Using AssertJ to Verify a Complex Exception

9:01 AM 0 Comments

AssertJ just went up another notch in my book today

For a particular unit test, I needed to verify the contents of an exception. Originally, I figured said contents where just part of the exception message:

assertThatCode(() -> doMyExceptionalThing())
    .isInstanceOf(SomeException.class)
    .hasMessage("This is the error message")

However, that assertion failed because, in the case that this particular exception has a cause, it has two messages to pick from (more on that in a moment) and it chooses the message from the cause instead of the message I wanted.

I could do something like this to get it to work:

try {
    doMyExceptionalThing();
} catch ( SomeException ex ) {
    ErrorDomainObject o = ex.getError();

    // this is the description I need,
    // not the cause's description!
    assertThat(o.getDescription())
        .isEqualTo("This is the error message");
}

But that would break my heart. (And we wouldn't want that... just to clarify)

So, I tried some of AssertJ's matching features:

assertThatCode(() -> doMyExceptionalThing())
    .isInstanceOf(SomeException.class)
    .matches(ex -> "This is the error message"
        .equals(
            ((SomeException) ex)
                .getError()
                .getDescription()));

Yuck!

And then, I came upon hasFieldOrPropertyWithValue. Hmm...

Could something like this possibly work?

assertThatCode(() -> doMyExceptionalThing())
    .isInstanceOf(SomeException.class)
    .hasFieldOrPropertyWithValue(
        "error.description",
        "This is the error message");

And it did! All hail AssertJ for helping me create a clean, readable assertion, even in this slightly more complex scenario.