Welcome!

Microservices Expo Authors: Liz McMillan, Karthick Viswanathan, Pat Romanski, Stackify Blog, Dalibor Siroky

Related Topics: Java IoT, Industrial IoT, Mobile IoT, Microservices Expo, IBM Cloud, Weblogic, Machine Learning

Java IoT: Article

Component Models in Java | Part 2

OSGi Component Model

OSGi is the latest component model to join the bandwagon of component models, which provides a platform for component oriented development and assembly. OSGi framework is a standards based platform whose specifications are provided by the OSGi Alliance (www.osgi.org, formerly OSGi was referred as Open Services Gateway Initiative). OSGi Alliance is an industry backed nonprofit organization which was founded in March 1999. The OSGi specification has gone through many releases and the current major version in use is 4 and version 5 has been introduced recently.

The OSGi defines a dynamic module system for Java. This offers help for Java's modularity problems by providing better control to the code structure, manage the lifecycle of the code and a complete loosely coupled approach that is much needed for component-oriented development.

The OSGi specification consists of two parts:

  • OSGi Framework
  • OSGi Standard services

The OSGi framework is the OSGi runtime environment that provides all the functionality as per the specifications. Applications are deployed and executed in the OSGi framework. The OSGi framework provides an API for the development of components. There are a number of framework implementations and some of the popular ones are Eclipse Equinox, Apache Felix and Knoplerfish. OSGi standard services define reusable services that should be provided as part of the development platform implementation. There are three conceptual layers in OSGi framework:

  • Module layer - Responsible for packaging and sharing code
  • Lifecycle layer - Responsible for managing the lifecycle of deployed module during runtime
  • Service layer - Responsible for dynamic service publication, searching and binding

OSGi Bundle
An OSGi bundle is a deployment module in the form of a JAR file. A module in OSGi parlance is known as a bundle. Bundles contain class files and resource files, similar to the regular JAR file in Java, but in addition they contain manifest information that contains metadata about the bundle. Apart from the regular JAR file's manifest contents, a bundle's manifest file has OSGi specific information such as module name, version number, dependencies, etc., thus giving better modularity and easy maintainability. Bundles are more powerful than JAR files in enforcing module boundaries, because a bundle needs to explicitly define what portion of its internal code is externally visible. Similarly, a bundle must explicitly declare any external dependencies that it has with the code exposed by other bundles. A bundle must have a unique identity - Bundle Name and Version.

The OSGi framework matches the exports and imports of deployed bundles to dynamically wire the entire application. This process of bundle resolution ensures consistency among the different bundles in terms of versions and other constraints. An application in OSGi is nothing but a collection of bundles with explicitly defined dependencies. A bundle is deployed in OSGi framework once it is developed.

OSGi Service Registry
The OSGi Service registry promotes service oriented programming. The service registry provides service publication service discovery and service binding. The bundles deployed in the OSGi framework can leverage the service registry later for publishing and consuming services. A bundle providing a service publishes the service in the OSGi Service Registry. A service is defined by a Java Interface, which represents a conceptual contract between the provider and consumer. A potential consumer can use the registry to search for providers of a particular service. Once if finds a service provider, it can bind and use the service. Services layer in OSGi facilitates one more level of dynamism other than bundles. Just as bundles can be added and removed in a running application, the Services can appear and go dynamically in a runtime application.

OSGi Component
As discussed earlier, a bundle is the deployment unit in OSGi component model. A bundle is a JAR file that contains:

  • Class files
  • Resource files
  • Manifest file (with additional metadata)

The class files are typically the interface and the implementation which constitutes the component. The manifest will have additional metadata as shown below:

Manifest-Version: 1.0

Bundle-ManifestVersion: 2

Bundle-Name: com.demo.helloWorld

Bundle-SymbolicName: com.demo.helloWorld

Bundle-Version: 1.0.0.qualifier

Bundle-Activator: com.demo.Activator

Bundle-Vendor: PIRAM

Bundle-RequiredExecutionEnvironment: JavaSE-1.7

Import-Package: org.osgi.framework;version="1.3.0"

