In OSGi, all components, Java classes, resources, and descriptors are deployed
via modules. The
MANIFEST.MF file describes the module’s physical
characteristics, such as the packages it exports and imports. The module’s
component description files specify its functional characteristics (i.e., the
services its components offer and consume). Also modules and their components
have their own lifecycles and administrative APIs. Declarative Services and
shell tools give you fine-grained control over module and component deployment.
Since a module’s contents depend on its activation, consider the activation steps:
Installation: Copying the module JAR into Liferay DXP’s
[Liferay Home]/deployfolder installs the module to the OSGi framework, marking the module
Resolution: Once all the module’s requirements are met (e.g., all packages it imports are available), the framework publishes the module’s exported packages and marks the module
Activation: Modules are activated eagerly by default. That is, they’re started in the framework and marked
ACTIVEon resolution. An active module’s components are enabled. If a module specifies a
lazyactivation policy, as shown in the manifest header below, it’s activated only after another module requests one of its classes.
The figure below illustrates the module lifecycle.
The Apache Felix Gogo Shell lets you manage the module lifecycle. You can install/uninstall modules and start/stop them. You can update a module and notify dependent modules to use the update. Liferay’s tools, including Liferay Dev Studio DXP, Liferay Workspace, and Blade CLI offer similar shell commands that use the OSGi Admin API.
On activating a module, its components are enabled. But only activated components can be used. Component activation requires all its referenced services be satisfied. That is, all services it references must be registered. The highest ranked service that matches a reference is bound to the component. When the container finds and binds all the services the component references, it registers the component. It’s now ready for activation.
Components can use delayed (default) or immediate activation policies. To
specify immediate activation, the developer adds the attribute
@Component( immediate = true, ... )
Unless immediate activation is specified, the component’s activation is delayed. That is, the component’s object is created and its classes are loaded once the component is requested. In this way, delayed activation can improve startup times and conserve resources.
Gogo Shell’s Service Component Runtime commands let you manage components:
scr:list [bundleID]: Lists the module’s (bundle’s) components.
scr:info [componentID|fullClassName]: Describes the component, including its status and the services it provides.
scr:enable [componentID|fullClassName]: Enables the component.
scr:disable [componentID|fullClassName]: Disables the component. It’s disabled on the server (or current server node in a cluster) until the server is restarted.
Service references are static and reluctant by default. That is, an injected
service remains bound to the referencing component until the service is
disabled. Alternatively, you can specify greedy service policies for
references. Every time a higher ranked matching service is registered, the
framework unbinds the lower ranked service from the component (whose service
policy is greedy) and binds the new service in its place automatically. Here’s a
@Reference annotation that uses a greedy policy:
@Reference(policyOption = ReferencePolicyOption.GREEDY)
Declarative Services annotations let you specify component activation and service policies. Gogo Shell commands let you control modules and components.