How to mock FileInputStream and other *Streams

Question

I have class that gets GenericFile as input argument reads data and does some additional processing. I need to test it:

public class RealCardParser {

    public static final Logger l = LoggerFactory.getLogger(RealCardParser.class);

    @Handler
    public ArrayList<String> handle(GenericFile genericFile) throws IOException {
        ArrayList<String> strings = new ArrayList<String>();
        FileInputStream fstream = new FileInputStream((File) genericFile.getFile());
        DataInputStream in = new DataInputStream(fstream);
        BufferedReader br =  new BufferedReader(new InputStreamReader(in));
        String strLine = br.readLine();//skip header
        while ((strLine = br.readLine()) != null) {
            l.info("handling in parser: {}", strLine);
            strings.add(strLine);
        }
        br.close();
        return strings;
    }
}

The issue is with new FileInputStream. I can mock GenericFile but it is useless cause FileInputStream checks if file exists. I changed my class so:

public class RealCardParser {

    public static final Logger l = LoggerFactory.getLogger(RealCardParser.class);

    protected BufferedReader getBufferedReader(GenericFile genericFile) throws FileNotFoundException {
        FileInputStream fstream = new FileInputStream((File) genericFile.getFile());
        DataInputStream in = new DataInputStream(fstream);
        return new BufferedReader(new InputStreamReader(in));
    }

    @Handler
    public ArrayList<String> handle(GenericFile genericFile) throws IOException {
        ArrayList<String> strings = new ArrayList<String>();
        BufferedReader br = getBufferedReader(genericFile);
        String strLine = br.readLine();//skip header
        while ((strLine = br.readLine()) != null) {
            l.info("handling in parser: {}", strLine);
            strings.add(strLine);
        }
        br.close();
        return strings;
    }
}

So now I can override method getBufferedReader and test method handler:

@RunWith(MockitoJUnitRunner.class)
public class RealCardParserTest {

    RealCardParser parser;

    @Mock
    GenericFile genericFile;

    @Mock
    BufferedReader bufferedReader;

    @Mock
    File file;

    @Before
    public void setUp() throws Exception {
        parser = new RealCardParser() {
            @Override
            public BufferedReader getBufferedReader(GenericFile genericFile) throws FileNotFoundException {
                return bufferedReader;
            }
        };

        when(genericFile.getFile()).thenReturn(file);
        when(bufferedReader.readLine()).thenReturn("header").thenReturn("1,2,3").thenReturn(null);
    }

    @Test
    public void testParser() throws Exception {
        parser.handle(genericFile);
        //do some asserts
    }
}

Handler method now is covered with tests, but I still have uncovered method getBufferedReader that leads to cobertura problems. How to test method getBufferedReader or maybe there is another solution of the problem?

Answer 1

You can mock FileInputStream by using PowerMockRunner and PowerMockito. See the below code for mocking-

@RunWith(PowerMockRunner.class)
@PrepareForTest({
        FileInputStream.class
})
public class A{

 @Test
        public void testFileInputStream ()
                    throws Exception
       {
           final FileInputStream fileInputStreamMock = PowerMockito.mock(FileInputStream.class);
           PowerMockito.whenNew(FileInputStream.class).withArguments(Matchers.anyString())
                               .thenReturn(fileInputStreamMock);
    //Call the actual method containing the new constructor of FileInputStream 

        }
}