Bundle-ActivationPolicy: lazy

The OSGi Framework provides an inbuilt API called BundleActivator which helps the bundle to hook its own lifecycle management. The BundleActivator interface has two methods - start() and stop() which are invoked when the bundle is started and stopped respectively. Any bundle can implement this interface to check its own life cycle. The bundle could perform actions as specified in the start and stop methods of the Activator class. The use of bundle as a component for building application on the OSGi framework does not just depend on the bundle doing the work whenever it is started or stopped. The bundle needs to be able to expose certain functionality as provided interfaces and it needs to consume functionalities as per the require interfaces. Thus a collection of bundles made into an assembly should be able to work together to form a system. Generally the provided interface will be created as a separate bundle and the implementations can be wired dynamically by the OSGi runtime from the implementation bundles. There can be more than one implementation, the wiring happens depending on the runtime.

Example to understand OSGi Component Model
The OSGi component model can be understood with the same shopping Cart example discussed in the earlier models.

Figure 3: OSGi Component Model - Cart Component Example

The Cart application in this example is created with the following bundles for better modularity and maintainability.

  1. Interface Bundle (com.online.shopping)
  2. Implementation Bundle (CartImpl)
  3. Client Bundle (CartClient)

The Cart component is comprised of interface bundle and implementation bundle. The interface bundle (com.online.shopping) defines an interface ICart. This interface will be used by the implementation bundle to invoke the exposed services. The client bundle will use the interface for invoking the required services which gets bounded to the implementation bundle by the service registry.

Interface Bundle
The interface bundle contains the interface ICart for the Cart component and is defined as below:

package com.online.shopping;

import java.util.Collection;

public interface ICart {
public void addItem(Product product, int quantity);
public Collection<Product> listItems();    
public double getTotalPrice();
public void clearCart();
}

This bundle has ONLY the interface and its helper class and it exports the com.online.shopping package as shown in the manifest file below:

Manifest-Version: 1.0
Bundle-ManifestVersion: 2
Bundle-Name: CartIntf
Bundle-SymbolicName: CartIntf
Bundle-Version: 1.0.0.qualifier
Bundle-ActivationPolicy: lazy
Bundle-RequiredExecutionEnvironment: JavaSE-1.6
Import-Package: org.osgi.framework;version="1.3.0"
Export-Package: com.online.shopping

The structure of the bundle jar file is as below:

Figure 4: Structure of Interface Bundle

Implementation Bundle
ICart interface is implemented by the class CartImpl, whose code as demonstrated below:

package com.online.shopping.impl;

import java.util.Collection;
import java.util.HashMap;
import java.util.Map;

import com.online.shopping.ICart;
import com.online.shopping.Product;

public class CartImpl implements ICart {
Map<Product, Integer> items = new HashMap<Product, Integer>();      

@Override
public void addItem(Product product, int quantity) {
if(items.containsKey(product)) {
quantity +=items.get(product);
}
items.put(product, quantity);
}

@Override

public Collection<Product> listItems() {

return items.keySet();

}

@Override
public double getTotalPrice() {
double totalPrice = 0;
for(Product product: items.keySet()) {
totalPrice+=product.getPrice()* items.get(product);           
}
return totalPrice;
}

@Override
public void clearCart() {
items.clear();
}

}

The CartImpl is the class in the implementation bundle that implements the ICart interface and provides the ICart service implementation. In the implementation bundle, the interface com.online.shopping.ICart is not added to the CLASSPATH, but imported by the OSGi framework. This bundle imports the interface bundle as explained in the MANIFEST.MF below:

Manifest-Version: 1.0
Bundle-ManifestVersion: 2
Bundle-Name: CartImpl
Bundle-SymbolicName: CartImpl
Bundle-Version: 1.0.0.qualifier
Bundle-RequiredExecutionEnvironment: JavaSE-1.6
Import-Package: com.online.shopping,
org.osgi.framework;version="1.6.0"
Service-Component: META-INF/component.xml

The implementation bundle is exposed as a declarative service component. From the manifest file, it is evident that the bundle is not exported as a package, but it is exposed as a service with the entry - Service-Component that this is exposed as a component and the component description is available in component.xml. With the help of such XML files, components declare their provided services. The OSGi framework helps to publish the CartImpl as a service in the OSGi service registry. The component.xml is as below:

