Netty Pipeline 源码分析 (2)

前面，我们分析了 Netty Pipeline 的初始化及节点添加与删除逻辑。接下来，我们将来分析 Pipeline 的事件传播机制。

Netty 版本：4.1.30

inBound 事件传播

示例

我们通过下面这个例子来演示 Netty Pipeline 的事件传播机制。

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);
        // 传播read事件到下一个channelhandler
        ctx.fireChannelRead(msg);
    }

}

class InboundHandlerB extends ChannelInboundHandlerAdapter {

    @Override
    public void channelRead(ChannelHandlerContext ctx, Object msg) throws Exception {
        System.out.println("InboundHandler B : " + msg);
        // 传播read事件到下一个channelhandler
        ctx.fireChannelRead(msg);
    }

    @Override
    public void channelActive(ChannelHandlerContext ctx) throws Exception {
        // channel激活，触发channelRead事件，从pipeline的heandContext节点开始往下传播
        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);
        // 传播read事件到下一个channelhandler
        ctx.fireChannelRead(msg);
    }
}

源码

通过 telnet 来连接上面启动好的 netty 服务，触发 channel active 事件：

1	$ telnet 127.0.0.1 8888

按照 InboundHandlerA、InboundHandlerB、InboundHandlerC 的添加顺序，控制台输出如下信息：

1
2
3

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

若是调用它们的添加顺序，则会输出对应顺序的信息，e.g:

...

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

...

输出如下信息：

1
2
3

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

源码分析

强烈建议 下面的流程，自己通过 IDE 的 Debug 模式来分析

待 netty 启动成功，通过 telnet 连接到 netty，然后通过 telnet 终端输入任意字符（这一步才开启 Debug 模式），进入 Debug 模式。

触发 channel read 事件，从下面的入口开始调用

public class DefaultChannelPipeline implements ChannelPipeline {
	
	...
	
    // 出发channel read事件
	@Override
    public final ChannelPipeline fireChannelRead(Object msg) {
        // 从head节点开始往下传播read事件
        AbstractChannelHandlerContext.invokeChannelRead(head, msg);
        return this;
    }
	
	...

}

调用 AbstractChannelHandlerContext 中的 invokeChannelRead(head, msg) 接口：

abstract class AbstractChannelHandlerContext extends DefaultAttributeMap
        implements ChannelHandlerContext, ResourceLeakHint {

	...

	// 调用channel read
    static void invokeChannelRead(final AbstractChannelHandlerContext next, Object msg) {
        // 获取消息
        final Object m = next.pipeline.touch(ObjectUtil.checkNotNull(msg, "msg"), next);
        // 获取 EventExecutor
        EventExecutor executor = next.executor();
        // true
        if (executor.inEventLoop()) {
        	// 调用下面的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():获取当前遍历到的channelHandler，第一个为HeandContext，最后为TailContext
            	// 调用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) {
        // 调回到上面的 invokeChannelRead(final AbstractChannelHandlerContext next, Object msg)
        invokeChannelRead(findContextInbound(), msg);
        return this;
    }
    
    ...
    
    // 遍历出下一个ChannelHandler
    private AbstractChannelHandlerContext findContextInbound() {
        AbstractChannelHandlerContext ctx = this;
        do {
        	//获取下一个inbound类型的节点
            ctx = ctx.next;
            // 必须为inbound类型
        } while (!ctx.inbound);
        return ctx;
    }
    
    ...
}

Pipeline 中的第一个节点为 HeadContext，它对于 channelRead 事件的处理，是直接往下传播，代码如下：

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

就这样一直循环下去，依次会调用到 InboundHandlerA、InboundHandlerB、InboundHandlerC 中的 channelRead(ChannelHandlerContext ctx, Object msg) 接口。

到最后一个 TailContext 节点，它对 channelRead 事件的处理如下：

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

        ...

    }
	
	...
    
    // 对未处理inbound消息做最后的处理
	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 {
            // 对msg对象的引用数减1，当msg对象的引用数为0时，释放该对象的内存
            ReferenceCountUtil.release(msg);
        }
    }
    
    ...
   
}

以上就是 pipeline 对 inBound 消息的处理流程。

SimpleChannelInboundHandler

在前面的例子中，假如中间有一个 ChannelHandler 未对 channelRead 事件进行传播，就会导致消息对象无法得到释放，最终导致内存泄露。

我们还可以继承 SimpleChannelInboundHandler 来自定义 ChannelHandler，它的 channelRead 方法，对消息对象做了 msg 处理，防止内存泄露。

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) {
                // 对msg对象的引用数减1，当msg对象的引用数为0时，释放该对象的内存
                ReferenceCountUtil.release(msg);
            }
        }
    }
    
    ...
    
}

outBound 事件传播

接下来，我们来分析 Pipeline 的 outBound 事件传播机制。代码示例如下：

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 {
        // 输出消息
        System.out.println("OutboundHandlerA: " + msg);
        // 传播write事件到下一个节点
        ctx.write(msg, promise);
    }
}

class OutboundHandlerB extends ChannelOutboundHandlerAdapter {

    @Override
    public void write(ChannelHandlerContext ctx, Object msg, ChannelPromise promise) throws Exception {
        // 输出消息
        System.out.println("OutboundHandlerB: " + msg);
        // 传播write事件到下一个节点
        ctx.write(msg, promise);
    }
	
    
    @Override
    public void handlerAdded(ChannelHandlerContext ctx) throws Exception {
        // 待handlerAdded事件触发3s后，模拟触发一个
        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 {
        // 输出消息
        System.out.println("OutboundHandlerC: " + msg);
        // 传播write事件到下一个节点
        ctx.write(msg, promise);
    }
}

源码

通过 telnet 来连接上面启动好的 netty 服务，触发 channel added 事件：

1	$ telnet 127.0.0.1 8888

按照 OutboundHandlerA、OutboundHandlerB、OutboundHandlerC 的添加顺序，控制台输出如下信息：

1
2
3

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

输出的顺序正好与 ChannelHandler 的添加顺序相反。

若是调用它们的添加顺序，则会输出对应顺序的信息，e.g:

...

