Integration tests are kind of tests which individual modules are combined and tested as a whole. Moreover integration tests might use system dependent values, accessing external systems like file system, database, web services, ..., and testing multiple aspects of one test case. We can say it is a high-level test.

This differs from unit test where only a single component is tested. Unit tests runs in isolation, mocking-out external components or using in-memory database in case of DAO layers. A unit test might be:

  • Repeatable.
  • Consistent.
  • In Memory.
  • Fast.
  • Self-validating.
  • Testing single concept

The problem when we are writing tests, is how to test rare (or untypical) conditions like "No disk space" in case of accessing file system, or "Connection lost" when executing a database query.
In unit testing this is not a problem you can mock up that component (database connection or filesystem access), generating required output like throwing IOException.

The problem becomes "harder" with integration tests. It would be strange to mock a component, when what you really want to do is validate the real system. So arrived at this point I see two possibilities:Creating a partial mock.Using fault injection.In this post I am going to show you how to use fault injection approach to test unusual erroneous situations. 


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