通用方法，等式约束

Question

您好我试图将通用方法实现为控制器基本方法，但我无法理解的问题发生在泛型方法签名上。

<T> ResponseEntity<T> makeApiCall(String path, HttpMethod httpMethod, T body, boolean isAdmin){

        String sender = isAdmin ? adminHash : userHash;
        HttpHeaders headers = new HttpHeaders();
        headers.add("Authorization", sender);
        headers.add("Content-Type", "application/json");
        HttpEntity<T> entity = new HttpEntity<>(body,headers);
        ResponseEntity<T> responseEntity = restTemplate.exchange(path, HttpMethod.POST, entity, body.getClass());
        return responseEntity;
}

我目前的编译错误如下：

Incompatible equality constraint: T and capture of ? extends Object

Answer 1

你没有确切地说出问题的确切位置，但我认为这将发生在restTemplate.exchange(调用，因为传递body.getClass()作为参数。这是因为body.getClass()的返回类型body.getClass()是Class<? extends Object> ， 就像在Javadoc中一样 ：

实际的结果类型是Class<? extends |X|> Class<? extends |X|> where |X| 是调用getClass的表达式的静态类型的擦除

问题是你无法保证body是特定的T - 它可能是T的子类。 因此， body.getClass()的结果可能不是Class<T> 。

如果您想要类型安全，则需要将其作为附加参数传递给方法。

<T> ResponseEntity<T> makeApiCall(
    String path, HttpMethod httpMethod, T body, Class<T> bodyClass, 
    boolean isAdmin){
  // ...
  ResponseEntity<T> responseEntity =
      restTemplate.exchange(
          path, HttpMethod.POST, entity, bodyClass);
  // ...
}

请注意，获取Class<T>的唯一方法是使用类文字，例如String.class如果T是String 。 这排除了使用通用主体类型，因为没有通用的类文字。

Answer 2

您需要cast body.getClass()为Class<T>

@SuppressWarnings("unchecked")
<T> ResponseEntity<T> makeApiCall(String path, HttpMethod httpMethod, T body, boolean isAdmin){

    String sender = isAdmin ? adminHash : userHash;

    HttpHeaders headers = new HttpHeaders();
    headers.add("Authorization", sender);
    headers.add("Content-Type", "application/json");
    HttpEntity<T> entity = new HttpEntity<>(body,headers);
    ResponseEntity<T> responseEntity = restTemplate.exchange(path, HttpMethod.POST, entity, (Class<T>) body.getClass());
    return responseEntity;
}

要使其类型安全，您需要将类对象显式地作为参数传递：

<T> ResponseEntity<T> makeApiCall(String path, HttpMethod httpMethod, T body, Class<T> clazz, boolean isAdmin){
String sender = isAdmin ? adminHash : userHash;

    HttpHeaders headers = new HttpHeaders();
    headers.add("Authorization", sender);
    headers.add("Content-Type", "application/json");
    HttpEntity<T> entity = new HttpEntity<>(body,headers);
    ResponseEntity<T> responseEntity = restTemplate.exchange(path, HttpMethod.POST, entity, clazz);
    return responseEntity;
}