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

...

输出如下信息：

1
2
3

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

源码分析

强烈建议 下面的流程，自己通过 IDE 的 Debug 模式来分析

从 channel 的 write 方法开始，往下传播 write 事件：

public abstract class AbstractChannel extends DefaultAttributeMap implements Channel {

	...

    @Override
    public ChannelFuture write(Object msg) {
        // 调用pipeline往下传播wirte事件
        return pipeline.write(msg);
    }
    
    ...

}

接着来看看 Pipeline 中的 write 接口：

public class DefaultChannelPipeline implements ChannelPipeline {
    
    ...
    
    @Override
    public final ChannelFuture write(Object msg) {
        // 从tail节点开始传播
        return tail.write(msg);
    }
    
    ...    

}

调用 ChannelHandlerContext 中的 write 接口：

abstract class AbstractChannelHandlerContext extends DefaultAttributeMap
        implements ChannelHandlerContext, ResourceLeakHint {
        
    ...
    
    @Override
    public ChannelFuture write(Object msg) {
   	 	// 往下调用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;
        }
        // 往下调用write接口
        write(msg, false, promise);

        return promise;
    }    
     
    ...
    
    private void write(Object msg, boolean flush, ChannelPromise promise) {
    	// 寻找下一个outbound类型的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 {
            	// 调用接口 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);
        }
    }
    
    // 寻找下一个outbound类型的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()) {
        	// 继续往下调用
            invokeWrite0(msg, promise);
        } else {
            write(msg, promise);
        }
    }
	
    private void invokeWrite0(Object msg, ChannelPromise promise) {
        try {
        	// 获取当前的channelHandler，调用其write接口
        	// handler()依次会返回 OutboundHandlerC OutboundHandlerB OutboundHandlerA
            ((ChannelOutboundHandler) handler()).write(this, msg, promise);
        } catch (Throwable t) {
            notifyOutboundHandlerException(t, promise);
        }
    }
    
    ...    
        
}

最终会调用到 HeadContext 的 write 接口：

@Override
public void write(ChannelHandlerContext ctx, Object msg, ChannelPromise promise) throws Exception {
    // 调用unsafe进行写数据操作
    unsafe.write(msg, promise);
}

异常传播

了解了 Pipeline 的入站与出站事件的机制之后，我们再来看看 Pipeline 的异常处理机制。

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);
        }
    }

}

源码

通过 telnet 来连接上面启动好的 netty 服务，并在控制台发送任意字符：

1	$ telnet 127.0.0.1 8888

触发 channel read 事件并抛出异常，控制台输出如下信息：

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

可以看到异常的捕获与我们添加的 ChannelHandler 顺序相同。

源码分析

在我们的示例中，InboundHandlerB 的 ChannelRead 接口抛出异常，导致从 InboundHandlerA 将 ChannelRead 事件传播到 InboundHandlerB 的过程中出现异常，异常被捕获。

abstract class AbstractChannelHandlerContext extends DefaultAttributeMap
        implements ChannelHandlerContext, ResourceLeakHint {
	
    ...
	
	@Override
    public ChannelHandlerContext fireExceptionCaught(final Throwable cause) {
    	//调用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()) {
        	// 调用下一个节点的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 {
                // 抛出异常
                ((ChannelInboundHandler) handler()).channelRead(this, msg);
            } catch (Throwable t) {
                // 异常捕获，往下传播
                notifyHandlerException(t);
            }
        } else {
            fireChannelRead(msg);
        }
    }
	
	// 通知Handler发生异常事件
    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;
        }
		// 往下调用invokeExceptionCaught接口
        invokeExceptionCaught(cause);
    }

	
    private void invokeExceptionCaught(final Throwable cause) {
        if (invokeHandler()) {
            try {
                // 调用当前ChannelHandler的exceptionCaught接口
                // 在我们的案例中，依次会调用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);
        }
    }
	
	...
		
}

最终会调用到 TailContext 节点的 exceptionCaught 接口，如果我们中途没有对异常进行拦截处理，做会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);
            }
        }
    }    
    
    ...
    
}

在实际的应用中，一般会定一个 ChannelHandler，放置 Pipeline 末尾，专门用来处理中途出现的各种异常。

最佳异常处理实践

单独定义 ExceptionCaughtHandler 来处理异常：


...
   
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());

...

输出：

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

Pipeline 回顾与总结

至此，我们对 Pipeline 的原理的解析就完成了。

Pipeline 是在什么时候创建的？
Pipeline 添加与删除节点的逻辑是怎么样的？
netty 是如何判断 ChannelHandler 类型的？
如何处理 ChannelHandler 中抛出的异常？
对于 ChannelHandler 的添加应遵循什么样的顺序？

参考资料

Java 读源码之 Netty 深入剖析