1. 什么是Callback其使用场景是什么?

在这个send方法中我们看到需要传入两个参数,而第二个参数是一个Callback
Callback是一个回调接口:用户可以实现的回调接口Callback，以允许在请求完成时执行实现接口方法的代码。

2.代码示例

import org.apache.kafka.clients.producer.Callback;
import org.apache.kafka.clients.producer.KafkaProducer;
import org.apache.kafka.clients.producer.ProducerRecord;
import org.apache.kafka.clients.producer.RecordMetadata;
import org.apache.log4j.Logger;

import java.util.Properties;
import java.util.concurrent.ExecutionException;

public class Producer extends Thread {
    private static Logger LOG = Logger.getLogger(Producer.class);
    private final KafkaProducer<Integer, String> producer;
    private final String topic;
    private final Boolean isAsync;

    /**
     * @param topic
     * @param isAsync
     */
    public Producer(String topic, Boolean isAsync) {
        Properties props = new Properties();
        props.put("bootstrap.servers", "192.168.83.100:9092");
        //props.put("bootstrap.servers", "192.168.83.100:9092,192.168.83.120:9092,192.168.83.130:9092");
        props.put("acks", "all");
        props.put("retries", 0);
        props.put("batch.size", 16384);
        props.put("linger.ms", 1);
        props.put("buffer.memory", 33554432);
        props.put("key.serializer", "org.apache.kafka.common.serialization.StringSerializer");
        props.put("value.serializer", "org.apache.kafka.common.serialization.StringSerializer");


        producer = new KafkaProducer<>(props);
        this.topic = topic;
        this.isAsync = isAsync;
    }

    public void run() {
        int messageNo = 1;
        while (true) {
            String messageStr = "Message_" + messageNo;
            LOG.warn("message:" + messageStr);
            long startTime = System.currentTimeMillis();
            if (isAsync) { // 异步发送 Send asynchronously
                producer.send(new ProducerRecord<>(topic,
                        messageStr), new DemoCallBack(startTime, messageNo, messageStr));//*********这里使用了Callback
            } else { // Send synchronously
                try {
                    producer.send(new ProducerRecord<>(topic,messageStr)).get();
                    LOG.warn("Sent message: (" + messageNo + ", " + messageStr + ")");
                } catch (InterruptedException | ExecutionException e) {
                    e.printStackTrace();
                }
            }
            ++messageNo;
        }
    }

    public static void main(String[] args) {
        //产生到那个主题下面
        Producer producer = new Producer("test_msg1",false);
        producer.start();

    }
}
//实现Callback接口的onCompletion方法
class DemoCallBack implements Callback {

    private final long startTime;
    private final int key;
    private final String message;
    
	//实现的是DemoCallBack的有参构造
    public DemoCallBack(long startTime, int key, String message) {
    //通过this把获取的参数内容传递到外层类中,这有这样当回调发生时onCompletion才可以获取类的参数
        this.startTime = startTime;
        this.key = key;
        this.message = message;
    }

    public void onCompletion(RecordMetadata metadata, Exception exception) {
        long elapsedTime = System.currentTimeMillis() - startTime;
        if (metadata != null) {
            System.out.println(
                    "message(" + key + ", " + message + ") sent to partition(" + metadata.partition() +
                            "), " +
                            "offset(" + metadata.offset() + ") in " + elapsedTime + " ms");
        } else {
            exception.printStackTrace();
        }
    }
}

当producer每生产一批数据send出去后,就会调用一次实现了Callback的DemocallBack类的onCompletion方法