Welcome!

Java Authors: Elizabeth White, Liz McMillan, Pat Romanski, Carmen Gonzalez, Robert Reeves

Related Topics: Java, SOA & WOA

Java: Article

Component Development and Assembly Using OSGi Services

Using components to build software systems will provide many architectural advantages apart from promoting ease of reuse

This article introduces the concepts of Component Oriented Development and Assembly (CODA) using the OSGi Service platform with an example application. The article starts with an introduction to software components, elaborates with an example application, followed by an overview of the OSGi Service platform, and an implementation of the example application using this platform.

Introduction to Software Components
Components are parts that can be assembled to form a larger system. Electronic components such as ICs (Integrated Circuits) are assembled together to build an electronic system; similarly software components are assembled together to build a software system. Software systems have a static form as well as a dynamic runtime form. Software components can be assembled either in static form or dynamic form. In either case, the software component is an independent unit of development, deployment, and assembly. Using components to build software systems will provide many architectural advantages apart from promoting ease of reuse.

Age Calculation - An Example Application
Consider an application meant for calculating the age of people. Assume this application prompts the user to provide her personal details such as name and date of birth. Once the user provides the details, the application calculates the age of the person as of today and displays it with a personal greeting. Such an application can be built from the following software components:

  • UserProfile Component: Responsible for collecting the personal details of the user such as name and date of birth
  • Age Component: Responsible for calculating the age, given the date of birth
  • Greeting Component: Responsible for displaying the greeting message along with age of the person; needs to make use of the UserProfile component to obtain input from the user and Age component to calculate the age
  • AgeCalculationApp Component: Responsible for invoking the Greeting service provided by Greeting component

The components can be represented in a UML component diagram as shown in Figure 1. The dotted arrow lines in the diagram denote dependency.

Figure 1: Components in the Age Calculation application

Interfaces of Software Components
A software interface is a collection of software operations. Software components interact with one another by invoking the software operations. To facilitate structured interactions across components, each component exposes one or more software interfaces through which other components can interact. The exposed interfaces of a component are called as its "provided interfaces." Internals of the component implement the service contracts exposed by the provided interfaces.

Apart from providing services, a component needs to consume services provided by other components. The set of services that a component wants to consume is specified through "required interfaces". A component can work with any other component that provides the required interfaces. Hence there is no tight coupling between one component and another; the dependency of a component is only with interfaces and not with other components.

Interfaces in the Age Calculation Application
Let us identify the interfaces provided and required by each of the four components in the Age Calculation application.

Age Component
The Age component is responsible for calculating the age, given the date of birth. Hence it needs to provide a service for calculating the age. Let's expose this service through a provided interface IAgeCalculation. The Java code for this interface is given in Figure 2.

Figure 2: AgeCalculation Java interface code

In the IAgeCalculation interface definition, Calendar is a data type defined in java.util package to represent the date. IAge is a custom-defined data type to hold the age. The Java code for IAge is shown in Figure 3.

Figure 3: Java code for data type IAge

The Age Component can be represented in a UML component diagram as shown in Figure 4. The lollipop connector from the component denotes provided interface.

Figure 4: UML representation of Age component

UserProfile Component
The UserProfile component is responsible for collecting the personal information from end user and passing it back to the consuming component. Let us define an interface called as IUserProfileCollection to expose this service. The Java code for this interface is shown in Figure 5.

Figure 5: Java code for IUserProfileCollection interface

When the method collectUserProfile() is called, the component starts an interactive session with the end user to collect her personal information. During the user interaction session, the method isProfileCollected() will return false; after the user interaction session has finished and the user profile has been collected, this method will return true. At this time, the user profile is ready for pick-up by the consuming component and it can do so by invoking giveUserProfile() method.

giveUserProfile() method returns the user profile information through a custom defined data type IUserProfile. Java code for IUserProfile data type is given in Figure 6.

Figure 6: Java code for IUserProfile data type

The UserProfile component in a UML component diagram notation is shown in Figure 7.

Figure 7: UML representation of UserProfile component

Greeting Component
The Greeting component is responsible for displaying the greeting message along with the age. We'll expose this service through the IGreeting interface. The Java code for IGreeting interface is shown in Figure 8.

