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From conflue...@apache.org
Subject [CONF] Apache Tapestry > IoC - injection
Date Fri, 01 Oct 2010 12:04:00 GMT
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    <h2><a href="https://cwiki.apache.org/confluence/display/TAPESTRY/IoC+-+injection">IoC
- injection</a></h2>
    <h4>Page <b>edited</b> by             <a href="https://cwiki.apache.org/confluence/display/~ccordenier">Christophe
Cordenier</a>
    </h4>
        <br/>
                         <h4>Changes (3)</h4>
                                 
    
<div id="page-diffs">
            <table class="diff" cellpadding="0" cellspacing="0">
            <tr><td class="diff-snipped" >...<br></td></tr>
            <tr><td class="diff-unchanged" >This step applies only to IoC layer
injection (not to injection into components). <br> <br></td></tr>
            <tr><td class="diff-changed-lines" >When the Inject annotation is
_not present_ at the point of injection, Tapestry checks to see if a resource can be injected.
When the Inject annotation is present, this step is skipped (this is necessary when the object
to be injected has a type that conflicts with a resource type, such as <span class="diff-deleted-words"style="color:#999;background-color:#fdd;text-decoration:line-through;">String).</span>
<span class="diff-added-words"style="background-color: #dfd;">List or Class).</span>
<br></td></tr>
            <tr><td class="diff-unchanged" > <br></td></tr>
            <tr><td class="diff-deleted-lines" style="color:#999;background-color:#fdd;text-decoration:line-through;">
org.slf4j.Logger The Logger of the service being constructed (or the logger of the Module
class being instantiated). String The service id of the service being constructed. [ObjectLocator|../apidocs/org/apache/tapestry5/ioc/ObjectLocator.html]
For contribute methods, used to locate additional objects. [ServiceResources|../apidocs/org/apache/tapestry5/ioc/ServiceResources.html]
For service builder methods, an extended version of ObjectLocator. Class The service interface
type. [OperationTracker|../apidocs/org/apache/tapestry5/ioc/OperationTracker.html] Used to
track deeply nested operations so that errors can be reported sensibly. Object, or service
interface type Passed to decorator methods. Collection, List, Map Assembled service configurations
passed to service builder methods (or service class constructors). Configuration, OrderedConfiguration,
MappedConfiguration Configuration passed to contribute methods, to build service configurations.If
field type does not match any of the available resource types, or the Inject annotation is
present, logic continues to the next step. <br></td></tr>
            <tr><td class="diff-added-lines" style="background-color: #dfd;">*
org.slf4j.Logger The Logger of the service being constructed (or the logger of the Module
class being instantiated). <br>* [ObjectLocator|../apidocs/org/apache/tapestry5/ioc/ObjectLocator.html]
For contribute methods, used to locate additional objects.  <br>* [ServiceResources|../apidocs/org/apache/tapestry5/ioc/ServiceResources.html]
For service builder methods, an extended version of ObjectLocator. Class The service interface
type.  <br>* [OperationTracker|../apidocs/org/apache/tapestry5/ioc/OperationTracker.html]
Used to track deeply nested operations so that errors can be reported sensibly.  <br>*
Object, or service interface type Passed to decorator methods.  <br>* Collection, List,
Map Assembled service configurations passed to service builder methods (or service class constructors).
<br>* Configuration, OrderedConfiguration, MappedConfiguration Configuration passed
to contribute methods, to build service configurations.If field type does not match any of
the available resource types, or the Inject annotation is present, logic continues to the
next step. <br></td></tr>
            <tr><td class="diff-unchanged" > <br>Injection of resources
into fields is triggered by the presence of the [InjectResource|../apidocs/org/apache/tapestry5/ioc/annotations/InjectResource.html]
annotation, whereas injection of resources into parameters occurs when the Inject or InjectService
annotation is _not_ present. These rules are slightly tricky, which reflects a desire to avoid
any annotations except when needed, and the fact that field injection came much later than
parameter injection. <br></td></tr>
            <tr><td class="diff-snipped" >...<br></td></tr>
        </table>
</div>                            <h4>Full Content</h4>
                    <div class="notificationGreySide">
        <h1><a name="IoC-injection-InjectioninDetail"></a>Injection in Detail</h1>

<p>Injection in Tapestry IoC can be a complicated subject for a number of reasons:</p>

<ul>
	<li>Injection can occur in many places: on fields, and on parameters to methods and
constructors of certain objects.</li>
	<li>Parts of Injection are themselves defined in terms of Tapestry IoC services, many
of which are extensible.<br/>
Despite this, injection generally <em>Just Works</em>: most of the time, you want
Tapestry to inject a service, and only a single service implements the service interface.</li>
</ul>


