System Programming - The Java Programming Language - page 998

23.3. Shutdown
Normally, an execution of a virtual machine terminates when the last
user thread terminates. A Runtime can also be shut down explicitly with its
exit method, passing an integer status code that can be communicated
to the environment executing the virtual machinezero to indicate suc-
cessful completion of a task and non-zero to indicate failure. This meth-
od abruptly terminates all threads in the runtime system no matter what
their state. They are not interrupted or even stopped. They simply cease
to exist as the virtual machine itself stops runningno finally clauses are
executed.
In either case, when exit is invoked or the last user thread terminates,
the shutdown sequence is initiated. The virtual machine can also be shut
down externally, such as by a user interrupting the virtual machine from
a keyboard (on many systems by typing control-C) or when the user logs
out or the computer is shut down.
All the methods related to shutting down the runtime system are security
checked and throw SecurityException if you don't have required permis-
sions.
23.3.1. Shutdown Hooks
An application can register a shutdown hook with the runtime system.
Shutdown hooks are threads that represent actions that should be taken
before the virtual machine exits. Hooks typically clean up external re-
sources such as files and network connections.
public void addShutdownHook(Thread hook)
Registers a new virtual-machine shutdown hook. If hook has
already been registered or has already been started an Illeg-
alArgumentException is thrown.
public boolean removeShutdownHook(Thread hook)