获取特定版本的资源

每个资源都有一个版本字段，该字段会在资源每次发生更改时发生变化，即 etag 字段。ETag 是 HTTP 的标准组成部分，Google 日历 API 在以下两种情况下支持 ETag：

  • 在资源修改时，确保在此期间没有对该资源进行其他写入（条件修改）。
  • 在资源检索时，仅在资源发生更改时检索数据（条件检索）。

条件修改

如果您只想在资源自上次检索以来未发生更改的情况下更新或删除资源，可以指定一个 If-Match 标头，其中包含上次检索的 ETag 值。这有助于防止资源丢失修改。客户端可以重新检索资源并重新应用更改。

如果条目（及其 ETag）自上次检索以来未发生更改，则修改成功，并且服务器会返回具有新 ETag 的资源的新版本。否则，服务器会返回 412 Precondition Failed 响应代码。

以下示例演示了如何使用条件 修改与 Java 客户端库：

  private static void run() throws IOException {
    // Create a test event.
    Event event = Utils.createTestEvent(client, "Test Event");
    System.out.println(String.format("Event created: %s", event.getHtmlLink()));

    // Pause while the user modifies the event in the Calendar UI.
    System.out.println("Modify the event's description and hit enter to continue.");
    System.in.read();

    // Modify the local copy of the event.
    event.setSummary("Updated Test Event");

    // Update the event, making sure that we don't overwrite other changes.
    int numAttempts = 0;
    boolean isUpdated = false;
    do {
      Calendar.Events.Update request = client.events().update("primary", event.getId(), event);
      request.setRequestHeaders(new HttpHeaders().setIfMatch(event.getEtag()));
      try {
        event = request.execute();
        isUpdated = true;
      } catch (GoogleJsonResponseException e) {
        if (e.getStatusCode() == 412) {
          // A 412 status code, "Precondition failed", indicates that the etag values didn't
          // match, and the event was updated on the server since we last retrieved it. Use
          // {@link Calendar.Events.Get} to retrieve the latest version.
          Event latestEvent = client.events().get("primary", event.getId()).execute();

          // You may want to have more complex logic here to resolve conflicts. In this sample we're
          // simply overwriting the summary.
          latestEvent.setSummary(event.getSummary());
          event = latestEvent;
        } else {
          throw e;
        }
      }
      numAttempts++;
    } while (!isUpdated && numAttempts <= MAX_UPDATE_ATTEMPTS);

    if (isUpdated) {
      System.out.println("Event updated.");
    } else {
      System.out.println(String.format("Failed to update event after %d attempts.", numAttempts));
    }
  }

条件检索

如果您只想在资源自上次检索以来发生更改的情况下检索资源，可以指定一个 If-None-Match 标头，其中包含上次检索的 ETag 值。如果条目（及其 ETag）自上次检索以来发生更改，则服务器会返回具有新 ETag 的资源的新版本。否则，服务器会返回 304 Not Modified 响应代码。

以下示例演示了如何使用条件 检索与 Java 客户端库：

  private static void run() throws IOException {
    // Create a test event.
    Event event = Utils.createTestEvent(client, "Test Event");
    System.out.println(String.format("Event created: %s", event.getHtmlLink()));

    // Pause while the user modifies the event in the Calendar UI.
    System.out.println("Modify the event's description and hit enter to continue.");
    System.in.read();

    // Fetch the event again if it's been modified.
    Calendar.Events.Get getRequest = client.events().get("primary", event.getId());
    getRequest.setRequestHeaders(new HttpHeaders().setIfNoneMatch(event.getEtag()));
    try {
      event = getRequest.execute();
      System.out.println("The event was modified, retrieved latest version.");
    } catch (GoogleJsonResponseException e) {
      if (e.getStatusCode() == 304) {
        // A 304 status code, "Not modified", indicates that the etags match, and the event has
        // not been modified since we last retrieved it.
        System.out.println("The event was not modified, using local version.");
      } else {
        throw e;
      }
    }
  }