Task Execution - Java Concurrency in Practice - page 146

There are several ways to create a Future to describe a task. The submit methods in
ExecutorService all return a Future , so that you can submit a Runnable or a
Callable to an executor and get back a Future that can be used to retrieve the result or
cancel the task. You can also explicitly instantiate a FutureTask for a given Runnable
or Callable . (Because FutureTask implements Runnable , it can be submitted to an
Executor for execution or executed directly by calling its run method.)
As of Java 6, ExecutorService implementations can override newTaskFor in Ab-
stractExecutorService to control instantiation of the Future corresponding to a
submitted Callable or Runnable . The default implementation just creates a new Fu-
tureTask , as shown in Listing 6.12 .
Listing 6.12. Default Implementation of newTaskFor in ThreadPoolExecutor .
Submitting a Runnable or Callable to an Executor constitutes a safe publication (see
Section 3.5 ) of the Runnable or Callable from the submitting thread to the thread that
will eventually execute the task. Similarly, setting the result value for a Future constitutes
a safe publication of the result from the thread in which it was computed to any thread that
retrieves it via get .
6.3.3. Example: Page Renderer with Future
As a first step towards making the page renderer more concurrent, let's divide it into two
tasks, one that renders the text and one that downloads all the images. (Because one task
is largely CPU-bound and the other is largely I/O-bound, this approach may yield improve-
ments even on single-CPU systems.)
Callable and Future can help us express the interaction between these cooperating
tasks. In FutureRenderer in Listing 6.13 , we create a Callable to download all the
images, and submit it to an ExecutorService . This returns a Future describing the
task's execution; when the main task gets to the point where it needs the images, it waits for
the result by calling Future.get . Ifwe're lucky, the results will already be ready by the
time we ask; otherwise, at least we got a head start on downloading the images.