Update KafkaApplication.java (#12424)

* Update KafkaApplication.java

Added partition offset to the listener so that it can start consuming the messages from the beginning of the topic.

* Revert "Update KafkaApplication.java"

This reverts commit 28801f8517.

* Counting Messages

1. Counting messages from Producer offset
2. Counting messages from Consumer
3. Rest APIs to view the count

* Refactoring

* Adding 3rd programmatic method to count number of messages in Kafka topic

* Removing irrelevant code

* Server port removed

spring-kafka/src/main/java/com/baeldung/countingmessages/Application.java

@@ -0,0 +1,11 @@
+package com.baeldung.countingmessages;
+
+import org.springframework.boot.SpringApplication;
+import org.springframework.boot.autoconfigure.SpringBootApplication;
+
+@SpringBootApplication
+public class Application {
+    public static void main(String[] args){
+        SpringApplication.run(Application.class,args);
+    }
+}

spring-kafka/src/main/java/com/baeldung/countingmessages/KafkaCountingMessagesComponent.java

@@ -0,0 +1,52 @@
+package com.baeldung.countingmessages;
+
+import org.apache.kafka.clients.consumer.ConsumerConfig;
+import org.apache.kafka.common.TopicPartition;
+import org.apache.kafka.common.serialization.StringDeserializer;
+import org.springframework.beans.factory.annotation.Value;
+import org.springframework.stereotype.Component;
+
+import javax.annotation.PostConstruct;
+import java.util.Collections;
+import java.util.HashMap;
+import java.util.List;
+import java.util.Map;
+import java.util.function.Function;
+import java.util.stream.Collectors;
+
+
+@Component
+public class KafkaCountingMessagesComponent {
+    @Value(value = "${kafka.bootstrapAddress}")
+    private String bootstrapAddress;
+
+    public static Map<String, Object> props = new HashMap<>();
+
+    @PostConstruct
+    public void init(){
+        System.out.println(getTotalNumberOfMessagesInATopic("baeldung"));
+    }
+
+    public Long getTotalNumberOfMessagesInATopic(String topic){
+        org.apache.kafka.clients.consumer.KafkaConsumer<String, String> consumer = new org.apache.kafka.clients.consumer.KafkaConsumer<>(getProps());
+        List<TopicPartition> partitions = consumer.partitionsFor(topic).stream()
+                .map(p -> new TopicPartition(topic, p.partition()))
+                .collect(Collectors.toList());
+        consumer.assign(partitions);
+        consumer.seekToEnd(Collections.emptySet());
+        Map<TopicPartition, Long> endPartitions = partitions.stream()
+                .collect(Collectors.toMap(Function.identity(), consumer::position));
+        return partitions.stream().mapToLong(p -> endPartitions.get(p)).sum();
+    }
+
+    public Map<String, Object> getProps() {
+        if (props.isEmpty()) {
+            props.put(ConsumerConfig.BOOTSTRAP_SERVERS_CONFIG, bootstrapAddress);
+            props.put(ConsumerConfig.KEY_DESERIALIZER_CLASS_CONFIG, StringDeserializer.class);
+            props.put(ConsumerConfig.VALUE_DESERIALIZER_CLASS_CONFIG, StringDeserializer.class);
+            props.put(ConsumerConfig.MAX_PARTITION_FETCH_BYTES_CONFIG, "20971520");
+            props.put(ConsumerConfig.FETCH_MAX_BYTES_CONFIG, "20971520");
+        }
+        return props;
+    }
+}