler()).channelRead(this, msg);
            } catch (Throwable t) {
                notifyHandlerException(t);
            }
        } else {
            fireChannelRead(msg);
        }
    }
    
    ...
    
    @Override
    public ChannelHandlerContext fireChannelRead(final Object msg) {
        // 調(diào)回到上面的 invokeChannelRead(final AbstractChannelHandlerContext next, Object msg)
        invokeChannelRead(findContextInbound(), msg);
        return this;
    }
    
    ...
    
    // 遍歷出下一個(gè)ChannelHandler
    private AbstractChannelHandlerContext findContextInbound() {
        AbstractChannelHandlerContext ctx = this;
        do {
            //獲取下一個(gè)inbound類型的節(jié)點(diǎn)
            ctx = ctx.next;
            // 必須為inbound類型
        } while (!ctx.inbound);
        return ctx;
    }
    
    ...
}

Pipeline中的第一個(gè)節(jié)點(diǎn)為HeadContext，它對(duì)于channelRead事件的處理，是直接往下傳播，代碼如下：

final class HeadContext extends AbstractChannelHandlerContext
    
    ...
    
    @Override
    public void channelRead(ChannelHandlerContext ctx, Object msg) throws Exception {
        // HeadContext往下傳播channelRead事件，
        // 調(diào)用HeandlerContext中的接口：fireChannelRead(final Object msg)
        ctx.fireChannelRead(msg);
    }
    
    ...
}

就這樣一直循環(huán)下去，依次會(huì)調(diào)用到 InboundHandlerA、InboundHandlerB、InboundHandlerC 中的 channelRead(ChannelHandlerContext ctx, Object msg) 接口。

到最后一個(gè)TailContext節(jié)點(diǎn)，它對(duì)channelRead事件的處理如下：

public class DefaultChannelPipeline implements ChannelPipeline {
        
    final class TailContext extends AbstractChannelHandlerContext implements ChannelInboundHandler {
        
        ...
                
        @Override
        public void channelRead(ChannelHandlerContext ctx, Object msg) throws Exception {
            // 調(diào)用onUnhandledInboundMessage接口
            onUnhandledInboundMessage(msg);
        }      

        ...

    }
    
    ...
    
    // 對(duì)未處理inbound消息做最后的處理
    protected void onUnhandledInboundMessage(Object msg) {
        try {
            logger.debug("Discarded inbound message {} that reached at the tail of the pipeline. Please check your pipeline configuration.", msg);
        } finally {
            // 對(duì)msg對(duì)象的引用數(shù)減1，當(dāng)msg對(duì)象的引用數(shù)為0時(shí)，釋放該對(duì)象的內(nèi)存
            ReferenceCountUtil.release(msg);
        }
    }
    
    ...
   
}

以上就是pipeline對(duì)inBound消息的處理流程。

SimpleChannelInboundHandler

在前面的例子中，假如中間有一個(gè)ChannelHandler未對(duì)channelRead事件進(jìn)行傳播，就會(huì)導(dǎo)致消息對(duì)象無(wú)法得到釋放，最終導(dǎo)致內(nèi)存泄露。

我們還可以繼承 SimpleChannelInboundHandler 來(lái)自定義ChannelHandler，它的channelRead方法，對(duì)消息對(duì)象做了msg處理，防止內(nèi)存泄露。

public abstract class SimpleChannelInboundHandler<I> extends ChannelInboundHandlerAdapter {
    ...

    @Override
    public void channelRead(ChannelHandlerContext ctx, Object msg) throws Exception {
        boolean release = true;
        try {
            if (acceptInboundMessage(msg)) {
                @SuppressWarnings("unchecked")
                I imsg = (I) msg;
                channelRead0(ctx, imsg);
            } else {
                release = false;
                ctx.fireChannelRead(msg);
            }
        } finally {
            if (autoRelease && release) {
                // 對(duì)msg對(duì)象的引用數(shù)減1，當(dāng)msg對(duì)象的引用數(shù)為0時(shí)，釋放該對(duì)象的內(nèi)存
                ReferenceCountUtil.release(msg);
            }
        }
    }
    
    ...
    
