Writing unittests for AEM (part 3): Mocking resources

I introduced SlingMocks in the recent blog posts (part 1, part 2), and till now we only covered some basics. Let’s dig deeper now and use one of its coolest features: Mocking resources.

Well, to be honest, we don’t mock resources. We do something much better: We build an in-memory structure which represent sling resources. We can even map that to an in-memory JCR-repository!

For this blog post I created a very simple Sling model, which supposed to build a classic navigation. For simplicity I omitted nearly all of the logic which makes up a production-ready navigation component, and want to add just the pages below the site root to it.

The relevant files in my github repo:

The interesting part is the unit test. This time I used the AemMocks library from wcm.io, because it provides all the magic of the SlingMocks, plus some AEM specific objects I would like to use. As the AemContext class inherits directly from the SlingContext class, we can just use it as a drop-in replacement.

Loading the test data from a JSON flle in the test resources

The setup method is very simple: It loads a JSON structure and populates a in-memory resource tree with it (adds everything below a resource “/content”). Using this approach there’s no longer the need to mock resources, properties/value maps, and mock the relation between these. This is all done by the SlingMocks/AemMocks framework.

Thus a testcase looks like this:

No boilerplate code anymore

This simple example shows how easy it can be to write unit tests. The test code is very concise and easy to read and understand. In my opinion the biggest issue when writing unit tests now is creating the test content 🙂

Some notes to the test content:

  • My personal style is to put the test content into the same structure than the java packages. This makes it very easy to find the testcontent and keeps the testcontent and the unittest together.
  • You can write the JSON manually. But you can also use CRXDE Lite to create some structure in your local AEM instance and then export it with “http://localhost:4502/content/myproject/testcontent.tidy.-1.json ” and paste it into the JSON file. I find this way much more convenient, especially if you already have most of the test content ready. But if you do that, please clean up the unnecessary properties in the test data. They are polluting the test content and make it harder to understand.

Writing unit tests for AEM (part 2): Maven Setup

In the previous post I showed you how easy it is to start using SlingContext. If you start to use this approach in your own project and just copy the code, your Maven build is likely to fail with messages like this:

[ERROR] testActivate_unconfiguredParams(de.joerghoh.cqdump.samples.unittest.ReplicationServletTest)  Time elapsed: 0.01 s  <<< ERROR!
org.apache.sling.testing.mock.osgi.NoScrMetadataException: No OSGi SCR metadata found for class de.joerghoh.cqdump.samples.unittest.ReplicationServlet at org.apache.sling.testing.mock.osgi.OsgiServiceUtil.injectServices(OsgiServiceUtil.java:381)

The problem here is not the use of the SlingMock library itself, but rather the fact that we use SlingMocks to test code which uses OSGI annotations. The fix itself is quite straight-forward: We need to create OSGI metadata also for the unittests and not only for the bundling (SlingMock is reading these metadata for the test execution).

That’s the reason why there’s the need to have a dedicated execution definition for the maven-bundle-plugin:


Also you need to instruct the maven-bundle-plugin to actually export the generated metadata to the filesystem using the “<exportScr> statements; this should be the default in my opinion, but you need to specify it explicitly. Also don’t forget to add the “_dsannotation” and “_metatypeannotations” statements to its instruction section:


And even then it will fail, if you don’t upgrade the maven-bundle-plugin to a version later than 4.0:


Ok, if you adapted your POMs in this way, your SlingMock based unittests for OSGI r6-based services should run fine. Now you can start exploring more features of SlingMocks.

Writing unit tests for AEM — using SlingMocks

Over the course of the last years the tooling for AEM development improved a lot. While some years ago there was hardly an IDE integration available, today we have dedicated tools for Eclipse and IntelliJ (https://helpx.adobe.com/experience-manager/6-4/sites/developing/using/aem-eclipse.html). Also packaging and validation support was poor, today we have the opensourced filevault and tools like oakpal (thanks Marc!)

But with SlingMocks we also have much better unittest tooling (thanks a lot to Stefan Seifert and the Sling people), so we much more and better/easier tooling than just Mockito and Powermock. SlingContext (and its extension AemContext) allows you create unit tests quite easily. Using them can help you get rid of mocking Sling Resources, repo access and many things more.

On top of that, SlingMock can easily work with the new OSGI r6 annotations, which allow you to define OSGI properties in Pojos. Mocking the @ObjectClassDefinition classes isn’t that easy, because they are essentially annotations …

To illustrate that, I have created a minimal demo with a servlet and testcases for it (source code at Github). Basically the functionality of the class itself is not relevant for this article, but we want to focus on aspects how you utilize the frameworks best to avoid boilerplate code.

