Saturday, September 08, 2018

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)
            k -> columnNames[k],
            (a, b) ->
              { throw new RuntimeException("Duplicate column " + a); },
            )); = data;
    this.rowIndex = -1;

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

    // mock
    doAnswer(invocation -> {
      return rowIndex < data.length;

    // 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];

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

    final var rsmd = mock(ResultSetMetaData.class);

    // mock rsmd.getColumnCount()

    // mock 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();

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