java - 我们可以同时从多个线程访问同一个实例的同步方法和非同步方法吗？

Question

这似乎是一个非常幼稚的问题，但我在任何地方都找不到任何具体的答案。我什至尝试过，但由于我们无法预测 Java 中线程资源分配的行为，因此很难确定。我只是想知道是否可以从该类的同一实例的两个不同线程同时访问该类的同步方法和非同步方法？

Answer 1

没发现任何问题。试试这个:

public class Main {

    public static final SyncNotsynced sn = new SyncNotsynced();

    public static void main(String[] args){
        Thread t1 = new Thread(sn::synced);
        Thread t2 = new Thread(sn::notsynced);
        t1.start();
        t2.start();
    }

    public static class SyncNotsynced {

        public synchronized void synced(){
            System.out.println(Thread.currentThread().getName() + " enter synced");
            try {
                Thread.sleep(2000);
            } catch (InterruptedException e) {
                Thread.currentThread().interrupt();
            }
            System.out.println(Thread.currentThread().getName() +  " exit synced");
        }

        public void notsynced(){
            System.out.println(Thread.currentThread().getName() + " enter notsynced");
            try {
                Thread.sleep(2000);
            } catch (InterruptedException e) {
                Thread.currentThread().interrupt();
            }
            System.out.println(Thread.currentThread().getName() +  " exit notsynced");
        }
    }
}

或者看看 live example :)。正如您所看到的，Thread 1 和 Thread 2 的进入都是在退出之前发生的:

Thread-0 enter synced
Thread-1 enter notsynced
Thread-0 exit synced
Thread-1 exit notsynced

有关正式解释，您可以阅读 JLS 17 ，但简而言之，只有一个线程可以进入同一个对象监视器上的同步块(synchronized block)。

顺便说一句，我使用了Thread.sleep because (强调我的):

Thread.sleep causes the currently executing thread to sleep (temporarily cease execution) for the specified duration, subject to the precision and accuracy of system timers and schedulers. The thread does not lose ownership of any monitors, and resumption of execution will depend on scheduling and the availability of processors on which to execute the thread.