The code is quite simple, but to make unittesting a bit more challenging, it uses the new OSGI r6 annotations for OSGI configuration (using the @Designate and the @ObjectClassDefinition annotations) plus a referenced service.


If you try to mock the ReplicationServlet.Config class the naive way, you will find out, that it’s an annotation, which is referenced in the activate() method. I always failed to mock it somehow, so I switched gear and started to use SlingMock for it. (I don’t want to say that it is not possible, but it’s definitly not straight-foward, and in my opinion writing unit-tests should be straight forward, otherwise they are not created at all.)

With SlingMocks the approach changes. I am not required to create mocks, but SlingMocks provides a mocked OSGI runtime we can use. That means, that we create the OSGI parameters as a map to tell the SlingContext object to register our service with these parameters (line 59).


Because SlingContext implements quite a bit of the OSGI semantics, it also requires that all referenced services are available (if these are static references). Therefor I use Mockito to mock the Replicator and I register the mock to provide the Replicator service. In realworld I could verify the interactions of my servlet with that mock.

This basic example illustrates how you can use SlingMocks to avoid a lot of mocking and stubbing. This example does not utilize the full power of SlingMocks yet, we are just scratching at the surface. But we already have some benefit : If you switch from SCR annotations to OSGI annotations, your SlingMock unittests don’t need any change, because it provides an OSGI-like environment, and there the way how metatypes are generated and injected are abstracted away.

How does Sling resolve an AEM Page to the correct resource type?

On the AEM forum there’s an interesting question:

Does this mean that all requests to the page node are internally redirected to _jcr_content for HTML and XML calls but not for other extensions ?

Without answering the question right here, it raises the question: How is a cq:Page node actually resolved? Or how does Sling know, that it should resolve a call to a cq:Page node to the resourcetype specified in its jcr:content node? Especially since the “cq:Page” does not have a “sling:resourceType” property at all?

A minimal page can look like this (JSON dump):

  "jcr:primaryType": "cq:Page",
  "jcr:createdBy": "admin",
  "jcr:created": "Mon Dec 03 2018 19:09:44 GMT+0100",
  "jcr:content": {
    "jcr:primaryType": "cq:PageContent",
    "jcr:createdBy": "admin",
    "cq:redirectTarget": "/content/we-retail/us/en",
    "jcr:created": "Mon Dec 03 2018 19:09:44 GMT+0100",
    "cq:lastModified": "Tue Feb 09 2016 00:05:48 GMT+0100",
    "sling:resourceType": "weretail/components/structure/page",
    "cq:allowedTemplates": ["/conf/we-retail/settings/wcm/templates/.*"]

A good tool to understand how a page is rendered is the “Recent Requests” tool available in the OSGI webconsole (/system/console/requests).

When I request /content/we-retail.html and check this request in the recent requests tool, I get this result (reduced to the relevant lines):

    1967 TIMER_END{65,ResourceResolution} URI=/content/we-retail.html resolves to Resource=JcrNodeResource, type=cq:Page, superType=null, path=/content/we-retail
1974 LOG Resource Path Info: SlingRequestPathInfo: path='/content/we-retail', selectorString='null', extension='html', suffix='null'
   1974 TIMER_START{ServletResolution}
   1976 TIMER_START{resolveServlet(/content/we-retail)}
   1990 TIMER_END{13,resolveServlet(/content/we-retail)} Using servlet /libs/cq/Page/Page.jsp
   1991 TIMER_END{17,ServletResolution} URI=/content/we-retail.html handled by Servlet=/libs/cq/Page/Page.jsp

The interesting part is here, that /content/we-retail resource has a type (= resource type) of “cq:Page”, and when we look down in the snippet, we see that it is resolved to /libs/cq/Page/Page.jsp; that means the resource type is actually “cq/Page”.

And yes, in case no resource type property is available, Sling’s first fallback strategy is to try the JCR nodetype as a resource type. And in case this fails as well, it goes back to the default servlets.

OK, now we have a point to investigate. The /libs/cq/Page/Page.jsp is very simple:

file="proxy.jsp" %>

And in the proxy.jsp there’s a call to the RequestDispatcher to include the jcr:content resource; and from there one the standard resolution starts which we all are used to.

