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Injection

Main article:  Injection

Contents

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:

...

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:

...

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:

...

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.

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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:

    /**
     * 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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