The following table demonstrates that most of annotations in Tapestry IoC and JSR-330 are interchangeable. However, there are few differences in semantics.
Tapestry marker annotations don’t need any qualifier annotations
By default all Tapestry services are singletons
Let’s start with field injection. In Tapestry the injection into fields is triggered by @Inject or @InjectService annotations. When @Inject annotation is present on a field, Tapestry tries to resolve the object to inject by the type of the field. If several implementations of the same service interface are available in the registry, you have to disambiguate which implementation you want to be injected. This can be done by placing the @InjectService annotation on the injection point.
Now let’s see the JSR-330 equivalent of the same service. As you can see the @Inject annotations are interchangeable. The difference is how to get a service by its unique id. For this purpose JSR-330 provides the @Named annotation which accompanies the @Inject annotation.
For constructor injection the @Inject annotations are interchangeable. You can use either JSR-330 or Tapestry annotation to mark a constructor for injection. Note that at most one constructor per class may be marked as injection point.
However, the semantics of constructor injection are different in JSR-330 and Tapestry IoC. In JSR-330 a constructor is injectable only if the @Inject annotation is present.
In Tapestry the @Inject annotation for constructors is optional. All available constructors are candidates for injection: the constructor with the most parameters will be invoked.
When several constructors are available and you don’t want the constructor with most parameters to be injectable, you need to place the @Inject annotation.
Injection Into Pages and Components
Inside Tapestry components, injection occurs exclusively on fields. So far the injection was triggered by the @Inject or @InjectService annotations. As of version 5.3 the injection points can also be marked with JSR-330 annotations. The following example demonstrates that.
Both JSR-330 and Tapestry IoC allow you to disambiguate services by marker or qualifier annotations, as shown in the following example.
Again, there is a slight difference. In JSR-330 a qualifier annotation like @French in the example above needs to be annotated by the @Qualifier annotation.
In Tapestry any annotation can be a marker annotation. You don’t need to place something like the @Qualifier annotation on your marker annotation.
Injectable methods is a next slight difference. In JSR-330 a method is injectable if the @Inject annotation is present. In Tapestry the @Inject annotation is optional. An ordinary setter method is a candidate to perform injection.
When building a Car instance, Tapestry IoC will try to resolve a service of type Engine. If available, Tapestry will perform injection by invoking the setter method.
Besides that, module methods are injectable. Again, there is no need to mark the methods with @Inject annotation as Tapestry explicitly knows which module methods to invoke. In the following example you can see how to use @Named annotation to inject a service by id into a contribute method.
By default, a JSR-330 injector creates an instance, uses the instance for one injection, and then forgets it. By placing the @Scope annotation you can tell the injector to retain the instance for possible reuse in a later injection. If you want a service to be a singleton, you need to use the @Singleton annotation.
In Tapestry, it is exactly the other way around. By default a service is a singleton. Once an instance is created, it is reused for injection. Another available scope is perthread, which exists primarily to help multi-threaded servlet applications. If a service has perthread scope, it is recreated for every incoming request.