OSGi

OSGi

OSGi Alliance logo
Year started 2000
Latest version 6.0
June 2015 (2015-06)[1]
Organization OSGi Alliance
Domain Java
License OSGi Specification License, Version 2.0
Website www.osgi.org

The OSGi Alliance, formerly known as the Open Services Gateway initiative, is an open standards organization founded in March 1999 that originally specified and continues to maintain the OSGi standard.

The OSGi specification describes a modular system and a service platform for the Java programming language that implements a complete and dynamic component model, something that does not exist in standalone Java/VM environments. Applications or components, coming in the form of bundles for deployment, can be remotely installed, started, stopped, updated, and uninstalled without requiring a reboot; management of Java packages/classes is specified in great detail. Application life cycle management is implemented via APIs that allow for remote downloading of management policies. The service registry allows bundles to detect the addition of new services, or the removal of services, and adapt accordingly.

The OSGi specifications have evolved beyond the original focus of service gateways, and are now used in applications ranging from mobile phones to the open-source Eclipse IDE. Other application areas include automobiles, industrial automation, building automation, PDAs, grid computing, entertainment, fleet management and application servers.

Specification process

The OSGi specification is developed by the members in an open process and made available to the public free of charge under the OSGi Specification License.[2] The OSGi Alliance has a compliance program that is open to members only. As of November 2010, there are seven certified OSGi framework implementations.[3] A separate page lists both certified and non-certified OSGi Specification Implementations, which include OSGi frameworks and other OSGi specifications.

Architecture

OSGi Service Gateway Architecture

OSGi (Open Service Gateway Initiative) is a Java framework for developing and deploying modular software programs and libraries. Each bundle is a tightly coupled, dynamically loadable collection of classes, jars, and configuration files that explicitly declare their external dependencies (if any).

The framework is conceptually divided into the following areas:

Bundles
Bundles are normal jar components with extra manifest headers.
Services
The services layer connects bundles in a dynamic way by offering a publish-find-bind model for Plain Old Java Interfaces (POJI) or Plain Old Java Objects (POJO).
Services Registry
The application programming interface for management services (ServiceRegistration, ServiceTracker and ServiceReference).
Life-Cycle
The application programming interface for life cycle management (install, start, stop, update, and uninstall) for bundles.
Modules
The layer that defines encapsulation and declaration of dependencies (how a bundle can import and export code).
Security
The layer that handles the security aspects by limiting bundle functionality to pre-defined capabilities.
Execution Environment
Defines what methods and classes are available in a specific platform. There is no fixed list of execution environments, since it is subject to change as the Java Community Process creates new versions and editions of Java. However, the following set is currently supported by most OSGi implementations:

Bundles

Classification: OSGi

A bundle is a group of Java classes and additional resources equipped with a detailed manifest MANIFEST.MF file on all its contents, as well as additional services needed to give the included group of Java classes more sophisticated behaviors, to the extent of deeming the entire aggregate a component.

Below is an example of a typical MANIFEST.MF file with OSGi Headers:

Bundle-Name: Hello World
Bundle-SymbolicName: org.wikipedia.helloworld
Bundle-Description: A Hello World bundle
Bundle-ManifestVersion: 2
Bundle-Version: 1.0.0
Bundle-Activator: org.wikipedia.Activator
Export-Package: org.wikipedia.helloworld;version="1.0.0"
Import-Package: org.osgi.framework;version="1.3.0"

The meaning of the contents in the example is as follows:[4]

Life-cycle

OSGi Bundle Life-Cycle

A Life Cycle layer adds bundles that can be dynamically installed, started, stopped, updated and uninstalled. Bundles rely on the module layer for class loading but add an API to manage the modules in run time. The life cycle layer introduces dynamics that are normally not part of an application. Extensive dependency mechanisms are used to assure the correct operation of the environment. Life cycle operations are fully protected with the security architecture.

Bundle State Description
INSTALLED The bundle has been successfully installed.
RESOLVED All Java classes that the bundle needs are available. This state indicates that the bundle is either ready to be started or has stopped.
STARTING The bundle is being started, the BundleActivator.start method has been called but the start method has not yet returned. When the bundle has an activation policy, the bundle will remain in the STARTING state until the bundle is activated according to its activation policy.
ACTIVE The bundle has been successfully activated and is running; its Bundle Activator start method has been called and returned.
STOPPING The bundle is being stopped. The BundleActivator.stop method has been called but the stop method has not yet returned.
UNINSTALLED The bundle has been uninstalled. It cannot move into another state.

