This topic describes how to use the SDK for Java to connect to the default endpoint of Message Queue for Apache Kafka from a Java client and send and subscribe to 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>0.10.2.2</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. That is, 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.

Prepare configurations

  1. Create a Log4j configuration file 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
    #
    # Unless required by applicable law or agreed to in writing, software
    # distributed under the License is distributed on an "AS IS" BASIS,
    # WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
    # See the License for the specific language governing permissions and
    # limitations under the License.
    
    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 Kafka configuration file.
    ## 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 the topic in the Message Queue for Apache Kafka console.
    topic=alikafka-topic-demo
    ## Configure the consumer group. You can create the consumer group in the Message Queue for Apache Kafka console.
    group.id=CID-consumer-group-demo
  3. Create a profile 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 a message

  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();
            // Specify the endpoint. We recommend that you obtain the endpoint of the topic from the Message Queue for Apache Kafka console.
            props.put(ProducerConfig.BOOTSTRAP_SERVERS_CONFIG, kafkaProperties.getProperty("bootstrap.servers"));
    
            // Specify the method for serializing 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");
            // Specify 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 retry interval for the client.
            props.put(ProducerConfig.RECONNECT_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 {
                // Obtaining future objects in batches can increase speed. Note that the batch size cannot be too 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 retrying, 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 the following methods.
  • Single consumer
    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();
              // Specify the endpoint. We recommend that you obtain the endpoint of the topic from the Message Queue for Apache Kafka console.
              props.put(ProducerConfig.BOOTSTRAP_SERVERS_CONFIG, kafkaProperties.getProperty("bootstrap.servers"));
              // Specify 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);
              // Specify 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);
              // Specify 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 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 message object, which is 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.
  • Multiple consumers
    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 demonstrates how to enable multiple consumers to simultaneously consume messages of the same topic in one process.
       * Ensure 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();
              // Specify the endpoint. We recommend that you obtain the endpoint of the topic from the Message Queue for Apache Kafka console.
              props.put(ProducerConfig.BOOTSTRAP_SERVERS_CONFIG, kafkaProperties.getProperty("bootstrap.servers"));
              // Specify 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);
              // Specify 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);
              // Specify 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 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.

Use the SDKs for other languages

For information about how to use the SDKs for other languages, see Overview.