This topic describes how a Java client uses SDK for Java to connect to the default endpoint of Message Queue for Apache Kafka in a virtual private cloud (VPC) and send and subscribe to messages.

Prerequisites

Install Java dependencies

Add the following dependencies to the pom.xml file:
<dependency>
    <groupId>org.apache.kafka</groupId>
    <artifactId>kafka-clients</artifactId>
    <version>2.4.0</version>
</dependency>
<dependency>
    <groupId>org.slf4j</groupId>
    <artifactId>slf4j-log4j12</artifactId>
    <version>1.7.6</version>
</dependency>
Note We recommend that you keep the version of the client consistent with that of the broker. This means that the client library version must be consistent with the major version of the Message Queue for Apache Kafka instance. You can obtain the major version of the Message Queue for Apache Kafka instance on the Instance Details page in the Message Queue for Apache Kafka console.

Preparations

  1. Create a Log4j configuration file named log4j.properties.
    # Licensed to the Apache Software Foundation (ASF) under one or more
    # contributor license agreements.  See the NOTICE file distributed with
    # this work for additional information regarding copyright ownership.
    # The ASF licenses this file to You under the Apache License, Version 2.0
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    #
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    # distributed under the License is distributed on an "AS IS" BASIS,
    # WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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    log4j.rootLogger=INFO, STDOUT
    
    log4j.appender.STDOUT=org.apache.log4j.ConsoleAppender
    log4j.appender.STDOUT.layout=org.apache.log4j.PatternLayout
    log4j.appender.STDOUT.layout.ConversionPattern=[%d] %p %m (%c)%n
  2. Create a Message Queue for Apache Kafka configuration file named kafka.properties.
    ## Set the endpoint to the default endpoint on the Instance Details page in the Message Queue for Apache Kafka console.
    bootstrap.servers=xxxxxxxxxxxxxxxxxxxxx
    ## Configure the topic. You can create it in the Message Queue for Apache Kafka console.
    topic=alikafka-topic-demo
    ## Configure the consumer group. You can create it in the Message Queue for Apache Kafka console.
    group.id=CID-consumer-group-demo
  3. Create a configuration file loader named JavaKafkaConfigurer.java.
    public class JavaKafkaConfigurer {
        private static Properties properties;
        public synchronized static Properties getKafkaProperties() {
           if (null ! = properties) {
               return properties;
           }
           // Obtain the content of the kafka.properties file.
           Properties kafkaProperties = new Properties();
           try {
               kafkaProperties.load(KafkaProducerDemo.class.getClassLoader().getResourceAsStream("kafka.properties"));
           } catch (Exception e) {
               // If the file cannot be loaded, exit the program.
               e.printStackTrace();
           }
           properties = kafkaProperties;
           return kafkaProperties;
        }
    }

Send messages

  1. Create a message sender named KafkaProducerDemo.java.
    import java.util.ArrayList;
    import java.util.List;
    import java.util.Properties;
    import java.util.concurrent.Future;
    
    import java.util.concurrent.TimeUnit;
    import org.apache.kafka.clients.CommonClientConfigs;
    import org.apache.kafka.clients.producer.KafkaProducer;
    import org.apache.kafka.clients.producer.ProducerConfig;
    import org.apache.kafka.clients.producer.ProducerRecord;
    import org.apache.kafka.clients.producer.RecordMetadata;
    
    public class KafkaProducerDemo {
    
        public static void main(String args[]) {
            // Load the kafka.properties file.
            Properties kafkaProperties =  JavaKafkaConfigurer.getKafkaProperties();
    
            Properties props = new Properties();
            // Set the endpoint. Obtain the endpoint of the corresponding topic in the Message Queue for Apache Kafka console.
            props.put(ProducerConfig.BOOTSTRAP_SERVERS_CONFIG, kafkaProperties.getProperty("bootstrap.servers"));
    
            // Set the method for deserializing  Message Queue for Apache Kafka  messages.
            props.put(ProducerConfig.KEY_SERIALIZER_CLASS_CONFIG, "org.apache.kafka.common.serialization.StringSerializer");
            props.put(ProducerConfig.VALUE_SERIALIZER_CLASS_CONFIG, "org.apache.kafka.common.serialization.StringSerializer");
            // Set the maximum time to wait for a request.
            props.put(ProducerConfig.MAX_BLOCK_MS_CONFIG, 30 * 1000);
            // Set the number of retries for the client.
            props.put(ProducerConfig.RETRIES_CONFIG, 5);
            // Set the interval between retries for the client.
            props.put(ProducerConfig.RETRY_BACKOFF_MS_CONFIG, 3000);
            // Construct a thread-safe producer object. One producer object can serve one process.
            // To improve performance, you can construct more objects. We recommend that you construct no more than five objects.
            KafkaProducer<String, String> producer = new KafkaProducer<String, String>(props);
    
            // Construct a  Message Queue for Apache Kafka  message.
            String topic = kafkaProperties.getProperty("topic"); // The topic of the message. Enter the topic that you created in the Message Queue for Apache Kafka console.
            String value = "this is the message's value"; // The content of the message.
    
