How to mock static method that returns final class

问题:

I want to mock next line:

Bigquery bigquery = Transport.newBigQueryClient(options).build();

The problem here is that newBigQueryClient method returns Bulder class, which is final. This means that I cannot mock it neither with mockito or powermockito(it returns such exception: Cannot subclass final class), but I need to return something suitable to mock build method on it. Any ideas how to do it?

回答1:

A suggestion to improve your code and making it much more testable:

First of all, you do not mock a statement like the assignment that you gave us as an example. You mock objects and assign their references to variables whose type represent a super type.

Additionally, if you feel you have to mock something away, you have obviously found a dependency in your code snippet that is an important concept.

Make this concept obvious!

In your case, you want to get a Bigquery object and assign its reference to a variable. The unclear concept is that someone has to provide such an object.

Make this concept clear with an interface:

interface BigqueryProvider {
    Bigquery provide(Object options);
}

In your class where you also have the statement

Bigquery bigquery = Transport.newBigQueryClient(options).build();

you now add the following instance variable

private final BigqueryProvider bigqueryProvider;

and change your constructors accordingly. As I made it final it must be set at construction time. Of course, you must also identify the code where you call the constructors. For that you also need a default implementation:

final class DefaultBigqueryProvider implements BigqueryProvider {
    @Override
    public Bigquery provide(Object options) {
        return Transport.newBigQueryClient(options).build();
    }
}

Then change the above mentioned assignment to

Bigquery bigquery = bigqueryProvider.provide(options);

Now to the tests: In your tests, you now can provide a test implementation for the provider interface. A mocking framework can easily do that for you. You are also able to return a Bigquery object of your choice, being it an object of that class or of a subclass, it doesn't matter.

Your test code then instantiates your class by providing them a mocked BigqueryProvider which you have under full control.