Why component scanning does not work for Spring Boot unit tests?

Question

The service class FooServiceImpl is annotated with @Service aka @Component which makes it eligible for autowiring. Why this class is not being pick

Answer 1

You should use @SpringBootTest(classes=FooServiceImpl.class).

As it mentioned on Annotation Type SpringBootTest:

public abstract Class[] classes

The annotated classes to use for loading an ApplicationContext. Can also be specified using @ContextConfiguration(classes=...). If no explicit classes are defined the test will look for nested @Configuration classes, before falling back to a SpringBootConfiguration search.

Returns: the annotated classes used to load the application context See Also: ContextConfiguration.classes()

Default: {}

This would load only necessary class. If don't specify, it may load a database configuration and other stuff which would make your test slower.

On the other hand, if you want really want unit test, you can test this code without Spring - then @RunWith(SpringRunner.class) and @SpringBootTest annotations are not necessary. You can test FooServiceImpl instance. If you have Autowired/injected properties or services, you set them via setters, constructors or mock with Mockito.