Figure 8: Java code for interface IGreeting

The Greeting component provides the IGreeting service, and it requires the IAgeCalculation and IUserProfileCollection services. The required and provided interfaces of the Greeting component can be represented using the UML component diagram as shown in Figure 9. The receptacle sockets in the component diagram denote required interfaces.

Figure 9: UML representation of Greeting component

AgeCalculationApp Component
The AgeCalculationApp component is responsible for invoking the IGreeting service of Greeting component. The AgeCalculationApp itself does not provide any service. The UML diagram for this component is shown in Figure 10.

Figure 10: UML representation of AgeCalculationApp

Application Assembly
Components are assembled together to build a software system. Components for an assembly should be chosen such a way that, amongst the chosen components the set of required interfaces is fulfilled by the set of provided interfaces. The four components of the example application can be assembled to result in Age Calculation application. UML component diagram denoting this assembly is shown in Figure 11. Connectors with a lollipop and a receptacle together show how two components assemble over a common interface (one component provides the interface and another component requires it).

Figure 11: UML representation of Age Calculation application assembly

Once an application is assembled, specific components from the assembly can be replaced with compatible newer components. Let us say we have two new components - GUIUserProfile and GUIGreeting, which have same interfaces as UserProfile and Greeting components, respectively. Then these new components can replace the old components to provide us with a GUI application. The application assembly would then look like Figure 12.

Figure 12: Age Calculation GUI Application Assembly

Introduction to the OSGi Service Platform
OSGi Service is a standards based software development platform. The platform standards are specified by the OSGi Alliance (http://www.osgi.org), formerly known as the Open Services Gateway initiative.  OSGi alliance is an industry backed nonprofit organization.

OSGi platform provides a modularity and component model on top of Java. Figure 13 illustrates how OSGi logically fits in a development / deployment stack.

Figure 13: OSGi From Development / Deployment Perspective

OSGi Framework
At the core of the OSGi platform is the OSGi framework defined by standards. OSGi framework is the runtime environment on which OSGi applications can be executed. OSGi framework specifies a common API using which application developers can develop OSGi applications. The OSGi framework specifications is used by OSGi framework developers also, who build implementations of the framework as per the specifications.

There are a number of framework implementations available from different developers. A few well known framework implementations are listed below:

The functionality of OSGi framework is divided into many layers. For our purposes, we shall focus on Modules Layer and Services Layer.  Figure 14 illustrates the layers we would like to focus.

The modules layer is the bottom most layer interacting with the runtime framework. The modules layer defines a modularity framework on top of standard Java so that modules expose and interact via well-defined APIs. Individual module is called as a "bundle" in OSGi. OSGi bundle is the atomic unit of deployment on an OSGi framework. An OSGi bundle is nothing but a Java Archive (JAR) File with some special manifest information that would be used by the OSGi framework when the bundle is deployed. Unlike a normal JAR file, which exposes all its code when it is deployed on a Java Virtual Machine (JVM), the bundle does not expose all the code contained within. Only code (Java packages) marked for explicit export is exposed outside the bundle.

Figure 14: OSGi Framework Functional Layers

Above the Module layer is the Services layer. An OSGi Service is defined by a Java interface. Any bundle can implement the service, and register the service with the OSGi Service Registry. OSGi Service Registry, defined as part of framework standards, supports a publish, subscribe, and look-up mechanism of exchanging services across components. A bundle can register the service that it has implemented with the OSGi Service Registry. A bundle requiring a service can query the OSGi Service Registry to look-up any registered service implementation. Instead of looking-up, a bundle can also subscribe to be notified on service registration / unregistration events. These mechanisms make the bundles that provide service and bundles that consume service unaware of each other, leading to independent development and deployment of these components. Once deployed, they can be  assembled dynamically using the OSGi Service Registry.

Implementing Age Calculation Using OSGi
The Age Calculation application has one bundle for each of the components. Each bundle has a Java package that is exposed for access from outside and an internal Java package. The exposed package defines the interfaces and the internal package contains implementation classes. The package diagram for all the bundles is shown in Figure 15.

