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JUnit: Testing for Specific Exceptions

Emma Brown
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#java #programming
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Tired of clunky exception testing in JUnit? Learn the elegant, idiomatic way to assert expected exceptions – no more boolean flags!

How do I idiomatically test exception throwing in JUnit? Is there a better approach than manually catching and checking a boolean flag?

Solution in a Nutshell

Asserting Exceptions in JUnit (Java, Spring Boot)

JUnit offers assertThrows (JUnit 5+) for concise exception testing. For older JUnit versions, use a try-catch block with assertTrue.

JUnit 5 (Recommended):

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

class MyServiceTest {
    @Test
    void testException() {
        assertThrows(IllegalArgumentException.class, () -> myService.doSomething(-1));
    }
}

This asserts IllegalArgumentException is thrown when myService.doSomething(-1) is called.

JUnit 4 (Older Versions):

import org.junit.Test;
import static org.junit.Assert.assertTrue;

public class MyServiceTest {
    @Test
    public void testException() {
        boolean exceptionThrown = false;
        try {
            myService.doSomething(-1);
        } catch (IllegalArgumentException e) {
            exceptionThrown = true;
        }
        assertTrue(exceptionThrown);
    }
}

This achieves the same but is less readable. Migrate to JUnit 5 for improved clarity and features. Always specify the exact exception type. Consider adding message checks for robustness: assertEquals("Specific error message", e.getMessage()); within the catch block for JUnit 4. Remember to handle potential exceptions appropriately in your production code.