有关Runtime.addShutdownHook(Thread)

今天在JDK的javadoc里瞎转的时候无意间发现这么一个有趣的回调方法：

addShutdownHook

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 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. Attempts to use other thread-based services such as the AWT event-dispatch thread, for example, may lead to deadlocks.

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(java.lang.Thread)

,

halt(int)

,

exit(int)

即可以在runtime对象上注册一个回调方法，该方法会在JVM退出时执行传入的线程对象。在mac下试了一下，当调用System.exit()、使用系统kill命令杀掉java进程时该方法都会被调用。但是需要注意的是，当调用kill -9杀进程时，方法不被调用，JVM直接退出，Ｗindows下还没试过，不知道结果如何

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