我是Java的新手(我在Scala上有一些经验),目前正在尝试了解Kafka。我在本教程中遇到了以下示例(主要是为了参考而添加代码):
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:
Thread myThread = new Thread(myConsumerRunnable);
它是在单个线程中运行还是在多个线程中运行?使用线程运行使用者的好处是什么?我(我以为Kafka无论如何都可以抽象化消费者之间的负载分配)
如您所见,使用者在run
方法中开始无限循环。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。这是myThread.start();
新线程开始执行的地方。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发出的关闭信号CountDownLatch latch
,因此可以避免使用用户线程。
为什么我们要通过单独的线程触发关闭挂钩?(从检查该方法的理解来看,它似乎更像是Java而不是Kafka)
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)
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