OSGi , Spring OSGi and the server-side. Part 3
Saturday 30 June 2007
Here is a part from comparison between Spring OSGi and OSGi services, Equinox extensions.
Comparison Matrix
| Extensions | Services | Declarative Services | Spring-OSGi | |
|---|---|---|---|---|
| What are registered? | XML declarations, optionally containing class names. | Java objects. | Placeholder proxies for Java objects, replaced by real Java objects on first use. | Placeholder proxies for Java objects, replaced by real Java objects on first use. |
| How are they registered? | All <extension> nodes in the plugin.xmlare automatically registered. | Through the BundleContext API | All service component XML files declared through the Service-Component manifest entry are automatically registered. | All Spring XML files contained in the META-INF/springfolder of a bundle, or declared through the Spring-Context |
| How are they consumed? | Queried by extension point ID. Registry returns all entries, consumer must iterate them to find which one(s) it wants. | Queried by interface name, plus a property filter. Usually the filter is specific enough to ensure just one match. | Internally uses same query over interface name plus filter, but the query is performed within SCR. Matching services are supplied to consumer code via JavaBean-style setter methods. | Same as for DS, but can additionally supply matching services via constructor arguments. |
| What is the cardinality? | Conventionally, one to many. One extension point has many extensions, but each extension has exacly one extension point. However another plug-in can query extension points that it doesn’t own. | Many to many. One service can be used by multiple consumers, and one consumer can use many services. | Same as for services | Same as for services |
| When are they loaded? | Extension declarations are loaded during start-up, or when a new plug-in is installed. Classes named in the extension are loaded lazily, i.e.only when needed. | The class that implements a service must be loaded and instantiated before the service can be registered. This happens only when the bundle is explicitly started. | For delayed services, the SCR registers a placeholder proxy service immediately when a DS-enabled bundle is resolved. The real class is loaded and instantiated when the service is first used. | Same as for DS. |
| How is dynamic installation/uninstallation dealt with? | Either query on each use or track with ExtensionTracker. | Either query on each use or track with ServiceTracker. | SCR calls methods on the component to supply it with the latest available matching service. It can also call an unset method when the service becomes unavailable. | Similarly to DS, supplies latest matching service through setter method on the bean. |
| Can caching references to extensions/services cause problems? | Yes, and as a legacy of pre-OSGi usage, some plug-ins still do this. | Yes, but this is strongly discouraged by the specification, and in practice happens rarely. | No, SCR does not do this. | No, Spring-OSGi does not do this. |
Comparison by EclipseZone
Posted byEugene at 09:57
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