Apache kafka - print from java application to console

Question

I am running ubuntu 18 and using java (intellij IDE) and writing a basic kafka application. I am trying to use the basic example from here , and to stream information to the application, and then print something to the screen, I am running the application using intellij "run" command.

When I connent the application to an output stream, it works fine and I manage to output information to the terminal.

I tried adding System.out.println() in the foreach methods, in the apply methods, it's not working, I am adding breakpoints inside them and running debug mode, and it's not getting there, I guess the flow does not get there during the run.

I am streaming information to the application using the proper topic, and prints that I put in the application outside the apply and foreach are working fine.

How can I get the application to print something each time I stream information to it? the main idea is to process something and print the results to the monitor and not to a kafka stream

Here is the code:

import org.apache.kafka.clients.producer.KafkaProducer;
import org.apache.kafka.clients.consumer.ConsumerConfig;
import org.apache.kafka.common.serialization.Serdes;

import org.apache.kafka.streams.KafkaStreams;
import org.apache.kafka.streams.StreamsBuilder;
import org.apache.kafka.streams.StreamsConfig;
import org.apache.kafka.streams.kstream.*;
import org.apache.kafka.streams.kstream.Printed;

import java.util.Arrays;
import java.util.Locale;
import java.util.Properties;
import java.util.concurrent.CountDownLatch;

public class main {
    public static void main(String[] args) throws Exception {
        Properties props = new Properties();
        props.put(StreamsConfig.APPLICATION_ID_CONFIG, "streams-wordcount");
        props.put(StreamsConfig.BOOTSTRAP_SERVERS_CONFIG, "localhost:9092");
        props.put(StreamsConfig.CACHE_MAX_BYTES_BUFFERING_CONFIG, 0);
        props.put(StreamsConfig.DEFAULT_KEY_SERDE_CLASS_CONFIG, Serdes.String().getClass().getName());
        props.put(StreamsConfig.DEFAULT_VALUE_SERDE_CLASS_CONFIG, Serdes.String().getClass().getName());

        // setting offset reset to earliest so that we can re-run the demo code with the same pre-loaded data
        // Note: To re-run the demo, you need to use the offset reset tool:
        // https://cwiki.apache.org/confluence/display/KAFKA/Kafka+Streams+Application+Reset+Tool
        props.put(ConsumerConfig.AUTO_OFFSET_RESET_CONFIG, "earliest");

        StreamsBuilder builder = new StreamsBuilder();

        KStream<String, String> source = builder.stream("streams-plaintext-input");

        source.foreach(new ForeachAction<String, String>() {
            @Override
            public void apply(String key, String value) {
                System.out.println("yeah");
            }
        });

        KTable<String, Long> counts = source
                .flatMapValues(new ValueMapper<String, Iterable<String>>() {
                    @Override
                    public Iterable<String> apply(String value) {
                        return Arrays.asList(value.toLowerCase(Locale.getDefault()).split(" "));
                    }
                })
                .groupBy(new KeyValueMapper<String, String, String>() {
                    @Override
                    public String apply(String key, String value) {
                        System.out.println("what");
                        return value;
                    }
                })
                .count();
        //System.exit(0);
        // need to override value serde to Long type
        System.out.println("what");
        //counts.toStream().to("streams-wordcount-output", Produced.with(Serdes.String(), Serdes.Long()));



        final KafkaStreams streams = new KafkaStreams(builder.build(), props);
        final CountDownLatch latch = new CountDownLatch(1);

        // attach shutdown handler to catch control-c
        Runtime.getRuntime().addShutdownHook(new Thread("streams-wordcount-shutdown-hook") {
            @Override
            public void run() {
                streams.close();
                latch.countDown();
            }
        });

        try {
            streams.start();
            latch.await();
        } catch (Throwable e) {
            System.exit(2);
        }
        System.exit(0);
    }


}

Answer 1

KafkaStreams provides output console print dsl

KStream<String, String> source = builder.stream("streams-plaintext-input");

source.print(Printed.toSysOut());
KTable<String, Long> counts = source
    .flatMapValues(new ValueMapper<String, Iterable<String>>() {
...

test result

./kafka-console-producer --broker-list localhost:9092 --topic streams-plaintext-input

input1
input2

Intellij Console Result

[KSTREAM-SOURCE-0000000000]: null, input1
[KSTREAM-SOURCE-0000000000]: null, input2