}

outBound事件傳播

接下來(lái)，我們來(lái)分析Pipeline的outBound事件傳播機(jī)制。代碼示例如下：

示例

public class NettyPipelineOutboundExample {

    public static void main(String[] args) {
        EventLoopGroup group = new NioEventLoopGroup(1);
        ServerBootstrap strap = new ServerBootstrap();
        strap.group(group)
                .channel(NioServerSocketChannel.class)
                .localAddress(new InetSocketAddress(8888))
                .childOption(ChannelOption.TCP_NODELAY, true)
                .childHandler(new ChannelInitializer<SocketChannel>() {
                    @Override
                    protected void initChannel(SocketChannel ch) throws Exception {
                        ch.pipeline().addLast(new OutboundHandlerA());
                        ch.pipeline().addLast(new OutboundHandlerB());
                        ch.pipeline().addLast(new OutboundHandlerC());
                    }
                });
        try {
            ChannelFuture future = strap.bind().sync();
            future.channel().closeFuture().sync();
        } catch (InterruptedException e) {
            e.printStackTrace();
        } finally {
            group.shutdownGracefully();
        }
    }
}

class OutboundHandlerA extends ChannelOutboundHandlerAdapter {

    @Override
    public void write(ChannelHandlerContext ctx, Object msg, ChannelPromise promise) throws Exception {
        // 輸出消息
        System.out.println("OutboundHandlerA: " + msg);
        // 傳播write事件到下一個(gè)節(jié)點(diǎn)
        ctx.write(msg, promise);
    }
}

class OutboundHandlerB extends ChannelOutboundHandlerAdapter {

    @Override
    public void write(ChannelHandlerContext ctx, Object msg, ChannelPromise promise) throws Exception {
        // 輸出消息
        System.out.println("OutboundHandlerB: " + msg);
        // 傳播write事件到下一個(gè)節(jié)點(diǎn)
        ctx.write(msg, promise);
    }
    
    
    @Override
    public void handlerAdded(ChannelHandlerContext ctx) throws Exception {
        // 待handlerAdded事件觸發(fā)3s后，模擬觸發(fā)一個(gè)
        ctx.executor().schedule(() -> {
//            ctx.write("Hello world ! ");
            ctx.channel().write("Hello world ! ");
        }, 3, TimeUnit.SECONDS);
    }
}

class OutboundHandlerC extends ChannelOutboundHandlerAdapter {

    @Override
    public void write(ChannelHandlerContext ctx, Object msg, ChannelPromise promise) throws Exception {
        // 輸出消息
        System.out.println("OutboundHandlerC: " + msg);
        // 傳播write事件到下一個(gè)節(jié)點(diǎn)
        ctx.write(msg, promise);
    }
}

源碼

通過(guò) telnet 來(lái)連接上面啟動(dòng)好的netty服務(wù)，觸發(fā)channel added事件：

$ telnet 127.0.0.1 8888

按照OutboundHandlerA、OutboundHandlerB、OutboundHandlerC的添加順序，控制臺(tái)輸出如下信息：

OutboundHandlerC: Hello world ! 
OutboundHandlerB: Hello world ! 
OutboundHandlerA: Hello world !

輸出的順序正好與ChannelHandler的添加順序相反。

若是調(diào)用它們的添加順序，則會(huì)輸出對(duì)應(yīng)順序的信息，e.g:

...

ch.pipeline().addLast(new InboundHandlerB());
ch.pipeline().addLast(new InboundHandlerA());
ch.pipeline().addLast(new InboundHandlerC());

...

輸出如下信息：

OutboundHandlerC: Hello world ! 
OutboundHandlerA: Hello world ! 
OutboundHandlerB: Hello world !

源碼分析

強(qiáng)烈建議 下面的流程，自己通過(guò)IDE的Debug模式來(lái)分析

從channel的write方法開(kāi)始，往下傳播write事件：

public abstract class AbstractChannel extends DefaultAttributeMap implements Channel {

    ...

    @Override
    public ChannelFuture write(Object msg) {
        // 調(diào)用pipeline往下傳播wirte事件
        return pipeline.write(msg);
    }
    
    ...

}

接著來(lái)看看Pipeline中的write接口：

public class DefaultChannelPipeline implements ChannelPipeline {
    
    ...
    
    @Override
    public final ChannelFuture write(Object msg) {
        // 從tail節(jié)點(diǎn)開(kāi)始傳播
        return tail.write(msg);
    }
    
    ...    

}

調(diào)用ChannelHandlerContext中的write接口：

abstract class AbstractChannelHandlerContext extends DefaultAttributeMap
        implements ChannelHandlerContext, ResourceLeakHint {
        
    ...
    
    @Override
    public ChannelFuture write(Object msg) {
        // 往下調(diào)用write接口
        return write(msg, newPromise());
    }

    @Override
    public ChannelFuture write(final Object msg, final ChannelPromise promise) {
        if (msg == null) {
            throw new NullPointerException("msg");
        }

        try {
            if (isNotValidPromise(promise, true)) {
                ReferenceCountUtil.release(msg);
                // cancelled
                return promise;
            }
        } catch (RuntimeException e) {
            ReferenceCountUtil.release(msg);
            throw e;
        }
        // 往下調(diào)用write接口
        write(msg, false, promise);

        return promise;
    }    
     
    ...
    
    private void write(Object msg, boolean flush, ChannelPromise promise) {
        // 尋找下一個(gè)outbound類型的channelHandlerContext
        AbstractChannelHandlerContext next = findContextOutbound();
        final Object m = pipeline.touch(msg, next);
        EventExecutor executor = next.executor();
        if (executor.inEventLoop()) {
            if (flush) {
                next.invokeWriteAndFlush(m, promise);
            } else {
                // 調(diào)用接口 invokeWrite(Object msg, ChannelPromise promise)
                next.invokeWrite(m, promise);
            }
        } else {
            AbstractWriteTask task;
            if (flush) {
                task = WriteAndFlushTask.newInstance(next, m, promise);
            }  else {
                task = WriteTask.newInstance(next, m, promise);
            }
            safeExecute(executor, task, promise, m);
        }
    }
    
    // 尋找下一個(gè)outbound類型的channelHandlerContext
    private AbstractChannelHandlerContext findContextOutbound() {
        AbstractChannelHandlerContext ctx = this;
        do {
            ctx = ctx.prev;
        } while (!ctx.outbound);
        return ctx;
    }
    
    private void invokeWrite(Object msg, ChannelPromise promise) {
        if (invokeHandler()) {
            // 繼續(xù)往下調(diào)用
            invokeWrite0(msg, promise);
        } else {
            write(msg, promise);
        }
    }
    
    private void invokeWrite0(Object msg, ChannelPromise promise) {
        try {
            // 獲取當(dāng)前的channelHandler，調(diào)用其write接口
            // handler()依次會(huì)返回 OutboundHandlerC OutboundHandlerB OutboundHandlerA
            ((ChannelOutboundHandler) handler()).write(this, msg, promise);
        } catch (Throwable t) {
            notifyOutboundHandlerException(t, promise);
        }
    }
    
    ...    
        
}

最終會(huì)調(diào)用到HeadContext的write接口：

@Override
public void write(ChannelHandlerContext ctx, Object msg, ChannelPromise promise) throws Exception {
    // 調(diào)用unsafe進(jìn)行寫數(shù)據(jù)操作
    unsafe.write(msg, promise);
}

異常傳播

了解了Pipeline的入站與出站事件的機(jī)制之后，我們?cè)賮?lái)看看Pipeline的異常處理機(jī)制。

示例

public class NettyPipelineExceptionCaughtExample {

    public static void main(String[] args) {
        EventLoopGroup group = new NioEventLoopGroup(1);
        ServerBootstrap strap = new ServerBootstrap();
        strap.group(group)
                .channel(NioServerSocketChannel.class)
                .localAddress(new InetSocketAddress(8888))
                .childOption(ChannelOption.TCP_NODELAY, true)
                .childHandler(new ChannelInitializer<SocketChannel>() {
                    @Override
                    protected void initChannel(SocketChannel ch) throws Exception {
                        ch.pipeline().addLast(new InboundHandlerA());
                        ch.pipeline().addLast(new InboundHandlerB());
                        ch.pipeline().addLast(new InboundHandlerC());
                        ch.pipeline().addLast(new OutboundHandlerA());
                        ch.pipeline().addLast(new OutboundHandlerB());
                        ch.pipeline().addLast(new OutboundHandlerC());
                    }
                });
        try {
            ChannelFuture future = strap.bind().sync();
            future.channel().closeFuture().sync();
        } catch (InterruptedException e) {
            e.printStackTrace();
        } finally {
            group.shutdownGracefully();
        }
    }

    static class InboundHandlerA extends ChannelInboundHandlerAdapter {

        @Override
        public void exceptionCaught(ChannelHandlerContext ctx, Throwable cause) throws Exception {
            System.out.println("InboundHandlerA.exceptionCaught:" + cause.getMessage());
            ctx.fireExceptionCaught(cause);
        }
    }

    static class InboundHandlerB extends ChannelInboundHandlerAdapter {

        @Override
        public void channelRead(ChannelHandlerContext ctx, Object msg) throws Exception {
            throw new Exception("ERROR !!!");
        }

