EJB之Timer

EJB Timer

• 要么: Annotation @Schedule 或者方法前声明@Timeout
• 要么: 在部署描述中定义timeout-method

• 如果是使用@Schedule, Timer在一个ejb中可以支持多个,如:

    @Schedule(second="*/2", minute="*",hour="*", persistent=false,info="timer1")
    public void doWork(){
        System.out.println( "Hi from the EJB timer example!" );
    }


    @Schedule(second="*/1", minute="*",hour="*", persistent=false,info="timer2")
    public void doWork2(){
        System.out.println( "Hi .... 2nd timer!" );    
    }
以上分别启动了2个timer,一个每隔2秒执行一次,一个每隔1秒执行一次. 执行时container做了并发访问控制.也就是说callback方法一个时刻只能执行一个方法.

• 如果是使用@Timeout, 也就是Programmatic Timers, callback方法有且只有一个.

All timers created via one of the TimerService timer creation methods for a particular component use a single timeout callback method. --From ejb spec 3.1 chapter 18
如前一个调用没有完成,而后面的timer过来了,在后面的timer在调用callback时会等待write lock(ejb方法默认), 如果超时就会出现如下异常, Jboss的行为, 还会retry一次,如果还是timeout,则会放弃.:

09:20:59,045 ERROR [org.jboss.as.ejb3] (EJB default - 2) JBAS014122: Error during retrying timeout for timer: [id=c2a31520-2c55-4b01-b362-d8dfdb73a2d0 timedObjectId=jboss-ejb-timer.jboss-ejb-timer.TimeoutExample auto-timer?:false persistent?:false timerService=org.jboss.as.ejb3.timerservice.TimerServiceImpl@65886ae8 initialExpiration=Tue Mar 29 09:20:48 CST 2016 intervalDuration(in milli sec)=0 nextExpiration=null timerState=RETRY_TIMEOUT: javax.ejb.ConcurrentAccessTimeoutException: JBAS014373: EJB 3.1 PFD2 4.8.5.5.1 concurrent access timeout on org.jboss.invocation.InterceptorContext$Invocation@7c49f17f - could not obtain lock within 5000MILLISECONDS
    at org.jboss.as.ejb3.concurrency.ContainerManagedConcurrencyInterceptor.processInvocation(ContainerManagedConcurrencyInterceptor.java:100) [jboss-as-ejb3-7.3.0.Final-redhat-14.jar:7.3.0.Final-redhat-14]



TimerService的createSingleActionTimer()和createIntervalTimer()

• createSingleActionTimer

只触发一次，如果触发时等不到锁，timeout， 则如上所述，再试一次，如果还不行，直接放弃。
注意: 如是Singleton的EJB, 默认的锁是Write lock, 除非主动声明为READ lock, 如:@Lock(LockType.READ)

• createIntervalTimer

周期性触发，到点触发发现上一个还没搞完，则直接skip，放弃了，接着等下一个，如下code

     @Timeout
    public void scheduler(Timer timer) {
        System.out.println("EJB Timer: Info=" + timer.getInfo() + " " + Thread.currentThread().getName());

        try {
            Thread.sleep(1000*30);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    }


    //
    @PostConstruct
    public void initialize( InvocationContext ctx ) {
        //...
        timerService.createIntervalTimer(2000, 10*1000, new TimerConfig("Single Timer #2", false));
        //....
    }

输出的log如下：
10:21:47,864 INFO  [stdout] (EJB default - 7) EJB Timer: Info=Single Timer #2 EJB default - 7

