How to test exceptions

Testing exceptions in JUnit is straightforward and a core part of unit testing—you want to ensure your code fails the right way when given bad input.

Here’s how to test exceptions using JUnit 5 and JUnit 4, with a few best practices along the way.

✅ JUnit 5: Using assertThrows

🔥 Recommended Approach

import org.junit.jupiter.api.Test;
import static org.junit.jupiter.api.Assertions.*;

public class CalculatorTest {

    @Test
    void testDivideByZeroThrowsException() {
        Calculator calc = new Calculator();

        ArithmeticException exception = assertThrows(
            ArithmeticException.class,
            () -> calc.divide(10, 0)
        );

        assertEquals("/ by zero", exception.getMessage());
    }
}

✅ JUnit 4: Using ExpectedException Rule or try-catch

✔ Option 1: Use assertThrows (if using JUnit 4.13+)

import static org.junit.Assert.assertThrows;

@Test
public void testDivideByZero() {
    Calculator calc = new Calculator();

    ArithmeticException ex = assertThrows(
        ArithmeticException.class,
        () -> calc.divide(10, 0)
    );

    assertEquals("/ by zero", ex.getMessage());
}

✔ Option 2: Use ExpectedException Rule (older style, JUnit 4.12)

import org.junit.Rule;
import org.junit.Test;
import org.junit.rules.ExpectedException;

public class CalculatorTest {

    @Rule
    public ExpectedException thrown = ExpectedException.none();

    @Test
    public void testDivideByZeroThrows() {
        Calculator calc = new Calculator();

        thrown.expect(ArithmeticException.class);
        thrown.expectMessage("/ by zero");

        calc.divide(10, 0);
    }
}

❌ Not Recommended: Plain try-catch (works, but verbose)

@Test
public void testDivideByZero() {
    Calculator calc = new Calculator();

    try {
        calc.divide(10, 0);
        fail("Expected ArithmeticException");
    } catch (ArithmeticException ex) {
        assertEquals("/ by zero", ex.getMessage());
    }
}

📌 Summary

💡 Bonus: Custom Exceptions

Let’s say you have a custom exception:

public class InvalidUserException extends RuntimeException {
    public InvalidUserException(String msg) {
        super(msg);
    }
}

Test it like so:

@Test
void testThrowsCustomException() {
    UserService service = new UserService();

    InvalidUserException ex = assertThrows(
        InvalidUserException.class,
        () -> service.loadUser("invalid-user")
    );

    assertTrue(ex.getMessage().contains("not found"));
}