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| |
| <h1>Schedule CDI Events</h1> |
| </div> |
| </div> |
| </div> |
| <div class="row"> |
| |
| <div class="col-md-12"> |
| <div id="preamble"> |
| <div class="sectionbody"> |
| <div class="paragraph"> |
| <p>This example uses a nice CDI/EJB combination to schedule CDI Events. This is useful if you want CDI Events that fire regularly or at a specific time or calendar date.</p> |
| </div> |
| <div class="paragraph"> |
| <p>Effectively this is a simple wrapper around the <code>BeanManager.fireEvent(Object,Annotations...)</code> method that adds <code>ScheduleExpression</code> into the mix.</p> |
| </div> |
| </div> |
| </div> |
| <div class="sect1"> |
| <h2 id="_scheduleexpression_and_timeout">ScheduleExpression and @Timeout</h2> |
| <div class="sectionbody"> |
| <div class="paragraph"> |
| <p>The logic here is simple, we effecitvely expose a method identical to <code>BeanManager.fireEvent(Object, Annotations...)</code> and wrap it as <code>scheduleEvent(ScheduleExpression, Object, Annotation...)</code></p> |
| </div> |
| <div class="paragraph"> |
| <p>To do that we use the EJB <code>TimerService</code> (under the covers this is Quartz) and create an <code>@Timeout</code> method which will be run when the <code>ScheduleExpression</code> activates.</p> |
| </div> |
| <div class="paragraph"> |
| <p>The <code>@Timeout</code> method, simply called <code>timeout</code>, takes the event and fires it.</p> |
| </div> |
| <div class="listingblock"> |
| <div class="content"> |
| <pre class="highlight"><code class="language-java" data-lang="java">@Singleton |
| @Lock(LockType.READ) |
| public class Scheduler { |
| |
| @Resource |
| private TimerService timerService; |
| |
| @Resource |
| private BeanManager beanManager; |
| |
| public void scheduleEvent(ScheduleExpression schedule, Object event, Annotation... qualifiers) { |
| |
| timerService.createCalendarTimer(schedule, new TimerConfig(new EventConfig(event, qualifiers), false)); |
| } |
| |
| @Timeout |
| private void timeout(Timer timer) { |
| final EventConfig config = (EventConfig) timer.getInfo(); |
| |
| beanManager.fireEvent(config.getEvent(), config.getQualifiers()); |
| } |
| |
| // Doesn't actually need to be serializable, just has to implement it |
| private final class EventConfig implements Serializable { |
| |
| private final Object event; |
| private final Annotation[] qualifiers; |
| |
| private EventConfig(Object event, Annotation[] qualifiers) { |
| this.event = event; |
| this.qualifiers = qualifiers; |
| } |
| |
| public Object getEvent() { |
| return event; |
| } |
| |
| public Annotation[] getQualifiers() { |
| return qualifiers; |
| } |
| } |
| }</code></pre> |
| </div> |
| </div> |
| <div class="paragraph"> |
| <p>Then to use it, have <code>Scheduler</code> injected as an EJB and enjoy.</p> |
| </div> |
| <div class="listingblock"> |
| <div class="content"> |
| <pre class="highlight"><code class="language-java" data-lang="java">public class SomeBean { |
| |
| @EJB |
| private Scheduler scheduler; |
| |
| public void doit() throws Exception { |
| |
| // every five minutes |
| final ScheduleExpression schedule = new ScheduleExpression() |
| .hour("*") |
| .minute("*") |
| .second("*/5"); |
| |
| scheduler.scheduleEvent(schedule, new TestEvent("five")); |
| } |
| |
| /** |
| * Event will fire every five minutes |
| */ |
| public void observe(@Observes TestEvent event) { |
| // process the event |
| } |
| |
| }</code></pre> |
| </div> |
| </div> |
| </div> |
| </div> |
| <div class="sect1"> |
| <h2 id="_test_case">Test Case</h2> |
| <div class="sectionbody"> |
