java - 使用参数创建模板类型的新对象

Question

我正在用java编写一些JSON解析代码，并且我有几个方法，它们之间的唯一区别是它们是否返回 JSONObject或JSONArray 。我正在尝试摆脱这个:

  private JSONArray getJsonArray(String path) throws IOException {
    HttpGet httpget = new HttpGet(path);
    httpget.setConfig(requestConfig);

    try (CloseableHttpClient httpClient = httpClientBuilder.build()) {
      try (CloseableHttpResponse result = httpClient.execute(apiHost, httpget)) {
        return new JSONArray(new JSONTokener(result.getEntity().getContent()));
      }
    }
  }

  private JSONObject getJsonObject(String path) throws IOException {
    HttpGet httpget = new HttpGet(path);
    httpget.setConfig(requestConfig);

    try (CloseableHttpClient httpClient = httpClientBuilder.build()) {
      try (CloseableHttpResponse result = httpClient.execute(apiHost, httpget)) {
        return new JSONObject(new JSONTokener(result.getEntity().getContent()));
      }
    }
  }

对此(无效代码):

  private <T> get(String path, Class<T> type) throws IOException {
    HttpGet httpget = new HttpGet(path);
    httpget.setConfig(requestConfig);

    try (CloseableHttpClient httpClient = httpClientBuilder.build()) {
      try (CloseableHttpResponse result = httpClient.execute(apiHost, httpget)) {
        return new T(new JSONTokener(result.getEntity().getContent()));
      }
    }
  }

如何使用参数正确初始化 T 类型的新对象？我可以以某种方式将 T 的可能值限制为 JSONObject/JSONArray 吗？我知道<T extends Something>形式，但这两个似乎直接继承自 Object没有通用接口(interface):(

Answer 1

您可以使用反射来获取并调用匹配的构造函数(如果有)，并在不存在此类构造函数时引发异常。

private <T> T get(String path, Class<T> type) throws IOException {
    HttpGet httpget = new HttpGet(path);
    httpget.setConfig(requestConfig);

    try (CloseableHttpClient httpClient = httpClientBuilder.build()) {
        try (CloseableHttpResponse result = httpClient.execute(apiHost, httpget)) {
            Constructor<T> constructor = type.getConstructor(JSONTokener.class);
            return constructor.newInstance(new JSONTokener(result.getEntity().getContent()));
        } catch (ReflectiveOperationException e) {
            throw new IllegalArgumentException("Provided result class does not accept JSONTokener parameter.");
        }
    }
}

请注意，这有点像 duck typing ，即您并不真正将类型限制为 JSONObject 或 JSONArray，而是提供相应构造函数的所有内容都可以。