Netty源码分析之ChannelPipeline—异常事件的传播

ChannelHandler中异常的获取与处理是经过继承重写exceptionCaught方法来实现的，本篇文章咱们对ChannelPipeline中exceptionCaught异常事件的传播进行梳理分析java

一、出站事件的传播示例

首先咱们继续在以前的代码上进行改造，模拟异常事件的传播bootstrap

public class ServerApp { public static void main(String[] args) { EventLoopGroup boss = new NioEventLoopGroup(); EventLoopGroup work = new NioEventLoopGroup(2); try { ServerBootstrap bootstrap = new ServerBootstrap(); bootstrap.childOption(ChannelOption.SO_SNDBUF,2); bootstrap.group(boss, work).channel(NioServerSocketChannel.class) .childHandler(new ChannelInitializer<SocketChannel>() { @Override public void initChannel(SocketChannel ch) throws Exception { ChannelPipeline p = ch.pipeline(); // p.addLast(new LoggingHandler(LogLevel.INFO)); // 向ChannelPipeline中添加自定义channelHandler
                            p.addLast(new OutHandlerA()); p.addLast(new ServerHandlerA()); p.addLast(new ServerHandlerB()); p.addLast(new ServerHandlerC()); p.addLast(new OutHandlerB()); p.addLast(new OutHandlerC()); } }); bootstrap.bind(8050).sync(); } catch (Exception e) { // TODO: handle exception
 } } } public class OutHandlerA extends ChannelOutboundHandlerAdapter { @Override public void exceptionCaught(ChannelHandlerContext ctx, Throwable cause) throws Exception { System.err.println(this.getClass().getName()+"---"+cause.getMessage()); ctx.fireExceptionCaught(cause); } } public class OutHandlerB extends ChannelOutboundHandlerAdapter { @Override public void exceptionCaught(ChannelHandlerContext ctx, Throwable cause) throws Exception { System.err.println(this.getClass().getName()+"---"+cause.getMessage()); ctx.fireExceptionCaught(cause); } } public class OutHandlerC extends ChannelOutboundHandlerAdapter { @Override public void exceptionCaught(ChannelHandlerContext ctx, Throwable cause) throws Exception { System.err.println(this.getClass().getName()+"---"+cause.getMessage()); ctx.fireExceptionCaught(cause); } } public class ServerHandlerB extends ChannelInboundHandlerAdapter { @Override public void exceptionCaught(ChannelHandlerContext ctx, Throwable cause) throws Exception { System.err.println(this.getClass().getName()+"---"+cause.getMessage()); ctx.fireExceptionCaught(cause); } } public class ServerHandlerC extends ChannelInboundHandlerAdapter { @Override public void exceptionCaught(ChannelHandlerContext ctx, Throwable cause) throws Exception { System.err.println(this.getClass().getName()+"---"+cause.getMessage()); ctx.fireExceptionCaught(cause); } }

而后咱们在ServerHandlerA的channelRead方法中执行ctx的write方法，模拟异常事件的发生。微信

 @Override public void channelRead(ChannelHandlerContext ctx, Object object) { ctx.fireExceptionCaught(new Throwable("出现异常")); //ctx.pipeline().fireExceptionCaught(new Throwable("出现异常"));
 }

咱们首先看下运行结果ide

ctx.fireExceptionCaughtoop

io.netty.example.echo.my.ServerHandlerB---出现异常 io.netty.example.echo.my.ServerHandlerC---出现异常 io.netty.example.echo.my.OutHandlerB---出现异常 io.netty.example.echo.my.OutHandlerC---出现异常 18:34:17.147 [nioEventLoopGroup-3-1] WARN  i.n.channel.DefaultChannelPipeline - 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. java.lang.Throwable: 出现异常 at io.netty.example.echo.my.ServerHandlerA.channelRead(ServerHandlerA.java:39) at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:363) at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:348) at io.netty.channel.AbstractChannelHandlerContext.fireChannelRead(AbstractChannelHandlerContext.java:338) at io.netty.channel.DefaultChannelPipeline$HeadContext.channelRead(DefaultChannelPipeline.java:1424) at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:363) at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:348) at io.netty.channel.DefaultChannelPipeline.fireChannelRead(DefaultChannelPipeline.java:944) at io.netty.channel.nio.AbstractNioByteChannel$NioByteUnsafe.read(AbstractNioByteChannel.java:163) at io.netty.channel.nio.NioEventLoop.processSelectedKey(NioEventLoop.java:709) at io.netty.channel.nio.NioEventLoop.processSelectedKeysOptimized(NioEventLoop.java:639) at io.netty.channel.nio.NioEventLoop.processSelectedKeys(NioEventLoop.java:553) at io.netty.channel.nio.NioEventLoop.run(NioEventLoop.java:510) at io.netty.util.concurrent.SingleThreadEventExecutor$5.run(SingleThreadEventExecutor.java:912) at io.netty.util.concurrent.FastThreadLocalRunnable.run(FastThreadLocalRunnable.java:30) at java.lang.Thread.run(Thread.java:748)