            try {
                // Batch obtaining future objects can speed up the process. Take note that the batch size cannot be excessively large.
                List<Future<RecordMetadata>> futures = new ArrayList<Future<RecordMetadata>>(128);
                for (int i =0; i < 100; i++) {
                    // Send the message and obtain a future object.
                    ProducerRecord<String, String> kafkaMessage =  new ProducerRecord<String, String>(topic, value + ": " + i);
                    Future<RecordMetadata> metadataFuture = producer.send(kafkaMessage);
                    futures.add(metadataFuture);
    
                }
                producer.flush();
                for (Future<RecordMetadata> future: futures) {
                    // Synchronize the future object.
                    try {
                        RecordMetadata recordMetadata = future.get();
                        System.out.println("Produce ok:" + recordMetadata.toString());
                    } catch (Throwable t) {
                        t.printStackTrace();
                    }
                }
            } catch (Exception e) {
                // If the client does not send data even after retries, catch and handle the error.
                System.out.println("error occurred");
                e.printStackTrace();
            }
        }
    }
  2. Compile and run KafkaProducerDemo.java to send a message.

Subscribe to messages

You can subscribe to messages by using one of the following methods:
  • Enable only one consumer to subscribe to messages
    1. Create a single-consumer subscription program named KafkaConsumerDemo.java.
      import java.util.ArrayList;
      import java.util.List;
      import java.util.Properties;
      
      import org.apache.kafka.clients.consumer.ConsumerConfig;
      import org.apache.kafka.clients.consumer.ConsumerRecord;
      import org.apache.kafka.clients.consumer.ConsumerRecords;
      import org.apache.kafka.clients.consumer.KafkaConsumer;
      import org.apache.kafka.clients.producer.ProducerConfig;
      
      public class KafkaConsumerDemo {
      
          public static void main(String args[]) {
              // Load the kafka.properties file.
              Properties kafkaProperties =  JavaKafkaConfigurer.getKafkaProperties();
      
              Properties props = new Properties();
              // Set the endpoint. Obtain the endpoint of the corresponding topic in the Message Queue for Apache Kafka console.
              props.put(ProducerConfig.BOOTSTRAP_SERVERS_CONFIG, kafkaProperties.getProperty("bootstrap.servers"));
              // Set the maximum interval between two polling cycles.
              // The default value is 30s. If the consumer does not return a heartbeat message within the interval, the broker determines that the consumer is not alive. The broker removes the consumer from the consumer group and triggers rebalancing.
              props.put(ConsumerConfig.SESSION_TIMEOUT_MS_CONFIG, 30000);
              // Set the maximum number of messages that can be polled at a time.
              // Do not set this parameter to an excessively large value. If polled messages are not all consumed before the next poll starts, load balancing is triggered and performance may deteriorate.
              props.put(ConsumerConfig.MAX_POLL_RECORDS_CONFIG, 30);
              // Set the method for deserializing messages.
              props.put(ConsumerConfig.KEY_DESERIALIZER_CLASS_CONFIG, "org.apache.kafka.common.serialization.StringDeserializer");
              props.put(ConsumerConfig.VALUE_DESERIALIZER_CLASS_CONFIG, "org.apache.kafka.common.serialization.StringDeserializer");
              // Set the consumer group of the current consumer instance. You must create the consumer group in the Message Queue for Apache Kafka console.
              // The instances in a consumer group consume messages in load balancing mode.
              props.put(ConsumerConfig.GROUP_ID_CONFIG, kafkaProperties.getProperty("group.id"));
              // Construct a consumer object. This generates a consumer instance.
              KafkaConsumer<String, String> consumer = new org.apache.kafka.clients.consumer.KafkaConsumer<String, String>(props);
              // Set one or more topics to which the consumer group subscribes.
              // We recommend that you configure consumer instances with the same GROUP_ID_CONFIG value to subscribe to the same topics.
              List<String> subscribedTopics =  new ArrayList<String>();
              // If you want to subscribe to multiple topics, add the topics here.
              // You must create the topics in the Message Queue for Apache Kafka console in advance.
              String topicStr = kafkaProperties.getProperty("topic");
              String[] topics = topicStr.split(",");
              for (String topic: topics) {
                  subscribedTopics.add(topic.trim());
              }
              consumer.subscribe(subscribedTopics);
      
              // Consume messages in a loop.
              while (true){
                  try {
                      ConsumerRecords<String, String> records = consumer.poll(1000);
                      // All messages must be consumed before the next polling cycle starts. The total duration cannot exceed the timeout interval specified by SESSION_TIMEOUT_MS_CONFIG.
                      // We recommend that you create a separate thread pool to consume messages and then asynchronously return the results.
                      for (ConsumerRecord<String, String> record : records) {
                          System.out.println(String.format("Consume partition:%d offset:%d", record.partition(), record.offset()));
                      }
                  } catch (Exception e) {
                      try {
                          Thread.sleep(1000);
                      } catch (Throwable ignore) {
      
