Question

I have method for which I need to create a JUnit test:

public class MyClass {
  private String file1;
  private String file2;

  public void myMethodSpaceCheck(){
    if (new File(file1).size() > new File(file2).size() {
       throw new Exception .....  
    } 
  }
}

Is it possible to use Mockito to create that JUnit test?

Answer 1

When dealing with files in Java, my preferred option is to go with Apache VFS, as I can then treat them as any other POJO. Obviously, that’s a lot of work when you are already stuck with the File API.

Another option is to forget Mockito entirely and write those files on the system. I usually avoid that, as it sometimes make it harder to have tests run in parallel on some systems.

For this specific situation, my solution is generally to provide a special class, say FileBuilder, that can instantiate new Files:

public class FileBuilder {
  public java.io.File newFile(String pathname) {
    return new java.io.File(pathname);
  }
}

I then mock this class before passing it to MyClass, and instrument it as appropriate:

@Test(expected = Exception.class)
public void should_fail_when_file1_is_bigger_than_file2() {
  FileBuilder mockFile1 = file(2L);
  FileBuilder mockFile2 = file(1L);
  FileBuilder mockFileBuilder = mock(FileBuilder.class);
  when(mockFileBuilder.newFile("file1").thenReturn(mockFile1);
  when(mockFileBuilder.newFile("file2").thenReturn(mockFile2);

  new MyClass(mockFileBuilder).myMethodSpaceCheck();
}

private static File file(long length) {
    File mockFile = mock(File.class);
    when(mockFile.length()).thenReturn(length);
    return mockFile;
}

(your example mentions File.size(); I assumed you meant File.length())

The actual implementation of MyClass would look like this:

public class MyClass {
  private String file1;
  private String file2;
  private final FileBuilder fileBuilder;

  public MyClass() {
    this(new FileBuilder());
  }

  @VisibleForTesting
  MyClass(FileBuilder fileBuilder) {
    this.fileBuilder = fileBuilder;
  }

  public void myMethodSpaceCheck() //...
}