How to Assert Exceptions in JUnit 5 using Assertions.assertThrows()?

Asserting Exceptions in JUnit 5 with Assertions.assertThrows()

In JUnit 5, asserting that a method throws an exception can be achieved through the Assertions.assertThrows() method. This provides a convenient and expressive way to test various exception scenarios, particularly when dealing with multiple exceptions in a single test.

Consider the following test case:

import static org.junit.jupiter.api.Assertions.assertThrows;

@Test
void exceptionTesting() {
    MyException thrown = assertThrows(
        MyException.class,
        () -> myObject.doThing(),
        "Expected doThing() to throw, but it didn't"
    );

    assertTrue(thrown.getMessage().contains("Stuff"));
}

Here's how Assertions.assertThrows() works:

1. Exception Type: The first argument to assertThrows() is the expected exception type. In this case, we expect MyException to be thrown.
2. Executable Lambda: The second argument is a lambda expression representing the executable code under test. It should invoke the method that is expected to throw an exception.
3. Message (Optional): The third argument (optional) allows you to specify a custom error message in case the assertion fails.

Benefits of Using Assertions.assertThrows():

• Concise and Readable: Assertions.assertThrows() simplifies exception testing compared to using a @Rule, making your tests more compact and readable.
• Multiple Exceptions Testing: This method allows you to test multiple exceptions within the same test case.
• Cleaner Assertions: Instead of manually checking for the thrown exception, assertThrows() provides a more concise and declarative way to assert exception conditions.

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