How Do I Assert Exception Handling in JUnit Tests?

Asserting Exception Handling in JUnit Tests

In JUnit, testing code that should throw an exception requires a clean and concise approach. While manually checking for exceptions is possible, it's not the idiomatic way.

JUnit 5 and 4.13

In JUnit versions 5 and 4.13, you can use @Test(expected = ExceptionClass.class) annotation on the test method. This expects the specified exception to be thrown.

Example:

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

AssertJ and Google-Truth

If using libraries like AssertJ or Google-Truth, you can employ their assertions for validating exceptions.

AssertJ:

import static org.assertj.core.api.Assertions.assertThatThrownBy;

@Test
public void testFooThrowsIndexOutOfBoundsException() {
    assertThatThrownBy(() -> foo.doStuff()).isInstanceOf(IndexOutOfBoundsException.class);
}

Google-Truth:

import static com.google.common.truth.Truth.assertThat;

@Test
public void testFooThrowsIndexOutOfBoundsException() {
    assertThat(assertThrows(IndexOutOfBoundsException.class, foo::doStuff)).isNotNull();
}

JUnit

In JUnit versions less than or equal to 4.12, you can use Rule or TryCatch to handle exceptions.

Using Rule:

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

@Test
public void testIndexOutOfBoundsException() {
    thrown.expect(IndexOutOfBoundsException.class);
    ArrayList emptyList = new ArrayList();
    emptyList.get(0);
}

Using TryCatch:

import static org.junit.Assert.assertEquals;

@Test
public void testIndexOutOfBoundsException() {
    try {
        ArrayList emptyList = new ArrayList();
        emptyList.get(0);
        fail("IndexOutOfBoundsException was expected");
    } catch (IndexOutOfBoundsException e) {
        assertEquals(e.getClass(), IndexOutOfBoundsException.class);
    }
}

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