服务器返回 HTTP 响应代码：openConnection() 后 400

我正在尝试使用 Firebase Cloud Messaging HTTP v1，但是当我在调用 openConnection() 后获得响应代码时，会返回“404”错误代码

URL url = new URL(FCM_SEND_ENDPOINT);
HttpURLConnection httpURLConnection = (HttpURLConnection) url.openConnection();
System.out.println("status:" + httpURLConnection.getResponseCode());

我检查了 URL 中的项目 ID，应该是 Firebase 控制台中的项目 ID（设置 --> 常规 --> 项目 ID）

Firebase 控制台：

https://fcm.googleapis.com/v1/projects/xxxxxxxxx/messages:send

更新于 29-11-2017

错误的：
{"message":{"notification":{"title":"notification title","body":"message body"},"token":"123"}}

正确的：
{"message":{"notification":{"title":"notification title","body":"message body"},"token":"dwB0YqF......"}}

这里是示例代码

public static HttpURLConnection getHTTPConn() throws Exception{
    URL url = new URL(FCM_SEND_ENDPOINT);
    HttpURLConnection httpURLConnection = (HttpURLConnection) url.openConnection();
    String token = getAccessToken();
    httpURLConnection.setRequestProperty("Authorization", "Bearer " + token);
    httpURLConnection.setRequestProperty("Content-Type", "application/json; UTF-8");
    httpURLConnection.setDoOutput(true);
    return httpURLConnection;
}
//The following code to retrieve the token with Google API Client Library <https://firebase.google.com/docs/cloud-messaging/auth-server>
private static String getAccessToken() throws IOException {
    GoogleCredential googleCredential = GoogleCredential
    .fromStream(new FileInputStream("service-account.json"))
    .createScoped(Arrays.asList(SCOPES));
    googleCredential.refreshToken();
    return googleCredential.getAccessToken();
}
public static void sendPushNotificationV1(){
    String result = "";
    try{
        HttpURLConnection conn = getHTTPConn();
        OutputStreamWriter wr = new OutputStreamWriter(conn.getOutputStream());
        wr.write(json.toString());
        wr.flush();
        BufferedReader br = new BufferedReader(new InputStreamReader((conn.getInputStream())));

        String output;
        while ((output = br.readLine()) != null) {
            System.out.println(output);
        }
        result = "SUCCESS";
    } catch (Exception e) {
        e.printStackTrace();
    }
}

享受~