And to get back to the forum question: If you look around in /libs/cq/Page, you can find more scripts to deal with extensions and also some selectors. And they all include only the call to the proxy.jsp. If your selector & extension combination do not work as expected, you might want to add an overlay there.

OSGi DS & Metatype & SCR properties

When I wrote the last blog post on migrating to OSGi annotations, I already mentioned that the property annotations you used to use with SCR annotations cannot be migrated 1:1, but instead you have to decide if you add it the OCD or if you add them as properties to the @Component annotation.

I was remembered of that when I worked on the fixing the process label properties for the workflow steps contained in ACS AEM Commons (PR 1645). And the more I think about it, the more I get the impression that this might be causing some confusion in the adoption of the OSGI annotations.

Excursion to the OSGi specification

First of all, in OSGi there 2 specifications, which are important in this case: Declarative Services (DS, chapter 112 in the OSGI r6 enterprise specification, sometimes also referenced as Service Component Model) and the Metatype Services (chapter 105 in the OSGI r6 enterprise specification). See the OSGI website for download (unfortunately there is no HTML version available for r6, only PDFs).

Declarative Services deals with the services and components, their relations and the required things around it. Quoting from the spec (112.1):

The service component model uses a declarative model for publishing, finding and binding to OSGi services. This model simplifies the task of authoring OSGi services by performing the work of registering the service and handling service dependencies. This minimizes the amount of code a programmer has to write; it also allows service components to be loaded only when they are needed.

Metatype Services care about configuration of services. Quoting from chapter 105.1

The Metatype specification defines interfaces that allow bundle developers to describe attribute types in a computer readable form using so-called metadata. The purpose of this specification is to allow services to specify the type information of data that they can use as arguments. The data is based on attributes, which are key/value pairs like properties.

Ok, how does this relate to the @Property annotation in the Felix SCR annotations? I would say, that this annotation cannot be clearly attributed to either DS or Metatype, but it served both.

You could add @Properties as annotation to class, or you could add it as with an @Property to a field. You could add a property “metatype=true” to the annotation, and then it appeared in the OSGI console (then it was a “real” metatype property in the sense of the Metatype specification).

But in either way, all the properties were provided through the ServiceContext.getProperties() method; that means, in reality it never really made a difference how you defined the property, if a the class level or to properties, if you added the metatype=true property or not. That was nice and in most times also very convenient.

This changed with the OSGI annotations. Because now the properties are defubed a class annotated with the @ObjectClassDefinition annotation. Type-safe and named. But here it’s clearly a Metatype thing (it’s configuration), and it cannot be used with the Declarative Service (the services and components thing) in parallel. Now you have to make the decision: Is it a configuration item (something I use in the code) or is it a property which influences the component itself?

As an example, with SCR annotations you could write

@Component @Service
public class HeaderServlet implements SlingSafeMethodsServet { ...

Now, as these properties were visible via metatype as well, you could also overwrite them using some OSGI configuration and register the servlet on a different selector just by configuration. Or you could read these properties from the ServiceContext. That was not a problem (and hopefully noone ever really used it …).

With OSGI annotations this is no longer possible. You have configuration and component properties. You can change the configuration, but not the component properties any more.

What does that mean for you?

Mostly, don’t change all properties blindly to properties of the @ObjectClassDefinition object. For example the label of a workflow step is not configuration, but rather a property. That means there you should use something like this:

@Component(properties= {
"process.label=My process label"
public class MyWorkflowProcess implements WorkflowProcess {...

Disclaimer: I am not an OSGI expert, this is just my impression from dealing with that stuff a lot. Carsten, David, feel free to to correct me 🙂

From SCR annotations to OSGI annotations

Since the beginning of AEM development we used annotations to declare OSGI services; @Component, @Service, @Property and @Reference should be known to everyone how has ever developed backend stuff for AEM. The implementation behind these annotations came from the Apache Felix project, and they were called the SCR annotations (SCR = Service Component Runtime). But unlike the Service Component Runtime, which is part of the OSGI standard for quite some, these annotations were not standardized. This changed with OSGI Release 6.

With this release annotations were also standardized, but they are 100% compatible to the SCR annotations. And there are a lot of resources out there, which can help to explain the differences:

I recently worked on migrating a lot of the code from ACS AEM Commons from SCR annotations to OSGI annotations, and I want to share some learning I gained on the way. Because in some subtle areas the conversion isn’t that easy.

Mixed use of SCR annotations and OSGI annotations

You can mix SCR annotations and OSGI annotations in a project, you don’t need to migrate them all at once. But you can to be consistent on a class level, you cannot mix SCR and OSGI annotations in a single class. This is achieved by an extension to the maven-bundle-plugin (see below).

