Testing for Exceptions in JUnit

When writing unit tests, it’s essential to verify that your code behaves as expected when encountering exceptional conditions. In this tutorial, we’ll explore how to test for exceptions using JUnit.

Introduction to Exception Testing

Exception testing is a critical aspect of ensuring the reliability and robustness of your code. By verifying that your methods throw the correct exceptions under specific circumstances, you can guarantee that your application handles errors correctly and provides meaningful feedback to users.

Using @Test(expected = Exception.class)

One way to test for exceptions in JUnit is by using the expected attribute on the @Test annotation. This approach allows you to specify the expected exception type, which will be verified during test execution.

@Test(expected = IndexOutOfBoundsException.class)
public void testIndexOutOfBoundsException() {
    ArrayList emptyList = new ArrayList();
    Object o = emptyList.get(0);
}

However, this method has limitations, as it only verifies that the method under test throws an exception of the specified type. It doesn’t provide a way to inspect the exception’s message or other properties.

Using ExpectedException Rule

Another approach is to use the ExpectedException rule, which provides more flexibility and control over exception testing. This rule allows you to specify the expected exception type, message, and other properties.

public class FooTest {
    @Rule
    public final ExpectedException exception = ExpectedException.none();

    @Test
    public void doStuffThrowsIndexOutOfBoundsException() {
        Foo foo = new Foo();
        exception.expect(IndexOutOfBoundsException.class);
        foo.doStuff();
    }
}

Using Assertions.assertThrows()

In JUnit 5 and later, you can use the assertThrows() method to test for exceptions. This approach provides a more concise and expressive way to verify that your code throws exceptions as expected.

@Test
void exceptionTesting() {
    ArithmeticException exception = assertThrows(ArithmeticException.class, () -> calculator.divide(1, 0));
    assertEquals("/ by zero", exception.getMessage());
}

Using Try-Catch Block

You can also use a try-catch block to test for exceptions. This approach provides more control over the testing process and allows you to inspect the exception’s properties.

@Test
public void testFooThrowsIndexOutOfBoundsException() {
    try {
        foo.doStuff();
        fail("Expected exception was not thrown.");
    } catch (IndexOutOfBoundsException e) {
        // Verify exception properties
    }
}

Best Practices

When testing for exceptions, keep the following best practices in mind:

• Be specific: Use the most specific exception type possible to ensure that your test is accurate.
• Inspect exception properties: Verify that the exception’s message, cause, and other properties match your expectations.
• Keep tests concise: Use the assertThrows() method or ExpectedException rule to keep your tests concise and expressive.

By following these best practices and using the techniques outlined in this tutorial, you can ensure that your code is robust, reliable, and handles exceptions correctly.