MustRevalidateServer.java

/*
 * Copyright (C) 2018 Square, Inc.
 *
 * Licensed 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 okhttp3;

import java.io.IOException;
import java.text.DateFormat;
import java.text.SimpleDateFormat;
import java.util.Date;
import java.util.Locale;
import java.util.TimeZone;
import java.util.concurrent.TimeUnit;
import okhttp3.mockwebserver.Dispatcher;
import okhttp3.mockwebserver.MockResponse;
import okhttp3.mockwebserver.MockWebServer;
import okhttp3.mockwebserver.RecordedRequest;

/**
 * This server returns the current time on every request. The response is forged as 100 seconds old,
 * and HTTP conditional caching rules suggest is should remain fresh for 10 seconds.
 *
 * Firefox 61 and Safari 11 both cache this response for 10 seconds. You can see this by opening the
 * URL (http://localhost:8123) and clicking the link repeatedly. It should change after 10 seconds.
 *
 * Chrome does not cache the response. Every time you click the link you should see the current
 * time.
 */
public class MustRevalidateServer {
  public void run() throws IOException {
    MockWebServer server = new MockWebServer();
    server.setDispatcher(new Dispatcher() {
      @Override public MockResponse dispatch(RecordedRequest request) {
        return new MockResponse()
            .addHeader("content-type: text/html; charset=utf-8")
            .addHeader("cache-control", "private, must-revalidate")
            .addHeader("last-modified: " + formatDate(-100, TimeUnit.SECONDS))
            .addHeader("date: " + formatDate(0, TimeUnit.SECONDS))
            .setBody("<a href=\"\">" + formatDate(0, TimeUnit.SECONDS) + "</a>");
      }
    });

    server.start(8123);
  }

  private String formatDate(long delta, TimeUnit timeUnit) {
    return formatDate(new Date(System.currentTimeMillis() + timeUnit.toMillis(delta)));
  }

  private String formatDate(Date date) {
    DateFormat rfc1123 = new SimpleDateFormat("EEE, dd MMM yyyy HH:mm:ss zzz", Locale.US);
    rfc1123.setTimeZone(TimeZone.getTimeZone("GMT"));
    return rfc1123.format(date);
  }

  public static void main(String[] args) throws IOException {
    new MustRevalidateServer().run();
  }
}

The 100 seconds / 10 seconds explanation is here:
https://publicobject.com/2015/03/26/how-do-http-caching-heuristics-work/