Does anyone know how a message could be discarded in Netty? By discard I mean that it would not be further forwarded on the channel to other handlers in the pipeline.
Is this the way that I can do it?
public class DiscardInboundHandler extends ChannelInboundHandlerAdapter {
#Override
public void channelRead(ChannelHandlerContext ctx,
Object msg) {
ReferenceCountUtil.release(msg);
}
}
It is the responsibility of the user to call ReferenceCountUtil.release(message) if the message is
consumed and not passed to the next ChannelOutboundHandler in the ChannelPipleline. Once the message is passed over to the actual Transport it will be released automatically by it once the message was written or the Channel was closed.
Source: Netty in Action book
Related
This is a simple question. Netty seems to be a great tool for sending information between servers and clients. I want to send informations, and also events..or rather called instructions what do to.
public class PojoServerHandler extends ChannelInboundHandlerAdapter {
private Logger logger = LoggerFactory.getLogger(getClass());
#Override
public void channelRead(ChannelHandlerContext ctx, Object msg) throws Exception {
Message body = (Message) msg;
logger.info("server read msg id:{}, body:{}", body.getId(), body.getBody());
Message response = new Message();
response.setId(1024);
response.setFrom("server");
response.setBody("hello from server");
ctx.writeAndFlush(response);
}
#Override
public void exceptionCaught(ChannelHandlerContext ctx, Throwable cause) throws Exception {
logger.error("server caught exception", cause);
ctx.close();
}
}
I have this code here and it describes a handler for a server. When the client send its message to the server. The method channelRead going to be called directly.
Is it smart to implements a switch case inside channelRead if I want to use other functions around my application, or can Netty do that in another way?
It is possible to have multiple handlers each doing a different task instead of having a switch case. You can simply call
ctx.fireChannelRead(msg);
if you plan not to handle the message in a particular handler, then it will trigger the next handler in the pipeline.
I am writing a client using netty for handling custom protocol. I have defined a handler which extends SimpleChannelInboundHandler which handles both sending and receiving message.
public class ClientHandler extends SimpleChannelInboundHandler {
#Override
protected void channelRead0(ChannelHandlerContext channelHandlerContext, Object o) throws Exception {
log.info("Client received: " + ((ByteBuf)o).toString(CharsetUtil.UTF_8));
System.out.println("Client received: " + ((ByteBuf)o).toString(CharsetUtil.UTF_8));
}
#Override
public void channelActive(ChannelHandlerContext channelHandlerContext){
log.info("Client sent: $"+ new MessageRequest().toString() +"$");
channelHandlerContext.writeAndFlush(Unpooled.copiedBuffer((new MessageRequest().toString()), CharsetUtil.UTF_8));
}
#Override
public void exceptionCaught(ChannelHandlerContext channelHandlerContext, Throwable cause){
cause.printStackTrace();
channelHandlerContext.close();
}
}
This handler is able to print the response to the console. But since I am writing an client which will be used by another service, I need to send the response to the service which is calling my client.
Kindly help me in sending the response received to the calling service.
You can store the reference of your listening service in your ClientHandler class and call a setMessage method of the service class to give it the message from the channelRead0 method of your handler.
A better approach would be to use the Observer pattern
I want to send more than one response to client based on back end process. But in Netty examples I saw echo server is sending back the response at the same time.
My requirement is, I need to validate the client and send him OK response, then send him the DB updates when available.
How can I send more responses to client? Pls direct me to an example or any guide?
at every point in your pipeline you can get the pipeline Channel object from the MessageEvent object (or ChannelEvent) which is passed from handler to handler. you can use this information to send multiple responses at different points in the pipeline.
if we take the echo server example as a base, we can add a handler which send the echo again (that can be done also in the same handler, but the example is to show that multiple handlers can respond).
public class EchoServerHandler extends ChannelHandlerAdapter {
public void messageReceived(ChannelHandlerContext ctx, MessageEvent e) {
Channel ch = e.getChannel();
// first message
ch.write(e.getMessage());
}
// ...
}
public class EchoServerHandler2 extends ChannelHandlerAdapter {
public void messageReceived(ChannelHandlerContext ctx, MessageEvent e) {
Channel ch = e.getChannel();
// send second message
ch.write(e.getMessage());
}
// ...
}
You can do that as long as you have the reference to the relevant Channel (or ChannelHandlerContext). For example, you can do this:
public class MyHandler extends ChannelHandlerAdapter {
...
public void channelRead(ctx, msg) {
MyRequest req = (MyRequest) msg;
ctx.write(new MyFirstResponse(..));
executor.execute(new Runnable() {
public void run() {
// Perform database operation
..
ctx.write(new MySecondResponse(...));
}
}
}
...
}
You can do this as long as Netty doesn't close the Channel. Its better you call close() yourself when you're done.
Here's a sample: https://stackoverflow.com/a/48128514/2557517
for (int i = 1; i <= 100; i++) {
ctx.writeAndFlush(Unpooled.copiedBuffer(Integer.toString(i).getBytes(Charsets.US_ASCII)));
}
ctx.writeAndFlush(Unpooled.copiedBuffer("ABCD".getBytes(Charsets.US_ASCII))).addListener(new ChannelFutureListener() {
#Override
public void operationComplete(ChannelFuture future) throws Exception {
ctx.channel().close();
}
});
I write this in the channelRead() mehtod of my netty server handler, it will reponse "12345...100ABCD" back to the client as soon as the server receive a request.
