How to write Junit tests for this code with 100% code coverage? - spring-boot

Question

Junit tests for Spring Boot application.
Does anyone have any suggestions on how I might achieve this using JUnit and Mockito?
#Autowired
JdbcTemplate jdbcTemplate;
public List<Student> getStudentDetails(String department) {
List<Student> results = new LinkedList<String>();
results = jdbcTemplate.query("SELECT * FROM STUDENT WHERE DEPARTMENT = ?", new PreparedStatementSetter() {
#Override
public void setValues(PreparedStatement preparedStatement) throws SQLException {
preparedStatement.setString(1, department);
preparedStatement.setFetchSize(10);
}
}, new ResultSetExtractor<List<Student>>() {
#Override
public List<Student> extractData(ResultSet rs) throws SQLException {
List<Student> students = new ArrayList<>();
while (rs.next()) {
Student student = new Student<>();
student.setDepartment(rs.getString("NAME"));
student.setName(rs.getString("DEPARTMENT"));
students.add(student);
}
return students;
}
});
return results
}

Answer 1

Code that you have is related to database and in my opinion it should be tested using some database. Usual practice is to use embedded database ( e.g. h2 ). People do it, since using unit tests it's not possible to check if query really works, since you don't actually run it. So I would combine integration and unit tests for testing of this class.
Unit test would be something like this:
#ExtendWith(MockitoExtension.class)
class StubTest {
#Mock
JdbcTemplate jdbcTemplate;
#InjectMocks
Stub stub;
#Test
void whenExecuteQuery_thenExtractDataCorrectly() throws SQLException {
//GIVEN
ArgumentCaptor<PreparedStatementSetter> setterCaptor = ArgumentCaptor.forClass(PreparedStatementSetter.class);
ArgumentCaptor<ResultSetExtractor> extractorCaptor = ArgumentCaptor.forClass(ResultSetExtractor.class);
//WHEN
stub.getStudentDetails("TEST");
//THEN
verify(jdbcTemplate).query(anyString(), setterCaptor.capture(), extractorCaptor.capture());
//AND
PreparedStatementSetter setter = setterCaptor.getValue();
PreparedStatement preparedStatement = Mockito.mock(PreparedStatement.class);
setter.setValues(preparedStatement);
verify(preparedStatement).setString(1, "TEST");
verify(preparedStatement).setFetchSize(10);
verifyNoMoreInteractions(preparedStatement);
//AND
ResultSetExtractor extractor = extractorCaptor.getValue();
ResultSet rs = Mockito.mock(ResultSet.class);
when(rs.next()).thenReturn(true).thenReturn(false);
when(rs.getString(anyString())).thenReturn("TEST","name");
verifyNoMoreInteractions(rs);
List<Student> students = (List<Student>) extractor.extractData(rs);
assertThat(students.get(0).getName()).isEqualTo("name");
assertThat(students.get(0).getDepartment()).isEqualTo("TEST");
}
}
Here I'm just capturing arguments with business logic that we send to query method. Then I run overridden methods of arguments and verify that they work as we expect.