Figure 15: Age Calculation Application bundles

The service provided by each bundle is exposed by using the OSGi Declarative Services framework. Using the same framework, each bundle also specifies the list of services it requires. The OSGi Declarative Services framework (also known as Service Component Runtime), will perform the work of registering the provided service with the OSGi Service Registry and obtaining the required service through look-up into OSGi Service Registry. Thus, the Declarative Services framework provides automatic assembly of deployed components.

As an example, let us consider the Greeting bundle. The Declarative Services specification of the components in this bundle is done through an XML file, which is given in Figure 16.

Figure 16: Greeting component Declarative Service Definition

The XML file contains following metadata about the Greeting component:

  • A component by name com.demo.greeting is defined
  • The component is implemented by the class com.demo.greeting.internal.Greeting
  • The component requires two interfaces - IuserProfileCollection and IAgeCalculation. These required interfaces should be dynamically wired to this component using setUserProfileCollection() and setAgeCalculation() methods on the component implementation class.
  • The component provides a service through the provided interface com.demo.greeting.IGreeting.

To understand how the Greeting component works, some portions of the implementation class is presented in Figure 17. The Greeting class has two field variables referring to the services required by this component. In the component Declarative Service metadata setter methods of these two variables are linked to the service reference (required interface). Hence when the Service Component Runtime (SCR) activates this component, it would call the setter methods to point these variables to objects which provide the service. For example, an instance of AgeCalculator object from Age component would be passed to ageCalculation_ member variable.

Figure 17: Implementation of Greeting component

The Greeting class's  implementation of greet() method obtains the user profile by invoking IUserProfileCollection service. From the obtained profile, date of birth is extracted and IAgeCalculation service is used to calculate the age. Finally the method displays the age along with a greeting to the user.

Output from the Example Application
The Age Calculation application, with console mode user interaction components, has the following output.

osgi> What is your first name?

Albert

What is your last name?

Einstein

What is your title (Mr./Ms./Mrs./Prof./Dr.)?

Mr.

What is your year of birth?

1879

What is your month of birth (1-12)?

03

What is your date of birth (1-31)?

14

Hello Mr. Albert Einstein, you are 132 years, 10 months, and 3 days old.

If we replace the Greeting and UserProfile components in the application assembly with GUIGreeting and GUIUserProfile components, the application output is as below:

The source code of the application can be downloaded as a zip file containing an Eclipse workspace folder. To open the source code and run the application simply unzip the folder and switch the Eclipse workspace to point to the unzipped folder. Choose the appropriate components in the Run Configuration to run either console version or the GUI version of the assembly. For example, for the GUI version, the following run configuration will be applicable:

More Stories By Piram Manickam

Piram Manickam works at Infosys Limited. He would like to acknowledge and thank Sangeetha S, a beloved colleague and friend, for her invaluable contributions in this work.

More Stories By Subrahmanya SV

Subrahmanya SV works at Infosys Limited. He would like to acknowledge and thank Sangeetha S, a beloved colleague and friend, for her invaluable contributions in this work.

Comments (1) View Comments

Share your thoughts on this story.

Add your comment
You must be signed in to add a comment. Sign-in | Register

In accordance with our Comment Policy, we encourage comments that are on topic, relevant and to-the-point. We will remove comments that include profanity, personal attacks, racial slurs, threats of violence, or other inappropriate material that violates our Terms and Conditions, and will block users who make repeated violations. We ask all readers to expect diversity of opinion and to treat one another with dignity and respect.


Most Recent Comments
newuniverse 01/31/12 03:10:00 AM EST

Hi,
This is a very simple and descriptive example. Good job.

