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

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
    # (the "License"); you may not use this file except in compliance with
    # the License.  You may obtain a copy of the License at
    #
    #    http://www.apache.org/licenses/LICENSE-2.0
    #
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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.
    ## Specify the default endpoint displayed on the Instance Details page in the Message Queue for Apache Kafka console. 
    bootstrap.servers=xxxxxxxxxxxxxxxxxxxxx
    ## Specify a topic. You can create a topic in the Message Queue for Apache Kafka console. 
    topic=alikafka-topic-demo
    ## Specify a consumer group. You can create a consumer group in the Message Queue for Apache Kafka console.
        Group 
       
        Group 
        
    group.id=CID-consumer-group-demo
  3. Create a program named JavaKafkaConfigurer.java to load the configuration files.
    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 producer program named KafkaProducerDemo.java that contains the following code:
    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();
            // Specify an endpoint. Obtain the default endpoint of the corresponding instance 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 maximum number of retries allowed for the client. 
            props.put(ProducerConfig.RETRIES_CONFIG, 5);
            // Set the interval between two consecutive retries for the client. 
            props.put(ProducerConfig.RETRY_BACKOFF_MS_CONFIG, 3000);
            // Create a thread-safe producer object. One producer object can serve one process. 
            // To improve performance, you can create multiple objects. We recommend that you create no more than five objects. 
            KafkaProducer<String, String> producer = new KafkaProducer<String, String>(props);
    
            // Create 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 {
                // To improve the efficiency, obtain multiple future objects at a time. Do not obtain a large number of future objects at a time. 
                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 message still fails to be sent after retries, troubleshoot the error. 
                System.out.println("error occurred");
                e.printStackTrace();
            }
        }
    }
  2. Compile and run KafkaProducerDemo.java to send messages.

Consume messages

You can consume messages by using one of the following methods:
  • Use a single consumer to consume messages
    1. Create a single-consumer program named KafkaConsumerDemo.java that contains the following code:
      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();
              // Specify an endpoint. Obtain the default endpoint of the corresponding instance in the Message Queue for Apache Kafka console. 
              props.put(ProducerConfig.BOOTSTRAP_SERVERS_CONFIG, kafkaProperties.getProperty("bootstrap.servers"));
              // Set the maximum interval between two consecutive polling cycles. 
              // The default interval is 30s. If the consumer does not return a heartbeat message within the interval, the broker determines that the consumer is not alive. In this case, the broker removes the consumer from the consumer group and triggers load balancing.
          Group 
          
              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 the messages polled are not all consumed before the next polling cycle 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");
              // Specify the consumer group of the current consumer. You must create the consumer group in the Message Queue for Apache Kafka console. 
              // The consumers in a consumer group consume messages in load balancing mode. 
              props.put(ConsumerConfig.GROUP_ID_CONFIG, kafkaProperties.getProperty("group.id"));
              // Create a consumer object. 
              KafkaConsumer<String, String> consumer = new org.apache.kafka.clients.consumer.KafkaConsumer<String, String>(props);
              // Specify one or more topics to which the consumer group subscribes. 
              // We recommend that you configure consumers 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 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.
  • Use multiple consumers to consume messages
    1. Create a multi-consumer program named KafkaMultiConsumerDemo.java that contains the following code:
      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 use multiple consumers to simultaneously consume messages 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 subscribe. 
       */
      public class KafkaMultiConsumerDemo {
      
          public static void main(String args[]) throws InterruptedException {
              // Load the kafka.properties file. 
              Properties kafkaProperties = JavaKafkaConfigurer.getKafkaProperties();
      
              Properties props = new Properties();
              // Specify an endpoint. Obtain the default endpoint of the corresponding instance in the Message Queue for Apache Kafka console. 
              props.put(ProducerConfig.BOOTSTRAP_SERVERS_CONFIG, kafkaProperties.getProperty("bootstrap.servers"));
              // Set the maximum interval between two consecutive polling cycles. 
              // The default interval is 30s. If the consumer does not return a heartbeat message within the interval, the broker determines that the consumer is not alive. In this case, the broker removes the consumer from the consumer group and triggers load balancing.
          Group 
          
              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 the messages polled are not all consumed before the next polling cycle 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");
              // Specify the consumer group of the current consumer. You must create the consumer group in the Message Queue for Apache Kafka console. 
              // The consumers 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 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 the exception. 
                      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.