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Difference Between Unit Testing and Integration Testing?

Unit testing and integration testing are two of the key forms of software testing that have different intentions, remaining as an integral part of the software development cycle. The following compares these two practices thoroughly.

Aspect	Unit Testing	Integration Testing
Definition	Tests a single component or unit of code in isolation	Tests the interoperation of multiple components or modules.
Purpose	To provide an assurance that the discrete tested unit(s) operate correctly in isolation.	To provide assurance that multiple tested units, as they are, operate correctly in combination with each other.
Granularity	Fine-grained; regarding the functioning of a single function, method, or module at a time.	Coarse-grained; regarding the functioning of multiple modules or operations applied together.
Dependencies	Uses mocks or stubs to replace external dependencies.	Uses a real external database during testing or real API.
Testing Type	Whitebox testing. Test requires knowledge of underlying code.	Blackbox testing. Test does not require knowledge of underlying code.
Execution Speed	Faster than integration testing. Unit testing focuses on the functioning of single, isolated units from the code, so executes faster than integration testing.	Slower than unit testing. Integration test requires testing the functioning of discrete unit(s) and whether they function together presents more complexity in how the tests are conducted.
Responsibility	Primarily performed by developers during development.	Test is typically conducted by testers after unit testing has completed.
Cost and Maintenance	Unit test is relatively cheaper and easier to maintain the limited scope of the test.	Integration testing is expensive and complex to maintain due to the wider scope of the test and dealing with limits of scope.
Error Detection	Unit tests reveal bugs at the code level, as early as possible while developing.	Integration testing reveals errors in integration or communication between modules.
When Performed	Unit testing is performed early, often each time code is changed or committed.	Integration testing is performed after unit testing and before the system testing phase of the software life cycle.