Netty源码分析之ChannelPipeline(五)—异常事件的传播
ChannelHandler中异常的获取与处理是通过继承重写exceptionCaught方法来实现的，本篇文章我们对ChannelPipeline中exceptionCaught异常事件的传播进行梳理分析
1、出站事件的传播示例
首先我们继续在之前的代码上进行改造，模拟异常事件的传播
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); } }
然后我们在ServerHandlerA的channelRead方法中执行ctx的write方法，模拟异常事件的发生。
@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")); }
我们首先看下运行结果
ctx.fireExceptionCaught
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$5.run(SingleThreadEventExecutor.java:912) at io.netty.util.concurrent.FastThreadLocalRunnable.run(FastThreadLocalRunnable.java:30) at java.lang.Thread.run(Thread.java:748)
ctx.pipeline().fireExceptionCaught
io.netty.example.echo.my.OutHandlerA---出现异常 io.netty.example.echo.my.ServerHandlerA---出现异常 io.netty.example.echo.my.ServerHandlerB---出现异常 io.netty.example.echo.my.ServerHandlerC---出现异常 io.netty.example.echo.my.OutHandlerB---出现异常 io.netty.example.echo.my.OutHandlerC---出现异常 20:08:53.723 [nioEventLoopGroup-3-1] WARN i.n.channel.DefaultChannelPipeline - An exceptionCaught() event was fired, and it reached at the tail of the pipeline. It usually means the last handler in the pipeline did not handle the exception. java.lang.Throwable: 出现异常 at io.netty.example.echo.my.ServerHandlerA.channelRead(ServerHandlerA.java:40) at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:363) at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:348) at io.netty.channel.AbstractChannelHandlerContext.fireChannelRead(AbstractChannelHandlerContext.java:338) at io.netty.channel.DefaultChannelPipeline$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)
根据输出结果可以看出ctx.fireExceptionCaught 会从异常产生的ChannelHandler一直往后传播到tail尾节点，ctx.pipeline().fireExceptionCaught会从管道中第一个节点一直往后传播到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。
2、异常事件传播的分析
ctx.pipeline().fireExceptionCaught与ctx.fireExceptionCaught两种传播异常方法
前者调用的是DefaultChannelPipeline 的 fireExceptionCaught方法
@Override public final ChannelPipeline fireExceptionCaught(Throwable cause) { AbstractChannelHandlerContext.invokeExceptionCaught(head, cause); return this; }
后者调用的是AbstractChannelHandlerContext 的 fireExceptionCaught方法
@Override public ChannelHandlerContext fireExceptionCaught(final Throwable cause) { invokeExceptionCaught(next, cause); return this; }
可以看到DefaultChannelPipeline的fireExceptionCaught方法中默认传入了head头部节点，所以ctx.pipeline().fireExceptionCaught会从管道中第一个节点开始向后传播。
我们进入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() {//如果线程不一致，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); } }
3、异常处理机制的设计
通过上面的分析我们可以看到如果通过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); } }
在实际项目中我们可以在ChannelPipeline尾部增加一个异常处理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()); }
通过以上三点内容我们对异常信息在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，其中如有不足与不正确的地方还望指出与海涵。
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