The document discusses three puzzles related to OSGi concepts. The first puzzle involves determining which bundle fragments attach to host bundles based on their version ranges. The second puzzle involves a potential race condition between threads when using wait/notify to signal the availability of a service. The third puzzle involves determining what is printed by an activation method in a component implementation, based on the accessibility of the method and DS specification version. Each puzzle includes multiple choice answers and an explanation of the correct answer.

1. Pet
er K
rien
s, a
Qu
te
grav e, IBM
B J Har
OSGi Puzzlers
Tuesday, March 22, 2011

2. 1. Attachment
H1 H2
Bundle-SymbolicName: foo.host Bundle-SymbolicName: foo.host
Bundle-Version: 1.0 Bundle-Version: 2.0
F
Bundle-SymbolicName: foo.fragment
Fragment-Host: foo.host; version=”[1.0,2.0)”
Tuesday, March 22, 2011

3. To what host(s) does fragment F attach when
resolved?
H1 H2
Bundle-SymbolicName: foo.host Bundle-SymbolicName: foo.host
Bundle-Version: 1.0 Bundle-Version: 2.0
F
Bundle-SymbolicName: foo.fragment
Fragment-Host: foo.host; version=”[1.0,2.0)”
(a) F attaches to only H1
(b) F attaches to only H2
(c) F attaches to both H1 and H2
(d) F does not attach to either H1 or H2
Tuesday, March 22, 2011

4. To what host(s) does fragment F attach when
resolved?
(a) F attaches to only H1
(b) F attaches to only H2
(c) F attaches to both H1 and H2
(d) F does not attach to either H1 or H2
bundle-version is the proper attribute! Note: in
4.3 attribute matching is now supported, so the
answer will be (d)!
Tuesday, March 22, 2011

5. Solution
H1 H2
Bundle-SymbolicName: foo.host Bundle-SymbolicName: foo.host
Bundle-Version: 1.0 Bundle-Version: 2.0
F
Bundle-SymbolicName: foo.fragment
Fragment-Host: foo.host; bundle-version=”[1.0,2.0)”
Tuesday, March 22, 2011

6. 2. Waiting For Service
Thread 1
// a service in which the tracker is interested is registered
...
Waiter waiter;
public T addingService(ServiceReference<S> reference) {
synchronized (waiter) {
waiter.notify(); // tell waiter about the service
}
return super.addingService(reference);
}
Thread 2
// waiter object
ServiceTracker tracker;
void waitingForService {
synchronized (this) {
wait(); // waiting here for a service
}
T service = tracker.getService();
System.out.println(service);
}
Tuesday, March 22, 2011

7. What is printed by Thread 2?
Thread 1
// a service in which the tracker is interested is registered
...
Waiter waiter;
(a) nothing; thread
public T addingService(ServiceReference<S> reference) {
synchronized (waiter) { 2 does not reach
waiter.notify(); // tell waiter about the service
}
return super.addingService(reference);
println
}
(b) the service
Thread 2 object
// waiter object
ServiceTracker tracker; (c) null
(d) none of the
void waitingForService {
synchronized (this) {
wait(); // waiting here for a service
}
T service = tracker.getService();
above
System.out.println(service);
}
Tuesday, March 22, 2011

8. What is printed by Thread 2?
(a) nothing; thread 2 does not reach println
(b) the service object
(c) null
(d) none of the above: race condition
The answer can be (b) or (c) depending on how the
race works out.
Tuesday, March 22, 2011

9. Another Look
Thread 1
Waiter waiter;
public T addingService(ServiceReference<S> reference) {
synchronized (waiter) {
waiter.notify(); // tell waiter about the service
}
return super.addingService(reference);
// until this method returns, the tracker is not actually tracking the service!
}
Thread 2
// waiter object
ServiceTracker tracker;
void waitingForService {
synchronized (this) {
wait(); // waiting here for a service
}
T service = tracker.getService();
System.out.println(service);
}
Tuesday, March 22, 2011

10. 3. Activator
Component Description
<component name="example.activator">
<implementation class="com.acme.Activator"/>
</component>
Component Implementation
public class Activator {
private String data = null;
public Activator() {
data = “initialized”;
}
private void activate(ComponentContext context) {
System.out.println(data);
}
}
Tuesday, March 22, 2011

11. What is printed by the activate method?
Component Description
<component name="example.activator">
(a) null
<implementation class="com.acme.Activator"/>
</component>
(b) initialized
Component Implementation (c) throws
public class Activator { exception
private String data = null;
public Activator() {
data = “initialized”;
(d) none of the
}
private void activate(ComponentContext context) {
above
System.out.println(data);
}
}
Tuesday, March 22, 2011

12. What is printed by the activate method?
(a) null
(b) initialized
(c) throws exception
(d) none of the above: activate method is not
called
The component description does not specify a
namespace and thus is the DS 1.0 namespace. DS
1.0 required the activate method to be at least
protected!
Tuesday, March 22, 2011

13. One Solution: Change accessibility
Component Description
<component name="example.activator">
<implementation class="com.acme.Activator"/>
</component>
Component Implementation
public class Activator {
private String data = null;
public Activator() {
data = “initialized”;
}
protected void activate(ComponentContext context) {
System.out.println(data);
}
}
Tuesday, March 22, 2011

14. Better Solution: Specify the DS 1.1 namespace
Component Description
<scr:component name="example.activator"
xmlns:scr="http://www.osgi.org/xmlns/scr/v1.1.0">
<implementation class="com.acme.Activator"/>
</scr:component>
Component Implementation
public class Activator {
private String data = null;
public Activator() {
data = “initialized”;
}
private void activate(ComponentContext context) {
System.out.println(data);
}
}
Tuesday, March 22, 2011

15. Tuesday, March 22, 2011