<?xml version="1.0" encoding="UTF-8"?>
<scr:component xmlns:scr="http://www.osgi.org/xmlns/scr/v1.1.0" name="CartImpl">
<implementation class="com.online.shopping.impl.CartImpl"/>
<service>
<provide interface="com.online.shopping.ICart"/>
</service>
</scr:component>

The ICart is exposed as a service and the service is implemented by the CartImpl implementation class. Looking at the component.xml, it is clear that the component provides the ICart service. The component declares the implementation class and the provided interface. The declarative services in the OSGi framework publish the service at the execution time after the bundle is activated. The structure of the JAR file of the bundle is as follows:

Figure 5: Structure of Implementation Bundle

Client Bundle
The client bundle is supposed to consume the services exposed by the ICart service implementation and consume it. The client bundle is another component that imports the com.online.shopping package and consumes the service through OSGi service registry. The client bundle's manifest looks as below:

Manifest-Version: 1.0
Bundle-ManifestVersion: 2
Bundle-Name: CartClient
Bundle-SymbolicName: CartClient
Bundle-Version: 1.0.0.qualifier
Bundle-RequiredExecutionEnvironment: JavaSE-1.6
Import-Package: com.online.shopping,
org.osgi.framework;version="1.6.0",
org.osgi.service.component;version="1.1.0"
Service-Component: META-INF/component.xml

The client is also a component which consumes the services provided by the ICart component.

Figure 6: Structure of Client Bundle

The component.xml in client bundle has reference to the ICart service interface.

<?xml version="1.0" encoding="UTF-8"?>
<scr:component xmlns:scr="http://www.osgi.org/xmlns/scr/v1.1.0" name="CartClient">
<implementation class="com.client.CartClient"/>
<reference bind="gotService" cardinality="1..1" interface="com.online.shopping.ICart" name="ICart" policy="dynamic" unbind="lostService"/>
</scr:component>

Apart from the interface reference, the component.xml also refers to some methods called ‘gotService' and ‘lostService' during binding and unbinding of service references. These are the methods defined in the client class which will be invoked with the associated service references into the service object. This allows the component to find out the services without retrieving them. The declarative specifications in OSGi framework defines the methods where the service reference will be injected. The component service policy may be static or dynamic. In static policy, the service reference is injected once and not changed until the component is deactivated. Where as in the dynamic policy, the component is notified whenever the service comes or goes utilizing the true dynamism. In the example, it is dynamic. The client invokes the ICart service as follows:

package com.client;

import java.util.Collection;

import org.osgi.framework.ServiceReference;
import org.osgi.service.component.ComponentContext;

import com.online.shopping.ICart;
import com.online.shopping.Product;

public class CartClient {

ComponentContext context;
ServiceReference reference;
ICart cart;

public void activate(ComponentContext context) {
System.out.println("Activate Component");

if(reference!= null) {
cart = (ICart)context.locateService("ICart", reference);

Product product = new Product();
product.setName("OSGi");
product.setPrice(550.00);
cart.addItem(product, 20);

Product newProduct = new Product();
newProduct.setName("Enterprise OSGi");
newProduct.setPrice(400.00);
cart.addItem(newProduct, 10);

Collection<Product> productItems = cart.listItems();
for(Product items: productItems) {
System.out.println(items.getName()+"******"+ items.getPrice());             
}            

System.out.println("Total Price of Cart Items: "+cart.getTotalPrice());

cart.clearCart();
}

}

public void gotService(ServiceReference reference) {
System.out.println("Bind Service");
this.reference = reference;
}

public void lostService(ServiceReference reference) {
System.out.println("unbind Service");
this.reference = null;           
}

}

The client has defined three methods:

  • activate - part of declarative services API. This method is invoked when this component is activated. The ComponentContext is used to locate the ICart with the injected service reference.
  • gotService - user defined method as available in the component.xml, this method is invoked with the service reference (using dependency injection) when the service object is binded.
  • lostService - user defined method as mentioned in the client component.xml, this method is invoked with the injected service reference when the service object is unbinded.

Figure 7: Cart Component Bundles Deployment in OSGi Container

The client is not even aware of the implementation bundle. If there are multiple implementations available for the same service, the service is bounded dynamically by the environment. If there is any change in the implementation, only the implementation bundle will undergo change. A revised bundle can provide additional services which can be consumed by clients. So replacing components is easier and will not affect any other component. This way, OSGi gives good modularity by de-coupling components and a pluggable dynamic service model which are much needed features of a component model.