Below is an example of a typical Java class implementing the BundleActivator interface:

package org.wikipedia;

import org.osgi.framework.BundleActivator;
import org.osgi.framework.BundleContext;

public class Activator implements BundleActivator {
	private BundleContext context;

	@Override
	public void start(BundleContext context) throws Exception {
		System.out.println("Starting: Hello World");
		this.context = context;
	}

	@Override
	public void stop(BundleContext context) throws Exception {
		System.out.println("Stopping: Goodbye Cruel World");
		this.context = null;
	}
}

Services

Standard services

The OSGi Alliance has specified many services. Services are specified by a Java interface. Bundles can implement this interface and register the service with the Service Registry. Clients of the service can find it in the registry, or react to it when it appears or disappears.

The table below shows a description of OSGi System Services:

System Services Description
Logging The logging of information, warnings, debug information or errors is handled through the Log Service. It receives log entries and then dispatches these entries to other bundles that subscribed to this information.
Configuration Admin This service allows an operator to set and get the configuration information of deployed bundles
Device Access Facilitates the coordination of automatic detection and attachment of existing devices. This is used for Plug and Play scenarios.
User Admin This service uses a database with user information (private and public) for authentication and authorization purposes.
IO Connector The IO Connector Service implements the CDC/CLDC javax.microedition.io package as a service. This service allows bundles to provide new and alternative protocol schemes.
Preferences Offers an alternative, more OSGi-friendly mechanism to using Java’s default Properties for storing preferences.
Component Runtime The dynamic nature of services—they can come and go at any time—makes writing software harder. The Component Runtime specification can simplify handling these dynamic aspects by providing an XML based declaration of the dependencies.
Deployment Admin Standardizes access to some of the responsibilities of the management agent.
Event Admin Provides an inter-bundle communication mechanism based on a publish-and-subscribe model.
Application Admin Simplifies the management of an environment with many different types of applications that are simultaneously available.

The table below shows a description of OSGi Protocol Services:

Protocol Services Description
HTTP Service Allows information to be sent and received from OSGi using HTTP.
UPnP Device Service Specifies how OSGi bundles can be developed to interoperate with Universal Plug and Play (UPnP) devices.
DMT Admin Defines an API for managing a device using concepts from the Open Mobile Alliance (OMA) device management specifications.

The table below shows a description of OSGi Miscellaneous Services:

Miscellaneous Services Description
Wire Admin Allows the connection between a Producer service and a Consumer service.
XML Parser The XML Parser service allows a bundle to locate a parser with desired properties and compatibility with JAXP.
Measurement and State The Measurement and State service allows and simplifies the correct handling of measurements in an OSGi service platform.

Organization

The OSGi Alliance was founded by Ericsson, IBM, Motorola, Sun Microsystems and others in March 1999. Before incorporating as a nonprofit corporation it was called the Connected Alliance.

Among its members are (as of February 2015) more than 35 companies from quite different business areas, for example Adobe Systems, Deutsche Telekom, Hitachi, IBM, Liferay, Makewave (formerly Gatespace Telematics), NEC, NTT, Oracle, Orange S.A., ProSyst, Salesforce.com, Siemens, Software AG and TIBCO Software.[5]

The Alliance has a board of directors that provides the organization's overall governance. OSGi officers have various roles and responsibilities in supporting the alliance. Technical work is conducted within Expert Groups (EGs) chartered by the board of directors, and non-technical work is conducted in various working groups and committees. The technical work conducted within Expert Groups include developing specifications, reference implementations, and compliance tests. These Expert Groups have produced five major releases of the OSGi specifications (As of 2012).

Dedicated Expert Groups exist for the enterprise, mobile, vehicle and the core platform areas.

The Enterprise Expert Group (EEG) is the newest EG and is addressing Enterprise / Server-side applications. In November 2007 the Residential Expert Group (REG) started to work on specifications to remotely manage residential/home-gateways. In October 2003, Nokia, Motorola, IBM, ProSyst and other OSGi members formed a Mobile Expert Group (MEG) that will specify a MIDP-based service platform for the next generation of smart mobile phones, addressing some of the needs that CLDC cannot manage - other than CDC. MEG became part of OSGi as with R4.

Specification versions

Projects using OSGi

Current framework implementations

Name License
Apache Felix Open source
Concierge OSGi Open source
Equinox OSGi Open source
Hitachi SuperJ Commercial
Knopflerfish Open source
ProSyst mBS Commercial
Eclipse Gemini Open Source

See also

References

  1. ↑ "OSGi Alliance | Specifications / HomePage". osgi.org. 2015. Retrieved June 30, 2016.
  2. ↑ OSGi Specification License
  3. ↑ "Certified Products". OSGi Alliance. Retrieved 2010-11-01.
  4. ↑ Creating OSGi bundles by Costin Leau
  5. ↑ OSGi Alliance | About / Members
  6. ↑ "Adempiere Branch_GlobalQSS_361". http://www.adempiere.com/. Retrieved 27 September 2014. External link in |website= (help)

Further reading

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