How to mock FileInputStream and other *Streams

问题:

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?

回答1:

Maybe this is a bad idea, but my first approach would have been creating an actual test-file rather than mocking the stream object.

One could argue that this would test the GenericFile class rather than the getBufferedReader method.

Maybe an acceptable way would be to return an actually existing test-file through the mocked GenericFile for testing the getBufferedReader?

回答2:

I would first extract the creation of the Stream into a dependency. So your RealCardParser gets a StreamSource as a dependency.

Now you can take appart your problem:

1. for your current test provide a mock (or in this case I would prefer a fake) implementation returning a Stream constructed from a String.

2. Test the actual StreamSource with a real file, ensuring that it returns the correct content and what not.

回答3:

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 

        }
}

回答4:

I know this isn't the answer that you want.

The idea of unit testing is to make sure your logic is correct. Unit tests catch bugs where incorrect logic has been written. If a method contains no logic (that is, no branching, looping or exception handling), then it is uneconomical to unit test it. By that, I mean that a unit test costs money - time to write it, and time to maintain it. Most unit tests pay us back for that investment, either by finding bugs, or re-assuring us that there are no bugs in the domain of what is being tested.

But a unit test for your getBufferedReader method would not pay you back for our investment. It has a finite cost, but zero benefit, because there is no actual logic that can go wrong. Therefore, you should NOT write such a unit test. If your Cobertura settings or your organisational standards require the existence of such a unit test, then those settings or standards are WRONG and should be changed. Otherwise, your employer's money is being spent on something that has an infinite cost:benefit ratio.

I strongly recommend that your standards are changed so that you only write unit test for methods that contain branching, looping or exception handling.