ctx.pipeline().fireExceptionCaughtthis

io.netty.example.echo.my.OutHandlerA---出现异常 io.netty.example.echo.my.ServerHandlerA---出现异常 io.netty.example.echo.my.ServerHandlerB---出现异常 io.netty.example.echo.my.ServerHandlerC---出现异常 io.netty.example.echo.my.OutHandlerB---出现异常 io.netty.example.echo.my.OutHandlerC---出现异常 20:08:53.723 [nioEventLoopGroup-3-1] WARN  i.n.channel.DefaultChannelPipeline - 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. java.lang.Throwable: 出现异常 at io.netty.example.echo.my.ServerHandlerA.channelRead(ServerHandlerA.java:40) at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:363) at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:348) at io.netty.channel.AbstractChannelHandlerContext.fireChannelRead(AbstractChannelHandlerContext.java:338) at io.netty.channel.DefaultChannelPipeline$HeadContext.channelRead(DefaultChannelPipeline.java:1424) at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:363) at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:348) at io.netty.channel.DefaultChannelPipeline.fireChannelRead(DefaultChannelPipeline.java:944) at io.netty.channel.nio.AbstractNioByteChannel$NioByteUnsafe.read(AbstractNioByteChannel.java:163) at io.netty.channel.nio.NioEventLoop.processSelectedKey(NioEventLoop.java:709) at io.netty.channel.nio.NioEventLoop.processSelectedKeysOptimized(NioEventLoop.java:639) at io.netty.channel.nio.NioEventLoop.processSelectedKeys(NioEventLoop.java:553) at io.netty.channel.nio.NioEventLoop.run(NioEventLoop.java:510) at io.netty.util.concurrent.SingleThreadEventExecutor$5.run(SingleThreadEventExecutor.java:912) at io.netty.util.concurrent.FastThreadLocalRunnable.run(FastThreadLocalRunnable.java:30) at java.lang.Thread.run(Thread.java:748)

根据输出结果能够看出ctx.fireExceptionCaught 会从异常产生的ChannelHandler一直日后传播到tail尾节点，ctx.pipeline().fireExceptionCaught会从管道中第一个节点一直日后传播到tail尾节点，而上面结果中打印的异常信息则是在TailContext尾节点中统一处理的。spa

二、异常事件传播的分析

ctx.pipeline().fireExceptionCaught与ctx.fireExceptionCaught两种传播异常方法.net

前者调用的是DefaultChannelPipeline 的 fireExceptionCaught方法线程

 @Override public final ChannelPipeline fireExceptionCaught(Throwable cause) { AbstractChannelHandlerContext.invokeExceptionCaught(head, cause); return this; }

后者调用的是AbstractChannelHandlerContext 的 fireExceptionCaught方法debug

 @Override public ChannelHandlerContext fireExceptionCaught(final Throwable cause) { invokeExceptionCaught(next, cause); return this; }

能够看到DefaultChannelPipeline的fireExceptionCaught方法中默认传入了head头部节点，因此ctx.pipeline().fireExceptionCaught会从管道中第一个节点开始向后传播。

咱们进入invokeExceptionCaught方法内部看下具体实现

static void invokeExceptionCaught(final AbstractChannelHandlerContext next, final Throwable cause) { ObjectUtil.checkNotNull(cause, "cause");//检查异常是否为空
        EventExecutor executor = next.executor(); if (executor.inEventLoop()) {//判断是否与当前线程一直
            next.invokeExceptionCaught(cause);//触发回调，触发下一个AbstractChannelHandlerContext节点中handler的异常处理事件
        } else { try { executor.execute(new Runnable() {//若是线程不一致，由其绑定的executor执行
 @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); } } }

invokeExceptionCaught方法内部实现

private void invokeExceptionCaught(final Throwable cause) { if (invokeHandler()) {//判断当前handler的状态
            try { handler().exceptionCaught(this, cause);//调用exceptionCaught方法实现
            } 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); } }

三、异常处理机制的设计

经过上面的分析咱们能够看到若是经过ctx.fireExceptionCaught一直向后传递异常事件，最终会触发尾节点的exceptionCaught事件打印异常日志；

 @Override public void exceptionCaught(ChannelHandlerContext ctx, Throwable cause) { 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); } }

在实际项目中咱们能够在ChannelPipeline尾部增长一个异常处理handle用来统一处理异常信息;

public void initChannel(SocketChannel ch) throws Exception { ChannelPipeline p = ch.pipeline(); // p.addLast(new LoggingHandler(LogLevel.INFO)); // 向ChannelPipeline中添加自定义channelHandler
                            p.addLast(new OutHandlerA()); p.addLast(new ServerHandlerA()); p.addLast(new ServerHandlerB()); p.addLast(new ServerHandlerC()); p.addLast(new OutHandlerB()); p.addLast(new OutHandlerC()); p.addLast(new ExceptionHandler()); }

经过以上三点内容咱们对异常信息在ChannelPipeline中的传播进行了模拟，梳理事件的传播流程以及应该怎样统一处理异常信息，其中若有不足与不正确的地方还望指出与海涵。

关注微信公众号，查看更多技术文章。