Click here to close now.

Welcome!

Java Authors: AppDynamics Blog, Pat Romanski, Elizabeth White, Liz McMillan, Esmeralda Swartz

Related Topics: Java, MICROSERVICES

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
SYS-CON Events announced today that Cisco, the worldwide leader in IT that transforms how people connect, communicate and collaborate, has been named “Gold Sponsor” of SYS-CON's 16th International Cloud Expo®, which will take place on June 9-11, 2015, at the Javits Center in New York City, NY. Cisco makes amazing things happen by connecting the unconnected. Cisco has shaped the future of the Internet by becoming the worldwide leader in transforming how people connect, communicate and collaborate. Cisco and our partners are building the platform for the Internet of Everything by connecting the...
Temasys has announced senior management additions to its team. Joining are David Holloway as Vice President of Commercial and Nadine Yap as Vice President of Product. Over the past 12 months Temasys has doubled in size as it adds new customers and expands the development of its Skylink platform. Skylink leads the charge to move WebRTC, traditionally seen as a desktop, browser based technology, to become a ubiquitous web communications technology on web and mobile, as well as Internet of Things compatible devices.
SYS-CON Events announced today that robomq.io will exhibit at SYS-CON's @ThingsExpo, which will take place on June 9-11, 2015, at the Javits Center in New York City, NY. robomq.io is an interoperable and composable platform that connects any device to any application. It helps systems integrators and the solution providers build new and innovative products and service for industries requiring monitoring or intelligence from devices and sensors.
The WebRTC Summit 2014 New York, to be held June 9-11, 2015, at the Javits Center in New York, NY, announces that its Call for Papers is open. Topics include all aspects of improving IT delivery by eliminating waste through automated business models leveraging cloud technologies. WebRTC Summit is co-located with 16th International Cloud Expo, @ThingsExpo, Big Data Expo, and DevOps Summit.
Docker is an excellent platform for organizations interested in running microservices. It offers portability and consistency between development and production environments, quick provisioning times, and a simple way to isolate services. In his session at DevOps Summit at 16th Cloud Expo, Shannon Williams, co-founder of Rancher Labs, will walk through these and other benefits of using Docker to run microservices, and provide an overview of RancherOS, a minimalist distribution of Linux designed expressly to run Docker. He will also discuss Rancher, an orchestration and service discovery platf...
Wearable technology was dominant at this year’s International Consumer Electronics Show (CES) , and MWC was no exception to this trend. New versions of favorites, such as the Samsung Gear (three new products were released: the Gear 2, the Gear 2 Neo and the Gear Fit), shared the limelight with new wearables like Pebble Time Steel (the new premium version of the company’s previously released smartwatch) and the LG Watch Urbane. The most dramatic difference at MWC was an emphasis on presenting wearables as fashion accessories and moving away from the original clunky technology associated with t...
SYS-CON Events announced today that Vitria Technology, Inc. will exhibit at SYS-CON’s @ThingsExpo, which will take place on June 9-11, 2015, at the Javits Center in New York City, NY. Vitria will showcase the company’s new IoT Analytics Platform through live demonstrations at booth #330. Vitria’s IoT Analytics Platform, fully integrated and powered by an operational intelligence engine, enables customers to rapidly build and operationalize advanced analytics to deliver timely business outcomes for use cases across the industrial, enterprise, and consumer segments.
SYS-CON Events announced today that Solgenia will exhibit at SYS-CON's 16th International Cloud Expo®, which will take place on June 9-11, 2015, at the Javits Center in New York City, NY, and the 17th International Cloud Expo®, which will take place on November 3–5, 2015, at the Santa Clara Convention Center in Santa Clara, CA. Solgenia is the global market leader in Cloud Collaboration and Cloud Infrastructure software solutions. Designed to “Bridge the Gap” between Personal and Professional Social, Mobile and Cloud user experiences, our solutions help large and medium-sized organizations dr...
SYS-CON Events announced today that Liaison Technologies, a leading provider of data management and integration cloud services and solutions, has been named "Silver Sponsor" of SYS-CON's 16th International Cloud Expo®, which will take place on June 9-11, 2015, at the Javits Center in New York, NY. Liaison Technologies is a recognized market leader in providing cloud-enabled data integration and data management solutions to break down complex information barriers, enabling enterprises to make smarter decisions, faster.
@ThingsExpo has been named the Top 5 Most Influential M2M Brand by Onalytica in the ‘Machine to Machine: Top 100 Influencers and Brands.' Onalytica analyzed the online debate on M2M by looking at over 85,000 tweets to provide the most influential individuals and brands that drive the discussion. According to Onalytica the "analysis showed a very engaged community with a lot of interactive tweets. The M2M discussion seems to be more fragmented and driven by some of the major brands present in the M2M space. This really allows some room for influential individuals to create more high value inter...
