Java的ReentrantLock没有按预期工作

Question

The question is simple, I just expect to see the demo.i to be 70000 since I have 7 threads and using a Lock, or it will be less than 70000

public class Sy {

    public static void main(String[] args) {
        Lock lock = new ReentrantLock();
        Demo demo = new Demo();
        Thread[] threads = new Thread[7];
        for(int i=0;i<threads.length;i++) {
            int finalI = i;
            Thread thread = new Thread(() ->  {
                System.out.println("Thread " + finalI + " started!");
                for(int j=0;j<10000;j++){
                    lock.lock();
                    demo.i++;
                    lock.unlock();
                }
                System.out.println("Thread " + finalI + " ended!");
            });
            threads[i] = thread;
            thread.start();
        }

        System.err.println(demo.i);
    }
}


class Demo {

    public int i = 0;

}

Answer 1

You need to wait for the threads to finish before printing:

for (Thread thread : threads) {
    thread.join();
}

Answer 2

The threads need to finish

Try this:

public class Sy {

    public static void main(String[] args) {
        Lock lock = new ReentrantLock();
        Demo demo = new Demo();
        Thread[] threads = new Thread[7];
        for(int i=0;i<threads.length;i++) {
            int finalI = i;
            Thread thread = new Thread(() ->  {
                System.out.println("Thread " + finalI + " started!");
                for(int j=0;j<10000;j++){
                    lock.lock();
                    demo.i++;
                    lock.unlock();
                }
                System.out.println("Thread " + finalI + " ended!");
            });
            threads[i] = thread;
            thread.start();
        }

        for(int i=0;i<threads.length;i++) {
          try { 
            threads[i].join();
          } catch (Exception e) {
            e.printStackTrace();
          }
        }

        System.out.println(demo.i);
    }
}


class Demo {

    public int i = 0;

}