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Injection

Main article: Injection

What's the difference between the

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@Component and

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@InjectComponent annotations?

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

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

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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

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@InjectPage and

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@InjectContainer annotations?

The InjectPage @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:

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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 @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.

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

What's the difference between

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@Inject and

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@Environmental?

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

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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.

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:

    /**
     * Builds a proxy to the current {@link JavaScriptSupport} inside this thread's {@link Environment}.
     * 
     * @since 5.2.0
     */
    public JavaScriptSupport buildJavaScriptSupport()
    {
        return environmentalBuilder.build(JavaScriptSupport.class);
    }

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:

    public Request buildRequest()
    {
        return shadowBuilder.build(requestGlobals, "request", Request.class);
    }

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).

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