<p>This document discusses what to do when you hit a case that doesn't Just Work, or
when you want to extend the injection logic in some way.</p>

<p>Some aspects of this discussion reflect Tapestry IoC used within a Tapestry web application:
the tapestry-core module makes some extensions to injection.</p>

<h1><a name="IoC-injection-InjectionTriggers"></a>Injection Triggers</h1>

<p>Injection is triggered in a number of ways:</p>

<ul>
	<li>A field in a component class, autobuilt object, or service implementation class
the <span class="error">&#91;Inject|../apidocs/org/apache/tapestry5/ioc/annotations/Inject.html&#93;</span>
annotation.</li>
	<li>A method parameter to a service builder method, a decorator method, or a contribute
method (in a Tapestry IoC module class).</li>
	<li>A constructor parameter to an autobuilt object, or a service implementation class.</li>
	<li>Any of the above with an <span class="error">&#91;InjectService|../apidocs/org/apache/tapestry5/ioc/annotations/InjectService.html&#93;</span>
annotation.<br/>
Injection also covers a related matter: providing special resources to a service or component.
For a service, the service's id (as a string) or extensible configuration (as a Collection,
List or Map) may be provided. For a component, the component's id, locale, message catalog,
or component resources may be provided.</li>
</ul>


<h1><a name="IoC-injection-StandardInjectionProcessing"></a>Standard Injection
Processing</h1>

<p>This section describes standard injection, which applies at the IoC layer: autobuild
objects and service implementations. The steps for injection into Tapestry components are
slightly different and are covered later.</p>

<p>So a the point of injection, Tapestry has identified a field or parameter that should
be injected. At this point, Tapestry knows the following:</p>

<ul>
	<li>The field name (if field injection). The parameter name is not available.</li>
	<li>The field or parameter type, as a Java class. In many cases, this will be enough
to identify what object shall be injected.</li>
	<li>Any additional annotations on the field or parameter.<br/>
Tapestry proceeds with this information.</li>
</ul>


<h2><a name="IoC-injection-CheckforInjectService"></a>Check for InjectService</h2>

<p>Tapestry checks first for the InjectService annotation. The value of this annotation
is the service id to inject. When InjectService is present at the point of injection, that
process is done, though it can fail if the service id indicated does not exist, or if the
service's interface is not compatible with the field's type.</p>

<h2><a name="IoC-injection-Checkforserviceresources"></a>Check for service
resources</h2>

<p>This step applies only to IoC layer injection (not to injection into components).</p>

<p>When the Inject annotation is <em>not present</em> at the point of injection,
Tapestry checks to see if a resource can be injected. When the Inject annotation is present,
this step is skipped (this is necessary when the object to be injected has a type that conflicts
with a resource type, such as List or Class).</p>

<ul>
	<li>org.slf4j.Logger The Logger of the service being constructed (or the logger of
the Module class being instantiated).</li>
	<li><span class="error">&#91;ObjectLocator|../apidocs/org/apache/tapestry5/ioc/ObjectLocator.html&#93;</span>
For contribute methods, used to locate additional objects.</li>
	<li><span class="error">&#91;ServiceResources|../apidocs/org/apache/tapestry5/ioc/ServiceResources.html&#93;</span>
For service builder methods, an extended version of ObjectLocator. Class The service interface
type.</li>
	<li><span class="error">&#91;OperationTracker|../apidocs/org/apache/tapestry5/ioc/OperationTracker.html&#93;</span>
Used to track deeply nested operations so that errors can be reported sensibly.</li>
	<li>Object, or service interface type Passed to decorator methods.</li>
	<li>Collection, List, Map Assembled service configurations passed to service builder
methods (or service class constructors).</li>
	<li>Configuration, OrderedConfiguration, MappedConfiguration Configuration passed to
contribute methods, to build service configurations.If field type does not match any of the
available resource types, or the Inject annotation is present, logic continues to the next
step.</li>
</ul>


<p>Injection of resources into fields is triggered by the presence of the <span class="error">&#91;InjectResource|../apidocs/org/apache/tapestry5/ioc/annotations/InjectResource.html&#93;</span>
annotation, whereas injection of resources into parameters occurs when the Inject or InjectService
annotation is <em>not</em> present. These rules are slightly tricky, which reflects
a desire to avoid any annotations except when needed, and the fact that field injection came
much later than parameter injection.</p>

<h2><a name="IoC-injection-ServiceLookupbyTypeandAnnotations"></a>Service
Lookup by Type and Annotations</h2>

<p>Tapestry attempts to find a matching <em>service</em>.</p>

<p>First, it generates a set of services whose service interface is compatible with
the injection type. This is based on assignability.</p>

