netty应该怎样处理异常

netty应该怎样处理异常

netty 在ChannelInBouindHandler中为我们提供了exceptionCaught()方法，看下源码来看看他在什么情况下会被调用

首先点开 ctx.fireChannelRead(msg);的源码查看,发现

@Override
   public ChannelHandlerContext fireChannelRead(final Object msg) {
       invokeChannelRead(findContextInbound(MASK_CHANNEL_READ), msg);
       return this;
   }

   static void invokeChannelRead(final AbstractChannelHandlerContext next, Object msg) {
       final Object m = next.pipeline.touch(ObjectUtil.checkNotNull(msg, "msg"), next);
       EventExecutor executor = next.executor();
       if (executor.inEventLoop()) {
           next.invokeChannelRead(m);
       } else {
           executor.execute(new Runnable() {
               @Override
               public void run() {
                   next.invokeChannelRead(m);
               }
           });
       }
   }

   private void invokeChannelRead(Object msg) {
       if (invokeHandler()) {
           try {
               ((ChannelInboundHandler) handler()).channelRead(this, msg);
           } catch (Throwable t) {
               notifyHandlerException(t);
           }
       } else {
           fireChannelRead(msg);
       }
   }

上面代码会在pinline中查找下一个inHandler，并回掉他的ChannelRead方法,且用try catch处理回掉过程。
如果报错会调用notifyHandlerException(t)方法

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;
       }

       invokeExceptionCaught(cause);
   }

   private void invokeExceptionCaught(final Throwable cause) {
       if (invokeHandler()) {
           try {
               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);
       }
   }

notifyHandlerException(t)方法会调用handler()即当前handler的exceptionCaught(this, cause);方法

如果当前handler实现ChannelInboundHandlerAdapter且没实现exceptionCaught方法的话，默认会调用 ctx.fireExceptionCaught(cause),即下一个handler的exceptionCaught方法。

总结

​ netty中执行handler报错时，会先调用当前handler的exceptionCaught方法，如果没有当前没重写exceptionCaught方法会调用下一个handler的exceptionCaught方法。
​ 所以如果错误需要在当前handler处理，则重写exceptionCaught方法，自己选择要不要将异常传递下去，
如果当前handler不能处理，可以在pipline链最后一位放置一个重写exceptionCaught方法的handler
，放在pipline最后一位即可。

​

​ 有一种情况下，ChannelFutureListener.FIRE_EXCEPTION_ON_FAILURE，这是一个监听器，它会将错误从head节点向下传播。