1.
OSGi Puzzlers
Neil Bartlett and Peter Kriens

2.
Overlapping Concerns...

3.
What Does it Print?
MyService
P1
P2
Example
Component
NOW STOP BUNDLE P1...
MyService

4.
CKFU!
(OBVIOUSLY)

5.
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)

6.
Beware this Pattern
@Component
public 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!
}
}

7.
Better...
@Component
public 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;
}
}

8.
Even Better...
@Component
public 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);
}
}

9.
Stooopid Hoooman...

10.
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

11.
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

12.
WTF?

13.
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

14.
You Gotta Start Me Up...

15.
What Does It Print?
A. “Constructed...Started.”
B. “Constructed...”
C. Nothing
D. IllegalAccessException
E. “Started.”

16.
What Does It Print?
A. “Constructed...Started.”
B. “Constructed...”
C. Nothing
D. IllegalAccessException
E. “Started.”

17.
WTF?
DS Services are Lazy
Not an error... just waiting for a consumer!

18.
REMEMBER:
OSGi is Your Friend!