取得特定版本的資源

每個資源都有一個版本欄位，這個欄位會在資源變更時變更，也就是 etag 欄位。Etag 是 HTTP 的標準部分，日曆 API 支援兩種情況：

  • 資源修改作業，確保在此期間內沒有其他寫入作業 (條件式修改)
  • 在資源擷取作業中，僅在資源有變更時才擷取資源資料 (條件式擷取)

條件式修改

如果您只想在資源自上次擷取後未變更的情況下更新或刪除該資源，可以指定 If-Match 標頭，其中包含上次擷取時的 etag 值。這對於避免資源遺失修改內容非常有用。用戶端可以選擇重新擷取資源，並重新套用變更。

如果自上次擷取後，項目 (及其 etag) 並未變更，則修改作業會成功，並傳回含有新 etag 的資源新版本。否則，您會收到 412 (Precondition failed) 回應代碼。

以下程式碼片段範例示範如何使用 Java 用戶端程式庫執行條件修改作業。

  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));
    }
  }
ConditionalModificationSample.java

條件式擷取

如果您只想在資源自上次擷取後有所變更時才擷取，可以指定 If-None-Match 標頭，其中包含上次擷取時的 etag 值。如果自上次擷取後，項目 (以及相應的 etag) 已變更，系統會傳回含有新 etag 的資源新版本。否則，您會收到 304 (未修改) 回應碼。

以下程式碼片段範例說明如何使用 Java 用戶端程式庫執行條件式擷取作業。

  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;
      }
    }
  }
ConditionalRetrievalSample.java