Java线程代码流

Question

I'm really puzzled by the following pieces of code :

@Component
public class DrawingFileExplorer {

    private final ExecutorService pool = Executors.newFixedThreadPool(10);

        public void explore(File drawingFolder) throws InterruptedException {       
            for(File each : drawingFolder.listFiles(DrawingFileFilter.getInstance())) {
                if(each.isDirectory()) {
                    explore(each);
                } else {
                    //pool.execute(new DrawingFileReviewer(each));
                }
            }
            System.out.println("THIS LINE OF CODE SHOULD BE INVOKED ONCE");

            pool.shutdown();
            pool.awaitTermination(Integer.MAX_VALUE, TimeUnit.SECONDS);
        }

}

// Trigger of DrawingFileExplorer
public class DrawingFileExplorerTest {

    private static File baseFolder = new File("C:\\Users\\Jake\\Desktop\\baseFolder\\02. Current Drawings");

    public static void main(String[] args) throws InterruptedException {
        ApplicationContext ctx = new ClassPathXmlApplicationContext("spring/applicationContext.xml");
        DrawingFileExplorer drawingFileExplorer = (DrawingFileExplorer) ctx.getBean("drawingFileExplorer");
        drawingFileExplorer.explore(baseFolder);
    }

}

As of i know the line System.out.println(..) after foreach should be invoked only once. but the output of the code is following.

THIS LINE OF CODE SHOULD BE INVOKED ONCE
THIS LINE OF CODE SHOULD BE INVOKED ONCE

Could anyone explain how it can be invoked twice? my thread between first output and second line of output are not executed due to the shutdown().

Answer 1

您的方法是递归的，为什么您不希望一遍又一遍地调用它（每个目录一次）？

Answer 2

There's a recursive call after checking if each is a directory, that is -

 if(each.isDirectory()) {
   explore(each); // <-- HERE

So you will get multiple(s) of your line if (and only if) there are sub-directories.

Answer 3

Your explore is recursive method, which can be invoked even multiple times based on number of directories.

for(File each : drawingFolder.listFiles(DrawingFileFilter.getInstance())) {
                if(each.isDirectory()) {
                    explore(each);  // this call is inside for loop and will be invoke recursively.
                } else {
                    //pool.execute(new DrawingFileReviewer(each));
                }
            }

For eg, given directory structure will print the line for 5 times,

DraginFolder

SubFolder1

  SubFolder2 

SubFolder3

SubFolder4

Answer 4

You have to move

System.out.println("THIS LINE OF CODE SHOULD BE INVOKED ONCE");
pool.shutdown();
pool.awaitTermination(Integer.MAX_VALUE, TimeUnit.SECONDS);

outside of explore

When the code executed on a directory only contain files, the above 3 lines code will be executed. And because you shutdown the pool, you will cannot explore all files.

Answer 5

@Component
public class DrawingFileExplorer {

    private final ExecutorService pool = Executors.newFixedThreadPool(10);

    public void explore(File drawingFolder) throws InterruptedException {   
        _explore(drawingFolder);

        pool.shutdown();
        pool.awaitTermination(Integer.MAX_VALUE, TimeUnit.SECONDS);
    }

    private void _explore(File drawingFolder) {
        for(File each : drawingFolder.listFiles(DrawingFileFilter.getInstance())) {
            if(each.isDirectory()) {
                _explore(each);
            } else {
                pool.execute(new DrawingFileReviewer(each));
            }
        }
    }

}

made another _explorer(..) method to solve it thanks.