How to get "Delegation Pattern for Sling Models" working? - aem

Question

I'm trying to create a proxy component for AEM Core Component with custom Sling Model which delegates part of functionality back to the Core Component's Sling Model.
I'm following the example from https://github.com/adobe/aem-core-wcm-components/wiki/Delegation-Pattern-for-Sling-Models but it results in java.lang.IllegalArgumentException: Can not set com.example.core.models.Title field.
Setup
AEM version 6.4.4
core.wcm.components.content version 2.4.0
Sources
/apps/myproject/components/pageHeadline/.content.xml
<?xml version="1.0" encoding="UTF-8"?>
<jcr:root xmlns:sling="http://sling.apache.org/jcr/sling/1.0" xmlns:cq="http://www.day.com/jcr/cq/1.0" xmlns:jcr="http://www.jcp.org/jcr/1.0"
jcr:primaryType="cq:Component"
jcr:title="Page Headline"
jcr:description="Display Page Heading"
sling:resourceSuperType="core/wcm/components/title/v2/title"
componentGroup="My Project"/>
com.example.core.models.PageHeadline
package com.example.core.models;
import com.adobe.cq.wcm.core.components.models.Title;
import com.day.cq.wcm.api.Page;
import org.apache.sling.api.SlingHttpServletRequest;
import org.apache.sling.models.annotations.Model;
import org.apache.sling.models.annotations.Via;
import org.apache.sling.models.annotations.injectorspecific.ScriptVariable;
import org.apache.sling.models.annotations.injectorspecific.Self;
import org.apache.sling.models.annotations.via.ResourceSuperType;
#Model(adaptables = SlingHttpServletRequest.class, adapters = Title.class, resourceType = "myproject/components/pageHeadline")
public class PageHeadline implements Title {
#ScriptVariable
private Page currentPage;
#Self #Via(type = ResourceSuperType.class)
private Title delegate;
#Override
public String getText() {
return currentPage.getTitle();
}
#Override
public String getType() {
return delegate.getType();
}
}
Result
org.apache.sling.api.SlingException: Cannot get DefaultSlingScript: Identifier com.example.core.models.Title cannot be correctly instantiated by the Use API
...
Caused by: org.apache.sling.scripting.sightly.SightlyException: Identifier com.example.core.models.Title cannot be correctly instantiated by the Use API
at org.apache.sling.scripting.sightly.impl.engine.extension.use.UseRuntimeExtension.call(UseRuntimeExtension.java:78) [org.apache.sling.scripting.sightly:1.0.54.1_4_0]
at org.apache.sling.scripting.sightly.impl.engine.runtime.RenderContextImpl.call(RenderContextImpl.java:69) [org.apache.sling.scripting.sightly:1.0.54.1_4_0]
at org.apache.sling.scripting.sightly.apps.core.wcm.components.title.v2.title.title_html.render(title_html.java:41)
at org.apache.sling.scripting.sightly.java.compiler.RenderUnit.render(RenderUnit.java:48) [org.apache.sling.scripting.sightly.compiler.java:1.0.26.1_4_0]
at org.apache.sling.scripting.sightly.impl.engine.SightlyCompiledScript.eval(SightlyCompiledScript.java:61) [org.apache.sling.scripting.sightly:1.0.54.1_4_0]
at org.apache.sling.scripting.core.impl.DefaultSlingScript.call(DefaultSlingScript.java:386) [org.apache.sling.scripting.core:2.0.54]
at org.apache.sling.scripting.core.impl.DefaultSlingScript.eval(DefaultSlingScript.java:184) [org.apache.sling.scripting.core:2.0.54]
at org.apache.sling.scripting.core.impl.DefaultSlingScript.service(DefaultSlingScript.java:491) [org.apache.sling.scripting.core:2.0.54]
... 269 common frames omitted
