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In Eclipse 3.0, infrastructure was added to support running application code 
concurrently.  This support is useful when your plug-in has to perform
some CPU-intensive work and you want to allow the user to continue
working while it goes on.  For example, a Web browser or mail client
typically fetches content from a server in the background, allowing the user
to browse existing content while waiting.

The basic unit of concurrent activity in Eclipse is provided by the <tt>Job</tt> class. 
Jobs are a cross between the interface <tt>java.lang.Runnable</tt>
and the class <tt>java.lang.Thread</tt>.  Jobs are similar to runnables
because they encapsulate behavior inside a <tt>run</tt> method and 
are similar to threads because they are not  executed in the
calling thread.  This example illustrates how a job is used:

<pre>
   Job myJob = new Job("Sample Job") {
      public IStatus run(IProgressMonitor monitor) {
         System.out.println("This is running in a job");
         return Status.OK_STATUS;
      }
   };
   myJob.schedule();
</pre>

When <tt>schedule</tt> is called, the job is added to a queue of jobs waiting to be run.  Worker threads then remove them from the queue and invoke their <tt>run</tt> method.  This system has a number of advantages over creating and starting a Java thread:

* <i>Less overhead</i>.  Creating a new thread every time you want to run something can be expensive.  The job infrastructure uses a thread pool that reuses the same threads for many jobs.

* <i>Support for progress and cancellation</i>.  Jobs are provided with a progress monitor object, allowing them to respond to cancellation requests and to report how much work they have done. The UI can listen to these progress messages and display feedback as the job executes.

* <i>Support for priorities and mutual exclusion</i>.  Jobs can be configured with varying priorities and with scheduling rules that describe when jobs can be run concurrently with other jobs.

* <i>Advanced scheduling features</i>.  You can schedule a job to run at any time in the future and to reschedule itself after completing.

Note that the same job instance can be rerun as many times as you like, but you cannot schedule a job that is already sleeping or waiting to run. Jobs are often written as singletons, both to avoid the possibility of the same job being scheduled multiple times and to  avoid the overhead of creating a new object every time it is run.

== See Also: ==
*[[FAQ How do I show progress for things happening in the background?]]
*[[FAQ How do I switch from using a Progress dialog to the Progress view?]]
*[[FAQ Actions, commands, operations, jobs: What does it all mean?]]
*[http://www.eclipse.org/articles/Article-Concurrency/jobs-api.html On the Job: The Eclipse Jobs API]
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