Pitfalls with “Convention over Configuration”

The paradigm Convention over Configuration is a great way to develop software (web) application. I like it…IFF it is implemented well.


Unfortunately, many frameworks which rely on this paradigm are hard to work with. One example is Spring Boot. Users who are new to Spring Boot must read many pages of documentation to learn the “sensible defaults”. These defaults are often not intutive to new users because the users do not yet know the underlying concepts for which the defaults are ment to be. Hence, the first steps with Spring Boot take time – more time than necessary.

Moreover, the defaults seem to work in a magical way – just by putting some new annotations at a class or a method. The javadoc often does not explain how the annotations work. It’s so easy to add a short text that describes that Spring searches the classpath for annotations.

Furthermore, annotations can also cause incompatibility issues with other annotations and conventions. For example, the annotation @DataJpaTest disables full auto-configuration such that you wonder why auto-wiring is not working anymore. Although the javadoc mentions this issue, it is unnecessarily complicated. I wish that adding annotations only adds behavior.

Finally, you can often do one thing in more than one way. Usually, basic annotations are provided which are used by aggregated annotations to cover recurring use cases. This approach often results in annotated classes which have several basic annotations twice or thrice. Although this approach does not influence the functionality, it is really confusing for (new) users who need to write some piece of code: when should I use which (aggregated) annotations? Users who read the code could ask themselves: why did the author of this code add these annotations more than once?

Proposed Solution

  1. Uncover all conventions, e.g., by a configuration file with default values, such that the conventions are visible for (new) users. In this way, the user can read through the config file and learn what features and conventions are provided by the framework. We follow this approach with our framework Kieker.
  2. Each annotation should explain how it is processed. If you have a bunch of similar annotations, either copy and paste the same text or add a javadoc link to the processing engine where the approach is described once at a central place.
  3. Let annotations only add new behavior. Let them not disable other features.
  4. Only provide one way to do something to reduce confusion and time to read/understand. When the (new) user has several possibilities to do the same thing, it is laborious to understand when to use which approach. If there is no other choice, provide at least recommendations when to use which approach.

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