When to use mock objects in unit tests

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As you’re writing new code (along with the new unit tests) or refactoring existing code, you want to be able to run the unit tests over and over to be reasonably confident existing functionality was not broken. Therefore, the unit tests must be stable and fast.

Suppose the class to be tested depends on some external resource such as a database. You make a code change and the unit tests are suddenly failing. Did the unit tests break because of a bug you just introduced, or because the external resource is not available? There is no guarantee the external resource will always be available so the unit tests are unstable. Mock the external resource.

Also, connecting to an external resource can take too much time. When you eventually have thousands of tests which connect to various external resources, the milliseconds to connect to the external resource add up, which slows you down. Mock the external resource.

Now add a CI/CD pipeline. During the build the unit tests fail. Is the external resource down or did your code change break something? Perhaps the build server doesn’t have access to an external resource? Mock the external resource.

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