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();
 }
}