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| <article class="doc"> |
| <h1 class="page">Dependency Manager - Service Scopes</h1> |
| <div id="preamble"> |
| <div class="sectionbody"> |
| <div class="paragraph"> |
| <p>By default service providers are registered once in the osgi registry, and all service consumers share the same service provider instance. |
| Now, you can control the scope of the provided services: From the provider side, a "scope" parameter is available for all types of DM components and allows to define the scope of the registered service.</p> |
| </div> |
| <div class="paragraph"> |
| <p>The <code>scope</code> attribute has three enum values: SINGLETON, BUNDLE, PROTOTYPE</p> |
| </div> |
| <div class="ulist"> |
| <ul> |
| <li> |
| <p>SINGLETON: it’s as before: your registered service is a singleton, meaning that the service must be instantiated and registered once, and all using services will share the same service instance of your component.</p> |
| </li> |
| <li> |
| <p>BUNDLE: the service will be registered as a ServiceFactory, meaning an instance of the component must be created for each bundle using the service.</p> |
| </li> |
| <li> |
| <p>PROTOTYPE: the service will be registered as a PrototypeServiceFactory, meaning the service is registered as a prototype scope service and an instance of the component must be created for each distinct service requester.</p> |
| </li> |
| </ul> |
| </div> |
| <div class="paragraph"> |
| <p>Scoped Services are supported by all kind of DM service components:</p> |
| </div> |
| <div class="ulist"> |
| <ul> |
| <li> |
| <p>Component</p> |
| </li> |
| <li> |
| <p>Aspects</p> |
| </li> |
| <li> |
| <p>Adapters</p> |
| </li> |
| <li> |
| <p>Bundle Adapters</p> |
| </li> |
| <li> |
| <p>Resource Adapters</p> |
| </li> |
| </ul> |
| </div> |
| <div class="paragraph"> |
| <p>When a consumer requests a service (using ServiceDependency), then DM will automatically dereference the service like this:</p> |
| </div> |
| <div class="ulist"> |
| <ul> |
| <li> |
| <p>if the service has a SERVICE_SCOPE service property matching SCOPE_PROTOTYPE, the DM will internally derefence the service using the ServiceObject API: so, the consumer will get its own copy of the requested service instance.</p> |
| </li> |
| <li> |
| <p>else, DM will internally dereference the service, as before. |
| When defining scoped component implementation, you can optionally define two special class fields in order to get injected with the client Bundle requesting the service, and the ServiceRegisgtration Object. |
| Just use @Inject annotations in order to get injected with the client Bundle or the ServiceRegistration. |
| You can also define a constructor which takes as argument the client bundle as well as the service registration.</p> |
| </li> |
| </ul> |
| </div> |
| </div> |
| </div> |
| <div class="sect1"> |
| <h2 id="_examples"><a class="anchor" href="#_examples"></a>Examples</h2> |
| <div class="sectionbody"> |
| <div class="paragraph"> |
| <p>So, here is a MyService component with PROTOTYPE scope, and each requester will get its own copy of MyService instance (the MyServiceImpl.start() method will be called for each MyServiceImpl instance):</p> |
| </div> |
| <div class="listingblock"> |
| <div class="content"> |
| <pre class="highlightjs highlight"><code class="language-java hljs" data-lang="java">public class Activator extends DependencyActivatorBase { |
| @Override |
| public void init(BundleContext context, DependencyManager dm) throws Exception { |
| dm.add(createComponent() |
| .setScope(ServiceScope.PROTOTYPE) |
| .setInterface(MyService.class.getName(), null) |
| .setImplementation(MyServiceImpl.class)); |
| } |
| } |
| |
| public class MyServiceImpl implements MyService { |
| void start() { |
| // called on each MyService instance |
| } |
| }</code></pre> |
| </div> |
| </div> |
| <div class="paragraph"> |
| <p>The MyServiceImpl, like with annotations, can define a constructor in order to be injected with the client bundle invoking the service and also the service Registration:</p> |
| </div> |
| <div class="listingblock"> |
| <div class="content"> |
| <pre class="highlightjs highlight"><code class="language-java hljs" data-lang="java"> public class MyServiceImpl implements MyService { |
| public MyServiceImpl(Bundle clientBundle, ServiceRegistration reg) { ... } |
| void start() { |
| // called on each MyService instance |
| } |
| }</code></pre> |
| </div> |
| </div> |
| <div class="paragraph"> |
| <p>(if you want to auto configure the client Bundle, and the ServiceRegistration, then simply define class fields, they will be auto injected, unless you disable auto configuraiton for Bundle/ServiceRegistration using Component.setAutoConfig(Class, boolean) methods.</p> |
| </div> |
| <div class="paragraph"> |
| <p>Here is a Client component which simply depends on the MyService service using a basic DM activator (nothing special to do):</p> |
| </div> |
| <div class="listingblock"> |
| <div class="content"> |
| <pre class="highlightjs highlight"><code class="language-java hljs" data-lang="java">public class Activator extends DependencyActivatorBase { |
| @Override |
| public void init(BundleContext context, DependencyManager dm) throws Exception { |
| dm.add(createComponent() |
| .setImplementation(Client.class) |
| .add(createServiceDependency() |
| .setService(MyService.class, null).setRequired(true).setCallbacks("bind", "unbind")); |
| } |
| } |
| |
| public class Client { |
| void bind(MyService service) { |
| // our client is injected with a specific instance of the MyService component |
| // that is created for our Client. |
| // If another component defines a service dependency on MyService, then the other |
| // component will get its own private copy of MyService component instance |
| } |
| }</code></pre> |
| </div> |
| </div> |
| </div> |
| </div> |
| <div class="sect1"> |
| <h2 id="_example_using_serviceobjects_api"><a class="anchor" href="#_example_using_serviceobjects_api"></a>Example using ServiceObjects API</h2> |
| <div class="sectionbody"> |
| <div class="paragraph"> |
| <p>If now you want to control the creation of the MyService using raw OSGI ServiceObjects API, you can also do it like this:</p> |
| </div> |
| <div class="listingblock"> |
| <div class="content"> |
| <pre class="highlightjs highlight"><code class="language-java hljs" data-lang="java"> public class Client { |
| void bind(ServiceObjects<MyService> so) { |
| MyService s1 = so.getService(); |
| MyService s2 = so.getService(); |
| ... |
| so.ungetService(s1); // will deactivate the MyService s1 instance |
| so.ungetService(s2); // will deactivate the MyService s2 instance |
| } |
| }</code></pre> |
| </div> |
| </div> |
| </div> |
| </div> |
| <div class="sect1"> |
| <h2 id="_scoped_services_and_initdestroy_callbacks"><a class="anchor" href="#_scoped_services_and_initdestroy_callbacks"></a>Scoped services and init/destroy callbacks</h2> |
| <div class="sectionbody"> |
| <div class="paragraph"> |
| <p>When you need to specify dynamic required dependencies from your component.init() method, the following mechanism will be used:</p> |
| </div> |
| <div class="paragraph"> |
| <p>First, if your component defines an init callback, then one single component prototype instance singleton is created, as if the component is declared with SCOPE=SINGLETON. |
| So, the prototype instance will be invoked in the init callback, but won’t be called in start()/stop(). |
| And when all dependencies are satisfied and injected (including the dynamic dependencies defined in the init method), then the ServiceFactory (or PrototypeServiceFactory) is registered. |
| And when one client will request a component instance, then a clone will be created and returned.</p> |
| </div> |
| <div class="paragraph"> |
| <p>Example of a scoped component which defines an init method:</p> |
| </div> |
| <div class="listingblock"> |
| <div class="content"> |
| <pre class="highlightjs highlight"><code class="language-java hljs" data-lang="java">public class Activator extends DependencyActivatorBase { |
| @Override |
| public void init(BundleContext context, DependencyManager dm) throws Exception { |
| dm.add(createComponent() |
| .setScope(ServiceScope.PROTOTYPE) |
| .setInterface(MyService.class.getName(), null) |
| .setImplementation(MyServiceImpl.class)); |
| } |
| } |
| |
| public static class MyServiceImpl implements MyService { |
| void init(Component comp) { |
| // add required dependencies dynamically |
| } |
| |
| void start() { |
| // only called on clone, not on the prototype instance singleton |
| } |
| |
| void stop() { |
| // called on each clone, not on the prototype instance singleton |
| } |
| }</code></pre> |
| </div> |
| </div> |
| <div class="paragraph"> |
| <p>So, if you don’t specify an init callback then the prototype instance singleton won’t be instantiated. |
| Also,</p> |
| </div> |
| </div> |
| </div> |
| <div class="sect1"> |
| <h2 id="_limitation_when_using_dm_servicedependency_from_api_and_serviceobjects"><a class="anchor" href="#_limitation_when_using_dm_servicedependency_from_api_and_serviceobjects"></a>Limitation when using DM ServiceDependency from API and ServiceObjects</h2> |
| <div class="sectionbody"> |
| <div class="paragraph"> |
| <p>When using DependencyManager ServiceDependency from the DM API (not using annotations), you have to know that the ServiceDependency always internally dereferences the service dependency, even if you specify a ServiceObjecs parameter in your bind method. |
| If now you really want to disable the auto-deref ServiceDependency (because you want to directly use the ServiceObjects API), you must then use the "setDereference(false") method on your ServiceDependency: in this way, you tell DM to never dereference internally the scoped service. |
| Here is an example:</p> |
| </div> |
| <div class="listingblock"> |
| <div class="content"> |
| <pre class="highlightjs highlight"><code class="language-java hljs" data-lang="java">public class Activator extends DependencyActivatorBase { |
| @Override |
| public void init(BundleContext context, DependencyManager dm) throws Exception { |
| dm.add(createComponent() |
| .setImplementation(Client.class) |
| .add(createServiceDependency() |
| .setService(MyService.class, null).setRequired(true).setCallbacks("bind", "unbind") |
| .setDereference(false)); |
| } |
| } |
| |
| public class Client { |
| void bind(ServiceObjects<MyService> so) { |
| MyService s1 = so.getService(); |
| MyService s2 = so.getService(); |
| ... |
| so.ungetService(s1); // will deactivate the MyService s1 instance |
| so.ungetService(s2); // will deactivate the MyService s2 instance |
| } |
| }</code></pre> |
| </div> |
| </div> |
| <div class="paragraph"> |
| <p>In the above example, the Activator defines the ServiceDependency using the ServiceDependency.setDereference(false) method because it’s the Client.bind method which will create the MyService instances manually.</p> |
| </div> |
| <div class="paragraph"> |
| <p>In the future, I will try to auto detect the signatures of the Client.bind method in order to never auto-dereference the injected service in case the bind method takes as argument a ServiceObjects (or a ServiceReference) method.</p> |
| </div> |
| </div> |
| </div> |
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