Sling Model Exporter: What is exported into the JSON?

Last week we came across a strange phenomenon, when in the AEM release validation process the process broke in an unexpected situation. Which is indeed a good thing, because it covered an aspect I have never thought of.

The validation broke because during a request the serialization of a Sling Model failed with an exception. The short version: It tried to serialize a ResourceResolver(!) into JSON (more details in SLING-11924). Why would anyone serialize a ResourceResolver into a JSON to be consumed by an SPA? I clearly believe that this was not done intentionally, but happened by accident. But nevertheless, it broke the improvement we intended to make, so we had to rollback it and wait for SLING-11924 being implemented.

But it gives me the opportunity to explain, which fields of a Sling Model are exported by the SlingModelExporter. As it is backed by the Jackson data-bind framework, the same rules apply:

• All public fields are serialized
• all public available getter methods, which do not expect a parameter are serialized.

It is not too hard to check this, but there are a few subtle aspect to consider in the context of Sling Models.

• Injections: make sure that you make only these injections as public, which you want to be dealt with by the SlingModelExporter. Make everything else private.
• I see often Lombok used to create getters for SlingModels (because you need them for the use in HTL). This is especially problematic, when the annotation @Getter is done on a class-level, because now for every field (not matter the visibility) a getter is created, which is then picked up by the SlingModelExporter.

My call to action: Validate your SlingModels and check them that you don’t export a ResourceResolver by accident. (If you are a AEM as a Cloud Service customer and affected by this problem, you will probably get an email from us, telling you to do exactly that.)