"由于并发，Java 代码在几秒钟后退出"

Question

I am writing the same code on tutorial. But in tutorial the program never exits, my in my computer it exits after 4 seconds. Why? tutorial with exact time where this code is shown: https:\/\/youtu.be\/vzBw1LPupnA?t=169<\/a>

public class Main {
    private static boolean stopRequested;

    public static void main(String[] args) throws InterruptedException {
        Thread backgroundThread = new Thread(() -> {
            int i = 0;
            while (!stopRequested) {
                i++;
                System.out.println("i = " + i);
            }
        });
        backgroundThread.start();
        TimeUnit.SECONDS.sleep(1);
        stopRequested = true;
    }
}

Answer 1

The reason that you are seeing different behavior on your machine and in the video is because the program has unspecified behavior.

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Answer 2

As the excellent Answer by Stephen C says, your code is not thread-safe .

Establishing an AtomicBoolean early on addresses the visibility problem explained in that other Answer. This class is a thread-safe wrapper around its payload boolean value.

The volatile keyword is another solution. But I find the Atomic… classes simpler and more obvious.

Also, in modern Java we rarely need to address the Thread class directly. Instead, use the Executors framework. Define your task as a Runnable or Callable , and submit to an executor service.

Something like this untested code.

public class Main {
    private static final AtomicBoolean stopRequested = new AtomicBoolean( false ) ;

    public static void main(String[] args) throws InterruptedException {
        Runnable task = () -> {
            int i = 0;
            while ( ! stopRequested.get() ) {
                i++;
                System.out.println("i = " + i);
                TimeUnit.MILLISECONDS.sleep(100); // Don’t spin too fast.
            }
        };
        ExecutorService es = Executors.newSingleThreadedExecutorService() ;
        es.submit( task ) ;
        TimeUnit.SECONDS.sleep(1);
        stopRequested.set( true ) ;
        TimeUnit.SECONDS.sleep(1);

   // Shut down here executor service. Boilerplate taken from Javadoc.
   es.shutdown(); // Disable new tasks from being submitted
   try {
     // Wait a while for existing tasks to terminate
     if (!es.awaitTermination(60, TimeUnit.SECONDS)) {
       es.shutdownNow(); // Cancel currently executing tasks
       // Wait a while for tasks to respond to being cancelled
       if (!es.awaitTermination(60, TimeUnit.SECONDS))
           System.err.println("Executor service did not terminate");
     }
   } catch (InterruptedException ex) {
     // (Re-)Cancel if current thread also interrupted
     es.shutdownNow();
     // Preserve interrupt status
     Thread.currentThread().interrupt();
   }
    }
}