Injection FAQ

Injection

Main article: Injection

What's the difference between the @Component and @InjectComponent annotations?

The @Component annotation is used to define the type of component, and its parameter bindings. When using @Component, the template must not define the type, and any parameter bindings are merged in:

Here the type of component is defined by the field type. The field name is matched against the t:id in the template. The page parameter is set in the Java class, and the informal class parameter is set in the template. If the tag in the template was <t:pagelink>, or if the template tag included the attribute t:type="pagelink", then you would see an exception.

By contrast, @InjectComponent expects the component to be already defined, and doesn't allow any configuration of it:

Again, we're matching the field name to the component id, and you would get an error if the component is not defined in the template.

What's the difference between the @InjectPage and @InjectContainer annotations?

The @InjectPage annotation is used to inject some page in the application into a field of some other page. You often see it used from event handler methods:

This code pattern is used to configure peristent properties of a page before returning it; Tapestry will send a client redirect to the page to present the data.

@InjectContainer can be used inside a component or a mixin. In a component, it injects the immediate container of the component; this is often the top-level page object.

In a mixin, it injects the component to which the mixin is attached.

I get an exception because I have two services with the same interface, how do I handle this?

It's not uncommon to have two or more services that implement the exact same interface. When you inject, you might start by just identifying the type of service to inject:

Which results in the error: Service interface org.apache.tapestry5.services.ComponentEventResultProcessor is matched by 3 services: AjaxComponentEventResultProcessor, ComponentEventResultProcessor, ComponentInstanceResultProcessor. Automatic dependency resolution requires that exactly one service implement the interface.

We need more information than just the service interface type in order to identify which of the three services to inject. One possibility is to inject with the correct service id:

This works ... but it is clumsy. If the service id, "ComponentEventResultProcessor", ever changes, this code will break. It's not refactoring safe.

Instead, we should use marker annotations. If we look at TapestryModule, where the ComponentEventResultProcessor service is defined, we'll see it identifies the necessary markers:

When a service has marker annotations, the annotations present at the point of injection (the field, method parameter, or constructor parameter) are used to select a matching service. The list of services that match by type is then filtered to only include services that have all of the marker annotations present at the point of injection.

The two marker annotations, @Traditional and @Primary, ensure that only a single service matches.

What's the difference between @Inject and @Environmental?

@Inject is relatively general; it can be used to inject resources specific to a page or component (such as ComponentResources, Logger, or Messages), or it can inject services or other objects obtained from the Tapestry IoC container. Once the page is loaded, the values for these injections never change.

@Environmental is different; it exposes a request-scoped, dynamically bound value:

  • "Request scoped": different threads (processing different requests) will see different values when reading the field.
  • "Dynamically bound": the value is explicitly placed into the Environment, and can be overridden at any time.

Environmentals are a form of loosely connected communication between an outer component (or even a service) and an inner component. Example: the Form component places a FormSupport object into the environment. Other components, such as TextField, use the FormSupport when rendering to perform functions such as allocate unique control names or register client-side validations. The TextField doesn't require that the Form component be the immediate container component, or even an ancestor: a Form on one page may, indirectly, communicate with a TextField on some entirely different page. Neither component directly links to the other, the FormSupport is the conduit that connects them.

The term "Environmental" was chosen as the value "comes from the environment".

But wait ... I see I used the @Inject annotation and it still worked. What gives?

In certain cases, Tapestry exposes a service (which can be injected) that is a proxy to the environmental; this is primarily for common environmentals, such as JavaScriptSupport, that may be needed outside of component classes. You can see this in TapestryModule:

TapestryModule.java (partial)

This kind of logic is based on the EnvironmentalShadowBuilder service.

Ok, but Request is a singleton service, not an environmental, and I can inject that. Is Tapestry really thread safe?

Yes, of course Tapestry is thread safe. The Request service is another special case, as seen in TapestryModule:

TapestryModule.java (partial)

RequestGlobals is a per-thread service. The Request service is a global singleton created by the PropertyShadowBuilder service, but is just a proxy. It has no internal state; invoking a method on the Request service just turns around and extracts the Request object from the per-thread RequestGlobals and invokes the same method there.

I use @Inject on a field to inject a service, but the field is still null, what happened?

This can happen when you use the wrong @Inject annotation; for example, com.google.inject.Inject instead of org.apache.tapestry5.ioc.annotations.Inject. This can occur when you have TestNG on the classpath, for example, and your IDE is too helpful. Double check your imports when things seem weird.

Also remember that @Inject on fields works for components and for service implementations or other objects that Tapestry instantiates, but not on arbitrary objects (that are created via Java's new keyword).