As far as I see, the order of the message client received from the netty server is always "12345...100ABCD".
I don't know is this just by chance? Maybe sometime it would be "32451...ABCD100" (out of the server write order)?
Is it possible that the server execute
clientChannel.writeAndFlush(msg1);
clientChannel.writeAndFlush(msg2);
clientChannel.writeAndFlush(msg3);
but the client received msg2-msg1-msg3 or msg3-msg1-msg2 but not the write order msg1-msg2-msg3
In the proxy sample of netty project, https://github.com/netty/netty/tree/master/example/src/main/java/io/netty/example/proxy
the HexDumpProxyBackendHandler writes:
#Override
public void channelRead(final ChannelHandlerContext ctx, Object msg) throws Exception {
inboundChannel.writeAndFlush(msg).addListener(new ChannelFutureListener() {
#Override
public void operationComplete(ChannelFuture future) throws Exception {
if (future.isSuccess()) {
ctx.channel().read();
} else {
future.channel().close();
}
}
});
}
It makes sure that it trigger next channelRead() (That is inboundChannel.writeAndFlush(msg) in channelRead()) only if the wirteAndFlush() operation is finished.
So what's the purpose to write ctx.channel().read() in the listener and execute it when future.isSuccess() ? Isn't it to make sure that the messages writes to the client are received in a right order?
If I change it to
#Override
public void channelRead(final ChannelHandlerContext ctx, Object msg) throws Exception {
inboundChannel.writeAndFlush(msg);
ctx.channel().read();
}
Will it cause some issues?
No it is not possible. TCP ensures that.
As EJP states either technique should guarantee the ordering. The difference between the example and how you've changed it is a question of flow control.
In the original example the inbound channel will only be read after the data has been successfully flushed to the network buffers. This guarantees that it only reads data as fast as it can send it, preventing Netty's send queue from building up and thus preventing out of memory errors.
The altered code reads as soon as the write operation is queued. If the outbound channel is unable to keep up there's a chance you could see out of memory errors if you're transferring a lot of data.
I am trying to post messages to some existing actors like show below but there may be a chance to to refer non existing actor and i would like to know before posting the message.
Thanks in advance
actor = getContext().actorFor("actorSystem/user/" + nameOfActor);
actor.tell("message",getSelf());
You can send them Identify message prior to sending your actual message. All actors understand it and should reply with Self(). Alternatively use resolveOne method:
You can acquire an ActorRef for an ActorSelection with the
resolveOne method of the ActorSelection. It returns a Future of the
matching ActorRef if such an actor exists. It is completed with
failure [[akka.actor.ActorNotFound]] if no such actor exists or the
identification didn't complete within the supplied timeout.
The only possible way of knowing that an actor is alive (without DeathWatch) is by receiving a message from it. And that only proves that the actor was alive at some point in time (when it has sent the message).
The following is snippet of my code how i implemented used DeadLettersHandler actor to handle DeadLetters
public class MyActor extends UntypedActor
{
#Override
public void onReceive(Object message) throws Exception
{
System.out.println("MyActor received : "+message.toString());
}
}
public class DeadLettersHandler extends UntypedActor
{
public void onReceive(Object deadLetter) throws Exception
{
System.out.println("DeadLettersHandler received : "+deadLetter.toString());
}
}
public class DeadLetterTest
{
public static void main(String[] args)
{
ActorSystem MyActorSystem = ActorSystem.create("MyActorSystem");
ActorRef existingActor = MyActorSystem.actorOf(Props.create(MyActor.class),"ExistingActor");
ActorRef DLH = MyActorSystem.actorOf(Props.create(DeadLettersHandler.class), "DeadLetterHandler");
MyActorSystem.eventStream().subscribe(DLH, DeadLetter.class);
ActorSelection nonExist = MyActorSystem.actorSelection("akka://user/MyActorSystem/NonExistingActor");
existingActor.tell("Hello Akka", existingActor);
nonExist.tell("Hello Akka", DLH);
MyActorSystem.shutdown();
}
}
output:
MyActor received : Hello Akka
DeadLettersHandler received : DeadLetter(Hello Akka,Actor[akka://MyActorSystem/user/DeadLetterHandler#-3707992],Actor[akka://MyActorSystem/deadLetters])
[INFO] [10/10/2013 15:43:43.343] [MyActorSystem-akka.actor.default-dispatcher-6] [akka://MyActorSystem/deadLetters] Message [java.lang.String] from Actor[akka://MyActorSystem/user/DeadLetterHandler#-3707992] to Actor[akka://MyActorSystem/deadLetters] was not delivered. [1] dead letters encountered. This logging can be turned off or adjusted with configuration settings 'akka.log-dead-letters' and 'akka.log-dead-letters-during-shutdown'.
Actually I need to handle the every message received, if a actor not exists then that message has to be handled separately which i achied using Deadletter implementation
final ActorRef actor = actorSystem.actorOf(new Props(DeadLetterHandlerActor.class));
actorSystem.eventStream().subscribe(actor, DeadLetter.class);