Mockito | JUnit | Avro: Mocking a void method does not work

I wrote a method for avro serialization, which I want to test.

Method

@SneakyThrows
@Override
public byte[] serialize(String topic, T data) {
    try (ByteArrayOutputStream stream = new ByteArrayOutputStream()) {
        writeSerializedAvro(stream, data, data.getSchema());
        return stream.toByteArray();
    } catch (IOException e) {
        throw new AvroSerializationException(e.getMessage(), e);
    }
}

private void writeSerializedAvro(ByteArrayOutputStream stream, T data, Schema schema) throws IOException {
    binaryEncoder = EncoderFactory.get().binaryEncoder(stream, binaryEncoder);
    DatumWriter<T> datumWriter = new GenericDatumWriter<>(schema);
    datumWriter.write(data, binaryEncoder);
    binaryEncoder.flush();
}

datumWriter.write or binaryEncoder.flush() can throw IOException.

In the test, I want to see if IOException is thrown, my method catchers it and wraps it in AvroSerializationException (a class written by me).

Unit Test

    @Test(expected = AvroSerializationException.class)
public void testSerializeException() throws IOException {
    // Given 
// need to mock here

    // When
    serializer.serialize("", avroObject);
}

I have tried mocking GenericDatumWriter, BufferedBinaryEncoder and EncoderFactory with doThrow().when() methods but nothing worked for me.

BufferedBinaryEncoder binaryEncoder = Mockito.mock(BufferedBinaryEncoder.class);
Mockito.doThrow(new IOException()).when(binaryEncoder).flush();

And

    GenericDatumWriter datumWriter = Mockito.mock(GenericDatumWriter.class);
    Mockito.doThrow(new IOException()).when(datumWriter).write(any(), any());

But again, the question comes back to, how am I supposed to pass these objects to my serializer, keeping the best practices in mind?

You need first to mock the object using Mockito and use it in your class and then the exception will be thrown, like below:

Your code:

private DatumWriter<T> datumWriter = new GenericDatumWriter<>(schema);

@SneakyThrows
@Override
public byte[] serialize(String topic, T data) {
    try (ByteArrayOutputStream stream = new ByteArrayOutputStream()) {
        writeSerializedAvro(stream, data, data.getSchema());
        return stream.toByteArray();
    } catch (IOException e) {
        throw new AvroSerializationException(e.getMessage(), e);
    }
}

private void writeSerializedAvro(ByteArrayOutputStream stream, T data, Schema schema) throws IOException {
    binaryEncoder = EncoderFactory.get().binaryEncoder(stream, binaryEncoder);
    datumWriter.write(data, binaryEncoder);
    binaryEncoder.flush();
}

public void setDatumWriter (DatumWriter datumWriter){
   this.datumWriter=datumWriter;
}

Your test code:

@Test(expected = AvroSerializationException.class)
public void testSerializeException() throws IOException {
    // Given 
    DatumWriter<YourClass> datumWriter = Mockito.mock(DatumWriter<YourClass>.class);
    when(datumWriter.write(anyObject(), anyObject())).thenThrow(IOException.class);
    
    serializer.setDatumWriter(datumWriter);  
   
    serializer.serialize("", avroObject);
}