How can custom annotations be used in Java?

Custom annotations can be used in various scenarios in Java programming, here are a few common use cases:

1. Annotation: can be used to mark specific classes, methods, or fields, so that other programs can process them accordingly based on the annotation. For example, the @Test annotation in the JUnit framework is used to mark test methods.
2. Configuration annotations can be used to set properties of a class, method, or field. For example, the @Autowired annotation in the Spring framework is used for automatically wiring dependency objects.
3. Validation annotations: can be used to validate method parameters or return values. For example, the @NotEmpty annotation in the Hibernate framework is used to ensure that a field is not empty.
4. Log annotation: used to generate log information and simplify log code writing. For example, the @Log annotation in the Lombok framework is used to generate log objects.
5. Authorization annotation: can be used to control the access permissions of methods or classes. For example, the @PreAuthorize annotation in the Spring Security framework is used to control method access permissions.
6. Performance annotation: can be used to record the execution time of a method or other performance-related information, such as the @VisibleForTesting annotation in the Guava framework, which is used to mark a method as only for testing purposes.

The above are just some common usage scenarios. In fact, the usage scenarios of custom annotations can be flexibly defined according to specific needs. Custom annotations can be defined based on business requirements to enhance code readability, maintainability, and scalability.