我是一个学生，试图学习apache Kafka。我试图在使用java的spring Boot 中使用kafka将JSON对象作为消息发送。但是当我试图发送它时，它抛出了一个错误，说我的模型类不能被转换为字符串，即使我在www.example.com文件中提到了Json序列化器application.properties。例外是：

java.lang.ClassCastException: class com.example.demo.model.BookES cannot be cast to class java.lang.String (com.example.demo.model.BookES is in unnamed module of loader 'app'; java.lang.String is in module java.base of loader 'bootstrap')

我的application.properties文件是

server.port=8081

spring.kafka.consumer.bootstrap-servers=localhost:9092
spring.kafka.consumer.group-id=myGroup
spring.kafka.consumer.auto-offset-reset=earliest
spring.kafka.consumer.key-deserializer=org.apache.kafka.common.serialization.StringDeserializer
spring.kafka.consumer.value-deserializer=org.springframework.kafka.support.serializer.JsonDeserializer

spring.kafka.producer.bootstrap-servers=localhost:9092
spring.kafka.producer.key-serializer=org.apache.common.serialization.StringSerializer
spring.kafka.producer.value-serializer=org.springframework.kafka.support.serializer.JsonSerializer

我的控制器类，我正尝试在其中发送消息

@PostMapping("/publish")
    public ResponseEntity<String> publish(@RequestBody BookES bookES){
        logger.info("in publish method");
        kafkaProducer.sendMessage(bookES);
        return ResponseEntity.ok("Json message sent to kafka topic");
    }

我的Kafka生产者类有sendMessage方法：

package com.example.demo.kafka;

import org.apache.kafka.clients.admin.NewTopic;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.kafka.core.KafkaTemplate;
import org.springframework.kafka.support.KafkaHeaders;
import org.springframework.messaging.Message;
import org.springframework.messaging.support.MessageBuilder;
import org.springframework.stereotype.Service;

import com.example.demo.controller.BookController;
import com.example.demo.model.Book;
import com.example.demo.model.BookES;

@Service
public class KafkaProducer {
    
    @Autowired
    private NewTopic topic;
    
    Logger logger = LoggerFactory.getLogger(KafkaProducer.class);
    
    private String topicName = "bookmanagement";
    
    @Autowired
    private KafkaTemplate<String, BookES> kafkaTemplate;

    
    
    public void sendMessage(BookES bookES) {
        
        logger.info("in sendMessage method");
        logger.info(String.format("Message sent -> %s",bookES.toString()));
        
        Message<BookES> message = MessageBuilder.withPayload(bookES).setHeader(KafkaHeaders.TOPIC, topic.name()).build();
        kafkaTemplate.send(message);
        
        
        
    }
    
    
}

我的模型类：

package com.example.demo.model;

import org.springframework.data.annotation.Id;
import org.springframework.data.elasticsearch.annotations.Document;

import lombok.AllArgsConstructor;
import lombok.Data;
import lombok.Getter;
import lombok.NoArgsConstructor;
import lombok.Setter;

@Document(indexName="my-application")
@Data
@AllArgsConstructor
@NoArgsConstructor
@Getter
@Setter
public class BookES{
    @Override
    public String toString() {
        return "BookES [bookId=" + bookId + ", bookName=" + bookName + ", description=" + description + "]";
    }
    @Id
    private String bookId;
    private String bookName;
    private String description;
    public String getBookId() {
        // TODO Auto-generated method stub
        return this.bookId;
    }
    public String getBookName() {
        return bookName;
    }
    public void setBookName(String bookName) {
        this.bookName = bookName;
    }
    public String getDescription() {
        return description;
    }
    public void setDescription(String description) {
        this.description = description;
    }
    public void setBookId(String bookId) {
        this.bookId = bookId;
    }

    
}

我的项目在github链接上：github link
我试着用不同的注解（如@JsonSerializer等）来旋转我的模型类，但没有成功。我在postman上得到的回应是：

{
    "timestamp": "2022-11-22T11:24:30.738+00:00",
    "status": 500,
    "error": "Internal Server Error",
    "message": "Can't convert value of class com.example.demo.model.BookES to class org.apache.kafka.common.serialization.StringSerializer specified in value.serializer",
    "path": "/books/publish"
}