code: // Licensed to the Software Freedom Conservancy (SFC) under one
  // or more contributor license agreements.  See the NOTICE file
  // distributed with this work for additional information
  // regarding copyright ownership.  The SFC licenses this file
  // to you under the Apache License, Version 2.0 (the
  // "License"); you may not use this file except in compliance
  // with the License.  You may obtain a copy of the License at
  //
  //   http://www.apache.org/licenses/LICENSE-2.0
  //
  // Unless required by applicable law or agreed to in writing,
  // software distributed under the License is distributed on an
  // "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
  // KIND, either express or implied.  See the License for the
  // specific language governing permissions and limitations
  // under the License.
  
  package org.openqa.selenium.net;
  
  import static java.util.concurrent.TimeUnit.MILLISECONDS;
  import static java.util.concurrent.TimeUnit.NANOSECONDS;
  
  import java.io.IOException;
  import java.net.HttpURLConnection;
  import java.net.URL;
  import java.util.Arrays;
  import java.util.concurrent.ExecutionException;
  import java.util.concurrent.ExecutorService;
  import java.util.concurrent.Executors;
  import java.util.concurrent.Future;
  import java.util.concurrent.TimeUnit;
  import java.util.concurrent.atomic.AtomicInteger;
  import java.util.logging.Logger;
  
  /** Polls a URL until a HTTP 200 response is received. */
  public class UrlChecker {
  
    private static final Logger LOG = Logger.getLogger(UrlChecker.class.getName());
  
    static final int CONNECT_TIMEOUT_MS = 500;
    private static final int READ_TIMEOUT_MS = 1000;
    private static final long MAX_POLL_INTERVAL_MS = 320;
    private static final long MIN_POLL_INTERVAL_MS = 10;
  
    private static final AtomicInteger THREAD_COUNTER = new AtomicInteger(1);
    private static final ExecutorService EXECUTOR =
        Executors.newCachedThreadPool(
            r -> {
              Thread t =
                  new Thread(
                      r, "UrlChecker-" + THREAD_COUNTER.incrementAndGet()); // Thread safety reviewed
              t.setDaemon(true);
              return t;
            });
  
    public void waitUntilAvailable(long timeout, TimeUnit unit, final URL... urls)
        throws TimeoutException {
      long start = System.nanoTime();
      LOG.fine("Waiting for " + Arrays.toString(urls));
      try {
        Future<Void> callback =
            EXECUTOR.submit(
                () -> {
                  HttpURLConnection connection = null;
  
                  long sleepMillis = MIN_POLL_INTERVAL_MS;
                  while (true) {
                    if (Thread.interrupted()) {
                      throw new InterruptedException();
                    }
                    for (URL url : urls) {
                      try {
                        LOG.fine("Polling " + url);
                        connection = connectToUrl(url);
                        if (connection.getResponseCode() == HttpURLConnection.HTTP_OK) {
                          return null;
                        }
                      } catch (IOException e) {
                        // Ok, try again.
                      } finally {
                        if (connection != null) {
                          connection.disconnect();
                        }
                      }
                    }
                    MILLISECONDS.sleep(sleepMillis);
                    sleepMillis =
                        (sleepMillis >= MAX_POLL_INTERVAL_MS) ? sleepMillis : sleepMillis * 2;
                  }
                });
        callback.get(timeout, unit);
      } catch (java.util.concurrent.TimeoutException e) {
        throw new TimeoutException(
            String.format(
                "Timed out waiting for %s to be available after %d ms",
                Arrays.toString(urls), MILLISECONDS.convert(System.nanoTime() - start, NANOSECONDS)),
            e);
      } catch (InterruptedException e) {
        Thread.currentThread().interrupt();
        throw new RuntimeException(e);
      } catch (ExecutionException e) {
        throw new RuntimeException(e);
      }
    }
  
    public void waitUntilUnavailable(long timeout, TimeUnit unit, final URL url)
        throws TimeoutException {
      long start = System.nanoTime();
      LOG.fine("Waiting for " + url);
      try {
        Future<Void> callback =
            EXECUTOR.submit(
                () -> {
                  HttpURLConnection connection = null;
  
                  long sleepMillis = MIN_POLL_INTERVAL_MS;
                  while (true) {
                    try {
                      LOG.fine("Polling " + url);
                      connection = connectToUrl(url);
                      if (connection.getResponseCode() != HttpURLConnection.HTTP_OK) {
                        return null;
                      }
                    } catch (IOException e) {
                      return null;
                    } finally {
                      if (connection != null) {
                        connection.disconnect();
                      }
                    }
  
                    MILLISECONDS.sleep(sleepMillis);
                    sleepMillis =
                        (sleepMillis >= MAX_POLL_INTERVAL_MS) ? sleepMillis : sleepMillis * 2;
                  }
                });
        callback.get(timeout, unit);
      } catch (java.util.concurrent.TimeoutException e) {
        throw new TimeoutException(
            String.format(
                "Timed out waiting for %s to become unavailable after %d ms",
                url, MILLISECONDS.convert(System.nanoTime() - start, NANOSECONDS)),
            e);
      } catch (InterruptedException | ExecutionException e) {
        throw new RuntimeException(e);
      }
    }
  
    private HttpURLConnection connectToUrl(URL url) throws IOException {
      HttpURLConnection connection = (HttpURLConnection) url.openConnection();
      connection.setConnectTimeout(CONNECT_TIMEOUT_MS);
      connection.setReadTimeout(READ_TIMEOUT_MS);
      connection.connect();
      return connection;
    }
  
    public static class TimeoutException extends Exception {
      public TimeoutException(String s, Throwable throwable) {
        super(s, throwable);
      }
    }
  }
  