How is an executor termination recursion in java?

Question

This is a program that reads information site for previous format it uses recursion and executor.It works fine,my problem is to test whether the program is completed and suc

Answer 1

The issue is that you're calling executor.execute after calling executor.shutdown(). The awaitTermination method only waits for tasks that were submitted before shutdown was called.

Here's why this is happening:

• Let's say the program starts on thread main.
• The first call to executor.execute is on main.
• Suppose the executor now runs on thread-pool-1, it also calls executor.execute.
• But before those new tasks execute, we jump back onto main and call executor.shutdown.
• The executor might start executing tasks again, maybe on thread-pool-2 this time, and it tries to call executor.execute. However, executor.shutdown() has already been called, so a java.util.concurrent.RejectedExecutionException is thrown.

There are a few ways to solve this:

1. Using an executor might be overkill in this instance; just doing it single threaded will probably be sufficient.
2. It may be possible to refactor the code so that the last executor.execute can call executor.shutdown. However, if there isn't a distinct last call, this won't be possible.

3. If you absolutely need to do this off the main thread, you could perform the entire recursive call in its own thread:

   public static void main(String[] args) {
       try {
           ExecutorService executor = Executors.newSingleThreadExecutor();
           executor.execute(new Runnable() {
               @Override
               public void run() {
                   NewClass.getRecursive("http://www.java2s.com/", 0);
               }
           });
           executor.shutdown();
           executor.awaitTermination(1, TimeUnit.HOURS);
           if (executor.isTerminated()) {
               JOptionPane.showMessageDialog(null, "Success");
           }
       }
       catch (Exception ex)
       {
           Logger.getLogger(NewClass.class.getName()).log(Level.SEVERE, null, ex);
       }
   }
   

   and

   private static class NewClass {
   
       static String levels[] = { "div.col-md-9 li a", "div#sidebar ul li a" };
       static String links = "";
   
       private static void getRecursive(String href, int level) {
   
           if (level > levels.length - 1) {
               return;
           }
   
           Document doc;
           try {
               doc = Jsoup.connect(href).get();
               Elements elements = doc.select(levels[level]);
               final int flevel = ++level;
               for (final Element element : elements) {
                   if (!element.attr("href").isEmpty()) {
                       links += element.attr("abs:href") + "\n";
                       System.out.println(links);
                       getRecursive(element.attr("abs:href"), flevel);
                   }
               }
           }
           catch (IOException e1) {
               e1.printStackTrace();
           }
       }
   }