How to mock Files.copy method in JUnit - spring

Question

I am using some of the methods of Files class like (delete, copy methods) to do upload and delete of file. Below is the code to perform these operations.
public String uploadFile(MultipartFile file) {
try {
String fileName = file.getOriginalFilename()
// Copy file to the target location (Replacing existing file with the same name)
Path targetLocation = Paths.get("uploadPath" + File.separator + StringUtils.cleanPath(fileName));
Files.copy(file.getInputStream(), targetLocation, StandardCopyOption.REPLACE_EXISTING);
return fileName;
} catch (IOException ex) {
throw new FileStorageException("Not able to upload", ex);
}
}
But for this source code I am not able to write JUnit tests because not able to mock Files class. For mocking final classes we can use PowerMock which supports to mock static and final methods. But here if I do using PowerMock still it is not mocking. I am using Spring Framework 5.2.1.RELEASE , Is there any change in JUnit with this version to mock final classes or methods? Or can any one help me on writing the unit tests for this code (versions I am using Spring Framework 5.2.1 and JUnit4.12).

Answer 1

Mocking static and final classes is indeed possible only with tools like PowerMock or PowerMockito, and its not related to JUnit or Spring frameworks.
I think you should not Mock Files.copy operation.
Instead consider the following strategy:
Define an interface for working with files, a kind of DAO but for file system:
public interface FileSystemDAO {
void copy(InputStream is, Path target, StandardCopyOption ... options);
}
public class FileSystemDAOImpl implements FileSystemDAO {
void copy(InputStream is, Path target, StandatadCopyOption ... options) {
Files.copy(...)
}
}
Now use dependency injection in all the places that work with files (if you're using spring as you've said - define FileSystemDAOImpl as a bean).
class MySampleUploadService {
private final FileSystemDAO fileSystemDao;
public MySampleUploadService(FileSystemDAO dao) {
this.fileSystemDao = dao;
}
public String uploadFile(MultipartFile file) {
try {
String fileName = file.getOriginalFilename()
// Copy file to the target location (Replacing existing file with the same name)
Path targetLocation = Paths.get("uploadPath" + File.separator +
StringUtils.cleanPath(fileName));
fileSystemDao.copy(file.getInputStream(), targetLocation, StandardCopyOption.REPLACE_EXISTING);
return fileName;
} catch (IOException ex) {
throw new FileStorageException("Not able to upload", ex);
}
}
}
Now with this approach you can easily test the Upload service by mocking the FileSystemDao interface.