线程中的死锁问题

Question

I have a method which uses a number to find if it is prime or not. I have made 10 threads which would seek a number from the counter and print if it is prime. When i run the below code there seems to be a deadlock in the program printing just 2 and 3. I am new to threads and so i am pretty sure i am doing a silly mistake. Can anyone help me out?

public class Main{

public static int counter=0;

public static void main(String[] args) {

    Thread thread[] = new Thread[10]; //10 threads created

    for(int i=0;i<10;i++){
        thread[i]=new Thread(new NewThread());
        thread[i].start();
    }

}

public static int increment(){

    long count=0;

    count=++counter;

    if(count>1000000000){
        return -1;
    }

    return counter;
}

}


public class NewThread implements Runnable{

    Lock lock=new ReentrantLock();

    public boolean isPrime(int number) throws ThresholdReachedException{

        if(number<0){
            throw new ThresholdReachedException("Limit Reached");
        }

        if(number==1){
            return false;
        }

        double limit=Math.pow(number, 0.5);

        for(int i=1;i<(int)limit;i++){
            if(number%i==0){
                return false;
            }
        }
        return true;
    }

    @SuppressWarnings("static-access")
    @Override
    public void run() {

        //System.out.println(Thread.currentThread().getName()+" running");
        int number=0;
        try{
            while(true){
                if(lock.tryLock()){
                //lock.lock();
                number=Main.increment();
                lock.unlock();
                }else{
                    Thread.currentThread().sleep(2000);;
                }

                if(isPrime(number)){
                    System.out.println(Thread.currentThread().getName()+" Prime number : "+number);
                }
            }
        }catch(ThresholdReachedException e){

        } catch (InterruptedException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        } 

    }

}

Answer 1

Few things to note:

1. You're declaring lock as an instance variable, not a class variable ( static ). As a result, each thread will have its own copy of it, which defeats the purpose.

2. You don't need to do a tryLock and then sleep -- simply calling .lock() will put the thread to sleep until the lock is available. Alternatively, you can bypass using a lock entirely by using an AtomicInteger : https://docs.oracle.com/javase/8/docs/api/java/util/concurrent/atomic/AtomicInteger.html

3. (the actual bug) In your prime checker itself, you're starting the for loop at 1 : n % 1 is always equal to 0, since all integers are (by definition) divisible by 1. The only reason 2 and 3 worked presumably is that limit will be equal to 1 and so the for loop wouldn't run (correct behavior). You should start i at 2 instead.