Introducing new async tasks extensions.
by Mike Brock
One of the things that we've come to realize while developing Errai is that when you work with a framework such as Errai that push messaging is basically free, easy and awesome -- you end up wanting to do things like stream live data across the wire.
A lot of our demos have involved creating threads and pushing data across completely asynchronously. And it's always messy code. You need to worry about managing those threads, making sure they die when the session dies, or when the subject is unsubscribed, etc. Having to worry about all this creates security problems, resource management issues, and it makes your code messy.
Well, worry no longer! The latest commit into trunk introduces a new comprehensive (and simple) way of creating asynchronously running tasks -- as part of the the standard MessageBuilder API.
Take our new TimeDisplay demo, where we stream a bunch of updates from the server to the client containing System.currentTimeMillis() results. Up until now, the demo consisted of a thread that looped around and around and built new messages to send. Through the addition of a new API extensions, this demo is greatly simplified.
The first addition is some helper classes that help you create managed contexts to store stuff in the session. One is called SessionContext, and the other is called LocalContext. SessionContext allows you to create session scoped attributes, and LocalContext lets you create locally-scoped, as in, page-scoped. So if a user has multiple browser windows open, or multiple tabs, each window or tab is it's own LocalContext. This is a pretty powerful little tool.
The second addition is the implementation of a provided message parts. Unlike regular message parts, these parts are resolved via providers at the time of transmission. This is a key aspect of what we're about to show below.
The third addition is the implementation of a repeating and delayed message transmission calls as part of the standard messaging API.
Let's take a look at the example:
AsyncTask task = MessageBuilder.createConversation(message)
.toSubject("TimeChannel").signalling()
.withProvided(TimeServerParts.TimeString, new ResourceProvider<String>() {
public String get() {
return String.valueOf(System.currentTimeMillis());
}
}).noErrorHandling().replyRepeating(TimeUnit.MILLISECONDS, 100);
In this example, we create a conversational message which replies not just once, but replies continuously. Once every 100 milliseconds as it would turn out. The replyRepeating and replyDelayed, sendRepeating and sendDelayed methods all return an instance of AsyncTask, which is a handle on the task being performed. You can use this object to cancel the task.
task.cancel(true);
But let's look at the total example now:
@Service("TimeServer")
@RequireAuthentication
public class TimeDisplay implements MessageCallback {
private MessageBus bus;
@Inject
public TimeDisplay(MessageBus bus) {
this.bus = bus;
}
public void callback(final Message message) {
if (message.getCommandType() == null) return;
/**
* Create a local context to store state that is unique to this client instance. (not session wide).
*/
final LocalContext context = LocalContext.get(message);
/**
* Switch on the TimeServerCommand type provided
*/
switch (TimeServerCommands.valueOf(message.getCommandType())) {
case Start:
/**
* We want to start streaming.
*/
AsyncTask task = MessageBuilder.createConversation(message)
.toSubject("TimeChannel").signalling()
.withProvided(TimeServerParts.TimeString, new ResourceProvider<String>() {
public String get() {
return String.valueOf(System.currentTimeMillis());
}
}).noErrorHandling().replyRepeating(TimeUnit.MILLISECONDS, 100);
/**
* Store the task as an attribute uniquely identified by it's class type.
*/
context.setAttribute(AsyncTask.class, task);
/**
* Create a listener that will kill the task gracefully if the subject is unsubscribed. This
* isn't 100% necessary, as the task will be auto-killed ungracefully. But this provides
* and opportunity to clean up after ourselves.
*/
bus.addUnsubscribeListener(new UnsubscribeListener() {
public void onUnsubscribe(SubscriptionEvent event) {
if ("TimeChannel".equals(event.getSubject())) {
/**
* Delete this listener after this execution.
*/
event.setDisposeListener(true);
/**
* Stop the task from running.
*/
context.getAttribute(AsyncTask.class).cancel(true);
/**
* Destroy the local context. Sort of unnecessary, but helps reduce memory usage.
*/
context.destroy();
}
}
});
break;
case Stop:
/**
* Access our stored AsyncTask from this instance and cancel it.
*/
context.getAttribute(AsyncTask.class).cancel(true);
/**
* Destroy the local context. Sort of unnecessary, but helps reduce memory usage.
*/
context.destroy();
break;
}
}
}
That's all there is to it. It's pretty sweet. This API works on both the client and the server side. All the thread scheduling is all transparently managed by an executor service on the server, and by a simple Timer based implementation in the client. I'm hoping people will find this a welcome addition to Errai-land.
All the best,
Mike.
16 years
Ways around @ExposeEntity
by Asgeir Frimannsson
Hi all,
>From the documentation, one of the requirements for serialization is
the use of an @ExposeEntity annotation. Is there any ways around this,
so that I can send POJOs generated by frameworks which doesn't know
about Errai (but still follow GWT RPC serialization rules)?
If not, would it be possible to get rid of it? ;-)
For instance, by e.g.:
a) Following a similar 'white-list' approach that GWT uses, or
b) Define an annotation where I could add my exposed entities, such as
@ExposedEntities({
MyPOJO.class
MyOtherPojo.class
})
Any hints or approaches appreciated!
cheers,
asgeir
16 years, 1 month
Has anyone tried hooking up Seam 2.x events to Errai MessageBus?
by Asgeir Frimannsson
Hi all,
I've played with the Weld example in the past, but just wondered if
anyone had - with some level of success - tried developing a bridge
for Seam 2.x?
I guess a simple approach would be to define a bean that listens to
the events you are interested in, and adapts these to the errai bus,
and similarly the other way around.
cheers,
asgeir
16 years, 1 month
JBoss 4.2.3 JBossErraiServlet
by john destefano
Hi,
Sorry if this already got posted. I am trying to deploy a gwt application
that uses Errai to JBoss 4.2.3. I configured the APR stuff and according to
the start up log it is working correctly. When I deploy my app to the server
I get the following exception:
java.lang.ClassNotFoundException: org.jboss.servlet.http.HttpEventServlet
Looks like it might be related to the version of JBoss Web that is used in
JBoss 4.2.3.
Will Errai run on 4.2.3 with the comet servlet? Do you need to use 5.1? If
so, are there any sample apps that show the appropriate configuration? This
would include what jar files are deployed in the web-inf/lib directory.
Thanks
16 years, 1 month
JBossCometServlet
by John DeStefano
Hi Guys,
Having a hard time getting my app deployed to JBoss 5.10. I had issues with the servlet api being part of gwt-user.jar. I removed that package from the jar and got a little further. Now I' getting the following:
*************************************************************************************************
** PROBLEM!
** It appears something has been incorrectly configured. In order to use ErraiBus
** on JBoss, you must ensure that you are using the APR connector. Also make sure
** hat you have added these lines to your WEB-INF/web.xml file:
** ---
**
** JBossErraiServlet
** org.jboss.errai.bus.server.servlet.JBossCometServlet
** 1
**
**
**
** JBossErraiServlet
** *.erraiBus
**
** ---
** If you have the following lines in your WEB-INF/web.xml, you must comment or remove them:
** ---
**
** org.jboss.errai.bus.server.ErraiServletConfig
**
I have the JBossCometServlet servlet configured as above. How do I configure the AS to use the APR connector? Is there any documentation on how to configure an Errai application with the JBossErraiServlet?
I'm using the 1.1 snap shot. Also, what versions of JBoss have you guys used for testing?
Thx
16 years, 1 month
