- The Always On feature allows applications to pay and keep 3 instances of their application always running, which can significantly reduce application latency.
- Developers can now enable Warmup Requests. By specifying a handler in an app's appengine-web.xml, App Engine will attempt to send a Warmup Request to initialize new instances before a user interacts with it. This can reduce the latency an end-user sees for initializing your application.
- The Channel API is now available for all users.
- Task Queue has been officially released, and is no longer an experimental feature. The API import paths that use 'labs' have been deprecated. Task queue storage will count towards an application's overall storage quota, and will thus be charged for.
- The deadline for Task Queue and Cron requests has been raised to 10 minutes. Datastore and API deadlines within those requests remain unchanged.
- For the Task Queue, developers can specify task retry-parameters in their queue.xml.
- Apps that have enabled billing are allowed up to 100 queues with the Task Queue API.
- Metadata Queries on the datastore for datastore kinds, namespaces, and entity properties are available.
- URL Fetch allowed response size has been increased, up to 32 MB. Request size is still limited to 1 MB.
- The request and response sizes for the Images API have been increased to 32 MB.
- The total size of Memcache batch operations is increased to 32 MB. The 1 MB limit on individual Memcache objects still applies.
- The attachment size for outgoing emails has been increased from 1 MB to 10 MB. The size limit for incoming emails is still 10 MB.
- Size and quantity limits on datastore batch get/put/delete operations have been removed. Individual entities are still limited to 1 MB, but your app may batch as many entities together for get/put/delete calls as the overall datastore deadline will allow for.
- When iterating over query results, the datastore will now asynchronously prefetch results, reducing latency in many cases by 10-15%.
- The Admin Console Blacklist page lists the top blacklist rejected visitors.
- The automatic image thumbnailing service supports arbitrary crop sizes up to 1600px.
- Overall average instance latency in the Admin Console is now a weighted average over QPS per instance.
- Added a low-level AysncDatastoreService for making calls to the datastore asynchronously.
- Added a getBodyAsBytes() method to QueueStateInfo.TaskStateInfo, this returns the body of the task state as a pure byte-string.
- The whitelist has been updated to include all classes from javax.xml.soap.
- Fixed an issue sending email to multiple recipients.
- Revert the default logging level during GWT hosted mode back to INFO.
- Fixed an issue with OpenId over SSL.
转自：http://code.google.com/p/googleappengine/wiki/SdkForJavaReleaseNotes#Version_1.4.0_-_December_02,_2010
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