Chapter 4

Question 1

Name the three implementation layers of component framework?

Answer 1

1. Platform
2. Container
3. Components

Question 2

From a conceptual point of view, software component C consists of three items

Answer 2

• A component identifier id
• Component specification spc, and
• Component body bdy

C = <id, spc, bdy>

Question 3

Explain composition mechanism

Answer 3

Composition mechanisms explain how components are composed and deployed in a framework.

Question 4

Explain the main composition mechanisms of component frameworks

Answer 4

1. Bundling: it means constructing new component C from the given components C1, …Cn before deployment. C is a component with its name, specification and body.
   Bundling is sometimes called Aggregation.
2. Deploying: it means making the name and body of components C available within a container D.
3. Connecting: it means making the services of a component available to another component at runtime. This is done by passing references from one component to another.

Question 5

Explain OSGi

Answer 5

It is a component framework of Java that supports dynamic installation and management of components. Complete applications and single components can be remotely installed, started, stopped, updated and uninstalled without requiring a reboot of the host process.

Question 6

OSGi framework basics to know

Answer 6

• OSGi defines the structure and syntax of deployable components which are called bundle in OSGi context
• OSGi provides a container together with a sophisticated API for installing, uninstalling, and dynamically managing components.

Question 7

Services in OSGi

Answer 7

They are identified by Java interfaces.

Question 8

What does Activator class mean in OSGi component?

Answer 8

It contains the code for initial startup and final cleanup just like Glue code.

Question 9

What is the important aspect of OSGi container?

Answer 9

It stores its current state in a persistent representation when the container is closed. If the container is relaunched, it sets up the state it was in before closing.

Question 10

Components in general and OSGi bundles have two functions:

Answer 10

They provide services and manage code.

Question 11

With the dynamic runtime environment of OSGi framework, at runtime, the when the framework/container is running :

Answer 11

1. Bundles can be installed and resolved and uninstalled
2. Resolved bundles can be started, and stopped
3. Services can be registered and unregistered

Question 12

Limitation of service listeners

Answer 12

It is more suitable for a small systems, for a complex system and much services, a more suitable alternative Service trackers or Declarative Services that OSGi provides.

Question 13

Difference between declarative services and service listeners.

Answer 13

Instead of describing in the program how service listeners reacts to changes, declarative services describes the dependencies and properties of services in an additional XML file.

Question 14

Explain the two kinds of components the OSGi framework supports

Answer 14

1. Immediate components: it is activated as soon as its dependencies are satisfied
2. Delayed components: it is activated as soon as it’s service is requested.

All components providing services are delayed components while others are immediate components.
You can override this by setting the parameter immediate of the @Component annotation to false.

Question 15

OSGi service listeners are based on

Answer 15

Events i.e publish-subscribe principle.

Question 16

What do you understand by Topics in publish-subscribe events?

Answer 16

Topics is the common signal names that senders and receivers have to agree on, its a precondition for the communication between senders and receivers.

Question 17

Events main components?

Answer 17

• Event publisher: it sends out events for a particular topic.
• Event subscriber: it receives all the messages for a particular topic.

Question 18

Limitations of OSGi framework

Answer 18

it provides no support or guidelines for handling concurrency and synchronisation issues.
It is a weakness to component developers who do not know the execution context in which their components will be used.

Question 19

Hierarchical composition provides

Answer 19

better control of the interface of the new component and better encapsulation of the components used.

Question 20

Let C1 = <id1, spc1,bdy1> and C2 = <id2,spc2,bdy2> be two components. When are C1 and C2 considered to be equal?
is it sufficient that the specification is the same?

Answer 20

Two components are equal if they have the same name, specification and body.

It is not sufficient that they have or satisfy the same specifications.

Question 21

A bundle can be in different states in the OSGi container. What corresponds to the transition from INSTALLED to RESOLVED in traditional software construction?

Answer 21

The transition from INSTALLED to RESOLVED corresponds to linking in software construction with compiler, linker and loader.

Question 22

In OSGi program code is managed in the form of ?

Answer 22

Java packages

Question 23

What are the static and dynamic aspects of bundles? How are they related?

Answer 23

Bundles can provide packages (static aspect) and services (dynamic aspect)

The dynamic aspect reflects the provided interfaces and services of components, what the clients want to have. The static aspect is necessary to share and effectively structure the code that components need to provide the services.

Question 24

The goal of component frameworks is?

Answer 24

black-box reuse

Question 25

How many layers do we have in software systems and in CBSD with component frameworks?

Answer 25

We have two layers in software systems
1. The platform
2. System of interest

We have three layers in Component frameworks
1. The platform
2. The container
3. The components

Question 26

Why does a bundle have to list the OSGi framework package as imports?

Answer 26

It is because the activator class needs access to the container API.

Question 27

What is the function of the activator class in OSGi?

Answer 27

it is the glue code that sets up the component and its service.

Question 28

OSGi bundles and components have two functions

Answer 28

They provide services and manage code.

Question 29

Limitation of OSGi framework

Answer 29

it provides no support or guidelines for handling concurrency and synchronisation issues.