Testing: jUnit's TemporaryFolder

Testing: jUnit's TemporaryFolder

with Simple CSV generator

The problem

Lately I had a question in my head: how to test a method, which output is a file? 

Should I divide creation of file from creation of content and test only the latter part?

Well, usually I would like to call a method "generateReport" or retrieveFileByHash(), which encapsulate all file-connected operations and return simple java.io.File. In other words, my service classes looks like this:

• Interface (like ReportGenerator)
• one method
• returns: File
• Concrete class (like DefaultReportGenerator)
• one public implementation of interface method, (like generateReport())
• returns: File
• set of private methods
• returns: whatever is needed to create File

With such services, file's content creation is separated from File creation:

• Interface - ReportGenerator
• method: generateReport()
• returns File
• Concrete Class - DefaultReportGenerator
• method: implementation of generateReport()
• public
• return File
• method: retriveSystemDataFromCurrentMonth()
• private
• returns List of data for report

But since retrieving data is isolated in private method, I cannot simply test it - I would have to call public method anyway and create file, then save it somewhere, confirm its content is valid and proper and then remove it at the end of a test.

The solution

To test such a behaviour, I had to create proper intergration test, since unit tests should not touch file system, according to Michael Feather's. And then, use special thing in jUnit 4 - @Rule TemporaryFolder. It handles all file's operations and guarantees to delete files created through the test.

So to play with this functionality, I created very basic CSV file generator. It works on any entity implementing special proper interface. Let's check integration test.

	@Rule
	public TemporaryFolder testFolder = new TemporaryFolder();
	private String filePath;
	private final String FILENAME = "testCsvFile.csv";
	@Test
	public void generate___shouldSucceed() throws IOException {
	// given
	filePath = this.generateFilePath();
	SystemUser systemUser = new SystemUser(NAME, EMAIL);
	CsvFileGenerator csvFileGenerator = new SimpleCsvFileGenerator(
	filePath);
	// when
	File file = csvFileGenerator.generate(List.of(systemUser));
	// then
	assertTrue(file.exists());
	assertTrue(file.isFile());
	assertEquals(FILENAME, file.getName());
	List<String> content = Files.lines(Paths.get(file.getAbsolutePath()))
	.collect(Collectors.toList());
	assertEquals(2, content.size());
	assertEquals("Identificator,User's name,User's email", content.get(0));
	assertEquals("1,Test name,Test email", content.get(1));
	}

view raw temporaryFolder.java hosted with ❤ by GitHub

I create CSV file inside the TemporaryFolder and I do not have to care about any other thing :)

Full sample project: https://github.com/toficzak/SimpleCsvGenerator

In the following posts I will try to implement proper abstraction in CSV generator, allowing it to create file's in production environment and mock it in test environment.

Some resources:

• https://stackoverflow.com/questions/3381801/how-do-i-unit-test-saving-file-to-the-disk
• https://www.artima.com/weblogs/viewpost.jsp?thread=126923
• https://garygregory.wordpress.com/2010/01/20/junit-tip-use-rules-to-manage-temporary-files-and-folders/