Java中的firebase的HTTP Post消息

Question

我想从我的Java程序向Firebase发送通知。但我不知道该怎么做。

使用curl发送通知：

# api_key=YOUR_SERVER_KEY

# curl --header "Authorization: key=$api_key" \
   --header Content-Type:"application/json" \
   https://fcm.googleapis.com/fcm/send \
   -d "{\"registration_ids\":[\"ABC\"]}"

我希望在Java中具有相同的功能。我试过以下但是不正确

String rawData =  "{\"registration_ids\":[\"ABC\"]}";
String encodedData = URLEncoder.encode( rawData, "UTF-8" ); 

URL u = new URL("https://fcm.googleapis.com/fcm/send");
HttpURLConnection conn = (HttpURLConnection) u.openConnection();
conn.setDoOutput(true);
conn.setRequestMethod("POST");
conn.setRequestProperty("Authorization", "key="+api_key);
conn.setRequestProperty("Content-Type", "application/json");
OutputStream os = conn.getOutputStream();
os.write(encodedData.getBytes());