在Android中通过POST方法发送json，但始终收到resonponse代码400。服务器正常，如果在后台执行任务

URL url = new URL("http://example.com/api");
HttpURLConnection connection = (HttpURLConnection) url.openConnection();
connection.setRequestMethod("POST");
connection.setRequestProperty("Content-Type", "application/json");
connection.setDoOutput(true);

String jsonInputString = "{\"key1\": \"value1\", \"key2\": \"value2\"}";

try(OutputStream outputStream = connection.getOutputStream()) {
    byte[] input = jsonInputString.getBytes("utf-8");
    outputStream.write(input, 0, input.length);
}

int responseCode = connection.getResponseCode();