GWT, Google App Engine, TimerTask or Thread in ServiceImpl throw exception

Question

I am using GWT and Google App Engine. I have array of records and I want to update them every 30 mins. In the ServiceImpl I have the fallowing code :

new Timer().schedule(new TimerTask(){
    @Override
    public void run() {
        try {
            Thread.sleep(30000);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
        result = updateFeeds();
    }
}, 30000,Long.MAX_VALUE);

When I run the application and when I get :

com.google.gwt.user.server.rpc.UnexpectedException:
Service method 'public abstract java.util.List org.joke.planet.client.FeedService.loadFeeds()' threw an unexpected exception:
java.security.AccessControlException: access denied (java.lang.RuntimePermission modifyThreadGroup)

at the first line or pasted code.

My Question is HOW I can make a background worker that works in GWT+AppEngine Service ?

Answer 1

you cannot - a background worker implies a thread, and thread creation in gae does not work.

The answer to your task is not to create a thread or background worker, but to use this http://code.google.com/appengine/docs/java/config/cron.html

Answer 2

You can't use java.util.Timer because it creates an ordinary thread, which is not allowed on AppEngine. However, you can use the AppEngine Modules API's Background thread facility, as documented here: https://developers.google.com/appengine/docs/java/modules/#Java_Background_threads

Note that you can only use background threads on manually scaled instances. In this example, a background thread prints out a message every 30 seconds, forever:

Thread thread = ThreadManager.createBackgroundThread(new Runnable() {
  public void run() {
    try {
      while (true) {
        System.err.println("Doing something!");
        Thread.sleep(30_000);
      }
    } catch (InterruptedException ex) {
    }
  }
});
thread.start();