Why I would deprecate InjectionStrategy.OPTIONAL for Sling Models

Sling Models offer a very convenient abstraction, as they allow data from the repository being mapped into fields of Java POJO classes. One feature I find often used is the optional InjectionStrategy. By default if an injection is not working, the instantiation of the POJO fails. When the InjectionStrategy.OPTIONAL field is set in the model annotation (see the Sling docs), such a non-working injection will not fail the creation of the model, but instead the field is left with the default value of the respective type. Which is null for Strings and other complex types. And this setting is valid for the entire class, so when you want to write reliable code, you would have to assume that every injected String property could be null.

This comes with a few challenges, because now you can’t rely anymore on values being non-null, but you would need to test each field if a proper value has been provided. Which is sometimes done, but in the majority of cases it is just assumed, that the field is non-null.

I wonder, why this is done at all. Because normally you write your components in a way that the necessary properties are always available. And if you operate with defaults, you can guarantee with several ways that they are available as soon as the component is being created and authored for the very first time. And while for a few cases a missing property must be dealt with for whatever reason, it is never justified to treat all property injections as optional. Because that would mean, that this sling model is supposed to make sense of almost any resource it is adapted from. And that won’t work.

And if a property is really optional: we added some time back the feature to use something like this (if you really can’t give a default value, which would be a much better choice):

@ValueMapValue
Optional<String> textToDisplay;

With this you can express the optionality of this value with the Java type system, and in that case it’s quite unlikely to miss the validation.

But if it would be just be up to me, I would deprecate InjectionStrategy.OPTIONAL and ban it, because it’s one of the most frequent reasons for NullPointer exceptions in AEM.

I know that using InjectionStrategy.OPTIONAL saves you from asking yourself “is this property always present?”, but that’s a very poor excuse. Because with just a few more seconds of work you can make your Sling Model more robust by just providing default values for every injected field. So please:

• Avoid using optional injections when possible!
• When it’s required use the Optional type to express it!
• Don’t use InjectionStrategy.OPTIONAL!

Using “optional” (in all cases) can also come with a performance impact when used with the generic @Inject annotation; for that read my earlier blog posts on the performance of Sling Models: Sling Model Performance.