How can I check exception handling in JUnit?

Question

that if an error occurs, an exceptional situation will actually occur. How to make a test so that it checks that an exception really occurs?

 private String getTemplate(String templateName) {
    try {
        FileReader reader = new FileReader(
                getClass().getClassLoader().getResource("templates/" 
                   + templateName).getFile()
        );
        StringBuilder templateString = new StringBuilder();

        char[] symbol = new char[256];
        int readBytes;
        while((readBytes = reader.read(symbol)) > 0)
            for (int i = 0; i < readBytes; i++)
                templateString.append(symbol[i]);

        return templateString.toString();
    } catch (NullPointerException e) {
        e.printStackTrace();
        return "{'detail': 'internal server error: page not found'}";
    }  catch (IOException e) {
        e.printStackTrace();
        return "{'detail': 'internal server error: page cannot be read'}";
    }
}

Answer 1

Basically your problem is that you wrote hard to test code:

    FileReader reader = new FileReader(
            getClass().getClassLoader().getResource("templates/" 
               + templateName).getFile()

This call to new basically translates to: if at all, you need PowerMock(ito) or JMockit to gain control over your production code.

What you could do instead: pass a FileReader object to that method, so that it just calls method on that object. And then you can use Mockito to provide a mocked FileReader to that code. By doing so, you can then control what happens when you test your production code.

And then you could do something like:

@Test
public void testNPE() {
  Mockito.when(mockedFileReader.read(ArgumentsMatcher.any())).thenThrow(new NullPointerException("haha"));
  String result = objectUnderTest.callAPublicMethodThatCallsTheMethodUnderTest(foo);

  assertThat(result, is("{'detail': 'internal server error: page not found'}");
  }

The above instructs that mocked file reader to throw a NPE when its read() method is called. Then you would expect your production code to return that string.

As you can see, there are a lot of things you have to "fill":

• you can't call private methods, so obviously, you would have to call a public entry point to your class under test
• it only works when that string coming out of the private method somehow shows up in the response of that public method
• and, as said: somehow that mocked file reader needs to get into that objectUnderTest

Finally: it is a very bad idea to catch NPE, and assume that this means "page not found". A null pointer is just that: a reference to null. If at all, your code should be checking some condition to come to the conclusion "page not found". A NPE can happen for many reasons. There could be a code bug, and you go and tell the customer "page not found"?!

Thus the real answer is a different one: you should step back and do quite a bit of research how you do proper unit testing, maybe using a framework like Mockito. And then you learn how to write code that can be easily tested. As said: the code you wrote is basically untestable. You better fix that, instead of hoping that some magic framework helps you working around that inherent problem of your code base.