Spring项目中QuartZ定时服务的原理是什么

package demoinfo.spring.quartz;
public interface IPrintInfoService {
  public void print();
}

package demoinfo.spring.quartz;

import java.text.SimpleDateFormat;
import java.util.Calendar;
import java.util.Date;

import demoinfo.spring.quartz.IPrintInfoService;

public class PrintInfoServiceImpl implements IPrintInfoService{

  public void print() {
    Calendar now = Calendar.getInstance();
    System.out.println("现在是北京时间：" + this.format(now.getTime()));
  }

  public String format(Date date){
    SimpleDateFormat sdf = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss");
    return sdf.format(date);
  }

}

package demoinfo.spring.quartz;

import org.quartz.JobExecutionContext;
import org.quartz.JobExecutionException;
import org.springframework.scheduling.quartz.QuartzJobBean;

import demoinfo.spring.quartz.IPrintInfoService;

public class PrintInfoJob extends QuartzJobBean{
  
  private IPrintInfoService prinfInfoService = null;
  public IPrintInfoService getPrinfInfoService() {
    return prinfInfoService;
  }
  public void setPrinfInfoService(IPrintInfoService prinfInfoService) {
    this.prinfInfoService = prinfInfoService;
  }
  @Override
  protected void executeInternal(JobExecutionContext arg0)
      throws JobExecutionException {
    this.prinfInfoService.print();
  }
}

<&#63;xml version="1.0" encoding="UTF-8"&#63;>
<beans xmlns="http://www.springframework.org/schema/beans"
  xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
  xmlns:context="http://www.springframework.org/schema/context"
  xmlns:tx="http://www.springframework.org/schema/tx"
  xsi:schemaLocation="http://www.springframework.org/schema/beans
        http://www.springframework.org/schema/beans/spring-beans-2.5.xsd
        http://www.springframework.org/schema/context
        http://www.springframework.org/schema/context/spring-context-2.5.xsd
        http://www.springframework.org/schema/tx
        http://www.springframework.org/schema/tx/spring-tx-2.5.xsd">


  <bean id="printInfoService" class="demoinfo.spring.quartz.PrintInfoServiceImpl" />
  <!-- 配置一个Job -->
  <bean id="printInfoJob" class="org.springframework.scheduling.quartz.JobDetailBean">
    <property name="jobClass" value="demoinfo.spring.quartz.PrintInfoJob" />
    <property name="jobDataAsMap">
      <map>
        <entry key="prinfInfoService" value-ref="printInfoService"></entry>
      </map>
    </property>
  </bean>

  <!-- 简单的触发器 -->
  <bean id="simplePrintInfoTrigger" class="org.springframework.scheduling.quartz.SimpleTriggerBean">
    <property name="jobDetail">
      <ref bean="printInfoJob" />
    </property>
    <property name="startDelay">
      <value>6000</value>
    </property>
    <property name="repeatInterval">
      <value>6000</value>
    </property>
  </bean>

  <!--复杂的触发器 -->
  <bean id="complexPrintInfoTrigger" class="org.springframework.scheduling.quartz.CronTriggerBean">
    <property name="jobDetail">
      <ref bean="printInfoJob" />
    </property>
    <property name="cronExpression">
      <value>00,05,10,15,20,25,30,35,40,45,50,55 * * * * &#63;</value>
    </property>
  </bean>

  <!-- spring触发工厂 -->
  <bean class="org.springframework.scheduling.quartz.SchedulerFactoryBean">
    <property name="triggers">
      <list>
        <ref bean="complexPrintInfoTrigger" />
      </list>
    </property>
  </bean>
</beans>

package demoinfo.spring.quartz;

import org.springframework.context.support.ClassPathXmlApplicationContext;

public class SpringQuartzDemo {

  public static void main(String[] args) {
    System.out.println("测试开始......");
    new ClassPathXmlApplicationContext(
        "classpath:demoinfo/spring/quartz/applicationContext.xml");
    System.out.println("测试结束......");
  }

}