10:21:57,864 WARN  [org.jboss.as.ejb3] (EJB default - 9) JBAS014143: A previous execution of timer [jboss-ejb-timer.jboss-ejb-timer.TimeoutExample 8ba38f5a-0da7-4993-8b0b-e69854f61ee4] is still in progress, skipping this overlapping scheduled execution at: Tue Mar 29 10:21:57 CST 2016
10:22:07,864 WARN  [org.jboss.as.ejb3] (EJB default - 6) JBAS014143: A previous execution of timer [jboss-ejb-timer.jboss-ejb-timer.TimeoutExample 8ba38f5a-0da7-4993-8b0b-e69854f61ee4] is still in progress, skipping this overlapping scheduled execution at: Tue Mar 29 10:22:07 CST 2016
10:22:17,864 WARN  [org.jboss.as.ejb3] (EJB default - 8) JBAS014143: A previous execution of timer [jboss-ejb-timer.jboss-ejb-timer.TimeoutExample 8ba38f5a-0da7-4993-8b0b-e69854f61ee4] is still in progress, skipping this overlapping scheduled execution at: Tue Mar 29 10:22:17 CST 2016
10:22:27,865 INFO  [stdout] (EJB default - 3) EJB Timer: Info=Single Timer #2 EJB default - 3

10:22:37,864 WARN  [org.jboss.as.ejb3] (EJB default - 10) JBAS014143: A previous execution of timer [jboss-ejb-timer.jboss-ejb-timer.TimeoutExample 8ba38f5a-0da7-4993-8b0b-e69854f61ee4] is still in progress, skipping this overlapping scheduled execution at: Tue Mar 29 10:22:37 CST 2016
10:22:47,864 WARN  [org.jboss.as.ejb3] (EJB default - 2) JBAS014143: A previous execution of timer [jboss-ejb-timer.jboss-ejb-timer.TimeoutExample 8ba38f5a-0da7-4993-8b0b-e69854f61ee4] is still in progress, skipping this overlapping scheduled execution at: Tue Mar 29 10:22:47 CST 2016
10:22:57,864 WARN  [org.jboss.as.ejb3] (EJB default - 1) JBAS014143: A previous execution of timer [jboss-ejb-timer.jboss-ejb-timer.TimeoutExample 8ba38f5a-0da7-4993-8b0b-e69854f61ee4] is still in progress, skipping this overlapping scheduled execution at: Tue Mar 29 10:22:57 CST 2016

Jboss quickstart中有timer的例子。    


更复杂的例子:

如果显示指定callback方法为read lock,情况如何呢?

1. 如果上一个callback方法没有完成,这时候下一个timer(第二次timer)触发的调用过来了.
2. 因为是read lock, 所以顺利进入到callback方法内部, 上面说的等锁而导致的timeout retry行为就不会发生.
3. ejb container从线程池中调度一个thread过来,执行callback,因为某种原因花了很长时间.
4. 第三次timer触发, ejb从线程池再拿一个thread,执行callback, ...
5. 到了第11次触发(线程池数量假设为10), 无线程可用,方法阻塞,等待(注意不会timeout,因为已经进入到callback方法体)
6. 其他ejb如果需要从线程池中启动, 发现无资源可用,也会继续等待, 注意是等待,
7. 如果积攒的需要启动的线程越来越多, 如果前面的callback方法突然返回,哈哈,后面的线程就会疯狂的run.
8. .....

所以搞timer, 最好不要将callback设置为read lock, 否则有可能thread leak, 试想你启动一个timer,每一秒钟执行一次, 执行的过程中hung住了, 而此时callback方法为read lock.

....

如:

//EJB A中

    @Timeout
    @Lock(LockType.READ)
    public void scheduler(Timer timer) {

        //Just test code, please DONT use sleep in your code!

        try {
            Thread.sleep(1000*30);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }

    }

//in initialize method

for(int i = 2; i < 20; i++)
            timerService.createSingleActionTimer(1000, new TimerConfig("Single Timer #1", false) );

//EJB B

   @Schedule(second="*/2", minute="*",hour="*", persistent=false)
    public void doWork(){
        System.out.println( "Hi from the EJB timer example!" );      //这些timer会一直积攒在那里, 因为所有的线程全部被EJB A占用,没有返回.

    }

所以线程池里没有线程可用会死等下去, 而如果timer调用的callback方法不会死等, 有timeout. 如果是read lock就会一直耗干线程池中的线程实例.

关于timer.cancel

如果一个timer已经开始被执行，但还没完成，这时候调用timer.cancel()会成功，但当先已经被触发的API会继续执行完毕， 不会stop掉。