Java Reference
In-Depth Information
machine to abortfor example, under
UNIX
systems a
SIGKILL
signal will
force the virtual machine to abort. When the virtual machine is forced to
terminate in this way, no guarantees can be made about whether or not
shutdown hooks will run.
23.3.3. Shutdown Strategies
Generally, you should let a program finish normally rather than forcibly
shut it down with
exit
. This is particularly so with multithreaded pro-
grams, where the thread that decides it is time for the program to exit
may have no idea what the other threads are doing.
Designing a multithreaded application in a way that makes it safe to exit
at arbitrary points in the program is either trivial or extremely complex.
It is trivial when none of the threads are performing actions that must
be completedfor example, having a thousand threads all trying to solve
a numerical problem. It is complex whenever any of the threads perform
actions that must be completed.
You can communicate the fact that shutdown is required by writing your
You can broadcast the shutdown request to all your threads by invok-
ing the
interrupt
method of the parent
ThreadGroup
of your application.
This is no guarantee that a program will terminate, however, because
libraries that you have used may have created user threads that do not
respond to interrupt requeststhe
AWT
graphics library is one well-known
example.
There are two circumstances when you must invoke
exit
: when it is the
only way to terminate some of the threads and hence your application,
and when your application must return a status code. In both cases you
need to delay the call to
exit
until all your application threads have had
a chance to terminate cleanly. One way is for the thread initiating the
termination to
join
the other threads and so know when those threads
have terminated. However, an application may have to maintain its own
list of the threads it creates because simply inspecting the
ThreadGroup
