Tim Cooke

Capturing Method Call Arguments with Mockito

Here's how to capture arguments being passed to a method in order to make assertions on them.

Take the following production code. You want to make assertions on the User object that is being passed into the migrationDao.migrate() method from your class under test.

public class User {
    private String name;
    private int age;
    
    public User(String name, int age) {
        this.name = name;
        this.age = age;
    }
    
    public String getName() { return this.name; }
    
    public int getAge() { return this.age; }
}

public class MigrationService {
    private MigrationDao migrationDao;
    
    public MigrationService(MigrationDao migrationDao) {
        this.migrationDao = migrationDao;
    }
    
    public void migrate(User user) {
        migrationDao.migrate(user);
    }    
}

Capture the argument using Mockito’s ArgumentCaptor in order to make specific assertions on the object passed.

public class MigrationServiceTest {

    private MigrationService MigrationService;
    private MigrationDao migrationDaoMock;

    @Before
    public void setup() {
        migrationDaoMock = mock(MigrationDao.class);
        migrationService = new MigrationService(migrationDaoMock);
    }

    @Test
    public void migrate_givenMigrateSingleUserNamedTimAged37_shouldReceiveUserNamedTimAged37() {
        User userToMigrate = new User("Tim", 37);
        migrationService.migrateUser(userToMigrate);
        
        ArgumentCaptor<User> userArgCaptor = ArgumentCaptor.forClass(User.class);
        verify(mockMigrationDao).migrate(userArgCaptor.capture());
        User userPassedToMigrationDao = userArgCaptor.getValue();
        
        assertThat(userPassedToMigrationDao.getName(), is("Tim"));
        assertThat(userPassedToMigrationDao.getAge(), is(37));
    }
}