Shulou(Shulou.com)06/02 Report--

This article introduces the realization and principle of Java elegant downtime. The content is very detailed. Interested friends can use it for reference. I hope it will be helpful to you.

Graceful downtime? I am convinced of this noun, if you put aside the major, what a good noun!

In fact, elegant downtime is not to shut down the service immediately, but to do some follow-up operations, such as shutting down threads, releasing connection resources, and so on.

For example, it will not be interrupted as soon as the processing of the caller's request is increased. After processing this time, the service will be stopped.

In Java, we can register hooks through the Runtime.getRuntime (). AddShutdownHook () method to ensure a smooth exit of the program. (other languages are similar)

Have a chestnut:

Public class ShutdownGraceFullTest {/ * use thread pool to process tasks * / public static ExecutorService executorService = Executors.newCachedThreadPool (); public static void main (String [] args) {/ / suppose there are five threads that need to execute task for (int I = 0; I < 5; iThreads +) {final int id = I Thread taski = new Thread (new Runnable () {@ Override public void run () {System.out.println (System.currentTimeMillis () + ": thread_" + id + "start..."); try {TimeUnit.SECONDS.sleep (id) } catch (InterruptedException e) {e.printStackTrace ();} System.out.println (System.currentTimeMillis () + ": thread_" + id + "finish!");}}); taski.setDaemon (true); executorService.submit (taski) } / / add a hook to handle unfinished tasks Runtime.getRuntime () .addShutdownHook (new Thread (new Runnable () {@ Override public void run () {System.out.println (System.currentTimeMillis () + ":" + Thread.currentThread () .getName () + "No1 shutdown hooking..."); boolean shutdown = true Try {executorService.shutdown (); System.out.println (System.currentTimeMillis () + ":" + Thread.currentThread (). GetName () + "shutdown signal got, wait threadPool finish."); executorService.awaitTermination (1500, TimeUnit.SECONDS) System.out.println (System.currentTimeMillis () + ":" + Thread.currentThread (). GetName () + "all thread's done.");} catch (InterruptedException e) {e.printStackTrace () } System.out.println (System.currentTimeMillis () + ":" + Thread.currentThread () .getName () + "No1 shutdown done...");}}) / / multiple closed hooks execute Runtime.getRuntime () .addShutdownHook concurrently (new Thread (new Runnable () {@ Override public void run () {try {System.out.println (System.currentTimeMillis () + ":" + Thread.currentThread () .getName () + "No2 shutdown hooking...")) Thread.sleep (1000);} catch (InterruptedException e) {e.printStackTrace ();} System.out.println (System.currentTimeMillis () + ":" + Thread.currentThread (). GetName () + "No2 shutdown done...");}})) System.out.println ("main method exit..."); / / deliberately call the jvm exit command to send a close signal, otherwise jvm will normally wait for a non-daemon thread to shut down before exiting System.exit (0);}}

The running results are as follows:

Obviously, it is indeed elegant, although * received a shutdown signal, but still ensured the completion of the task processing. Isn't that great!

So, how to reflect elegant downtime in practical application?

Kill-15 pid

Send a shutdown signal to jvm through this command, and then start executing Shutdown Hook. You can do a lot of things:

Close the socket link

Clean up temporary files

Send a message notification to the subscriber, telling yourself to go offline

Notify the child process of the message that it will be destroyed

The release of various resources

In our daily work, we see a lot of operation and maintenance students doing this:

Kill-9 pid

If you do this, jvm will not be able to do so. Kill-9 is equivalent to a system outage and a power outage. This will take the app by surprise, leaving no chance for the app to react.

So, in any case, it is not elegant.

Be elegant. It's the code.

Where the thread pool is closed as follows:

ExecutorService.shutdown (); executorService.awaitTermination (1500, TimeUnit.SECONDS)

ThreadPoolExecutor becomes SHUTDOWN after shutdown, cannot accept new tasks, and then waits for the execution of the ongoing task to complete. It means that shutdown is just issuing a command, and it's up to the thread to shut it down or not.

ThreadPoolExecutor treats shutdownNow differently. The method becomes STOP after execution, and the Thread.interrupt () method is called on the executing thread (but nothing happens if the thread doesn't handle the interrupt), so it doesn't mean "shut down immediately."

Shutdown (): startup sequence is off, where previously submitted tasks are performed, but new tasks are not accepted. If it is closed, the call has no additional effect. This method does not wait for the previously submitted task to be completed.

ShutdownNow (): attempts to stop all tasks that are in progress, stops processing of waiting tasks, and returns a list of tasks waiting to be executed. When this method returns, these tasks are exhausted (deleted) from the task queue. This method does not wait for the active task to be terminated.

Executor.awaitTermination (this.awaitTerminationSeconds, TimeUnit.SECONDS); control the waiting time to prevent tasks from running during the * period (as emphasized earlier, even shutdownNow does not guarantee that threads will stop running).

Note:

The virtual machine invokes multiple shutdownhook in an unknown order and exits after execution.

If you perform a blocking operation when you receive the kill-15 pid command, you can wait for the task to complete before closing the JVM. At the same time, it also explains some problems that the application execution kill-15 pid cannot exit, such as: the interrupt is blocked, or the hook runs the endless loop code.

On the implementation of Java elegant downtime and what is the principle of sharing here, I hope that the above content can be of some help to you, can learn more knowledge. If you think the article is good, you can share it for more people to see.

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