Java: Mocking a ResultSet using Mockito

This post shows how you can mock a java.sql.ResultSet using Mockito. It can be used to help unit test code which performs operations on ResultSets (such as a ResultSetExtractor) without relying on an external datasource.

You can create a MockResultSet by providing a list of column names and a 2D array of data. For example:

var rs = MockResultSet.create(
           new String[] { "name", "age" }, //columns
           new Object[][] { // data
             { "Alice", 20 },
             { "Bob", 35 },
             { "Charles", 50 }
           });

The code for MockResultSet is shown below (also available in my GitHub Repository). Note that I have only mocked a few methods such as next, getString and getObject but it is quite easy to mock the rest by following the same pattern.

public class MockResultSet {

  private final Map<String, Integer> columnIndices;
  private final Object[][] data;
  private int rowIndex;

  private MockResultSet(final String[] columnNames,
                        final Object[][] data) {
    // create a map of column name to column index
    this.columnIndices = IntStream.range(0, columnNames.length)
        .boxed()
        .collect(Collectors.toMap(
            k -> columnNames[k],
            Function.identity(),
            (a, b) ->
              { throw new RuntimeException("Duplicate column " + a); },
            LinkedHashMap::new
            ));
    this.data = data;
    this.rowIndex = -1;
  }

  private ResultSet buildMock() throws SQLException {
    final var rs = mock(ResultSet.class);

    // mock rs.next()
    doAnswer(invocation -> {
      rowIndex++;
      return rowIndex < data.length;
    }).when(rs).next();

    // mock rs.getString(columnName)
    doAnswer(invocation -> {
      final var columnName = invocation.getArgumentAt(0, String.class);
      final var columnIndex = columnIndices.get(columnName);
      return (String) data[rowIndex][columnIndex];
    }).when(rs).getString(anyString());

    // mock rs.getObject(columnIndex)
    doAnswer(invocation -> {
      final var index = invocation.getArgumentAt(0, Integer.class);
      return data[rowIndex][index - 1];
    }).when(rs).getObject(anyInt());

    final var rsmd = mock(ResultSetMetaData.class);

    // mock rsmd.getColumnCount()
    doReturn(columnIndices.size()).when(rsmd).getColumnCount();

    // mock rs.getMetaData()
    doReturn(rsmd).when(rs).getMetaData();

    return rs;
  }

  /**
   * Creates the mock ResultSet.
   *
   * @param columnNames the names of the columns
   * @param data
   * @return a mocked ResultSet
   * @throws SQLException
   */
  public static ResultSet create(
                         final String[] columnNames,
                         final Object[][] data)
                         throws SQLException {
    return new MockResultSet(columnNames, data).buildMock();
  }
}