<p>If the <span class="error">&#91;Local|../apidocs/org/apache/tapestry5/ioc/annotations/Local.html&#93;</span>
annotation is present, then services not from the module containing the service being constructed
will be eliminated.</p>

<p>Tapestry then works through the known marker annotations. For each marker annotation
that is present at the point of annotation, Tapestry eliminates services which do not have
the marker. Thus, if multiple marker annotations are present, the final service must have
<em>all of them</em>.</p>

<p>At the end, of this, Tapestry determines how many services match.</p>

<ul>
	<li>If there is a single matching service, then the value to inject as been identified.</li>
	<li>If there are no matches, and there were no marker annotations at the point of injection,
then the Tapestry continues to the next step.</li>
	<li>Otherwise there were either no matches, or too many matches: Tapestry will throw
a RuntimeException.
<h2><a name="IoC-injection-MasterObjectProviderLookup"></a>MasterObjectProvider
Lookup</h2></li>
</ul>


<p>This is the point at which Tapestry's extensibility comes into play. MasterObjectProvider
is a service, with a configuration of <span class="error">&#91;ObjectProvider|../apidocs/org/apache/tapestry5/ioc/ObjectProvider.html&#93;</span>s.</p>

<p>The MasterObjectProvider is also the point at which Tapestry's IoC layer injection,
and Tapestry's component injection, unite.</p>

<p>As a chain-of-command, each of the following ObjectProviders will be considered and
will attempt to identify the object to be injected.</p>

<p><b>Note:</b> a common problem when extending injection is that contributions
into the MasterObjectProvider configuration have to be handled carefully. Any dependencies
of the contributed objects should be resolvable using the early stages of the injection process,
otherwise MasterObjectProvider will have to be instantiated in order to handle its own injection:
Tapestry will detect this impossibility and throw an exception. In addition, the <a href="#IoC-injection-coerce.html">TypeCoercer</a>
service is used by several ObjectProvider implementations, so the same restrictions apply
to TypeCoercer service contributions.</p>

<h3><a name="IoC-injection-ValueObjectProvider"></a>Value ObjectProvider</h3>

<p>Checks for the presence of the <span class="error">&#91;Value|../apidocs/org/apache/tapestry5/ioc/annotations/Value.html&#93;</span>
annotation. If present, then the annotations value is evaluated (to expand any symbol references),
and the TypeCoercer coverts the resulting String to the injection type.</p>

<h3><a name="IoC-injection-SymbolObjectProvider"></a>Symbol ObjectProvider</h3>

<p>Similar to the Value ObjectProvider: the <span class="error">&#91;Symbol|../apidocs/org/apache/tapestry5/ioc/annotations/Symbol.html&#93;</span>
annotation's value (if present) is lookup up and converted to the injection type.</p>

<h3><a name="IoC-injection-AutobuildObjectProvider"></a>Autobuild ObjectProvider</h3>

<p>Checks to see if the <span class="error">&#91;Autobuild|../apidocs/org/apache/tapestry5/ioc/annotation/Autobuild.html&#93;</span>
annotation is present and, if so, autobuilds the value for the parameter. Of course, the object
being built will itself be configured via injection.</p>

<h3><a name="IoC-injection-ServiceOverrideObjectProvider"></a>ServiceOverride
ObjectProvider</h3>

<p>Checks any contributions to the <span class="error">&#91;ServiceOverride|../apidocs/org/apache/tapestry5/ioc/services/ServiceOverride.html&#93;</span>
service. Contributions map a type to an object of that type. Thus, ServiceOverrides will override
injections of services that are not qualified with a marker annotation.</p>

<h3><a name="IoC-injection-AliasObjectProvider%28tapestrycore%29"></a>Alias
ObjectProvider (tapestry-core)</h3>

<p>Uses the <span class="error">&#91;Alias|../apidocs/org/apache/tapestry5/services/Alias.html&#93;</span>
service to look for an object that can be injected.</p>

<p>This is commonly used to override a built-in service by contributing an object with
the exact same interface. This is an older and more complex version of the ServiceOverride
provider.</p>

<h3><a name="IoC-injection-AssetObjectProvider%28tapestrycore%29"></a>Asset
ObjectProvider (tapestry-core)</h3>

<p>Checks for the <span class="error">&#91;Path|../apidocs/org/apache/tapestry5/annotations/Path.html&#93;</span>
annotation.</p>

<p>If present, the annotation's value has embedded symbols expanded, and is converted
into an Asset (which must exist).</p>

