Waiting for a custom element to be ready in Java with Java.net

import java.net.*;
import java.io.*;

// You can get an API key at https://pdfshift.io
String apiKey = "sk_xxxxxxxxxxxx";

URL url = new URL("https://api.pdfshift.io/v3/convert/pdf");
HttpURLConnection connection = (HttpURLConnection) url.openConnection();
connection.setRequestMethod("POST");
connection.setRequestProperty("X-API-Key", apiKey);
connection.setRequestProperty("Content-Type", "application/json");
connection.setDoOutput(true);

String payload = "{\n" +
    "  \"source\": \"https://www.example.com\",\n" +
    "  \"wait_for\": \"#dynamic-content.loaded\"\n" +
    "}";

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

int responseCode = connection.getResponseCode();

// Handle errors:
if (responseCode >= 400) {
    throw new RuntimeException("Request failed with status code " + responseCode);
}

InputStream inputStream = connection.getInputStream();
FileOutputStream outputStream = new FileOutputStream("result.pdf");
byte[] buffer = new byte[1024];
int bytesRead;
while ((bytesRead = inputStream.read(buffer)) != -1) {
    outputStream.write(buffer, 0, bytesRead);
}
outputStream.close();
inputStream.close();

System.out.println("The PDF document was generated and saved to result.pdf");