A few days ago I found that interesting blog post at https://sourcedcode.com/blog/aem/aem-sling-model-field-injection-vs-constructor-injection-memory-consumption, which makes the claim that Constructor injection with Sling Models is much more memory efficient than the “standard” field-based injection. The claim is, that the constructor injection-approach “saves 1800% in bytes” (152 bytes vs 8 bytes in the example).
Well, that result is not correct, because the example implementations of the SlingModels used there are not identical. Because in the case of field-based injection the references are available during the complete lifetime of that SlingModel, not just during the @PostConstruct method call, thus these references consume memory.
While with the example of constructor-based injection, the references are just available during the constructor call; they are not available in any other method. If you want to achieve the same behavior as in the field-injection example, you have to store the references in the global fields and then the memory consumption of that SlingModel increases.
But Justin Edelson pointed out correctly, that you gain from constructor-based injection, if you need the references just in the constructor to compute some results (which are then stored in fields), and in no other method. That’s indeed a small optimization.
But let’s be honest: If we are talking about an additional memory overhead of 100 bytes per a complex SlingModel, that’s a negligible number. Because it’s not typical that hundreds of these models are created per second. And even in that case, when they are created to render a page, the models are garbage collected immediately after when the request is completed. It doesn’t matter if 100 bytes more or less are allocated and collected. Thus the overhead is normally not even measurable.
But well, you might hit the edge case, where this really makes a difference.
Update June 8th: I got informed that the referenced blog article has been updated. It now contains a more reasonable example which makes the sling models comparable. Basically it reflects now the optimization Justin already mentioned. And the difference in object size is now only 40 bytes vs 24 bytes.