Screaming Coder

Introducing Weld  Scheduling – the best way to run scheduled events in JBoss Weld, Seam 3 and possibly any JSR-299 implementation. It makes use of CDI’s typesafe event model for tying business logic to schedules. That is, you define your schedules using the provided qualifiers, which you apply to observer methods containing the business logic that you wish to be run at those times. In other words:

    public void onSchedule(@Observes @Scheduled("20 */2 * ? * *") Event e) {
        // do something every 2 minutes, at 20 seconds past the minute.
    }

The CDI container will fire the @Scheduled(“20 */2 * ? * *”) Event at 20 seconds past every second minute, causing the onSchedule method to be executed each time. When CDI starts up with this module on the classpath, all observers of the Event class are detected by the module using standard CDI APIs. The module then inspects each associated @Scheduled binding annotation and sets up a schedule to fire an Event with that binding at the schedule found. Currently Quartz is used as the underlying scheduler.

One obvious shortcoming of this is that we’ve managed to hard-code scheduling information in our Java code. The answer to this is to define the schedule as a property in the scheduler.properties file at the root of your classpath, for example:

# This schedule is named "test.one" and runs every 2 minutes
test.one=20 */2 * ? * *
# This schedule is named "after.hours" and runs in the wee hours every day
after.hours=0 0 2 ? * *

You can then observe that schedule like this:

    public void onNamedSchedule(@Observes @Scheduled("test.one") Event event) {
        // the schedule is defined in scheduler.properties
    }

This is getting better, but that “test.one” String is still setting off some refactoring alarm bells. No worries, we can deal with this pretty easily using meta-annotations. We just create a custom qualifier like so:

@Scheduled("after.hours")
@Qualifier
@Retention(RetentionPolicy.RUNTIME)
@Target( { ElementType.PARAMETER, ElementType.METHOD, ElementType.FIELD, ElementType.TYPE })
public @interface AfterHours {}

And now we can observe the event in a typesafe manner, in as many places as we want throughout our codebase with all the benefits of code-completion and none of the refactoring headaches:

    public void onTypesafeSchedule(@Observes @AfterHours Event e) {
        // do something after hours
    }

There are also some built-in convenience events for regular schedules:

    public void everySecond(@Observes @Every Second second) {
        // this gets executed every second
    }

    public void everyMinute(@Observes @Every Minute minute) {
        // this gets executed every minute
    }

    public void everyHour(@Observes @Every Hour hour) {
        // this gets executed every hour
    }

Note though that none of these built-in events will be scheduled, let alone fired, unless the module finds an observer for them on startup.

This project has been submitted to the Seam 3 sandbox (find it in seam/sandbox/modules). An early release of the Weld Scheduling module and Memory Grapher example app can be downloaded from here: WeldScheduling.tgz. They’re both built with Maven 2.0.10+. To run the example app, ‘mvn clean install‘ both projects (’scheduling’ first, then ‘MemoryGrapher’) and then run ‘mvn -Drun install’ from inside MemoryGrapher. It uses the Weld SE extension to run it without an app server (it’s a Swing app).

MNA542C29VE6

Finally! My copy of Nobody Knows Shoes arrived in the mail this week. As an adoring fan of Why’s Poignant Guide I had perhaps unfairly high expectations of NKS. As can be seen in the downloadable PDF version it’s not as long nor as entertaining as the Guide, seems targeted at a slightly younger crowd and is decidedly less poignant. Unlike the Guide, the comic strips in NKS make no sense whatsoever and have abs(zero) relevance to the actual subject matter. There’s not even any mention of Chunky Bacon. But this is not to say that NKS is somehow inferior compared to WPG, more that I really had no business comparing the two in the first place. After all, Shoes is just a tiny toolkit by it’s own admission, designed with new programmers in mind. For those with a passing interest I recommend simply downloading the on-line PDF version (it is printed at cost anyway so you’ll just be saving _why the effort). But I would recommend it to any teacher-types with a class full of wanna-be programmers. Just hand them each a copy and let _why’s deranged cosmo-babble do the rest.