Caused by: org.apache.sling.models.factory.MissingElementsException: Could not inject all required fields into class com.example.core.models.PageHeadline
at org.apache.sling.models.impl.ModelAdapterFactory.createObject(ModelAdapterFactory.java:765) [org.apache.sling.models.impl:1.4.10]
at org.apache.sling.models.impl.ModelAdapterFactory.internalCreateModel(ModelAdapterFactory.java:448) [org.apache.sling.models.impl:1.4.10]
at org.apache.sling.models.impl.ModelAdapterFactory.createModel(ModelAdapterFactory.java:314) [org.apache.sling.models.impl:1.4.10]
at org.apache.sling.scripting.sightly.models.impl.SlingModelsUseProvider.provide(SlingModelsUseProvider.java:126) [org.apache.sling.scripting.sightly.models.provider:1.0.6]
at org.apache.sling.scripting.sightly.impl.engine.extension.use.UseRuntimeExtension.call(UseRuntimeExtension.java:73) [org.apache.sling.scripting.sightly:1.0.54.1_4_0]
... 276 common frames omitted
Suppressed: org.apache.sling.models.factory.MissingElementException: Could not inject private com.example.core.models.Title com.example.core.models.PageHeadline.title
at org.apache.sling.models.impl.ModelAdapterFactory.createObject(ModelAdapterFactory.java:749) [org.apache.sling.models.impl:1.4.10]
... 280 common frames omitted
Caused by: org.apache.sling.models.factory.ModelClassException: Could not inject field due to reflection issues
at org.apache.sling.models.impl.model.InjectableField.set(InjectableField.java:48) [org.apache.sling.models.impl:1.4.10]
at org.apache.sling.models.impl.ModelAdapterFactory.setField(ModelAdapterFactory.java:989) [org.apache.sling.models.impl:1.4.10]
at org.apache.sling.models.impl.ModelAdapterFactory.access$200(ModelAdapterFactory.java:132) [org.apache.sling.models.impl:1.4.10]
at org.apache.sling.models.impl.ModelAdapterFactory$SetFieldCallback.inject(ModelAdapterFactory.java:500) [org.apache.sling.models.impl:1.4.10]
at org.apache.sling.models.impl.ModelAdapterFactory.injectElement(ModelAdapterFactory.java:595) [org.apache.sling.models.impl:1.4.10]
at org.apache.sling.models.impl.ModelAdapterFactory.createObject(ModelAdapterFactory.java:744) [org.apache.sling.models.impl:1.4.10]
... 280 common frames omitted
Caused by: java.lang.IllegalArgumentException: Can not set com.example.core.models.Title field com.example.core.models.PageHeadline.title to com.adobe.cq.wcm.core.components.internal.models.v1.TitleImpl
at sun.reflect.UnsafeFieldAccessorImpl.throwSetIllegalArgumentException(UnsafeFieldAccessorImpl.java:167)
at sun.reflect.UnsafeFieldAccessorImpl.throwSetIllegalArgumentException(UnsafeFieldAccessorImpl.java:171)
at sun.reflect.UnsafeObjectFieldAccessorImpl.set(UnsafeObjectFieldAccessorImpl.java:81)
at java.lang.reflect.Field.set(Field.java:764)
at org.apache.sling.models.impl.model.InjectableField.set(InjectableField.java:46) [org.apache.sling.models.impl:1.4.10]
... 285 common frames omitted

Answer 1

I had the same problem, where the delegation pattern was not working, specifically, the injected attribute was null if I used #Optional, or the same stack trace as posted if #Optional was left out.
The problem for me was that we are using the following Embed-Dependency code:
<Embed-Dependency>
*;scope=compile|runtime
</Embed-Dependency>
<Embed-Transitive>true</Embed-Transitive>
in the maven-bundle-plugin. The issue was that the core components dependency got set as an embedded dependency. Once I fixed this, the delegation pattern worked fine.
<Embed-Dependency>
*;scope=compile|runtime;artifactId=!core.wcm.components.core
</Embed-Dependency>