Using Mock class with dependency injection in a JUnit test - spring

Question

I have a basic Interface which another class is implementing.
package info;
import org.springframework.stereotype.Service;
public interface Student
{
public String getStudentID();
}
`
package info;
import org.springframework.stereotype.Component;
import org.springframework.stereotype.Service;
import org.springframework.beans.factory.annotation.Autowired;
#Service
public class StudentImpl implements Student
{
#Override
public String getStudentID()
{
return "Unimplemented";
}
}
I then have a service to inject that class into
package info;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.stereotype.Component;
import org.springframework.stereotype.Service;
#Service
public class InfoService {
#Autowired
Student student;
public String runProg()
{
return student.getStudentID();
}
}
What I want to know is, how do I set up a JUnit test so that a Mock class of the Student interface steps in with a stubbed method instead of the method in StudentImpl. The injection does work but I want to use amock class to simulate the results instead for the sake of testing. Any help would be greatly appreciated.

Answer 1

In my opinion, autowiring in unit tests is a sign that's it's an integration test rather than unit test, so I prefer to do my own "wiring", as you describe. It might require you to do some refactoring of your code, but it shouldn't be a problem. In your case, I would add a constructor to InfoService that get's a Student implementation. If you wish, you can also make this constructor #Autowired, and remove the #Autowired from the student field. Spring would then still be able to autowire it, and it's also more testable.
#Service
public class InfoService {
Student student;
#Autowired
public InfoService(Student student) {
this.student = student;
}
}
Then it will be trivial to pass mocks between your services in your tests:
#Test
public void myTest() {
Student mockStudent = mock(Student.class);
InfoService service = new InfoService(mockStudent);
}