如何解决Java resttemplate post具有map而不是有效json错误请求错误400

import org.springframework.http.HttpEntity;
import org.springframework.http.HttpHeaders;
import org.springframework.http.MediaType;
import org.springframework.http.ResponseEntity;
import org.springframework.web.client.RestTemplate;

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

public class RestTemplateExample {
    public static void main(String[] args) {
        // 创建RestTemplate对象
        RestTemplate restTemplate = new RestTemplate();

        // 设置请求头
        HttpHeaders headers = new HttpHeaders();
        headers.setContentType(MediaType.APPLICATION_JSON);

        // 创建请求体参数
        Map<String, Object> requestBody = new HashMap<>();
        requestBody.put("key1", "value1");
        requestBody.put("key2", "value2");

        // 创建HttpEntity对象，设置请求头和请求体
        HttpEntity<Map<String, Object>> httpEntity = new HttpEntity<>(requestBody, headers);

        // 发送POST请求
        String url = "http://example.com/api/endpoint";
        ResponseEntity<String> response = restTemplate.postForEntity(url, httpEntity, String.class);

        // 处理响应
        if (response.getStatusCode().is2xxSuccessful()) {
            String responseBody = response.getBody();
            System.out.println("Response: " + responseBody);
        } else {
            System.out.println("Request failed with status code: " + response.getStatusCodeValue());
        }
    }
}