Interesting way to assert thrown exceptions using Spring Boot and Assertj

Traditional exception handling in JUnit tests

You’re probably used to testing code that throws exceptions in either one of the following two ways:

• JUnit's @Test annotation expected parameter

@Test(expected = UserService.NoSuchUserException.class)
public void deletesExistingUser() {
    final User existingUser = spy(new User());
    given(userRepository.findOne(eq("123"))).willReturn(existingUser);
    given(userRepository.save(eq(existingUser))).willReturn(existingUser);

    userService.deleteUser("123");

    verify(existingUser).setState(eq(User.State.DELETED));
}

The disadvantage of this approach is that we can't be sure that the exception thrown was thrown by the code we expect to throw it

• JUnit’s ExpectedException @Rule:

To gain control over which part of test should throw an exception, we can use ExpectedException JUnit rule, here's an example:

@Rule
ExpectedException thrown = ExpectedException.none();

@Test
public void deletesExistingUser() {
    thrown.expect(UserService.NoSuchUserException.class);
    thrown.expectMessage(format("User with id %s doesn't exist!", userId));
    final User existingUser = spy(new User());
    given(userRepository.findOne(eq("123"))).willReturn(existingUser);
    given(userRepository.save(eq(existingUser))).willReturn(existingUser);

    userService.deleteUser("123");

    verify(existingUser).setState(eq(User.State.DELETED));
}

Whereas the second approach gives more control over expressing where the test code should throw an exception, there's even better approach to design your JUnit tests, using powerful AssertJ testing library.

AssertJ approach

The one I found quite elegant and easy to use and understand comes from powerful AssertJ library, using static Assertions.assertThatThrownBy static method, as shown in the following test:

@Test
public void failsDeletingUser() {
    final String userId = "123";
    given(userRepository.findOne(eq(userId))).willReturn(null);
    assertThatThrownBy(() -> userService.deleteUser(userId))
            .isInstanceOf(UserService.NoSuchUserException.class)
            .hasMessage(format("User with id %s doesn't exist!", userId));
}

It receives instance of ThrowingCallable functional interface (Java 8) which is quite handy since you can pass lambda implementation (as shown). Given interface has only one method, which we implemented as lambda above. The interface looks like:

public interface ThrowingCallable {
    void call() throws Throwable;
}

In upcoming version 5 of JUnit framework there’s no more option to use expected attribute in @Test annotation. Having two options there, my favourite will be option coming from AssertJ library.

Let me know your thoughts!

That was all for today! Hope you liked it!