An example of how to use doReturn When for a spy.

SpyDoReturn.java

@Test
public void getCustomerById_returnsCorrectCustomer_whenGivenValidId() {
    // Arrange
    doReturn(new Customer(123, "Sam"))
        .when(spyCustomerRepository).findById(123)

    // Act
    Customer result = customerService.getCustomerById(123);

    // Assert
    assertThat(result.name).isEqualTo("Sam");
}