How to stop my threads in java

Question

I have the following section of code for a small brute-forcer. I've been trying to learn multi-threading and I have executors that will not stop.

private void generateThreads(int cores){
    if (cores > Runtime.getRuntime().availableProcessors() || cores <= 0)
        cores = Runtime.getRuntime().availableProcessors();

    ArrayList<Future<?>> tasks = new ArrayList<Future<?>>(cores);
    ExecutorService executor = Executors.newFixedThreadPool(cores);
    final long step = 2000;

    for (long i = 0; i < Long.MAX_VALUE; i += step){
        while(tasks.size() > cores) {
            for(int w = 0; w < tasks.size();w++){
                if(tasks.get(w).isDone()){
                    tasks.remove(w);
                    break;
                }
            }

            try{
                Thread.sleep(0);
            }

            catch (InterruptedException e){
                e.printStackTrace();
            }
        }
        {
            if (passwordFound == false){
                tasks.add(executor.submit(new Runnable(){

                    public void run(){
                        String attempt;
                        while(true){
                            try {

                                Thread.sleep(0);
                            } catch (InterruptedException e) {
                                e.printStackTrace();
                            }
                            attempt = bf.toString();
                            bf.increment();
                            if (passwordCrack(attempt)){
                                crackedPassword.append(attempt);
                                passwordFound = true;
                                break;
                            }

                        }
                    }
                }));
            }
            else
                break;
        }
    }
    executor.shutdownNow();
}

I thought that the Thread.sleep(0) would give the thread the ability to catch the InterruptedException, but my threads continue to go forever. What can I do to make sure that once a password is found, the threads stop correctly?

I usually just use a boolean flag. When you want it to stop, set it to false. — Mike G, Dec 11 '12 at 16:08
Is this what you are looking for? http://stackoverflow.com/questions/671049/how-do-you-kill-a-thread-in-java — Andrew Campbell, Dec 11 '12 at 16:09

Andrew Logvinov · Accepted Answer · 2012-12-11T16:18:30.983

2

You have to also check passwordFound variable:

// inside your run() method
while(!Thread.currentThread().isInterrupted() && !passwordFound) {
  ...
}

Thread.sleep() is not needed, as far as I can see.

P.S. Corrected my answer a bit to make it more clear what I meant.

edited Dec 11 '12 at 16:18

answered Dec 11 '12 at 16:11

Andrew Logvinov

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Argh... you are correct. I see that in the javadoc now. Thank you. you'll get the correct answer check mark in ~ 2 mins. – tophersmith116 Dec 11 '12 at 16:19
@tophersmith116 see note below from @PeterLawrey. `passwordFound` should indeed be `volatile` to be visible to other threads. – Andrew Logvinov Dec 11 '12 at 16:25

score 1 · Answer 2 · answered Dec 11 '12 at 16:13

1

It seems you want a behavior like this:

public void run() {
  /* do something */
  while(!passwordFound) {
      /* do something */
  }      
}

Be careful to take the lock on the variable where is stored password!

answered Dec 11 '12 at 16:13

Luca Davanzo

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How to stop my threads in java

2 Answers2