Task Execution - Java Concurrency in Practice - page 144

ument, going back and downloading the images and drawing them into the associated place-
holder. This approach is shown in SingleThreadRenderer in Listing 6.10 .
Downloading an image mostly involves waiting for I/O to complete, and during this time the
CPU does little work. So the sequential approach may underutilize the CPU, and also makes
the user wait longer than necessary to see the finished page. We can achieve better utilization
and responsiveness by breaking the problem into independent tasks that can execute concur-
rently.
Listing 6.10. Rendering Page Elements Sequentially.
6.3.2. Result-bearing Tasks: Callable and Future
The Executor framework uses Runnable as its basic task representation. Runnable is a
fairly limiting abstraction; run cannot return a value or throw checked exceptions, although
it can have side effects such as writing to a log file or placing a result in a shared data struc-
ture.
Many tasks are effectively deferred computations—executing a database query, fetching a
resource over the network, or computing a complicated function. For these types of tasks,
Callable is a better abstraction: it expects that the main entry point, call , will re-
turn a value and anticipates that it might throw an exception. [7] Executors includes
several utility methods for wrapping other types of tasks, including Runnable and
java.security.PrivilegedAction , with a Callable .
Runnable and Callable describe abstract computational tasks. Tasks are usually finite:
they have a clear starting point and they eventually terminate. The lifecycle of a task executed