[OCPJP]使用ExecutorService處理檔案

Given the code fragment:

public class FileThread implements Runnable {

    String fName;

    public FileThread(String fName) {
        this.fName = fName;
    }

    public void run() {
        System.out.println(fName);
    }

    public static void main(String[] args) throws IOException, InterruptedException {
        ExecutorService executor = Executors.newCachedThreadPool();
        Stream<Path> listOfFiles = Files.walk(Paths.get("Java Projects"));
        listOfFiles.forEach(line -> {
            executor.execute(new FileThread(line.getFileName().toString())); //line n1
        });
        executor.shutdown();
        executor.awaitTermination(5, TimeUnit.DAYS); //line n2
    }
}

The Java Projects directory exists and contains a list of files.

What is the result?

A. The program throws a runtime exception at line n2.
B. The program prints files names concurrently.
C. The program prints files names sequentially.
D. A compilation error occurs at line n1.

題解

Files類別的walk方法可以使用深度優先(depth-first)來走訪傳入的檔案路徑。

ExecutorService物件的execute方法可以使用新的執行緒來執行傳入的Runnable物件。shutdown方法可以關閉ExecutorService物件使其不再接受新的執行物件(Runnable或是Callable)，但並不會等待目前正在執行和正在排隊的工作執行完畢。awaitTermination方法可以指定一個上限時間來等待ExecutorService物件將所有的工作處理完成。

所以這題答案是選項B。