use mockito to write Kafka producer unit test.

Sure, here's an example of how you can use Mockito to write a unit test for a Kafka producer:

import static org.mockito.Mockito.times;
import static org.mockito.Mockito.verify;
import static org.mockito.Mockito.when;

import java.util.Properties;

import org.apache.kafka.clients.producer.KafkaProducer;
import org.apache.kafka.clients.producer.ProducerRecord;
import org.junit.Before;
import org.junit.Test;
import org.mockito.InjectMocks;
import org.mockito.Mock;
import org.mockito.MockitoAnnotations;

public class KafkaProducerUnitTest {

    @Mock
    private KafkaProducer<String, String> kafkaProducer;

    @InjectMocks
    private KafkaService kafkaService = new KafkaService();

    @Before
    public void setUp() {
        MockitoAnnotations.initMocks(this);
    }

    @Test
    public void testSendMessage() {
        Properties props = new Properties();
        props.put("bootstrap.servers", "localhost:9092");

        when(kafkaService.createProducer(props)).thenReturn(kafkaProducer);

        String topic = "test-topic";
        String message = "Hello, Kafka!";

        kafkaService.sendMessage(topic, message);

        verify(kafkaProducer, times(1)).send(new ProducerRecord<>(topic, message));
    }
}

In this example, we have a `KafkaService` class that wraps the Kafka producer, and we want to test its `sendMessage` method. We use Mockito to mock the Kafka producer and inject it into the `KafkaService`, and then we use the `when` method to specify the behavior of the `createProducer` method.

In the test method, we call the `sendMessage` method with a topic and a message, and then we verify that the `send` method of the Kafka producer is called once with the correct arguments.

Note that you'll need to add the Mockito and Kafka dependencies to your project for this example to work.