@ThingsExpo Stories
There is no doubt that Big Data is here and getting bigger every day. Building a Big Data infrastructure today is no easy task. There are an enormous number of choices for database engines and technologies. To make things even more challenging, requirements are getting more sophisticated, and the standard paradigm of supporting historical analytics queries is often just one facet of what is needed. As Big Data growth continues, organizations are demanding real-time access to data, allowing immediate and actionable interpretation of events as they happen. Another aspect concerns how to deliver ...
Scott Jenson leads a project called The Physical Web within the Chrome team at Google. Project members are working to take the scalability and openness of the web and use it to talk to the exponentially exploding range of smart devices. Nearly every company today working on the IoT comes up with the same basic solution: use my server and you'll be fine. But if we really believe there will be trillions of these devices, that just can't scale. We need a system that is open a scalable and by using the URL as a basic building block, we open this up and get the same resilience that the web enjoys.
The 3rd International Internet of @ThingsExpo, co-located with the 16th International Cloud Expo - to be held June 9-11, 2015, at the Javits Center in New York City, NY - announces that its Call for Papers is now open. The Internet of Things (IoT) is the biggest idea since the creation of the Worldwide Web more than 20 years ago.
In their session at @ThingsExpo, Shyam Varan Nath, Principal Architect at GE, and Ibrahim Gokcen, who leads GE's advanced IoT analytics, focused on the Internet of Things / Industrial Internet and how to make it operational for business end-users. Learn about the challenges posed by machine and sensor data and how to marry it with enterprise data. They also discussed the tips and tricks to provide the Industrial Internet as an end-user consumable service using Big Data Analytics and Industrial Cloud.
Things are being built upon cloud foundations to transform organizations. This CEO Power Panel at 15th Cloud Expo, moderated by Roger Strukhoff, Cloud Expo and @ThingsExpo conference chair, addressed the big issues involving these technologies and, more important, the results they will achieve. Rodney Rogers, chairman and CEO of Virtustream; Brendan O'Brien, co-founder of Aria Systems, Bart Copeland, president and CEO of ActiveState Software; Jim Cowie, chief scientist at Dyn; Dave Wagstaff, VP and chief architect at BSQUARE Corporation; Seth Proctor, CTO of NuoDB, Inc.; and Andris Gailitis, C...
How do APIs and IoT relate? The answer is not as simple as merely adding an API on top of a dumb device, but rather about understanding the architectural patterns for implementing an IoT fabric. There are typically two or three trends: Exposing the device to a management framework Exposing that management framework to a business centric logic Exposing that business layer and data to end users. This last trend is the IoT stack, which involves a new shift in the separation of what stuff happens, where data lives and where the interface lies. For instance, it's a mix of architectural styles ...
The Industrial Internet revolution is now underway, enabled by connected machines and billions of devices that communicate and collaborate. The massive amounts of Big Data requiring real-time analysis is flooding legacy IT systems and giving way to cloud environments that can handle the unpredictable workloads. Yet many barriers remain until we can fully realize the opportunities and benefits from the convergence of machines and devices with Big Data and the cloud, including interoperability, data security and privacy.
Technology is enabling a new approach to collecting and using data. This approach, commonly referred to as the "Internet of Things" (IoT), enables businesses to use real-time data from all sorts of things including machines, devices and sensors to make better decisions, improve customer service, and lower the risk in the creation of new revenue opportunities. In his General Session at Internet of @ThingsExpo, Dave Wagstaff, Vice President and Chief Architect at BSQUARE Corporation, discuss the real benefits to focus on, how to understand the requirements of a successful solution, the flow of ...
Cloud Expo 2014 TV commercials will feature @ThingsExpo, which was launched in June, 2014 at New York City's Javits Center as the largest 'Internet of Things' event in the world.
"People are a lot more knowledgeable about APIs now. There are two types of people who work with APIs - IT people who want to use APIs for something internal and the product managers who want to do something outside APIs for people to connect to them," explained Roberto Medrano, Executive Vice President at SOA Software, in this SYS-CON.tv interview at Cloud Expo, held Nov 4–6, 2014, at the Santa Clara Convention Center in Santa Clara, CA.
