How do you write unit and integration tests for a Grails application?

1. Use the standard Grails testing conventions, placing test classes in src/test/groovy mirroring the package structure of the code under test, built on top of Spock or JUnit depending on the project's chosen testing framework (Spock is the traditional Grails default, given its expressive, Groovy-native specification syntax).
2. Extend Grails' testing mixins/annotations for the artifact type being tested — a controller test gets access to mocked params, request, and response objects; a domain class test gets access to an in-memory or test-scoped GORM implementation without requiring a full running application.
3. Write unit tests for isolated logic — a service's business logic, a single domain class's constraints — using lightweight mocks for anything the class under test depends on, avoiding the overhead of starting the full Spring context.
4. Write integration tests for cross-layer behavior that genuinely needs the full application context wired up — verifying that a controller, service, and real GORM persistence layer actually work together correctly, not just in isolation.

Spock's given/when/then specification style tends to produce noticeably readable tests for Grails' Groovy-based codebase, which is part of why it remains the community's default choice even though standard JUnit is also fully supported for teams that prefer it.