OSGi is java modularity development framework. We can divide
complex application into multiple modules and OSGi will responsible to
communicate all modules and make it as single application.
OSGi is framework provides the run time environment so
that all these module can communicate each other. In OSGi terminology we can
call each unit as bundle. Bundle is independent component in OSGi and it can
run in OSGi container. OSGi container is responsible to handle lifecycle of
bundle.
Follow
below articles for Basics of OSGi and Bundles
In this article we are going to see develop basic bundle
development using eclipse and will run in OSGi run time environment.
Basic OSGi bundle development without eclipse and run
in Apache Felix OSGi run time environment was explained in below article.
Prerequisite.
Install Java 8 and set JAVA_HOME environment variable.
Download latest version of Eclipse from below link
We are using eclipse-jee-oxygen
for this example.
Any bundle in OSGi need activator java class and it is responsible for managing bundle
lifecycle in the OSGi container. So each activator class should extends “BundleActivator”.
When we create bundle with eclipse it will create
basic template with Activator class and if we observed the activator class, that
extends “BundleActivator” class.
Once you download the eclipse extract it local machine
then you can find “eclipse” directory. Navigate to “eclipse” then click on
eclipse icon then eclipse will be started and initially it will ask workspace
for your projects then select your workspace directory.
Create
OSGi sample bundle in Eclipse.
Go to File à
New à Project
It will open project selection Dialog then select “Plugin-in Project” click on next.
Once click on next then you can see another dialog
there we need to provide your project name then click on next.
Project Name: com.liferaysavvy.example.LiferaySavvyWorld
Plugin-in Targeted Run with Select OSGi framework à
Standard
Once click on next we can see other dialog there it
will ask information which will be used in the bundle MANIFEST file. Information like name, version, vendor and execution environment.
Click on Generate
Activator class check box then give your Activator class name.
Once click on Next it will ask you select OSGi template and click on Crete Plugin
using template then select “Hello OSGi”
so that it will generate bundle with activator class with lifecycle methods
Finally click on
finish button then bundle will create with basic activator class.
The following is basic OSGi bundle project view in
eclipse
Activator
Activator
is java class which handle the bundle lifecycle management in the OSGi container.
It will extends the “BundleActivator” class. We have different lifecycle
methods as follows
start(--)
: This method
will take the argument BundleContext
and container will invoke this method to start bundle.
stop(--):
This method also take BundleContext as
argument and OSGi container will invoke this method to stop bundle.
package
com.liferaysavvy.example.liferaysavvyworld;
import
org.osgi.framework.BundleActivator;
import
org.osgi.framework.BundleContext;
public class Activator implements BundleActivator {
/*
*
(non-Javadoc)
*
@see
org.osgi.framework.BundleActivator#start(org.osgi.framework.BundleContext)
*/
public void
start(BundleContext context) throws Exception {
System.out.println("Say Liferay
Savvy World!!");
}
/*
*
(non-Javadoc)
*
@see
org.osgi.framework.BundleActivator#stop(org.osgi.framework.BundleContext)
*/
public void stop(BundleContext
context) throws Exception {
System.out.println("Goodbye
Liferay Savvy World!!");
}
}
|
MANIFEST.MF:
MANIFEST.MF is deployment descriptor for the bundle
and have more information and it will be used by OSGi container. This is core
resource of the bundle.
Manifest-Version: 1.0
Bundle-ManifestVersion: 2
Bundle-Name: LiferaySavvyWorld
Bundle-SymbolicName:
com.liferaysavvy.example.LiferaySavvyWorld
Bundle-Version: 1.0.0.qualifier
Bundle-Activator:
com.liferaysavvy.example.liferaysavvyworld.Activator
Bundle-Vendor: LIFERAYSAVVY
Bundle-RequiredExecutionEnvironment: JavaSE-1.8
Import-Package:
org.osgi.framework;version="1.3.0"
|
Bundle-ManifestVersion:
This header
tells that bundle follows the OSGi specification and value 2 specified, it is
following SGi specification Release 4;
if it is value 1 its means it is following the compliant with Release 3 or
earlier.
Bundle-Name
Its human readable bundle name
Bundle-SymbolicName:
Its important header for bundle and it is unique name
used to refereeing it in other bundles when it communicate to other bundles.
Bundle-Version:
Its simple version number for the bundle. We can start
and run version specific bundles in the OSGi container.
Bundle-Activator
Bundle-Activator headers tells the Activator class
name and it will used to manage life cycle of bundle by OSGi container.
Bundle-Vendor
The Bundle-Vendor header is just human readable name
which tell the vendor who implemented the bundle.
Bundle-RequiredExecutionEnvironment:
Which tell the Jave environment is required to execute
the bundle.
Import-Package
This header is very important and it tells which packages
are required to execute the bundle. When the bundle depends on the other bundle
then we have specify the bundle symbolic name so that it will be handled by the
OSGi container. Basically each bundle must import OSGi implantation bundle it’s
like “org.osgi.framework;”
Executing
a bundle
Select bundle and right click on bundle
Run
As à Run Configuration
Select your bundle and Select “org.eclipse.osgi” for Target platform then Apply à
Run
You can see Output in the console as follows. Bundle
will be started by OSGi container then it will invoke activator start method start() then we can see message in the
console.
If you want execute bundle in Apache Felix environment then select Apache Felix OSGi implementation
for Targeted platform.
Just type Felix in the filter text input then you can
see Felix implementation jars.
If you want execute bundle in OSGi equinox then select
targeted platform equinox
We can also export bundle and execute in standalone
OSGi containers. Just export bundle as JAR file from eclipse and run jar file
in other OSGi containers
Here is example how to run bundle in Apache Felix
Environment. Look at “Install and Start
bundle in OSGi” Section.
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