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1. Overview

In this quick article, we’ll be looking at exception handling in Netty.

Simply put, Netty is a framework for building high-performance asynchronous and event-driven network applications. I/O operations are handled inside its life-cycle using callback methods.

More details about the framework and how to get started with it can be found in our previous article here.

2. Handling Exceptions in Netty

As mentioned earlier, Netty is an event-driven system and has callback methods for specific events. Exceptions are such events too.

Exceptions can occur while processing data received from the client or during I/O operations. When this happens, a dedicated exception-caught event is fired.

2.1. Handling Exceptions in the Channel of Origin

The exception-caught event, when fired, is handled by the exceptionsCaught() method of the ChannelInboundHandler or its adapters and subclasses.

Note that the callback has been deprecated in the ChannelHandler interface. It’s now limited to the ChannelInboudHandler interface.

The method accepts a Throwable object and a ChannelHandlerContext object as parameters. The Throwable object could be used to print the stack trace or get the localized error message.

So let’s create a channel handler, ChannelHandlerA and override its exceptionCaught() with our implementation:

public void exceptionCaught(ChannelHandlerContext ctx, Throwable cause) 
  throws Exception {
 
    logger.info(cause.getLocalizedMessage());
    //do more exception handling
    ctx.close();
}

In the code snippet above, we logged the exception message and also call the close() of the ChannelHandlerContext.

This will close the channel between the server and the client. Essentially causing the client to disconnect and terminate.

2.2. Propagating Exceptions

In the previous section, we handled the exception in its channel of origin. However, we can actually propagate the exception on to another channel handler in the pipeline.

Instead of logging the error message and calling ctx.close(), we’ll use the ChannelHandlerContext object to fire another exception-caught event manually.

This will cause the exceptionCaught() of the next channel handler in the pipeline to be invoked.

Let’s modify the code snippet in ChannelHandlerA to propagate the event by calling the ctx.fireExceptionCaught():

public void exceptionCaught(ChannelHandlerContext ctx, Throwable cause) 
  throws Exception {
 
    logger.info("Exception Occurred in ChannelHandler A");
    ctx.fireExceptionCaught(cause);
}

Furthermore, let’s create another channel handler, ChannelHandlerB and override its exceptionCaught() with this implementation:

@Override
public void exceptionCaught(ChannelHandlerContext ctx, Throwable cause) 
  throws Exception {
 
    logger.info("Exception Handled in ChannelHandler B");
    logger.info(cause.getLocalizedMessage());
    //do more exception handling
    ctx.close();
}

In the Server class, the channels are added to the pipeline in the following order:

ch.pipeline().addLast(new ChannelHandlerA(), new ChannelHandlerB());

Propagating exception-caught events manually is useful in cases where all exceptions are being handled by one designated channel handler.

3. Conclusion

In this tutorial, we’ve looked at how to handle exceptions in Netty using the callback method and how to propagate the exceptions if needed.

The complete source code is available over on Github.

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