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63. Life cycle

Plugins in SolarNode can be added to and removed from the platform at any time without restarting the SolarNode process, because of the Life Cycle process OSGi manages. The life cycle of a plugin consists of a set of states and OSGi will transition a plugin's state over the course of the plugin's life.

The available plugin states are:

State Description
INSTALLED The plugin has been successfully added to the OSGi framework.
RESOLVED All package dependencies that the bundle needs are available. This state indicates that the plugin is either ready to be started or has stopped.
STARTING The plugin is being started by the OSGi framework, but it has not finished starting yet.
ACTIVE The plugin has been successfully started and is running.
STOPPING The plugin is being stopped by the OSGi framework, but it has not finished stopping yet.
UNINSTALLED The plugin has been removed by the OSGi framework. It cannot change to another state.

The possible changes in state can be visualized in the following state-change diagram:

OSGi Life Cycle state change diagram

Faisal.akeel, Public domain, via Wikimedia Common

63.1 Activator

A plugin can opt in to receiving callbacks for the start/stop state transitions by providing an org.osgi.framework.BundleActivator implementation and declaring that class in the Bundle-Activator manifest attribute. This can be useful when a plugin needs to initialize some resources when the plugin is started, and then release those resources when the plugin is stopped.

public interface BundleActivator {
    /**
     * Called when this bundle is started so the Framework can perform the
     * bundle-specific activities necessary to start this bundle.
     *
     * @param context The execution context of the bundle being started.
     */
    public void start(BundleContext context) throws Exception;

    /**
     * Called when this bundle is stopped so the Framework can perform the
     * bundle-specific activities necessary to stop the bundle.
     *
     * @param context The execution context of the bundle being stopped.
     */
    public void stop(BundleContext context) throws Exception;
}

package com.example.activator;
import org.osgi.framework.BundleActivator;
import org.osgi.framework.BundleContext;
public class Activator implements BundleActivator {

    @Override
    public void start(BundleContext bundleContext) throws Exception {
        // initialize resources here
    }

    @Override
    public void stop(BundleContext bundleContext) throws Exception {
        // clean up resources here
    }
}

Manifest-Version: 1.0
Bundle-ManifestVersion: 2
Bundle-Name: Example Activator
Bundle-SymbolicName: com.example.activator
Bundle-Version: 1.0.0
Bundle-Activator: com.example.activator.Activator
Import-Package: org.osgi.framework;version="[1.3,2.0)"

Tip

Often making use of the component life cycle hooks available in Blueprint are sufficient and no BundleActivator is necessary.