                      }
                      e.printStackTrace();
                  }
              }
          }
      }
    2. Compile and run KafkaConsumerDemo.java to consume messages.
  • Enable multiple consumers to subscribe to messages
    1. Create a multi-consumer subscription program named KafkaMultiConsumerDemo.java.
      import java.util.ArrayList;
      import java.util.List;
      import java.util.Properties;
      import java.util.concurrent.atomic.AtomicBoolean;
      import org.apache.kafka.clients.consumer.ConsumerConfig;
      import org.apache.kafka.clients.consumer.ConsumerRecord;
      import org.apache.kafka.clients.consumer.ConsumerRecords;
      import org.apache.kafka.clients.consumer.KafkaConsumer;
      import org.apache.kafka.clients.producer.ProducerConfig;
      import org.apache.kafka.common.errors.WakeupException;
      
      /**
       * This tutorial shows you how to enable multiple consumers to simultaneously consume messages of the same topic in one process.
       * Make sure that the total number of consumers in the environment does not exceed the number of partitions of the topics to which the consumers are subscribed.
       */
      public class KafkaMultiConsumerDemo {
      
          public static void main(String args[]) throws InterruptedException {
              // Load the kafka.properties file.
              Properties kafkaProperties = JavaKafkaConfigurer.getKafkaProperties();
      
              Properties props = new Properties();
              // Set the endpoint. Obtain the endpoint of the corresponding topic in the Message Queue for Apache Kafka console.
              props.put(ProducerConfig.BOOTSTRAP_SERVERS_CONFIG, kafkaProperties.getProperty("bootstrap.servers"));
              // Set the maximum interval between two polling cycles.
              // The default value is 30s. If the consumer does not return a heartbeat message within the interval, the broker determines that the consumer is not alive. The broker removes the consumer from the consumer group and triggers rebalancing.
              props.put(ConsumerConfig.SESSION_TIMEOUT_MS_CONFIG, 30000);
              // Set the maximum number of messages that can be polled at a time.
              // Do not set this parameter to an excessively large value. If polled messages are not all consumed before the next poll starts, load balancing is triggered and performance may deteriorate.
              props.put(ConsumerConfig.MAX_POLL_RECORDS_CONFIG, 30);
              // Set the method for deserializing messages.
              props.put(ConsumerConfig.KEY_DESERIALIZER_CLASS_CONFIG, "org.apache.kafka.common.serialization.StringDeserializer");
              props.put(ConsumerConfig.VALUE_DESERIALIZER_CLASS_CONFIG, "org.apache.kafka.common.serialization.StringDeserializer");
              // Set the consumer group of the current consumer instance. You must create the consumer group in the Message Queue for Apache Kafka console.
              // The instances in a consumer group consume messages in load balancing mode.
              props.put(ConsumerConfig.GROUP_ID_CONFIG, kafkaProperties.getProperty("group.id"));
      
              int consumerNum = 2;
              Thread[] consumerThreads = new Thread[consumerNum];
              for (int i = 0; i < consumerNum; i++) {
                  KafkaConsumer<String, String> consumer = new KafkaConsumer<String, String>(props);
      
                  List<String> subscribedTopics = new ArrayList<String>();
                  subscribedTopics.add(kafkaProperties.getProperty("topic"));
                  consumer.subscribe(subscribedTopics);
      
                  KafkaConsumerRunner kafkaConsumerRunner = new KafkaConsumerRunner(consumer);
                  consumerThreads[i] = new Thread(kafkaConsumerRunner);
              }
      
              for (int i = 0; i < consumerNum; i++) {
                  consumerThreads[i].start();
              }
      
              for (int i = 0; i < consumerNum; i++) {
                  consumerThreads[i].join();
              }
          }
      
          static class KafkaConsumerRunner implements Runnable {
              private final AtomicBoolean closed = new AtomicBoolean(false);
              private final KafkaConsumer consumer;
      
              KafkaConsumerRunner(KafkaConsumer consumer) {
                  this.consumer = consumer;
              }
      
              @Override
              public void run() {
                  try {
                      while (! closed.get()) {
                          try {
                              ConsumerRecords<String, String> records = consumer.poll(1000);
                              // All messages must be consumed before the next polling cycle starts. The total duration cannot exceed the timeout interval specified by SESSION_TIMEOUT_MS_CONFIG.
                              for (ConsumerRecord<String, String> record : records) {
                                  System.out.println(String.format("Thread:%s Consume partition:%d offset:%d", Thread.currentThread().getName(), record.partition(), record.offset()));
                              }
                          } catch (Exception e) {
                              try {
                                  Thread.sleep(1000);
                              } catch (Throwable ignore) {
      
                              }
                              e.printStackTrace();
                          }
                      }
                  } catch (WakeupException e) {
                      // If the consumer is shut down, ignore exceptions.
                      if (! closed.get()) {
                          throw e;
                      }
                  } finally {
                      consumer.close();
                  }
              }
              // Implement a shutdown hook that can be called by another thread.
              public void shutdown() {
                  closed.set(true);
                  consumer.wakeup();
              }
          }
      }
    2. Compile and run KafkaMultiConsumerDemo.java to consume messages.