OSGi Puzzlers
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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.
OSGi Puzzlers
- 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