assertThrows doesn't fail if the thrown exception is of child type

[govi20]

Steps to reproduce

assertThrows(IOException.class, () -> service.readFile("sampleFile.txt"));

this assertion will pass even if the exception is FileNotFoundException.

I believe this is happening because we are using Class#isInstance it will return true for parent types also.

https://github.com/junit-team/junit5/blob/637a78393194737e77d6f9681ad93e9ddac1f399/junit-jupiter-api/src/main/java/org/junit/jupiter/api/AssertThrows.java#L58

Shouldn't this be replaced with if (expectedType == actualException.getClass()) ?

[sormuras]

Works for me:

  @Test
  void testAssertThrows() {
    assertThrows(FileNotFoundException.class, this::throwFileNotFoundException);
    assertThrows(IOException.class, this::throwFileNotFoundException);
    assertThrows(Exception.class, this::throwFileNotFoundException);
    assertThrows(Throwable.class, this::throwFileNotFoundException);

    // assertThrows(FileNotFoundException.class, this::throwIOException); // fails
    assertThrows(IOException.class, this::throwIOException);
    assertThrows(Exception.class, this::throwIOException);
    assertThrows(Throwable.class, this::throwIOException);
  }

  private void throwFileNotFoundException() throws FileNotFoundException {
    throw new FileNotFoundException();
  }

  private void throwIOException() throws IOException {
    throw new IOException();
  }

With assertThrows(FileNotFoundException.class, this::throwIOException) it yields:

Unexpected exception type thrown ==> expected: <java.io.FileNotFoundException> but was: <java.io.IOException>

[govi20]

@sormuras Seems, it's the reverse case, if the assertion expects Parent type and if the test code throws child type Exception it always passes.

    @Test
    void testAssertThrows() {
        assertThrows(IOException.class, this::throwCustomException);
     }

    private void throwCustomException() throws FileNotFoundException {
        throw new FileNotFoundException();
    }

As a workaround, I am checking the Class type explicitly

    Throwable throwable =  assertThrows(Throwable.class, () -> {
	service.readFile("sampleFile.txt");
    });
    assertEquals(FileNotFoundException.class, throwable.getClass());

[sormuras]

The current behaviour is intended. The API documentation of assertThrows could need a refinement pass, though.

Be as specific as possible, when using assertThrows.

[govi20]

@sormuras thanks for the feedback,
Even if I assert on the specific type, but if the specific type is RuntimeException and somebody changes the behavior and changes it to child type. shouldn't the test fail?

for e.g. below code:

    @Test
    void testHttpErrorCodeException() {
        assertThrows(HttpResponseException.class, () -> reThrowSpecificException(500));
        assertThrows(RuntimeException.class, () -> reThrowSpecificException(404));
     }

    private void reThrowSpecificException(int httpsStatusCode) {
        if(httpsStatusCode == 500) {
            throw new HttpResponseException();
        } else {
            throw new RuntimeException();
        }
    }

Now if someone modifies the code as below, I feel the second assertion should fail.

    @Test
    private void reThrowSpecificException(int httpsStatusCode) {
        throw new HttpResponseException();
    }

[marcphilipp]

The current behavior is designed to work like in a try-catch block. We could introduce another assertion, e.g. assertThrowsExactly.

[sormuras]

Yeah.

Perhaps a better name would have been: assertCatches(? extends Throwable, try-block)

[sormuras]

Also related to #2492, where an assertInstanceOf(...) is being introduced.

[marcphilipp]

@govi20 Could you please create a new issue, describe your use case and propose adding an assertThrowsExactly assertion method?

[govi20]

@marcphilipp hi, created an enhancement issue #2509

if you guys are okay with the new API, I am happy to work on the patch.