每个资源都有一个版本字段，即 etag 字段，每当资源发生更改时，版本字段都会更改。Etag 是 HTTP 的标准部分，在两种情况下在 calendar API 中受支持：
- 资源修改，以确保在此期间没有对此资源进行其他写入操作（条件修改）
- 检索资源，以便在资源发生变化时仅检索资源数据（条件检索）
条件修改
如果您只想更新或删除自上次检索后未更改的资源，则可以指定一个 If-Match 标头，其中包含上次检索的 etag 的值。这对于防止资源修改丢失非常有用。客户端可以选择重新检索资源并重新应用更改。
如果自上次检索后条目（及其 etag）未更改，则修改会成功，并返回具有新 etag 的新版资源。否则，您会收到 412（未满足前提条件）响应代码。
以下代码段演示了如何使用 Java 客户端库执行条件修改。
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
如果您只想检索自上次检索后发生了更改的资源，则可以指定 If-None-Match 标头，其中包含上次检索的 etag 的值。如果自上次检索后条目（及其 etag）发生了更改，则返回具有新 etag 的资源的新版本。否则，您会收到 304（未修改）响应代码。
以下代码段演示了如何使用 Java 客户端库执行条件检索。
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;
}
}
}