Mockito.doThrow() not throwing any exception

I am trying to stub a void method of a mocked object to return an exception. This mocked object passed as dependency to the service which I am writing tests for.

Service:

@Component
public class FileHandler {

    private static final Logger log = LoggerFactory.getLogger(FileHandler.class);

    private final SSHAccess sshAccess;
    @Value("${datastore.location}")
    private String dataStoreDir;

    public FileHandler(SSHAccess sshAccess){
        this.sshAccess = sshAccess;
    }

    public Either<Pair<Exception, FileRecord>, FileRecord> transferFile(FileRecord fileRecord){
        try {
            var sourceURI = new URI(fileRecord.getSourceURI());
            var digest = sshAccess.execute("md5sum " + sourceURI.getPath())
                    .replaceFirst("([^\\s]+)[\\d\\D]*", "$1");
            if (digest.equals(fileRecord.getSourceDigest())) {
                log.info(Thread.currentThread().getName() + ": Copying file: " + fileRecord.getId() + " of submission: " + fileRecord.getOwnedBy());
                sshAccess.download(sourceURI.getPath(),new File(mkdir(dataStoreDir, digest), digest));
                log.info(Thread.currentThread().getName() + ": Copying of file: " + fileRecord.getId() + " of submission: " + fileRecord.getOwnedBy() + " finished.");
                return Either.Right(fileRecord);
            }else{
                log.error("MD5 mismatch for source file {}", sourceURI.getPath());
                return Either.Left(Pair.of(new FileHandlerException("MD5 mismatch"), fileRecord));
            }

        } catch (URISyntaxException
                | IOException
                e) {
            return Either.Left(Pair.of(new FileHandlerException(e), fileRecord));
        }
    }

    private File mkdir(String dataStoreDir, String digest) throws IOException {
        File dir = new File(dataStoreDir, digest.substring(0, 3));
            if (!dir.exists() && !dir.mkdirs()) {
                log.error("Unable to create directory {}", dir);
                throw new IOException("Unable to create directory " + dir);
            }
        return dir;
    }
}

Test Class:

@SpringBootTest
@TestInstance(TestInstance.Lifecycle.PER_CLASS)
public class FileHandlerTest {
    private FileHandler fileHandler;

    @Mock
    private SSHAccess sshAccess;

    @BeforeAll
    public void init(){
       fileHandler = Mockito.spy(new FileHandler(sshAccess));
     
    }

        
    @Test
    public void transferFileShouldReturnFileHandlerExceptionOnEitherLeftWhenSSHclientThrowsIOException() throws IOException {
        FileRecord fileRecord = getFileRecord();
        var digest = "6e484ac23110fae10021e";
        when(sshAccess.execute(anyString())).thenReturn(digest);
        doThrow(IOException.class).when(sshAccess).download(anyString(), any(File.class));
        var result = fileHandler.transferFile(fileRecord);
        Assertions.assertTrue(result.getLeft().isPresent()
                && result.getLeft().get().getFirst() instanceof FileHandlerException);
    }

    private FileRecord getFileRecord() {
        var fileRecord = new FileRecord();
        fileRecord.setId(1L);
        fileRecord.setOwnedBy(1000);
        fileRecord.setSourceURI("scp:host/test/uri/filename");
        fileRecord.setSourceDigest("6e484ac23110fae10021e");
        return fileRecord;
    }
}

But when I run this test case, doThrow() doesn't throw any exception. Method executed without any exception and test failed. I am not sure what I am doing wrong here. Please help.

Not sure why are you using the @SpringBootTest annotation which will try to raise a similar context with the one when you are running the app. So in this case you could stop instantiating your FileHandler and just spy on it and on your SSHAccess beans or use @MockBean instead of @Mock.

Basically you should have something like this

@SpyBean
private FileHandler fileHandler;

@MockBean
private SSHAccess sshAccess;

You are using Junit 5. For Junit 5 you don't need the method @SpringBootTest, you need to use @ExtendWith(MockitoExtension.class)

@SpringBootTest
@TestInstance(TestInstance.Lifecycle.PER_CLASS)
public class FileHandlerTest {
    private FileHandler fileHandler;

    @Mock
    private SSHAccess sshAccess;

    @BeforeAll
    public void init(){
       fileHandler = Mockito.spy(new FileHandler(sshAccess));
     
    }
.....
.....
.....
}

Also instead of Mockito.spy(new FileHandler(sshAccess)) you can try Mockito.mock(new FileHandler(sshAccess))