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.

More Stories By S Sangeetha

S Sangeetha is a Senior Technical Architect at the E-Commerce Research Labs at Infosys Limited. She has over 15 years of experience in architecture, design and development of enterprise Java applications. She is also involved in enhancing the technical skills of Architects at Infosys. She has co-authored a book on ‘J2EE Architecture’ and also has written numerous articles on Java for various online Java forums like JavaWorld, java.net, DevX.com and internet.com. She can be reached at [email protected]

Comments (0)

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.


@MicroservicesExpo Stories
"As we've gone out into the public cloud we've seen that over time we may have lost a few things - we've lost control, we've given up cost to a certain extent, and then security, flexibility," explained Steve Conner, VP of Sales at Cloudistics,in this SYS-CON.tv interview at 20th Cloud Expo, held June 6-8, 2017, at the Javits Center in New York City, NY.
From manual human effort the world is slowly paving its way to a new space where most process are getting replaced with tools and systems to improve efficiency and bring down operational costs. Automation is the next big thing and low code platforms are fueling it in a significant way. The Automation era is here. We are in the fast pace of replacing manual human efforts with machines and processes. In the world of Information Technology too, we are linking disparate systems, softwares and tool...
You know you need the cloud, but you’re hesitant to simply dump everything at Amazon since you know that not all workloads are suitable for cloud. You know that you want the kind of ease of use and scalability that you get with public cloud, but your applications are architected in a way that makes the public cloud a non-starter. You’re looking at private cloud solutions based on hyperconverged infrastructure, but you’re concerned with the limits inherent in those technologies.
Is advanced scheduling in Kubernetes achievable?Yes, however, how do you properly accommodate every real-life scenario that a Kubernetes user might encounter? How do you leverage advanced scheduling techniques to shape and describe each scenario in easy-to-use rules and configurations? In his session at @DevOpsSummit at 21st Cloud Expo, Oleg Chunikhin, CTO at Kublr, answered these questions and demonstrated techniques for implementing advanced scheduling. For example, using spot instances and co...
It has never been a better time to be a developer! Thanks to cloud computing, deploying our applications is much easier than it used to be. How we deploy our apps continues to evolve thanks to cloud hosting, Platform-as-a-Service (PaaS), and now Function-as-a-Service. FaaS is the concept of serverless computing via serverless architectures. Software developers can leverage this to deploy an individual "function", action, or piece of business logic. They are expected to start within milliseconds...
The nature of test environments is inherently temporary—you set up an environment, run through an automated test suite, and then tear down the environment. If you can reduce the cycle time for this process down to hours or minutes, then you may be able to cut your test environment budgets considerably. The impact of cloud adoption on test environments is a valuable advancement in both cost savings and agility. The on-demand model takes advantage of public cloud APIs requiring only payment for t...
As DevOps methodologies expand their reach across the enterprise, organizations face the daunting challenge of adapting related cloud strategies to ensure optimal alignment, from managing complexity to ensuring proper governance. How can culture, automation, legacy apps and even budget be reexamined to enable this ongoing shift within the modern software factory? In her Day 2 Keynote at @DevOpsSummit at 21st Cloud Expo, Aruna Ravichandran, VP, DevOps Solutions Marketing, CA Technologies, was jo...
While some developers care passionately about how data centers and clouds are architected, for most, it is only the end result that matters. To the majority of companies, technology exists to solve a business problem, and only delivers value when it is solving that problem. 2017 brings the mainstream adoption of containers for production workloads. In his session at 21st Cloud Expo, Ben McCormack, VP of Operations at Evernote, discussed how data centers of the future will be managed, how the p...
These days, APIs have become an integral part of the digital transformation journey for all enterprises. Every digital innovation story is connected to APIs . But have you ever pondered over to know what are the source of these APIs? Let me explain - APIs sources can be varied, internal or external, solving different purposes, but mostly categorized into the following two categories. Data lakes is a term used to represent disconnected but relevant data that are used by various business units wit...
