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The following exception:

java.lang.IllegalStateException: SRMSG00028: The subscription to ... has been cancelled
at io.smallrye.reactive.messaging.extension.AbstractEmitter.verify(AbstractEmitter.java:165)
at io.smallrye.reactive.messaging.extension.AbstractEmitter.emit(AbstractEmitter.java:144)
at io.smallrye.reactive.messaging.extension.EmitterImpl.send(EmitterImpl.java:29)
at airhacks.kafka.jaxrs.boundary.MessagesResource.send(MessagesResource.java:34)
at airhacks.kafka.jaxrs.boundary.MessagesResource_Subclass.send$$superaccessor1(MessagesResource_Subclass.zig:209)
at airhacks.kafka.jaxrs.boundary.MessagesResource_Subclass$$function$$3.apply(MessagesResource_Subclass$$function$$3.zig:33)    

is caused by an injected Emitter into a RequestScoped managed bean:

import javax.annotation.PreDestroy;
import javax.enterprise.context.ApplicationScoped;
import javax.enterprise.context.RequestScoped;
import javax.inject.Inject;
import javax.ws.rs.*;

import org.eclipse.microprofile.reactive.messaging.Channel;
import org.eclipse.microprofile.reactive.messaging.Emitter;

@RequestScoped
@Path("messages")
@Consumes(MediaType.TEXT_PLAIN)
public class MessagesResource {

    @Inject
    @Channel("messages")
    Emitter<String> messageEmitter;
    
    @POST
    public void send(String message) {
        this.messageEmitter.send(message);
    }

    @PreDestroy
    public void closeStream(){
        this.messageEmitter.complete();
    }
}

Changing the scope to ApplicationScoped solves the problem:

@ApplicationScoped
@Path("messages")
@Consumes(MediaType.TEXT_PLAIN)
public class MessagesResource{
    //...
}

Example was tested with: quarkus.io and configured SmallRye Apache Kafka connector (mp.messaging.outgoing.messages.connector=smallrye-kafka).

devdocs.io is a useful documentation / API search Progressive Web App (PWA) with offline search capabilities.

devdocs.io is not only usable as a search utility for Web Components or webstandards APIs, also Java 11 javadoc is available for offline search:

The following snippet converts a byte[] array to a Stream using ByteBuffer:

import java.nio.ByteBuffer;
import java.util.stream.Collectors;
import java.util.stream.Stream;

import org.junit.jupiter.api.Test;

public class ByteArrayTest {
    
    @Test
    public void byteArrayToStream(){
        byte bytes[] = new byte[]{0xC,0xA,0xF,0xE,0xB,0xA,0xB,0xE};
        var buffer = ByteBuffer.wrap(bytes);
        var message = Stream.generate(() -> buffer.get()).
                        limit(buffer.capacity()).
                        map(b -> Integer.toHexString(b)).
                        collect(Collectors.joining());
        System.out.println(message);
    }    

Output: cafebabe

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Quarkus started in development (quarkus:dev) mode with a JAX-RS endpoint exposed, opens two ports: (http) 8080 and (debug) 5005 per default.

You will have to assign distinct ports for every new launched instance. The http port is configurable in the application.properties: quarkus.http.port=8282 or environment entries (not suitable for our purpose), the debug port can be changed (-Ddebug=6000) or deactivated from the command line only: mvn compile -Ddebug=false quarkus:dev

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   conference session 20 Jan 2021
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2. airhacks.tv: #83rd airhacks.tv Questions and Answers [online event]
   live questions and answers show 8 Feb 2021
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   conference session 16 Mar 2021
   https://programm.javaland.eu/2021/#/scheduledEvent/606335
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   live, interactive, virtual workshop 16 Mar 2021
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5. airhacks.live: Building Event-Driven Applications with Streams, Logs and Messages [online event]
   live, interactive, virtual workshop 23 Mar 2021
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   live, interactive, virtual workshop 15 Apr 2021
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   live, interactive, virtual workshop 6 May 2021
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Update from Java 14 with --enable-preview to Java 15, fixing Apache CXFs:

java.lang.IllegalAccessError: class org.apache.cxf.microprofile.client.MicroProfileClientProviderFactory$$Lambda$345/0x0000000800c309b0 tried to access protected method 'int org.apache.cxf.jaxrs.provider.ProviderFactory.compareCustomStatus(org.apache.cxf.jaxrs.model.ProviderInfo, org.apache.cxf.jaxrs.model.ProviderInfo)' (org.apache.cxf.microprofile.client.MicroProfileClientProviderFactory$$Lambda$345/0x0000000800c309b0 and org.apache.cxf.jaxrs.provider.ProviderFactory are in unnamed module of loader 'app'

A free bonus video from the microprofile.training video course:

Also checkout the corresponding repository: github.com/adambien/microprofile.training.

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Building a Java backend service with familiar APIs on a next generation runtime: