Mocking DynamoDB mapper query using Mockito

Question

I am trying to mock a very simple line of code that is used to query the DynamoDB using Java. Here is some sample code for the query -

List<Pojo> result;
        try {
            if (filters == null) {
                this.queryExpression = new DynamoDBQueryExpression<Pojo>()
                        .withKeyConditionExpression(partitionKeyCondition)
                        .withExpressionAttributeValues(this.eav);
            } else {
                setFilterQueryExpression(filters);
            }
            result = this.dynamoDBMapper.query(Pojo.class, queryExpression);
        } catch (final Exception e) {
            throw new InternalServerException("Something went wrong with the database query: ", e);
        }

The above piece of code works and I am able to retrieve a List of rows that automatically get deserialized into the Pojo.

I am now trying to Mock the this.dynamoDBMapper.query call as follows -

@Mock
private DynamoDBMapper mapper;
List<Pojo> result = new ArrayList<>();

when(mapper.query(Pojo.class,Mockito.any(DynamoDBQueryExpression.class)).thenReturn(result);

I am unable to do that with error -

Cannot resolve method 'thenReturn(java.util.List<com.amazon.xxx.xxx.Pojo>)'

I also tried another way -

doReturn(result).when(mapper).query(Pojo.class, Mockito.any(DynamoDBQueryExpression.class));

That seems to compile but the test fails with error -

org.mockito.exceptions.misusing.WrongTypeOfReturnValue

I have looked at other sample where the expected output of the query is of type PaginatedQueryList , I have tried changing to that as well. But I am still not sure why the above throws an error.

Have you tried changing List to PaginatedQueryList or similar? According to the documentation the query methods return specific implementations of List. You can assign a specific implementation to a more general List variable but you cannot return a more general type where a specific implementation is requested per the signature. Hence, Mockito cannot resolve the method you are asking for. — Bastis Programming Corner, Jan 27 '22 at 22:24
I did try changing the return type from `List` to `PaginatedQueryList` but the `when` still does not accept it. It says `Cannot resolve method 'thenReturn(PaginatedQueryList)'`. I feel like I need to do something extra to make it work but I am not entirely sure how. — Samarth, Jan 27 '22 at 23:05

score 0 · Answer 1 · answered Jan 27 '22 at 22:13

0

Do you also get the error when you use ArgumentMatchers?

Mockito.when(mapper.query(ArgumentMatchers.any(Pojo.class),ArgumentMatchers.any(DynamoDBQueryExpression.class)).thenReturn(result));

Do you also get the error if you expand the ArgumentMatchers (temporarily)?

Mockito.when(mapper.query(ArgumentMatchers.any(),ArgumentMatchers.any()).thenReturn(result));

answered Jan 27 '22 at 22:13

dcnis

61
4

Yes, it does not change anything. I have already tried replacing the arguments with `any()`. – Samarth Jan 27 '22 at 22:20
@Samarth Can you go into the .query() method and check the return type? Is it List or is it a base class? WrongTypeOfReturnValue and Cannot resolve method List<...Pojo> hints somehow that the return value is false. Also check if the return value is something like List extends something> because then your Pojo need to extend "something" as well. – dcnis Jan 27 '22 at 22:29
The return type is `PaginatedQueryList` as described in the documentation. I have tried switching all the return types in my code from `List` to `PaginatedQueryList` but I still run into the error. See comment on question for more. – Samarth Jan 27 '22 at 23:07

score 0 · Answer 2 · answered Jan 30 '22 at 18:52

As it turns out, you are missing a parenthesis before .thenReturn in order to complete the when part. Once you add it and switch from return type List to PaginatedQueryList, it should compile. Also note that any is a matcher. Once you specifiy a matcher, all arguments need to be matchers, therefore use eq etc. for your Pojo type. Otherwise, Mockito will show a InvalidUseOfMatchersException during runtime. Here is a simplified example that works for me:

import com.amazonaws.services.dynamodbv2.datamodeling.DynamoDBMapper;
import com.amazonaws.services.dynamodbv2.datamodeling.DynamoDBQueryExpression;
import com.amazonaws.services.dynamodbv2.datamodeling.PaginatedQueryList;
import org.junit.jupiter.api.Test;
import org.mockito.Mockito;

import static org.junit.jupiter.api.Assertions.assertEquals;
import static org.mockito.ArgumentMatchers.eq;
import static org.mockito.Mockito.mock;
import static org.mockito.Mockito.when;

public class MockTest {

    @Test
    void test() {
        DynamoDBMapper mapperMock = mock(DynamoDBMapper.class);
        PaginatedQueryList<String> expected = mock(PaginatedQueryList.class);
        // Note that when needs to be completed before thenReturn can be called.
        when(mapperMock.query(eq(String.class), Mockito.any(DynamoDBQueryExpression.class))).thenReturn(expected);

        QueryService queryService = new QueryService(mapperMock);
        PaginatedQueryList<String> actual = queryService.query();

        assertEquals(expected, actual);
    }

    public class QueryService {
        private final DynamoDBMapper mapper;

        public QueryService(DynamoDBMapper mapper) {
            this.mapper = mapper;
        }

        public PaginatedQueryList<String> query() {
            DynamoDBQueryExpression<String> queryExpression = new DynamoDBQueryExpression<>();
            return mapper.query(String.class, queryExpression);
        }
    }
}

Mocking DynamoDB mapper query using Mockito

2 Answers2