DevOps is under attack because developers don’t want to mess with infrastructure. They will happily own their code into production, but want to use platforms instead of raw automation. That’s changing the landscape that we understand as DevOps with both architecture concepts (CloudNative) and process redefinition (SRE). Rob Hirschfeld’s recent work in Kubernetes operations has led to the conclusion that containers and related platforms have changed the way we should be thinking about DevOps and...
With continuous delivery (CD) almost always in the spotlight, continuous integration (CI) is often left out in the cold. Indeed, it's been in use for so long and so widely, we often take the model for granted. So what is CI and how can you make the most of it? This blog is intended to answer those questions. Before we step into examining CI, we need to look back. Software developers often work in small teams and modularity, and need to integrate their changes with the rest of the project code b...
"I focus on what we are calling CAST Highlight, which is our SaaS application portfolio analysis tool. It is an extremely lightweight tool that can integrate with pretty much any build process right now," explained Andrew Siegmund, Application Migration Specialist for CAST, in this SYS-CON.tv interview at 21st Cloud Expo, held Oct 31 – Nov 2, 2017, at the Santa Clara Convention Center in Santa Clara, CA.
"Cloud4U builds software services that help people build DevOps platforms for cloud-based software and using our platform people can draw a picture of the system, network, software," explained Kihyeon Kim, CEO and Head of R&D at Cloud4U, in this SYS-CON.tv interview at 21st Cloud Expo, held Oct 31 – Nov 2, 2017, at the Santa Clara Convention Center in Santa Clara, CA.
Kubernetes is an open source system for automating deployment, scaling, and management of containerized applications. Kubernetes was originally built by Google, leveraging years of experience with managing container workloads, and is now a Cloud Native Compute Foundation (CNCF) project. Kubernetes has been widely adopted by the community, supported on all major public and private cloud providers, and is gaining rapid adoption in enterprises. However, Kubernetes may seem intimidating and complex ...
DevOps is often described as a combination of technology and culture. Without both, DevOps isn't complete. However, applying the culture to outdated technology is a recipe for disaster; as response times grow and connections between teams are delayed by technology, the culture will die. A Nutanix Enterprise Cloud has many benefits that provide the needed base for a true DevOps paradigm. In their Day 3 Keynote at 20th Cloud Expo, Chris Brown, a Solutions Marketing Manager at Nutanix, and Mark Lav...
As many know, the first generation of Cloud Management Platform (CMP) solutions were designed for managing virtual infrastructure (IaaS) and traditional applications. But that's no longer enough to satisfy evolving and complex business requirements. In his session at 21st Cloud Expo, Scott Davis, Embotics CTO, explored how next-generation CMPs ensure organizations can manage cloud-native and microservice-based application architectures, while also facilitating agile DevOps methodology. He expla...
"Grape Up leverages Cloud Native technologies and helps companies build software using microservices, and work the DevOps agile way. We've been doing digital innovation for the last 12 years," explained Daniel Heckman, of Grape Up in this SYS-CON.tv interview at 21st Cloud Expo, held Oct 31 – Nov 2, 2017, at the Santa Clara Convention Center in Santa Clara, CA.
Some people are directors, managers, and administrators. Others are disrupters. Eddie Webb (@edwardawebb) is an IT Disrupter for Software Development Platforms at Liberty Mutual and was a presenter at the 2016 All Day DevOps conference. His talk, Organically DevOps: Building Quality and Security into the Software Supply Chain at Liberty Mutual, looked at Liberty Mutual's transformation to Continuous Integration, Continuous Delivery, and DevOps. For a large, heavily regulated industry, this task ...
Enterprises are adopting Kubernetes to accelerate the development and the delivery of cloud-native applications. However, sharing a Kubernetes cluster between members of the same team can be challenging. And, sharing clusters across multiple teams is even harder. Kubernetes offers several constructs to help implement segmentation and isolation. However, these primitives can be complex to understand and apply. As a result, it’s becoming common for enterprises to end up with several clusters. Thi...
Let's do a visualization exercise. Imagine it's December 31, 2018, and you're ringing in the New Year with your friends and family. You think back on everything that you accomplished in the last year: your company's revenue is through the roof thanks to the success of your product, and you were promoted to Lead Developer. 2019 is poised to be an even bigger year for your company because you have the tools and insight to scale as quickly as demand requires. You're a happy human, and it's not just...