Google App Engine is a Java (and Python) hosting service. You are provided with a servlet container to host your web application and several other features.
You can execute scheduled jobs by invoking an URL of your web application periodically via cron jobs.
However, there is a 30 second hard timeout for every
request the container serves. Any request that takes more than 30
seconds to serve will experience a
com.google.apphosting.api.DeadlineExceededException. This limitation
applies to cron started requests as well.
So if you thought about doing long running batch jobs by invoking a private URL of your web application via cron you will fail. Even worse, Google App Engine does not allow you to spawn a new thread in your Java application. So there is basically no option for doing long running batch operations.