java - 他のすべてのスレッドをただちに停止する Java マルチスレッド

Question

私はこのコードを持っています:

public class BruteForceThread implements Callable<String> {

private static volatile boolean stopped = false;        

public String call() {

    String password = this.getNextPassword();

    while (!stopped) {

        System.out.println(numberOfThreat + ": " + password);

        synchronized(this) {
            try {
                if (this.testPassword(password)) {
                    stopped = true;
                    return password;
                }
            } catch (IOException e) {
                e.printStackTrace();
            }
        }

        password = this.getNextPassword();

    }
    return "";

}
}

メインクラス:

public static void main(String[] args) throws IOException {

int numberOfThreads = 2;
ExecutorService executor = Executors.newFixedThreadPool(numberOfThreads);
CompletionService<String> pool = new ExecutorCompletionService<String>(executor);
for (int i = 1; i < numberOfThreads + 1; i++) {
  Callable<String> bruteForce = new BruteForceThread(...);
  pool.submit(bruteForce);
}

executor.shutdown();

    for(int i = 0; i < numberOfThreads; i++){
        try {
            String result = pool.take().get();
            if (result != "") {
                System.out.println("Your password: " + result);
            }
        } catch (InterruptedException e) {
            e.printStackTrace();
        } catch (ExecutionException e) {
            e.printStackTrace();
        }
    }

executor = null;
}

出力: ...

1: aa <- here first thread found a password
2: Fa <- second thread is continue
2: Fb
2: Fc
... many more ...
2: IZ
Your password: aa

1 つのスレッドがパスワードを検出し、stopped を true に設定した場合、別のスレッドはすぐには停止しません。なんで？