Migrating properties

SCR property annotations give you a lot of freedom. You can annotate them on top of the class (using the @Properties annotation as container with nested @Property annotations), you can annotate individual constant values to be properties. You can make them visible in the OSGI webconsole (technically you are creating a metatype for them), or you can mark them as private (not metatype is created).

With OSGI annotations this is different.

  • Metatype properties are handled in the dedicated configuration class marked with @ObjectClassDefinition. They cannot be private.
  • Properties which are considered to be private are attached to the @Component annotation. They cannot be changed anymore.

A limitation from a backward compatibility point of view: With SCR annotations you are not limited in the naming of properties, next to characters often the “.” (dot) and the “-” (dash, minus) was used. With OSGI r6 annotations you can easily create a property with a “.” in it

String before_after() default "something";

will result in the property with the name “before.after”; but with OSGI r6 annotations you cannot create properties with a “-” in it. Only OSGI r7 (which is supported in AEM 6.4 onwards) supports it with a construct like this:

String before$_$after() default "something";

If you want to keep compatibility with AEM 6.3, expect the breakage of property names or you need to investigate in workarounds (see #1631 of ACS AEM Commons). But my recommendation is to avoid the use of the “-” in property names alltogether and harmonize this in your project.

Update: I posted an additional blog post specifically on migrating SCR properties, mostly in the context of OSGI DS and OSGI Metatypes.

Labels & description

All the metatype stuff (that means, how OSGI configurations appear in the /system/console/configMgr view) is handled on the level of the @ObjectClassDefinition annotation and the method annotated with it. With the SCR annotations this was all mixed up between the @Component annotation and the @Property fields.

Update the tooling to make it work

If you want to work with OSGI annotations, you should update some elements in your POM as well:

  • Update the maven-bundle-plugin to 4.1.0
  • Remove the dependency to the maven-scr-plugin
  • Add a dependency to org.osgi:org.osgi.annotations:6.0.0 to your POM.
  • Then you need to add an additional execution to your maven-bundle-plugin (it’s called “generate-scr-metadata-for-unittests“) and update its configuration (see it on ACS AEM Commons POM).

The interesting part is here is the plugin to the maven-bundle-plugin, which can also handle SCR annotations; this statement allows you to mix both types of annotations.

This blog post should have given you some hints how you migrate the SCR annotations of an existing codebase to OSGI annotations. It’s definitly not a hard task, but some details can be tricky. Therefor it’s cool if you have the chance to mix both types of annotations, so you don’t need a big-bang migration for this.

Do I need a dedicated instance for page preview?

Every now and then there is this question about how to integrate a dedicated preview instance into the typical “author – publish” setup. Some seem to be confused why there is no such instance in the default setups, which allows you to preview content exactly as ob publish, but just not visible yet to the public.

The simple answer to this is: There should be no need to have such a preview instance.

When creating content in AEM, you work in an full WYSIWYG environment, which means that you always should have perfect view of the context your content lives in.Everything should be usable, and even more complex UI interfaces (like single page applications) should allow you to have a proper preview. Even most integrations should work flawlessly. So getting the full picture should alwaysbe possible on authoring itself, and this must not be the reason to introduce a preview publish.

 Another reason often brought up in these discussions are approvals. When authors finish their work, they need to get an approval by someone who is not familiar with AEM. The typical workflow is then outlined like “I drop herthe link, she clicks the link, checks the page and then responds with an OK or not. And then I either implement her remarks or activate the page directly”.

 The problem here is that this is an informal workflow, which happens on a differnet medium(chat, phone, email) and which is not tracked within AEM. You don’t use the ways which are offered by the product (approval workflows), which leaves you without any audit trail. One could ask the question if you have a valid approval process at all then…

Then there’s the aspect of “Our approvers are not familar and not trained with AEM!”.Well, you don’t have to train them much of AEM. If you have SSO configured and the approvers get email notifications, approving itself is very easy: Click to the link on the inbox, select the item you want to preview, open it, review it and then click approve or reject in the inbox for it. You can definitely explain that workflow in a 5 minute video.

Is there no reason to justify a dedicated preview instance? I won’t argue that there will be never the need for such a preview instance, but in most cases you don’t need it. I am not aware of any right now.

If you think you need a preview instance: Please create a post over at the AEM forum, describe your scenario, ping me and I will try to show you that you can do it easier without it 🙂