无法为我的void方法编写Mockito测试用例

Question

I need to test this code with Mockito (JUnit):

public class Calculation {

    public void logTimeTaken(String label, long estimatedTime, int size, boolean isDebug) {
       String out = label + " took " + TimeUnit.MILLISECONDS.convert(estimatedTime, TimeUnit.NANOSECONDS) + " milliseconds for " + size + " events!";
        if (isDebug) {
            System.out.println(out);
        } else {
            System.out.println(out);
        }
    }
}

I search so many examples google but still not getting any idea.

Answer 1

You can configure System with an instance of PrintStream which you can then assert against after invoking Calculation.logTimeTaken .

Here's an example:

@Test
public void canLogTimeTaken() {
    ByteArrayOutputStream bout = new ByteArrayOutputStream();
    PrintStream out = new PrintStream(bout);
    System.setOut(out);

    Calculation sut = new Calculation();
    sut.logTimeTaken("label", 20 , 2, false);
    assertEquals("if isDebug is false label took 0 milliseconds for 2 events!\n", bout.toString());
}

Note: there is no need for Mockito here, this is just vanilla JUnit, no mocking.

But , it might be a better design to refactor logTimeTaken into two distinct aspects:

• Deriving the log message
• Logging that message

For example:

public String createTimeTakenMessage(String label, long estimatedTime, int size, boolean isDebug) {
    return label + " took " + TimeUnit.MILLISECONDS.convert(estimatedTime, TimeUnit.NANOSECONDS) + " milliseconds for " + size + " events!";
}

public void logTimeTaken(String message) {
    System.out.println(message);
}

Then testing createTimeTakenMessage is trivial and you might even choose not to test logTimeTaken at all since all it does is invoke a System method. Or, perhaps you would hide the 'log action' behind an interface with an implementation using System.out now and perhaps, later, other implementations using a formal logging framework such as Logback.