问在Java中获取独占系统范围的锁

EN

public class Singleton {

    // **** members ****
    private static  Singleton   singleInstance = null;

    private static  Semaphore   sem = new Semaphore(1);

    public          int         val = 0;

    // **** constructor ****
    private Singleton() {
        val = 0;
    }

    // **** get instance of this class ****
    public static Singleton getInstance() {

        // **** request access ****
        try {
            sem.acquire();
        } catch (InterruptedException e) {
            e.printStackTrace();
        }

        // **** instantiate this class (if needed) ****
        if (singleInstance == null) {

            // **** instantiate Singleton ****
            singleInstance = new Singleton();

            // **** inform user what is going on ****
            System.out.println("hashCode: " + singleInstance.hashCode());
        }

        // **** release access ****
        sem.release();

        // **** return this class ****
        return singleInstance;
    }

}