Testing Exceptions in Kotlin With assertFailsWith

I wanted to write this short post to highlight the assertFailsWith function available to Kotlin that makes testing exceptions a bit easier. Testing exceptions isn’t something fancy or new to JVM languages (from now on, I will use Java for comparisons), but Kotlin comes with the nice extra benefit of providing this functionality as part of its standard library. Comparing this to Java, you are likely to bring AssertJ into the mix to achieve similar results.

The main purpose of this post is to make you aware of the assertFailsWith function. I personally did not know it existed for a while and defaulted to depending on AssertJ. Not that I have anything against AssertJ that is. There are many other features that the library provides, but for this specific instance, it might be possible to remove it (assuming you are not using it for anything else that is).

What is good about assertFailsWith and AssertJ in general? It provides better exception testing than the simple constructs that JUnit provides. More precisely, it allows you to specify which part of your test that you expect an exception to be thrown, instead of declaring that an exception will arise somewhere in the code. This could lead to exceptions being incorrectly swallowed by test at an incorrect point and tricking you into thinking it is working as you think it should.

Now that I have that brief point out of the way, let’s get on with the main content of this post. Below is what assertFailsWith looks like inside a test:

@Test
fun `calling hereIsAnException will return an exception no matter what`() {
  assertFailsWith<IllegalArgumentException> { hereIsAnException() }
}

In this example, hereIsAnException is placed inside the body of assertFailsWith, who checks that an IllegalArgumentException is thrown. If one is not raised, then the assertion will fail. If one does occur, then the assertion will pass and the exception is caught.

Catching the exception allows the execution of the test code to continue if needed as well as allowing you to make further assertions on the state of the exception.

For example, is it a wrapper around another exception (what is the type of its cause property)?

@Test
fun `original cause for exception was IndexOutOfBoundsException`() {
  val exception = assertFailsWith<IllegalArgumentException> { hereIsAnException() }
  assertTrue(exception.cause is IndexOutOfBoundsException)
}

Is the message what you expect (not the most sturdy of checks)?

@Test
fun `exception has the correct message`() {
  val exception = assertFailsWith<IllegalArgumentException> { hereIsAnException() }
  assertEquals("I am a failure...", exception.message)
}

Only exceptions that are of the same type or subtype, as specified by assertFailsWith, will be caught. Any others will cause the test to to fail. Since it catches subtypes, please don’t go around just specifying Exception or RuntimeException. Try to be precise, so your tests are as useful as possible.

As touched on earlier, assertFailsWith will only catch an exception that is thrown within the body of the function. Therefore, if this was written instead:

@Test
fun `calling hereIsAnException will return an exception no matter what`() {
  hereIsAnException()
  assertFailsWith<IllegalArgumentException> { hereIsAnException() }
}

The test would fail. hereIsAnException has thrown an exception, which has not been caught and leads to the test failing. I believe this is the best part of this sort of function over the previous ways this used to be done (e.g. asserting inside @Test that an exception would occur).

@Test
fun `calling hereIsAnException will return an exception no matter what`() {
  assertFailsWith<IllegalArgumentException>("This should throw an illegal argument exception") { hereIsANormalReturnValue() }
}

I personally have never really used the message part of an assertion. Maybe you do, so I thought I’d at least let you know.

Before I wrap up the little amount of content in this post, let’s have a quick look at AssertJ so that we can draw a comparison between the two. Again, this is only for the case of catching exceptions, which is only a small part of what AssertJ provides.

@Test
fun `calling hereIsAnException will return an exception no matter what`() {
  assertThatExceptionOfType(IllegalArgumentException::class.java).isThrownBy {
    hereIsAnException()
  }
}

This is slightly more “verbose” than the assertFailsWith version. But, that is made up for with the plethora of functions that AssertJ provides that makes any further checking of the returned exception much easier. More precisely, when using assertFailsWith, I needed to write another assertion to check the message. In AssertJ, this is just a function chained onto the end of the previous call.

To conclude, assertFailsWith is a nice little function to use in testing to ensure that a piece of code throws a specific type of exception. It is built into the Kotlin standard library, which removes the need to bring in an extra dependency to your project. That being said, it is a relatively simple function and does not bring the sort of functionality that a library like AssertJ would. It is likely to suffice until you want to write tests that contain a wide range or assertions as this is the point where it can get messy.

The official docs for assertFailsWith can be found here if you are interested Kotlin Docs – assertFailsWith.

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