OSGi PuzzlersNeil Bartlett and Peter Kriens

Overlapping Concerns...

What Does it Print?

NOW STOP BUNDLE P1...

P1

P2Example

ComponentMyService

MyService

CKFU!(OBVIOUSLY)

WTF?

OSGi Compendium, section 112.5.10 Dynamic Service Replacement

“SCR must first bind a replacement target service and then unbind the outgoing service” (emphasis added)

Beware this Pattern@Componentpublic class ExampleComponent { private MyService svc; @Reference(dynamic = true) public void setMyService(MyService svc) { this.svc = svc; } public void unsetMyService(MyService svc) { this.svc = null; // OOOPS! }

}

Better...@Componentpublic class ExampleComponent { private MyService svc; @Reference(dynamic = true) public synchronized void setMyService(MyService svc) { this.svc = svc; } public synchronized void unsetMyService(MyService svc) { if (this.svc == svc) this.svc = null; }

}

Even Better...@Componentpublic class ExampleComponent { private final AtomicReference<MyService> svcRef = new AtomicReference<>(); @Reference(dynamic = true) public void setMyService(MyService svc) { svcRef.set(svc); } public void unsetMyService(MyService svc) { svcRef.compareAndSet(svc, null); }

}

Stooopid Hoooman...

What Does It Print?

A. 0,1,2,3,4,5,6,7,8,9,...B. 1,C. 1,2,3,4,5,6,7,8,9,....D. NullPointerException

What Does It Print?

A. 0,1,2,3,4,5,6,7,8,9,...B. 1,C. 1,2,3,4,5,6,7,8,9,....D. NullPointerException

WTF?

Blacklisting

EventHandlers have to return quickly

If they don’t... BLACKLIST! No more events for you.

Humans are slow and stooopid.

Turn off blacklisting, OR use printf-style debugging

You Gotta Start Me Up...

What Does It Print?

A. “Constructed...Started.”B. “Constructed...”C. NothingD. IllegalAccessExceptionE. “Started.”

What Does It Print?

A. “Constructed...Started.”B. “Constructed...”C. NothingD. IllegalAccessExceptionE. “Started.”

WTF?

DS Services are Lazy

Not an error... just waiting for a consumer!

REMEMBER:

OSGi is Your Friend!