Singleton and unit testing

Question

The Effective Java has the following statement on unit testing singletons

Making a class a singleton can make it difficult to test its clients, as it’s im

Answer 1

The problem with singletons (and also with static methods) is that it makes it hard to replace the actual code with a mocked implementation.

For example consider the following code

public class TestMe() {
  public String foo(String data) {
    boolean isFeatureFlag = MySingletonConfig.getInstance().getFeatureFlag();
    if (isFeatureFlag)
      // do somethine with data
    else
      // do something else with the data
     return result;
  }
}

It is not easy to write a unit test for the foo method and verifying the correct behavior is performed. This is because you can't easily change the return value of getFeatureFlag.

The same problem exists for static methods - it's not easy to replace the actual target class method with a mock behavior.

Sure, there are workarounds like powermock, or dependency injection to the method, or reflection in tests. But it is much better not to use singletons in the first place