What happens when multiple aspects are applied to the same class? This problem is called aspect composition. This is a non-trivial problem, and Metalama solves this problem by providing a consistent and deterministic model for aspect composition.
There are three significant points of interest.
- Strong ordering of aspects and advice
- Code model versioning
- Safe composition of advice
1. Strong ordering of aspects and advice
Aspects are "things" that receive a code model as input and provide outputs such as advice, diagnostics, validators, and child aspects. The only relevant output for this discussion is the advice because other outputs do not modify the code. Most aspects have a single layer of advice, but it is possible to define multiple layers.
To make the order of execution of aspects and advice consistent, Metalama uses two ordering criteria.
Aspect layer. The order of execution of aspect layers must be specified by the aspect author or user. To learn about aspect layer ordering, see Ordering aspects.
Depth Level of target declarations. Every declaration in the compilation is assigned a depth level. Within the same aspect layer, declarations are processed by order of increasing depth, i.e., base classes are visited before derived classes, types before their members, and so on.
Aspects and advice in the same layer and applied to declarations of the same depth are executed in an undetermined order and may be executed concurrently on several threads.
2. Code model versioning
Since the code model only represents declarations but does not give access to implementations such as method bodies or initializers, the only kinds of advice that affect the code model are introductions and interface implementations. Overriding an existing method does not affect the code model because it merely overrides its implementation.
For each aspect layer and depth level, Metalama will create a new version of the code model that reflects the changes done by the previous aspect layer or depth level.
Therefore, if an aspect introduces a member into a type, the following aspects will see that new member in the code model and may advise it.
To ensure the consistency of this model, aspects cannot provide outputs to previous aspects or to declarations that are not below the current target.
3. Safe composition of advice
When several aspects are unaware of each other and add advice to the same declaration, Metalama guarantees that the resulting code will be correct.
For instance, if two aspects override the same method, both aspects are guaranteed to compose correctly. This is a challenging problem, but Metalama solves it, so you don't have to worry about it.