Question

我对Java比较陌生（我在Scala上有一些经验），目前正在尝试学习Kafka。我在此tutorial中遇到了以下示例（我主要添加代码以供参考）：

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.common.errors.WakeupException;
import org.apache.kafka.common.serialization.StringDeserializer;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;

import java.time.Duration;
import java.util.Arrays;
import java.util.Collections;
import java.util.Properties;
import java.util.concurrent.CountDownLatch;

public class ConsumerDemoWithThread {

    public static void main(String[] args) {
        new ConsumerDemoWithThread().run();
    }

    private ConsumerDemoWithThread() {

    }

    private void run() {
        Logger logger = LoggerFactory.getLogger(ConsumerDemoWithThread.class.getName());

        String bootstrapServers = "127.0.0.1:9092";
        String groupId = "my-sixth-application";
        String topic = "first_topic";

        // latch for dealing with multiple threads
        CountDownLatch latch = new CountDownLatch(1);

        // create the consumer runnable
        logger.info("Creating the consumer thread");
        Runnable myConsumerRunnable = new ConsumerRunnable(
                bootstrapServers,
                groupId,
                topic,
                latch
        );

        // start the thread
        Thread myThread = new Thread(myConsumerRunnable);
        myThread.start();

        // add a shutdown hook
        Runtime.getRuntime().addShutdownHook(new Thread(() -> {
            logger.info("Caught shutdown hook");
            ((ConsumerRunnable) myConsumerRunnable).shutdown();
            try {
                latch.await();
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
            logger.info("Application has exited");
        }

        ));

        try {
            latch.await();
        } catch (InterruptedException e) {
            logger.error("Application got interrupted", e);
        } finally {
            logger.info("Application is closing");
        }
    }

    public class ConsumerRunnable implements Runnable {

        private final CountDownLatch latch;
        private final KafkaConsumer<String, String> consumer;
        private final Logger logger = LoggerFactory.getLogger(ConsumerRunnable.class.getName());

        public ConsumerRunnable(String bootstrapServers,
                                String groupId,
                                String topic,
                                CountDownLatch latch) {
            this.latch = latch;

            // create consumer configs
            Properties properties = new Properties();
            properties.setProperty(ConsumerConfig.BOOTSTRAP_SERVERS_CONFIG, bootstrapServers);
            properties.setProperty(ConsumerConfig.KEY_DESERIALIZER_CLASS_CONFIG, StringDeserializer.class.getName());
            properties.setProperty(ConsumerConfig.VALUE_DESERIALIZER_CLASS_CONFIG, StringDeserializer.class.getName());
            properties.setProperty(ConsumerConfig.GROUP_ID_CONFIG, groupId);
            properties.setProperty(ConsumerConfig.AUTO_OFFSET_RESET_CONFIG, "earliest");

            // create consumer
            consumer = new KafkaConsumer<String, String>(properties);
            // subscribe consumer to our topic(s)
            consumer.subscribe(Collections.singletonList(topic));
        }

        @Override
        public void run() {
            // poll for new data
            try {
                while (true) {
                    ConsumerRecords<String, String> records =
                            consumer.poll(Duration.ofMillis(100)); // new in Kafka 2.0.0

                    for (ConsumerRecord<String, String> record : records) {
                        logger.info("Key: " + record.key() + ", Value: " + record.value());
                        logger.info("Partition: " + record.partition() + ", Offset:" + record.offset());
                    }
                }
            } catch (WakeupException e) {
                logger.info("Received shutdown signal!");
            } finally {
                consumer.close();
                // tell our main code we're done with the consumer
                latch.countDown();
            }
        }

        public void shutdown() {
            // the wakeup() method is a special method to interrupt consumer.poll()
            // it will throw the exception WakeUpException
            consumer.wakeup();
        }
    }
}

我主要是想了解：

使用线程来运行线程有什么好处消费者？我（我以为Kafka无论如何都要抽象消费者之间的负载分布）
当我们使用Thread myThread = new Thread(myConsumerRunnable);时是在单个线程中运行还是在多个线程中运行？
为什么我们要通过单独的线程触发关闭钩子？（从检查该方法的理解来看，它似乎更像是Java事物而不是Kafka事物）

Answer 1

使用线程运行使用者的好处是什么？我卡夫卡（Kafka）认为抽象了消费者之间的负载分配无论如何）

如您所见，使用者使用run方法启动了一个无限循环。作为新线程启动它，可以在使用者已经处于活动状态时在主线程中执行更多操作。

当我们使用线程时，myThread = new Thread（myConsumerRunnable）;是在单线程还是跨多个线程运行？

创建Thread对象尚未启动新线程。新线程开始在myThread.start();处执行。您的示例程序有一个主线程和一个使用者线程。主线程实际上正在等待通过CountDownLatch latch的关闭信号，因此可以避免使用使用者线程。

我们为什么要通过单独的线程触发关闭钩子？（据我了解从检查方法来看，它似乎更像是Java事物而不是Kafka事物）

这是java的东西。直到发生关闭，才会真正执行关闭挂钩线程。参见https://docs.oracle.com/javase/7/docs/api/java/lang/Runtime.html#addShutdownHook(java.lang.Thread)

为什么我们需要使用线程来运行Kafka使用者？我们需要多少个线程？

1 个答案: