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From Angelo van der Sijpt <angelo.vandersi...@luminis.eu>
Subject Re: Dynamically debugging services with aspects...
Date Sun, 23 Jan 2011 14:55:31 GMT
Ooh, nice method, I've had this problem before.

To add to that, you could ease debugging (or at least, tracing of calls to EventAdmin) by
attaching the method that calls this postEvent or sendEvent method, for instance by using
    result.append(Thread.currentThread().getStackTrace()[2])

I am aware that this will fail in certain environments, but in most situations it gives you
an idea of 'where this message came from'.

Angelo

On Jan 22, 2011, at 3:18 PM, Marcel Offermans wrote:

> 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
> 


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