Performance is the intersection of power, agility, control, and choice. If you value performance, and more specifically consistent performance, you need to look beyond simple virtualized compute. Many factors need to be considered to create a truly performant environment. In his General Session at 15th Cloud Expo, Harold Hannon, Sr. Software Architect at SoftLayer, discussed how to take advantage of a multitude of compute options and platform features to make cloud the cornerstone of your online presence.
In this Women in Technology Power Panel at 15th Cloud Expo, moderated by Anne Plese, Senior Consultant, Cloud Product Marketing at Verizon Enterprise, Esmeralda Swartz, CMO at MetraTech; Evelyn de Souza, Data Privacy and Compliance Strategy Leader at Cisco Systems; Seema Jethani, Director of Product Management at Basho Technologies; Victoria Livschitz, CEO of Qubell Inc.; Anne Hungate, Senior Director of Software Quality at DIRECTV, discussed what path they took to find their spot within the technology industry and how do they see opportunities for other women in their area of expertise.
DevOps Summit 2015 New York, co-located with the 16th International Cloud Expo - to be held June 9-11, 2015, at the Javits Center in New York City, NY - announces that it is now accepting Keynote Proposals. The widespread success of cloud computing is driving the DevOps revolution in enterprise IT. Now as never before, development teams must communicate and collaborate in a dynamic, 24/7/365 environment. There is no time to wait for long development cycles that produce software that is obsolete at launch. DevOps may be disruptive, but it is essential.
Wearable devices have come of age. The primary applications of wearables so far have been "the Quantified Self" or the tracking of one's fitness and health status. We propose the evolution of wearables into social and emotional communication devices. Our BE(tm) sensor uses light to visualize the skin conductance response. Our sensors are very inexpensive and can be massively distributed to audiences or groups of any size, in order to gauge reactions to performances, video, or any kind of presentation. In her session at @ThingsExpo, Jocelyn Scheirer, CEO & Founder of Bionolux, will discuss ho...
Almost everyone sees the potential of Internet of Things but how can businesses truly unlock that potential. The key will be in the ability to discover business insight in the midst of an ocean of Big Data generated from billions of embedded devices via Systems of Discover. Businesses will also need to ensure that they can sustain that insight by leveraging the cloud for global reach, scale and elasticity.
“With easy-to-use SDKs for Atmel’s platforms, IoT developers can now reap the benefits of realtime communication, and bypass the security pitfalls and configuration complexities that put IoT deployments at risk,” said Todd Greene, founder & CEO of PubNub. PubNub will team with Atmel at CES 2015 to launch full SDK support for Atmel’s MCU, MPU, and Wireless SoC platforms. Atmel developers now have access to PubNub’s secure Publish/Subscribe messaging with guaranteed ¼ second latencies across PubNub’s 14 global points-of-presence. PubNub delivers secure communication through firewalls, proxy ser...
We’re no longer looking to the future for the IoT wave. It’s no longer a distant dream but a reality that has arrived. It’s now time to make sure the industry is in alignment to meet the IoT growing pains – cooperate and collaborate as well as innovate. In his session at @ThingsExpo, Jim Hunter, Chief Scientist & Technology Evangelist at Greenwave Systems, will examine the key ingredients to IoT success and identify solutions to challenges the industry is facing. The deep industry expertise behind this presentation will provide attendees with a leading edge view of rapidly emerging IoT oppor...
Connected devices and the Internet of Things are getting significant momentum in 2014. In his session at Internet of @ThingsExpo, Jim Hunter, Chief Scientist & Technology Evangelist at Greenwave Systems, examined three key elements that together will drive mass adoption of the IoT before the end of 2015. The first element is the recent advent of robust open source protocols (like AllJoyn and WebRTC) that facilitate M2M communication. The second is broad availability of flexible, cost-effective storage designed to handle the massive surge in back-end data in a world where timely analytics is e...
"There is a natural synchronization between the business models, the IoT is there to support ,” explained Brendan O'Brien, Co-founder and Chief Architect of Aria Systems, in this SYS-CON.tv interview at the 15th International Cloud Expo®, held Nov 4–6, 2014, at the Santa Clara Convention Center in Santa Clara, CA.
The Internet of Things will put IT to its ultimate test by creating infinite new opportunities to digitize products and services, generate and analyze new data to improve customer satisfaction, and discover new ways to gain a competitive advantage across nearly every industry. In order to help corporate business units to capitalize on the rapidly evolving IoT opportunities, IT must stand up to a new set of challenges. In his session at @ThingsExpo, Jeff Kaplan, Managing Director of THINKstrategies, will examine why IT must finally fulfill its role in support of its SBUs or face a new round of...