NullPointer Exception when mocking java.nio.Files.class

Question

I am trying to write a unit test for the class:

public class ClassToTest
{
    public List<String> walk(URI path)
    {
        List<String> directories= new List<String>();

        Path share = Paths.get(path);
        try
        {
            Stream<Path> folders = Files.list(share).filter(Files::isDirectory);
            folders.forEach(Path -> {
                {
                    directories.add(Path.toUri());
                }
            });

        }
        catch (IOException | SecurityException e)
        {
           System.out.println("Exception in crawling folder:"+path.toString());
           e.printStackTrace();
        }
        return directories;
    }
}

Here is my unit test:

@RunWith(PowerMockRunner.class)
@PrepareForTest({Files.class, ClassToTest.class })
class ClassToTestUTest
{

    @Mock
    private Path folder1;
    @Mock
    private Path folder2;
    
    private ClassToTest underTest;
    @BeforeEach
    void setUp() throws Exception
    {
        PowerMockito.mockStatic(Files.class);
    }

    @Test
    void testWalk() throws IOException
    {
        String sharepath = "\\\\ip\\Share";
        Path temp = Paths.get(sharepath);
        
        Stream<Path> folders;
        ArrayList<Path> listOfFolders = new ArrayList<Path>();
        listOfFolders.add(folder1);
        listOfFolders.add(folder2);
        folders = listOfFolders.stream();
        
        when(Files.list(any())).thenReturn(folders);
        List<String> directories= underTest.walk(temp.toUri());
        
        //Check the directories list.
    }

}

When I run this, the mock isn't working. I get a exception from the actual Files.list() method. I want to mock the (Paths.class) Paths.get() call also, but leave that for now.

Junit error:

java.lang.NullPointerException at java.nio.file.Files.provider(Files.java:97) at java.nio.file.Files.newDirectoryStream(Files.java:457) at java.nio.file.Files.list(Files.java:3451) at com.ClassToTestUTest.testWalk(ClassToTestUTest.java:51)

I found many questions related to mocking this Files class. I am using PowerMock 2.0.0 RC4 and Mockito 2.23.4.

Where am I going wrong?

Answer 1

Faced a similar issue. Unable to understand why java.nio.file.Files mocking failing and other final static methods are mocked.

class ClassUnderTest {
    
    public void methodUnderTest() throws IOException {
        //bla bla
        Files.lines(null/*some path*/);
        //bla bla
        return;
    }
}

Change this to

class ClassUnderTest {
    
    public void methodUnderTest() throws IOException {
        //bla bla
        filesListWrapperMethod(null/*some path*/);
        //bla bla
        return;
    }
    
    @VisibleForTesting
    Stream<Path> filesListWrapperMethod(Path path) throws IOException {
        return Files.list(path);
    }

}

Now, you can use Mockito.spy() and mock the specific APIs.