How is the hierarchy of testing levels organized in the general approach?

The hierarchy of testing levels typically includes several stages, each of which tests the system with a different level of detail:

1. Unit tests — test individual functions or methods without external dependencies. Fast and isolated.
2. Integration tests — test the interaction between modules or components, such as working with a database or external services.
3. System tests — test the entire system as a whole under conditions as close to real-world as possible.
4. Acceptance tests — verify whether the system meets the customer's requirements, often involving end-users.

This approach allows early detection of errors and reduces the cost of fixing them. In Java projects, different tools and frameworks are often used for each level, such as JUnit for unit tests, Spring Test for integration tests, and Selenium for acceptance tests.