After making a doctor’s appointment via your mobile device, you receive a calendar invite. The day of your appointment, you get a reminder with the doctor’s location and contact information. As you enter the doctor’s exam room, the medical team is equipped with the latest tablet containing your medical history – he or she makes real time updates to your medical file. At the end of your visit, you receive an electronic prescription to your preferred pharmacy and can schedule your next appointment.
The world's leading Cloud event, Cloud Expo has launched Microservices Journal on the SYS-CON.com portal, featuring over 19,000 original articles, news stories, features, and blog entries. DevOps Journal is focused on this critical enterprise IT topic in the world of cloud computing. Microservices Journal offers top articles, news stories, and blog posts from the world's well-known experts and guarantees better exposure for its authors than any other publication. Follow new article posts on Twitter at @MicroservicesE
The list of ‘new paradigm’ technologies that now surrounds us appears to be at an all time high. From cloud computing and Big Data analytics to Bring Your Own Device (BYOD) and the Internet of Things (IoT), today we have to deal with what the industry likes to call ‘paradigm shifts’ at every level of IT. This is disruption; of course, we understand that – change is almost always disruptive.
SYS-CON Events announced today the IoT Bootcamp – Jumpstart Your IoT Strategy, being held June 9–10, 2015, in conjunction with 16th Cloud Expo and Internet of @ThingsExpo at the Javits Center in New York City. This is your chance to jumpstart your IoT strategy. Combined with real-world scenarios and use cases, the IoT Bootcamp is not just based on presentations but includes hands-on demos and walkthroughs. We will introduce you to a variety of Do-It-Yourself IoT platforms including Arduino, Raspberry Pi, BeagleBone, Spark and Intel Edison. You will also get an overview of cloud technologies s...
SYS-CON Events announced today that SafeLogic has been named “Bag Sponsor” of SYS-CON's 16th International Cloud Expo® New York, which will take place June 9-11, 2015, at the Javits Center in New York City, NY. SafeLogic provides security products for applications in mobile and server/appliance environments. SafeLogic’s flagship product CryptoComply is a FIPS 140-2 validated cryptographic engine designed to secure data on servers, workstations, appliances, mobile devices, and in the Cloud.
SOA Software has changed its name to Akana. With roots in Web Services and SOA Governance, Akana has established itself as a leader in API Management and is expanding into cloud integration as an alternative to the traditional heavyweight enterprise service bus (ESB). The company recently announced that it achieved more than 90% year-over-year growth. As Akana, the company now addresses the evolution and diversification of SOA, unifying security, management, and DevOps across SOA, APIs, microservices, and more.
GENBAND has announced that SageNet is leveraging the Nuvia platform to deliver Unified Communications as a Service (UCaaS) to its large base of retail and enterprise customers. Nuvia’s cloud-based solution provides SageNet’s customers with a full suite of business communications and collaboration tools. Two large national SageNet retail customers have recently signed up to deploy the Nuvia platform and the company will continue to sell the service to new and existing customers. Nuvia’s capabilities include HD voice, video, multimedia messaging, mobility, conferencing, Web collaboration, deskt...
SYS-CON Media announced today that @WebRTCSummit Blog, the largest WebRTC resource in the world, has been launched. @WebRTCSummit Blog offers top articles, news stories, and blog posts from the world's well-known experts and guarantees better exposure for its authors than any other publication. @WebRTCSummit Blog can be bookmarked ▸ Here @WebRTCSummit conference site can be bookmarked ▸ Here
SYS-CON Events announced today that Akana, formerly SOA Software, has been named “Bronze Sponsor” of SYS-CON's 16th International Cloud Expo® New York, which will take place June 9-11, 2015, at the Javits Center in New York City, NY. Akana’s comprehensive suite of API Management, API Security, Integrated SOA Governance, and Cloud Integration solutions helps businesses accelerate digital transformation by securely extending their reach across multiple channels – mobile, cloud and Internet of Things. Akana enables enterprises to share data as APIs, connect and integrate applications, drive part...
Cloud is not a commodity. And no matter what you call it, computing doesn’t come out of the sky. It comes from physical hardware inside brick and mortar facilities connected by hundreds of miles of networking cable. And no two clouds are built the same way. SoftLayer gives you the highest performing cloud infrastructure available. One platform that takes data centers around the world that are full of the widest range of cloud computing options, and then integrates and automates everything. Join SoftLayer on June 9 at 16th Cloud Expo to learn about IBM Cloud's SoftLayer platform, explore se...