Introduction

Testing a single unit (method/class) in isolation from other components.

• Ensure the smallest code piece works correctly.
• Catch bugs early.
• Document expected behavior.

Testing the real logic correct or not

• Business rules
• Calculations
• Conditions
• Branching logic

Characteristics

• Isolated → no DB, network, file I/O.
  • mocks for all external calls (DB, APIs, file system)
  • Not testing spring wiring
• Fast → milliseconds, can run hundreds of tests quickly.
• Deterministic → same input → same output.
• Focused → one scenario/assertion per test.

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