根据CronSequenceGenerator计算cron表达式的时间

        Spring提供了CronSequenceGenerator类以便计算cron表达式的具体时间点的next方法（Spring3.2版本）和cron是否合法的isValidExpression方法（Spring4.3版本）。

实战举例：

package construct;

import org.springframework.scheduling.support.CronSequenceGenerator;

import java.util.Date;

/**
 * Created by zhangzh on 2017/1/18.
 */
public class CronSequenceGeneratorTest {

    public static void main(String[] args) {

        String cron = "0 */5 * * * ?"; //每个五分钟执行一次

        CronSequenceGenerator cronSequenceGenerator = new CronSequenceGenerator(cron);

        Date currentTime = new Date();

        System.out.println("currentTime: " + currentTime);

        Date nextTimePoint = cronSequenceGenerator.next(currentTime); // currentTime为计算下次时间点的开始时间
        System.out.println("nextTimePoint: " + nextTimePoint);

        Date nextNextTimePoint = cronSequenceGenerator.next(nextTimePoint);

        System.out.println("nextNextTimePoint: " + nextNextTimePoint);
    }
}

currentTime: Wed Jan 18 19:45:25 CST 2017
nextTimePoint: Wed Jan 18 19:50:00 CST 2017
nextNextTimePoint: Wed Jan 18 19:55:00 CST 2017