| <div class="paragraph"> |
| <p>A working test case for the above would be as follows:</p> |
| </div> |
| <div class="listingblock"> |
| <div class="content"> |
| <pre class="highlight"><code class="language-java" data-lang="java">import org.junit.Assert; |
| import org.junit.Before; |
| import org.junit.Test; |
| |
| import jakarta.ejb.AccessTimeout; |
| import jakarta.ejb.EJB; |
| import jakarta.ejb.ScheduleExpression; |
| import jakarta.ejb.embeddable.EJBContainer; |
| import jakarta.enterprise.event.Observes; |
| import java.util.concurrent.CountDownLatch; |
| import java.util.concurrent.TimeUnit; |
| |
| /** |
| * @version $Revision$ $Date$ |
| */ |
| public class SchedulerTest { |
| |
| public static final CountDownLatch events = new CountDownLatch(3); |
| |
| @EJB |
| private Scheduler scheduler; |
| |
| @Test |
| public void test() throws Exception { |
| |
| final ScheduleExpression schedule = new ScheduleExpression() |
| .hour("*") |
| .minute("*") |
| .second("*/5"); |
| |
| scheduler.scheduleEvent(schedule, new TestEvent("five")); |
| |
| Assert.assertTrue(events.await(1, TimeUnit.MINUTES)); |
| } |
| |
| |
| @AccessTimeout(value = 1, unit = TimeUnit.MINUTES) |
| public void observe(@Observes TestEvent event) { |
| if ("five".equals(event.getMessage())) { |
| events.countDown(); |
| } |
| } |
| |
| public static class TestEvent { |
| private final String message; |
| |
| public TestEvent(String message) { |
| this.message = message; |
| } |
| |
| public String getMessage() { |
| return message; |
| } |
| } |
| |
| @Before |
| public void setup() throws Exception { |
| EJBContainer.createEJBContainer().getContext().bind("inject", this); |
| } |
| }</code></pre> |
| </div> |
| </div> |
| </div> |
| </div> |
| <div class="sect1"> |
| <h2 id="_you_must_know">You must know</h2> |
| <div class="sectionbody"> |
| <div class="ulist"> |
| <ul> |
| <li> |
| <p>CDI Events are not multi-treaded</p> |
| </li> |
| </ul> |
| </div> |
| <div class="paragraph"> |
| <p>If there are 10 observers and each of them take 7 minutes to execute, then the total execution time for the one event is 70 minutes. It would do you absolutely no good to schedule that event to fire more frequently than 70 minutes.</p> |
| </div> |
| <div class="paragraph"> |
| <p>What would happen if you did? Depends on the <code>@Singleton</code> <code>@Lock</code> policy</p> |
| </div> |
| <div class="ulist"> |
| <ul> |
| <li> |
| <p><code>@Lock(WRITE)</code> is the default. In this mode the <code>timeout</code> method would essentially be locked until the previous invocation completes. Having it fire every 5 minutes even though you can only process one every 70 minutes would eventually cause all the pooled timer threads to be waiting on your Singleton.</p> |
| </li> |
| <li> |
| <p><code>@Lock(READ)</code> allows for parallel execution of the <code>timeout</code> method. Events will fire in parallel for a while. However since they actually are taking 70 minutes each, within an hour or so we’ll run out of threads in the timer pool just like above.</p> |
| </li> |
| </ul> |
| </div> |
| <div class="paragraph"> |
| <p>The elegant solution is to use <code>@Lock(WRITE)</code> then specify some short timeout like <code>@AccessTimeout(value = 1, unit = TimeUnit.MINUTES)</code> on the <code>timeout</code> method. When the next 5 minute invocation is triggered, it will wait up until 1 minute to get access to the Singleton before giving up. This will keep your timer pool from filling up with backed up jobs — the "overflow" is simply discarded.</p> |
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