Question

I would like to mock a File and FileInputStream Object for an JUnit test.

Let us say we are having a Parser. From outside the Parser is just accessible via the parse(File myFile) method. The other methods like readMyStream….are private.

Example Code:

public class Parser {

   public HashMap<String, String> parse(File myFile) throws Exception {
         HashMap<String, String> myConfig;
         Config config;

         try {
             //this line gives me a headache
             FileInputStream myFileInputStream = new FileInputStream(myFile);

             configStream = readMyStream(myFileInputStream);
               ..........
         } catch (FileNotFoundException e) {
                throw e;
         }
         return myConfig;
   }
   //reads the stream 
   private Config readMyStream(FileInputStream myFileInputStream) {
      Config config;    

      ...JDOM stuff....    

      return config;
   }
}

The problems I face:

• how to assert a File Object an FileInputStream (PowerMockito) like this File belongs to this FileInputStream with the following content
• how to mock a private method (Mockito/PowerMockito)

an example of the Mocking/not working :)….

public class ParserTest {

   @Test
   public final void testParse() {
      File MOCKEDFILE = PowerMockito.mock(File.class);
      PowerMockito.when(MOCKEDFILE.exists()).thenReturn(true);
      PowerMockito.when(MOCKEDFILE.isFile()).thenReturn(true);
      PowerMockito.when(MOCKEDFILE.isDirectory()).thenReturn(false);
      PowerMockito.when(MOCKEDFILE.createNewFile()).thenReturn(true);
      PowerMockito.when(MOCKEDFILE.length()).thenReturn(11111111L);

      //what about the path of MOCKEDFILE which isn't existing
      PowerMockito.when(MOCKEDFILE.getPath()).thenReturn(?????);

      //how to assign a File an FileInputStream? (I thought something like)
      PowerMockito.mockStatic(FileInputStream.class);
      FileInputStream MOCKEDINPUTSTREAM = PowerMockito.mock(FileInputStream.class);
      PowerMockito.whenNew(FileInputStream.class).withArguments(File.class).thenReturn(MOCKEDINPUTSTREAM);

      //how to mock the private method readMyStream
   }

Answer 1

Rather than mocking File, I would suggest using the TemporaryFolder Rule and create / not create the file as appropriate.

In my previous projects, we have written a FileInputStreamSupplier class that is used to create a FileInputStream. This class can then be mocked to provide a mocked FileInputStream to allow for testing behavior. Generically, you could have your class take a Supplier<FileInputStream> (using Guava) and mock that.