void
java.lang.
.addShutdownHook(
hook)
Registers a new virtual-machine shutdown hook.
The Java virtual machine shuts down
in response to two kinds of
events:
- The program exits
normally, when the last non-daemon thread exits or
when the
exit
(equivalently, System.exit
)
method is invoked, or
- The virtual machine is terminated
in response to a user interrupt,
such as typing ^C
, or a system-wide event, such as user logoff or
system shutdown.
A shutdown hook
is simply an initialized but unstarted thread. When
the virtual machine begins its shutdown sequence it will start all registered
shutdown hooks in some unspecified order and let them run concurrently. When all
the hooks have finished it will then run all uninvoked finalizers if
finalization-on-exit has been enabled. Finally, the virtual machine will halt.
Note that daemon threads will continue to run during the shutdown sequence, as
will non-daemon threads if shutdown was initiated by invoking the exit
method.
Once the shutdown sequence has begun it can be stopped only by invoking the
halt
method, which forcibly terminates the virtual machine.
Once the shutdown sequence has begun it is impossible to register a new
shutdown hook or de-register a previously-registered hook. Attempting either of
these operations will cause an IllegalStateException
to be thrown.
Shutdown hooks run at a delicate time in the life cycle of a virtual machine
and should therefore be coded defensively. They should, in particular, be
written to be thread-safe and to avoid deadlocks insofar as possible. They
should also not rely blindly upon services that may have registered their own
shutdown hooks and therefore may themselves in the process of shutting down.
Shutdown hooks should also finish their work quickly. When a program invokes
exit
the expectation is that the virtual machine will promptly shut down and exit.
When the virtual machine is terminated due to user logoff or system shutdown the
underlying operating system may only allow a fixed amount of time in which to
shut down and exit. It is therefore inadvisable to attempt any user interaction
or to perform a long-running computation in a shutdown hook.
Uncaught exceptions are handled in shutdown hooks just as in any other
thread, by invoking the uncaughtException
method of the thread's ThreadGroup
object. The default implementation of this method prints the exception's stack
trace to System.err
and terminates the thread; it does not cause the virtual machine to exit or
halt.
In rare circumstances the virtual machine may abort
, that is, stop
running without shutting down cleanly. This occurs when the virtual machine is
terminated externally, for example with the SIGKILL
signal on Unix or
the TerminateProcess
call on Microsoft Windows. The virtual machine may
also abort if a native method goes awry by, for example, corrupting internal
data structures or attempting to access nonexistent memory. If the virtual
machine aborts then no guarantee can be made about whether or not any shutdown
hooks will be run.
Parameters:
hook
An initialized but unstarted Thread
object
Throws:
IllegalArgumentException
- If the specified hook has already been registered, or if it can be determined
that the hook is already running or has already been run
IllegalStateException
- If the virtual machine is already in the process of shutting down
SecurityException
- If a security manager is present and it denies RuntimePermission
("shutdownHooks")
Since:
1.3
See Also:
removeShutdownHook
halt(int)
exit(int)
public class ShutdownHook {
public static void main(String[] args) {
Thread hook= new Thread(){
@Override
public void run() {
System.out.println("shutdown hook end!");
//super.run();
}
};
Runtime.getRuntime().addShutdownHook(hook);
Object f =new Object(){
@Override
protected void finalize() throws Throwable {
System.out.println("free Object!!!");
//super.finalize();
}
};
f=null;
System.gc();
System.out.println(" main exit (0)!");
//cause shutdown hook invoking.
System.exit(0);
//Runtime.getRuntime().removeShutdownHook(hook);
System.out.println(" end main string ");
try {
Thread.sleep(5000);
} catch (InterruptedException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
//throw new RuntimeException("a");
//If you enter ^C it will cause shutdown too.
// D:\develop\workspace\test\src>java ShutdownHook
// main exit (0)!
// shutdown hook end!
}
}
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