        @Override
        public void exceptionCaught(ChannelHandlerContext ctx, Throwable cause) throws Exception {
            System.out.println("InboundHandlerB.exceptionCaught:" + cause.getMessage());
            ctx.fireExceptionCaught(cause);
        }
    }

    static class InboundHandlerC extends ChannelInboundHandlerAdapter {

        @Override
        public void exceptionCaught(ChannelHandlerContext ctx, Throwable cause) throws Exception {
            System.out.println("InboundHandlerC.exceptionCaught:" + cause.getMessage());
            ctx.fireExceptionCaught(cause);
        }
    }


    static class OutboundHandlerA extends ChannelOutboundHandlerAdapter {

        @Override
        public void exceptionCaught(ChannelHandlerContext ctx, Throwable cause) throws Exception {
            System.out.println("OutboundHandlerA.exceptionCaught:" + cause.getMessage());
            ctx.fireExceptionCaught(cause);
        }

    }

    static class OutboundHandlerB extends ChannelOutboundHandlerAdapter {

        @Override
        public void exceptionCaught(ChannelHandlerContext ctx, Throwable cause) throws Exception {
            System.out.println("OutboundHandlerB.exceptionCaught:" + cause.getMessage());
            ctx.fireExceptionCaught(cause);
        }
    }

    static class OutboundHandlerC extends ChannelOutboundHandlerAdapter {

        @Override
        public void exceptionCaught(ChannelHandlerContext ctx, Throwable cause) throws Exception {
            System.out.println("OutboundHandlerC.exceptionCaught:" + cause.getMessage());
            ctx.fireExceptionCaught(cause);
        }
    }

}

源碼

通過(guò) telnet 來(lái)連接上面啟動(dòng)好的netty服務(wù)，并在控制臺(tái)發(fā)送任意字符：

$ telnet 127.0.0.1 8888

觸發(fā)channel read事件并拋出異常，控制臺(tái)輸出如下信息：

InboundHandlerB.exceptionCaught:ERROR !!!
InboundHandlerC.exceptionCaught:ERROR !!!
OutboundHandlerA.exceptionCaught:ERROR !!!
OutboundHandlerB.exceptionCaught:ERROR !!!
OutboundHandlerC.exceptionCaught:ERROR !!!

可以看到異常的捕獲與我們添加的ChannelHandler順序相同。

源碼分析

在我們的示例中，InboundHandlerB的ChannelRead接口拋出異常，導(dǎo)致從InboundHandlerA將ChannelRead事件傳播到InboundHandlerB的過(guò)程中出現(xiàn)異常，異常被捕獲。

abstract class AbstractChannelHandlerContext extends DefaultAttributeMap
        implements ChannelHandlerContext, ResourceLeakHint {
    
    ...
    
    @Override
    public ChannelHandlerContext fireExceptionCaught(final Throwable cause) {
        //調(diào)用invokeExceptionCaught接口
        invokeExceptionCaught(next, cause);
        return this;
    }

    static void invokeExceptionCaught(final AbstractChannelHandlerContext next, final Throwable cause) {
        ObjectUtil.checkNotNull(cause, "cause");
        EventExecutor executor = next.executor();
        if (executor.inEventLoop()) {
            // 調(diào)用下一個(gè)節(jié)點(diǎn)的invokeExceptionCaught接口
            next.invokeExceptionCaught(cause);
        } else {
            try {
                executor.execute(new Runnable() {
                    @Override
                    public void run() {
                        next.invokeExceptionCaught(cause);
                    }
                });
            } catch (Throwable t) {
                if (logger.isWarnEnabled()) {
                    logger.warn("Failed to submit an exceptionCaught() event.", t);
                    logger.warn("The exceptionCaught() event that was failed to submit was:", cause);
                }
            }
        }
    }
    
    ...
    
