junit-support

Injecting extensions

Class InjectingExtension can be used as base class for JUnit extensions that can inject objects into fields, constructor parameters or method parameters.

To create such an extension:

• Add a no-argument constructor that passes a field predicate and the result of calling MethodHandles.lookup() to the InjectingExtension constructor.
• Implement validateTarget. This method takes the injection target (see below) and the extension context, and should return an empty Optional if the injection target is valid, or an Optional with a JUnitException otherwise. The exception should be provided by one of the injection target argument's createException methods.
• Implement resolveValue. This method takes the injection target and the extension context, and should return the value to inject.

The injection target is provided as an instance of class InjectionTarget, and provides an abstraction of the field, constructor parameter or method parameter. It offers methods to provide the injection target's type, find annotations, or create exceptions without having to know whether the target is a field, a constructor parameter or a method parameter.