Junit mocking files using jmockit

Question

I am trying to test my java code using jmockit for the first time and I am really confused. I have a method that reads a file and returns the line of strings that reads from the file as a list.

import java.io.BufferedReader;
import java.io.File;
import java.io.FileReader;
import java.io.IOException;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.List;

public class Reader {
    public static final int LIMIT = -1;
    public static final int EMPTY_FILE = 0;
    private String delimiter = ",";

    
    public Reader() {}

    public List<List<String>> readFile(String fileName, String delimiter) throws IOException {
        List<List<String>> rawData = new ArrayList<>();

        File input = new File(fileName);

        if (!delimiter.isEmpty())
            this.delimiter = delimiter;

        if (input.length() == EMPTY_FILE) {
            throw new IOException("File is empty. Check file and try again.");
        }

        BufferedReader reader = new BufferedReader(new FileReader(input));

        String line;

        while ((line = reader.readLine()) != null) {
            List<String> lineData = Arrays.asList(line.split(this.delimiter, LIMIT));
            rawData.add(lineData);
        }
        return rawData;
    }
}

I am trying to test this code using mocked readers and bufferedReader but without any luck. Obviously I am doing something wrong but I can't figure out how to do it properly.

What I want is to create a mocked file that will be read and test it like its empty or non empty.

What I have tried so far :

class ReaderTest {
    static final String FILENAME = "input.txt";

    @Injectable
    File mockedFile;

    @Mocked
    BufferedReader mockedBufferedReader;

    @Mocked
    FileReader mockedFileReader;

    @Test
    void readNonEmptyInputFileShouldDoNothing() throws FileNotFoundException {
        new Expectations(File.class) {{
            new File(anyString);
            result = mockedFile;
        }};

        new Expectations(BufferedReader.class) {{
            new FileReader(anyString);
            result = mockedFileReader;

            new BufferedReader(mockedFileReader);
            result = mockedBufferedReader;
        }};
        Reader reader = new Reader();

        Assertions.assertDoesNotThrow(() ->
                reader.readFile(FILENAME, FieldsConstants.DELIMITER));
    }
}

This test gives me an IllegalArgumentException error: Invalid Class argument for partial mocking (use a MockUp instead): class java.io.File

Answer 1

I managed to solve my issue using PowerMock and EasyMock API. The only issue i faced was that in the beginning i was using Junit5 and PowerMock wasn't working as intended. I switched to Junit4 and everything worked out just fine. Just some sample code if anyone is interested:

@RunWith(PowerMockRunner.class)
@PrepareForTest({Reader.class})
@PowerMockIgnore({"javax.management.*", "javax.script.*"})
public class ReaderTest {
    public static final String TEST_LINE = "2,Ned,Flanders,SELLER,,,LINE_A,Springfield,12.8,TRUE";
    public static final String FILENAME = "file_name";
    public static final int LIMIT = -1;

    private static List<List<String>> tmpList;

    @BeforeClass
    public static void beforeAll() {
        List<String> tmpLine = Arrays.asList(TEST_LINE.split(FieldsConstants.DELIMITER, LIMIT));
        tmpList = new ArrayList<>();
        tmpList.add(tmpLine);
    }

    @Test
    public void readNonEmptyInputFileShouldDoNothing() throws Exception {
        File mockFile = createMockAndExpectNew(File.class, FILENAME);
        FileReader mockFileReader = createMockAndExpectNew(FileReader.class, mockFile);
        BufferedReader mockBufferedReader = createMockAndExpectNew(BufferedReader.class, mockFileReader);

        Reader reader = new Reader();

        expect(mockFile.length()).andReturn((long) 100);

        expect(mockBufferedReader.readLine()).andReturn(TEST_LINE);
        expect(mockBufferedReader.readLine()).andReturn(null);

        replayAll();

        assertEquals(reader.readFile(FILENAME, ""), tmpList);
        verifyAll();
    }
}