<p>The TypeCoercer can then convert the Asset to the injection type.</p>

<h3><a name="IoC-injection-ServiceObjectProvider%28tapestrycore%29"></a>Service
ObjectProvider (tapestry-core)</h3>

<p>Looks for the <span class="error">&#91;Service|../apidocs/org/apache/tapestry5/annotations/Service.html&#93;</span>
annotation; if present, the annotation's value is the exact service id to inject. This is
necessary because injections into <em>component</em> fields are always triggered
by the Inject annotation.</p>

<h3><a name="IoC-injection-SpringBeanObjectProvider%28tapestryspring%29"></a>SpringBean
ObjectProvider (tapestry-spring)</h3>

<p>Attempts to resolve a Spring bean purely by object type (Spring qualifiers are not
supported). If no beans are assignable to the type, then processing continues. If exactly
one is assignable, it is used as the injection value. If more than one bean is assignable,
it is an error (and a list of matching beans names will be part of the thrown exception).</p>

<h2><a name="IoC-injection-ServiceLookup"></a>Service Lookup</h2>

<p>If none of the ObjectProviders can identify the value to inject, a last step occurs:
lookup by service type. If exactly <em>one</em> service matches the injection
type, then that service is injected.</p>

<p>Otherwise, the lookup fails because either no services match, or more than one matches.</p>

<h1><a name="IoC-injection-PostInjectionMethods"></a>Post-Injection Methods</h1>

<p>Autobuilt objects (services and the like, but <em>not</em> components)
may have post-injection methods.</p>

<p>Any public method may have the <span class="error">&#91;PostInjection|../apidocs/org/apache/tapestry5/annotations/PostInjection.html&#93;</span>
annotation.</p>

<p>Such methods are invoked after constructor and/or field injection. Only <b>public
methods</b> will be invoked. Any return value is ignored.</p>

<p>Further injections may take place using the parameters of the method.</p>

<p>Often this is used to perform additional setup, such as registerring a service as
a listener of events produced by another service:</p>

<div class="preformatted panel" style="border-width: 1px;"><div class="preformattedContent
panelContent">
<pre>public class MyServiceImpl implements MyService, UpdateListener
{
  @PostInjection
  public void registerAsListener(UpdateListenerHub hub)
  {
    hub.addUpdateListener(this);
  }
}</pre>
</div></div>

<h1><a name="IoC-injection-ComponentInjection"></a>Component Injection</h1>

<p>Inside Tapestry components, injection occurs exclusively on <em>fields</em>
and is always triggered by the Inject annotation.</p>

<p>Component field injection is very similar to IoC layer, but with a different set
of injectable resources.</p>

<p>Injection is the responsibility of the <span class="error">&#91;InjectionProvider|../apidocs/org/apache/tapestry5/services/InjectionProvider.html&#93;</span>
service, which is a chain-of-command across a number of implementations.</p>

<h2><a name="IoC-injection-BlockInjectionProvider"></a>Block InjectionProvider</h2>

<p>Checks if the field type is Block. If so, determines the block id to inject (either
from the field name, or from an <span class="error">&#91;Id|../apidocs/org/apache/tapestry5/annotations/Id.html&#93;</span>,
if present).</p>

<h2><a name="IoC-injection-DefaultInjectionProvider"></a>Default InjectionProvider</h2>

<p>Uses the MasterObjectProvider service to provide the injectable value. The Service
Lookup stage is skipped.</p>

<h2><a name="IoC-injection-ComponentResourcesInjectionProvider"></a>ComponentResources
InjectionProvider</h2>

<p>Injects fields of type ComponentResources.</p>

<h2><a name="IoC-injection-CommonResourcesInjectionProvider"></a>CommonResources
InjectionProvider</h2>

<p>Injects fields with common resources:</p>

<ul>
	<li>String: the components' complete id</li>
	<li>org.slf4j.Logger: Logger for the component (based on component class name)</li>
	<li>Locale: locale for the containing page (page locale is immutable)</li>
	<li>Messages: Component's message catalog
<h2><a name="IoC-injection-AssetInjectionProvider"></a>Asset InjectionProvider</h2></li>
</ul>


<p>Triggered by the Path annotation: the Path value has symbols expanded, and is then
converted to an Asset.</p>

<h2><a name="IoC-injection-ServiceInjectionProvider"></a>Service InjectionProvider</h2>

<p>Equivalent to the Service Lookup phase in an IoC layer injection.</p>

<h1><a name="IoC-injection-@InjectServiceinComponents"></a>@InjectService
in Components</h1>

<p>This is now supported; you may use the <span class="error">&#91;InjectService|../apidocs/org/apache/tapestry5/ioc/annotations/InjectService.html&#93;</span>
annotation on component fields.</p>

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