JVM鉤子

鉤子是什麼：是一種消息的攔截機制，能夠攔截單個進程的消息（線程鉤子），能夠攔截全部進程的消息（系統鉤子），並能夠對攔截消息進行自動以處理。

鉤子應用的狀況：鍵盤打字時的顯示，關閉線程池。

 

java自帶的JVM鉤子使用方法：

Runtime.getRuntime().addShutdownHook(Thread）；

Api的解釋：

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.

　　應用的兩種情況：1-程序正常退出，當最新的非守護線程退出或該退出方法被喚醒；

　　　　2-JVM被中斷時，會進行關閉資源，對象銷燬等操做；

 

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.

 

主要功能是:註冊一個JVM鉤子，當JVM關閉的時候，會執行系統中已經設置的全部經過方法addShutdownHook添加的鉤子，JVM纔會關閉。因此這些鉤子在JVM關閉的時候運行 內存清理，對象銷燬等操做。

 

建議： 同一個JVM最好只使用一個關閉鉤子，而不是每一個服務都使用一個不一樣的關閉鉤子，使用多個關閉鉤子可能會出現當前這個鉤子所要依賴的服務可能已經被另一個關閉鉤子關閉了。爲了不這種狀況，建議關閉操做在單個線程中串行執行，從而避免了再關閉操做之間出現競態條件或者死鎖等問題。

 

使用的例子：

1-公司會用鉤子去關閉鏈接hbase數據庫的線程池。代碼之後加上。

2-下面是一段zeppelin的源碼：

Runtime.getRuntime().addShutdownHook(new Thread(){
@Override public void run() {
LOG.info("Shutting down Zeppelin Server ... ");
try {
jettyWebServer.stop();
if (!conf.isRecoveryEnabled()) {
ZeppelinServer.notebook.getInterpreterSettingManager().close();
}
notebook.close();
Thread.sleep(3000);
} catch (Exception e) {
LOG.error("Error while stopping servlet container", e);
}
LOG.info("Bye");
}
}); 

 

參考文獻：https://www.cnblogs.com/yanlong300/p/8446261.html

　　　　Java HOOK - 鉤子機制掃盲

             Java8 API