关于ResponseEntity<String> response = restTemplate.postForEntity( url, request , String.class );中request参数的问题

 RestTemplate restTemplate = new RestTemplate();
 String url = "http://localhost:8080/test/v2";
 HttpHeaders headers = new HttpHeaders();
 headers.setContentType(MediaType.APPLICATION_FORM_URLENCODED);
 MultiValueMap<String, String> map= new LinkedMultiValueMap<>();
 map.add("email", "844072586@qq.com");
 HttpEntity<Map> request = new HttpEntity<>(map, headers);
 ResponseEntity<String> response = restTemplate.postForEntity( url, request , String.class);

代码如上，运行正常。但是当我将map变量替换为HashMap放入request中，运行时就会报错，编译不报错：

No HttpMessageConverter for java.util.HashMap

但是HttpEntity的构造方法对于map这个位置的类型要求是 T body，如下：

public HttpEntity(@Nullable T body, @Nullable MultiValueMap<String, String> headers) {
        this.body = body;
        HttpHeaders tempHeaders = new HttpHeaders();
        if (headers != null) {
            tempHeaders.putAll(headers);
        }

        this.headers = HttpHeaders.readOnlyHttpHeaders(tempHeaders);
    }

那为什么还要源码还要用泛型而不是MultiValueMap呢，这样为什么会报错呢？除了MultiValueMap类型，我还可以传什么类型从而运行成功呢？望大佬解答。

https://www.cnblogs.com/javazhiyin/p/9851775.html