1. public static Runtime getRuntime()
Returns the runtime object associated withthe current Java application. Most ofthe methods ofclass Runtime are instance methods and must be invoked with respect tothe current runtime object.
Returns:
the Runtime object associated withthe current Java application.
2. public void addShutdownHook(Thread 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 thelast non-daemon thread exits or when theexit (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 insome unspecified order and let them run concurrently. When all the hooks have finished it will thenrun all uninvoked finalizers if finalization-on-exit has been enabled. Finally, the virtual machine will halt. Note that daemon threads will continuetorun during the shutdown sequence, as will non-daemon threads if shutdown was initiated by invoking theexit 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 itis 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 runat a delicate timeinthe life cycle of a virtual machine and should therefore be coded defensively. They should, in particular, be written to be thread-safe andto 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 inthe process of shutting down. Attempts to use other thread-based services such asthe AWT event-dispatch thread, for example, may lead to deadlocks.
Shutdown hooks should also finish their work quickly. When a program invokes exitthe expectation isthatthe virtual machine will promptly shut down andexit. When the virtual machine is terminated due to user logoff or system shutdown the underlying operating system may only allow a fixed amount oftimein which to shut down andexit. It is therefore inadvisable to attempt any user interaction orto perform a long-running computation in a shutdown hook.
Uncaught exceptions are handled in shutdown hooks just asin any other thread, by invoking the uncaughtException method ofthe thread's ThreadGroup object. The default implementation of this method prints the exception's stack trace to System.err and terminates the thread; itdoesnot cause the virtual machine toexitor halt.
In rare circumstances the virtual machine may abort, thatis, stop runningwithout shutting down cleanly. This occurs when the virtual machine is terminated externally, for example withthe SIGKILL signal on Unix orthe 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 ornot any shutdown hooks will be run.
Parameters:
hook - An initialized but unstarted Thread object
Throws:
IllegalArgumentException - If the specified hook has already been registered, orifit can be determined thatthe hook is already runningor has already been run
IllegalStateException - If the virtual machine is already inthe process of shutting down
SecurityException - If a security manager is present andit denies RuntimePermission("shutdownHooks")
Since:
1.3
See Also:
removeShutdownHook(java.lang.Thread), halt(int), exit(int)
scala.sys.package.scala
defaddShutdownHook(body: ⇒ Unit): ShutdownHookThread
Register a shutdown hook to be run when the VM exits. The hook is automatically registered: the returned value can be ignored, but is available in case the Thread requires further modification. It can also be unregistered by calling ShutdownHookThread#remove().
Note that shutdown hooks are NOT guaranteed to be run.