For ACE-112 [1] I had to debug the events flowing through the system to see if
the events being sent were created and received correctly. Most events (in ACE)
are being sent asynchronously, which makes debugging by setting breakpoints a
bit more difficult, as setting breakpoints in handleEvent() will not reveal the
sender.
One way to get more information is to "intercept" all calls to EventAdmin, and
the technique to do this is what I wanted to share with you in this e-mail.
What we can do is use the Dependency Manager to create an aspect service on top
of EventAdmin. An aspect can intercept all calls to services and in your
interceptor you can still delegate the calls to the original service.
Technically, this is being implemented using service rankings, which means that
consumers will automatically react to aspects.
What I did was add the following to the Activator of the VaadinClient:
manager.add(createAspectService(EventAdmin.class, null, 10, null)
.setImplementation(new EventAdmin() {
volatile EventAdmin m_eventAdmin;
private String print(Event event) {
StringBuffer result = new StringBuffer();
result.append("Event[ ");
String[] names = event.getPropertyNames();
for (String name : names) {
result.append(name);
result.append('=');
result.append(event.getProperty(name));
result.append(' ');
}
result.append(']');
return result.toString();
}
public void postEvent(Event event) {
System.out.println("postEvent: " + print(event));
m_eventAdmin.postEvent(event);
}
public void sendEvent(Event event) {
System.out.println("sendEvent: " + print(event));
m_eventAdmin.sendEvent(event);
}
}));
When we create the aspect service, the first two parameters are the service
class and a filter condition (null here) we want to create aspects on top of.
For each service that matches the service name and filter, and aspect will be
instantiated of the class specified below. The third parameters is the service
ranking for this aspect. This is there because you can also chain aspects on
top of each other. The final option (null here) is the name of the member that
should be injected into the aspect service (null means any member of the right
class).
The implementation is just one that intercepts all calls, prints the event, and
then delegates them to the real EventAdmin (which is injected automatically).
So it's quite easy to debug a service this way, and it's fully dynamic. Even in
a running production system, you can install a new bundle that adds an aspect
on top of an existing service and you can instantly intercept all calls, and
stop or uninstall the bundle again once you're done.
Greetings, Marcel
[1] https://issues.apache.org/jira/browse/ACE-112