    private void invokeChannelRead(Object msg) {
        if (invokeHandler()) {
            try {
                // 拋出異常
                ((ChannelInboundHandler) handler()).channelRead(this, msg);
            } catch (Throwable t) {
                // 異常捕獲，往下傳播
                notifyHandlerException(t);
            }
        } else {
            fireChannelRead(msg);
        }
    }
    
    // 通知Handler發(fā)生異常事件
    private void notifyHandlerException(Throwable cause) {
        if (inExceptionCaught(cause)) {
            if (logger.isWarnEnabled()) {
                logger.warn(
                    "An exception was thrown by a user handler " +
                    "while handling an exceptionCaught event", cause);
            }
            return;
        }
        // 往下調(diào)用invokeExceptionCaught接口
        invokeExceptionCaught(cause);
    }

    
    private void invokeExceptionCaught(final Throwable cause) {
        if (invokeHandler()) {
            try {
                // 調(diào)用當(dāng)前ChannelHandler的exceptionCaught接口
                // 在我們的案例中，依次會(huì)調(diào)用InboundHandlerB、InboundHandlerC、
                // OutboundHandlerA、OutboundHandlerB、OutboundHandlC
                handler().exceptionCaught(this, cause);
            } catch (Throwable error) {
                if (logger.isDebugEnabled()) {
                    logger.debug(
                        "An exception {}" +
                        "was thrown by a user handler's exceptionCaught() " +
                        "method while handling the following exception:",
                        ThrowableUtil.stackTraceToString(error), cause);
                } else if (logger.isWarnEnabled()) {
                    logger.warn(
                        "An exception '{}' [enable DEBUG level for full stacktrace] " +
                        "was thrown by a user handler's exceptionCaught() " +
                        "method while handling the following exception:", error, cause);
                }
            }
        } else {
            fireExceptionCaught(cause);
        }
    }
    
    ...
        
}

最終會(huì)調(diào)用到TailContext節(jié)點(diǎn)的exceptionCaught接口，如果我們中途沒(méi)有對(duì)異常進(jìn)行攔截處理，做會(huì)打印出一段警告信息！

public class DefaultChannelPipeline implements ChannelPipeline {

    ...
    
    final class TailContext extends AbstractChannelHandlerContext implements ChannelInboundHandler {
        
        ...
            
        @Override
        public void exceptionCaught(ChannelHandlerContext ctx, Throwable cause) throws Exception {
            onUnhandledInboundException(cause);
        }
        
        ...
        
        protected void onUnhandledInboundException(Throwable cause) {
            try {
                logger.warn(
                    "An exceptionCaught() event was fired, and it reached at the tail of the pipeline. " +
                    "It usually means the last handler in the pipeline did not handle the exception.",
                    cause);
            } finally {
                ReferenceCountUtil.release(cause);
            }
        }
    }    
    
    ...
    
}

在實(shí)際的應(yīng)用中，一般會(huì)定一個(gè)ChannelHandler，放置Pipeline末尾，專門用來(lái)處理中途出現(xiàn)的各種異常。

最佳異常處理實(shí)踐

單獨(dú)定義ExceptionCaughtHandler來(lái)處理異常：


...
   
class ExceptionCaughtHandler extends ChannelInboundHandlerAdapter {

    @Override
    public void exceptionCaught(ChannelHandlerContext ctx, Throwable cause) throws Exception {
        if (cause instanceof Exception) {
            // TODO
            System.out.println("Successfully caught exception ! ");
        } else {
            // TODO
        }
    }
}

...
    
ch.pipeline().addLast(new ExceptionCaughtHandler());

...

輸出：

InboundHandlerB.exceptionCaught:ERROR !!!
InboundHandlerC.exceptionCaught:ERROR !!!
OutboundHandlerA.exceptionCaught:ERROR !!!
OutboundHandlerB.exceptionCaught:ERROR !!!
OutboundHandlerC.exceptionCaught:ERROR !!!
Successfully caught exception !  // 成功捕獲日志

Pipeline回顧與總結(jié)

至此，我們對(duì)Pipeline的原理的解析就完成了。

  • Pipeline是在什么時(shí)候創(chuàng)建的？
  • Pipeline添加與刪除節(jié)點(diǎn)的邏輯是怎么樣的？
  • netty是如何判斷ChannelHandler類型的？
  • 如何處理ChannelHandler中拋出的異常？
  • 對(duì)于ChannelHandler的添加應(yīng)遵循什么樣的順序？

參考資料

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