Micronaut MockBean Explained

The@MockBean annotation in the Micronaut framework is used to create replacements for beans in tests. These tests must be annotated with @MicronautTest. There is often confusion about the roles of the method or field annotated with @MockBean.

Each method or field specifies both the bean to be replaced and the new bean to be created:

• The bean to be replaced is specified as the value of @MockBean and must be the implementation type of the bean, typically it is the one annotated with @Singleton.
• The new bean is specified by the return type of the method or the type of the field.

Creating or replacing a named bean can be more complex:

• The named value inside the @MockBean annotation is used to replace the existing named bean.
• The @Named annotation is used to name the newly created bean

The most complicated scenario is replacing a bean with a specified qualifier, such as @Primary, which is particularly challenging:

• Use the @Replace annotation directly, specifying the qualifier and/or a factory bean.
• Any qualifier added to the method will once again apply to the new bean.