java 多线程同步 synchronized 为什么将vehicleSum换成vehicleSy就可以正确的进行同步了

Question

public class SaveDatabasePlanCache {

    private static Integer vehicleSum = new Integer(0);

    private static Integer vehicleSy = new Integer(0);

    public static Integer getVehicleSum() {
        return vehicleSum;
    }

    public static void setVehicleSum(Integer vehicleSum) {

        for (int i = 0; i < 100000; i++) {}
        SaveDatabasePlanCache.vehicleSum = vehicleSum;

    }

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

        for (int i = 0; i < 30; i++) {
            
            new Thread(new Runnable() {
                public void run() {
                    for (int i = 0; i < 20; i++) {

                        synchronized (vehicleSum) {
                            int nim = SaveDatabasePlanCache.getVehicleSum() + 1;
                            SaveDatabasePlanCache.setVehicleSum(nim);
                            System.out.println(SaveDatabasePlanCache.getVehicleSum());
                        }

                    }

                }
            }).start();

        }

    }
}

Answer 1

加锁对象不应该在同步块内被修改。

"You're synchronizing on an object that you're no longer holding a reference to. Consider another thread running this method: they may enter and try to hit the lock at the moment after the reference to obj has been updated to point to the new object. At that point, they're synchronizing on a different object than the first thread. "

参考：http://stackoverflow.com/questions/6711838/synchronizing-on-an-object-in-java-then-changing-the-value-of-the-synchronized