Question

The Jobs API in Eclipse RCP apparently works much differently than I expected. I thought that creating and scheduling multiple Jobs would actually cause multiple worker threads to be created, executing the Jobs in parallel unless there was an ISchedulingRule conflict.

I went back and read the documentation more closely, and also discovered this comment in the JobManager class:

/**
 * Returns a running or blocked job whose scheduling rule conflicts with the 
 * scheduling rule of the given waiting job.  Returns null if there are no 
 * conflicting jobs.  A job can only run if there are no running jobs and no blocked
 * jobs whose scheduling rule conflicts with its rule.
 */

Now it looks to me like the Job manager will only ever attempt to use one background worker thread. Am I completely wrong about this? If I’m right,

• what is the point of scheduling rules and locks? If there is only one worker thread, Jobs can never preemt each other. Wouldn’t these only ever be used in case a Job’s sleep() method is called (e.g. sleeping while holding a Lock)?
• does any part of the platform allow two Jobs to actually run concurrently, on multiple worker threads, thus making the above features useful somehow?

What am I missing here?

Answer 1

Take a look at the run method in the documentation, specifically this part:

Jobs can optionally finish their
execution asynchronously (in another
thread) by returning a result status
of ASYNC_FINISH. Jobs that finish
asynchronously must specify the
execution thread by calling setThread,
and must indicate when they are
finished by calling the method done.

ASYNC_FINISH there looks interresting.