我需要向apache kafka主题发送（post）json负载，但收到以下错误：-“message”：“无法将com.xyz.user类的值转换为value.serializer中指定的org.apache.kafka.common.serialization.stringserializer类”
spring还显示类转换异常：-java.lang.classcastexception:com.xyz.user不能转换为java.lang.string
下面是我的模态，Kafka配置和控制器

public class User {
    private String firstname;
    private String email;

    public User() {}
    public User(String firstname, String email) {
        super();
        this.firstname = firstname;
        this.email = email;
    }
    public String getFirstname() {
        return firstname;
    }
    public void setFirstname(String firstname) {
        this.firstname = firstname;
    }
    public String getEmail() {
        return email;
    }
    public void setEmail(String email) {
        this.email = email;
    }
    @Override
    public String toString() {
        return "UserModel [firstname=" + firstname + ", email=" + email + "]";
    }

}

Kafka构型

package config;

import java.util.HashMap;
import java.util.Map;

import org.apache.kafka.clients.producer.ProducerConfig;
import org.apache.kafka.common.serialization.StringSerializer;
import org.springframework.context.annotation.Bean;
import org.springframework.context.annotation.Configuration;
import org.springframework.kafka.core.DefaultKafkaProducerFactory;
import org.springframework.kafka.core.KafkaTemplate;
import org.springframework.kafka.core.ProducerFactory;
import org.springframework.kafka.support.serializer.JsonSerializer;

import com.xyz.User;

@Configuration
public class KafkaConfiguration {

    @Bean
    public ProducerFactory<String, User> producerFactory(){
        Map<String, Object> config = new HashMap<>();

        config.put(ProducerConfig.BOOTSTRAP_SERVERS_CONFIG, "127.0.0.1:9092");
        config.put(ProducerConfig.KEY_SERIALIZER_CLASS_CONFIG, StringSerializer.class);
        config.put(ProducerConfig.VALUE_SERIALIZER_CLASS_CONFIG, JsonSerializer.class);

        return new DefaultKafkaProducerFactory<>(config);

    }
     @Bean
        public KafkaTemplate<String, User> kafkaTemplate() {
            return new KafkaTemplate<>(producerFactory());
        }

}

控制器

@RestController
@RequestMapping("/kafka")
public class UserController {
    @Autowired
    private KafkaTemplate<String, User> kafkaTemplate;

    private static String TOPIC = "kafka-producer";
    @PostMapping("/publish")
    public void getUserId(@RequestBody User user) {

        kafkaTemplate.send(TOPIC, user);

Postman 发送的json有效负载

{
    "firstname" : "xyz",
    "email" : "xyz@gmail.com.com"

}