EJB2.1 & EJB3.0: Timer Service三部曲

对于J2EE中的调度任务, 在ejb2.1推出前, 似乎只能采用一些非常途径:比如在servlet中new一个Timer.
我们急切的需要EJB的标准实现.
最近我们就遇到了这样的需求. 还好websphere升级到了6.0, 支持ejb2.1规范,在ejb2.1标准中, 提供了一个Timer Service的标准实现.
要实现Timer Service, 通常的做法就是让一个无状态sessionbean实现TimedObject接口.
一个最简单的Timer Service如下(由一个无状态sessionbean实现, home和local接口省略)

public class DemoTimerServiceBean implements javax.ejb.SessionBean, TimedObject
{

private javax.ejb.SessionContext mySessionCtx;
/**
* getSessionContext
*/
public javax.ejb.SessionContext getSessionContext() {
return mySessionCtx;
}
/**
* setSessionContext
*/
public void setSessionContext(javax.ejb.SessionContext ctx) {
mySessionCtx = ctx;
}
/**
* ejbCreate
*/
public void ejbCreate() throws javax.ejb.CreateException
{
}
/**
* ejbActivate
*/
public void ejbActivate() {
}
/**
* ejbPassivate
*/
public void ejbPassivate() {
}
/**
* ejbRemove
*/
public void ejbRemove()
{
}


public void startTimer()
{
TimerService ts=this.getSessionContext().getTimerService();
Timer timer = ts.createTimer(new Date(), 1000, "timerInfo");
}
/* (non-Javadoc)
* @see javax.ejb.TimedObject#ejbTimeout(javax.ejb.Timer)
*/
public void ejbTimeout(Timer arg0)
{
// implements your business logic.
System.out.println("process your business logic");
TaskHandler.process();

}
}

其中startTimer()用来启动一个调度任务,当调度任务的timeout时间到来时, ejb容器回调ejbTimeout()方法.以上示例子每隔一秒执行一次ejbTimeout()方法.
需要注意的是, timer service的启动需要客户调用startTimer()方法; 并且timer service具有持久化特性, 也就是说: 如果一个timer service已经启动, 如果
服务器重新启动了, 这个timer service会继续执行(无须再次调用startTimer()方法).
其实我们可以把startTimer()方法放在ejbCreate(), 当ejb实例化的时候就启动调度. 但这需要修改一下启动调度的策略. 因为容器会根据需要调用sessionbean的
create()方法, 如果每实例化一个sessionbean,就产生一个新调度, 这显然不是我们需要的.
所以,修改的策略为:
如果已经有一个同名的timer service了, 就放弃, 否则产生一个新timer service.

public void startTimer()
{
TimerService ts=this.getSessionContext().getTimerService();
Iterator timers = ts.getTimers().iterator();
while(timers.hasNext())
{
Timer timer = (Timer)timers.next();
String info = (String)timer.getInfo();
System.out.println("timer info = " + info);
if (info.equals("timerInfo"))
{
System.out.println("there is already a timer = timerInfo, so return");
return;
}
}
ts.createTimer(new Date(), 1000, "timerInfo");
}

贴一个完整的timer service实现供有需要的人参考, 包括启动, 停止timer service(由一个无状态sessionbean实现, home和local接口省略)

public class DemoTimerServiceBean implements javax.ejb.SessionBean, TimedObject
{
private static final String CONGIG_BEAN_NAME = "configBean";

private javax.ejb.SessionContext mySessionCtx;
/**
* getSessionContext
*/
public javax.ejb.SessionContext getSessionContext() {
return mySessionCtx;
}
/**
* setSessionContext
*/
public void setSessionContext(javax.ejb.SessionContext ctx) {
mySessionCtx = ctx;
}
/**
* ejbCreate
*/
public void ejbCreate() throws javax.ejb.CreateException
{
}
/**
* ejbActivate
*/
public void ejbActivate() {
}
/**
* ejbPassivate
*/
public void ejbPassivate() {
}
/**
* ejbRemove
*/
public void ejbRemove()
{
}


public void stopAllTimer()
{
TimerService ts=this.getSessionContext().getTimerService();
Iterator timers = ts.getTimers().iterator();
while(timers.hasNext())
{
Timer timer = (Timer)timers.next();
timer.cancel();
}
}

public void stopTimer(String timerName)
{
TimerService ts=this.getSessionContext().getTimerService();
Iterator timers = ts.getTimers().iterator();
while(timers.hasNext())
{
Timer timer = (Timer)timers.next();
String info = (String)timer.getInfo();
System.out.println("timer info = " + info);
if (info.equals(timerName))
{
System.out.println("there is already a timer = " + timerName + ", so we cancel it!");
timer.cancel();
}
}
}

public ArrayList getAllTimerInfo()
{
ArrayList timerList = new ArrayList();
TimerService ts=this.getSessionContext().getTimerService();
Iterator timers = ts.getTimers().iterator();
while(timers.hasNext())
{
Timer timer = (Timer)timers.next();
String info = (String)timer.getInfo();
timerList.add(info);
}

return timerList;
}

public void startTimer()
{
Object configobj = MyBeanFactory.instance().getBean(CONGIG_BEAN_NAME);
if (configobj == null)
{
System.out.println("Can't get configBean from spring config file. config == null");
return;
}
Config config = (Config)configobj;
long interval = config.getInterval();
String timerInfo = config.getTimerInfo();
this.stopTimer(timerInfo);
System.out.println("start a timer and info = " + timerInfo + ", interval = " + interval);
TimerService ts=this.getSessionContext().getTimerService();
Timer timer = ts.createTimer(new Date(), interval, timerInfo);
}
/* (non-Javadoc)
* @see javax.ejb.TimedObject#ejbTimeout(javax.ejb.Timer)
*/
public void ejbTimeout(Timer arg0)
{
// TODO Auto-generated method stub
System.out.println("in ejbTimeout now.");
TaskHandler.process();

}
}