How to test that no exception is thrown?

java

assertions

junit

test-coverage

byAnton Shumikhin·Oct 12, 2024

// Using a try-catch construct. This is like doing math with a hammer:
try {
    targetMethod();
} catch (Exception e) {
    assert false : "Expected no surprises, but BOOM 💥: " + e;
}

In JUnit tests, your method is a little angel if no exception is thrown:

@Test
public void shouldNotThrowException() {
    targetMethod(); // Innocent until proven guilty!
}

Feeling fancy with AssertJ? Here's a more fluent style:

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

@Test
public void shouldNotThrowExceptionWithAssertJ() {
    assertThatCode(() -> targetMethod()).doesNotThrowAnyException(); // Swag by Fluent API University 🎩
}

Surefire ways to assert no exceptions

Expressing clear intention with assertDoesNotThrow

assertDoesNotThrow() gives your test readability a steroid shot💉:

@Test
public void testMethodWithoutException() {
    Assertions.assertDoesNotThrow(() -> targetMethod()); // "I've got 99 problems, but an exception ain't one!" 🎶
}

Keep test cases squat and muscular

Each test method should flex one single behavior. This keeps your test suite ripped and ready to fight off bugs 💪:

Break up is hard to do, thanks to assertAll():

@Test
public void testMultipleAspects() {
    assertAll("Should not throw exceptions and yet change state like a chameleon 🦎",
        () -> Assertions.assertDoesNotThrow(() -> targetMethod()),
        () -> assertEquals(expectedState, actualState)
    );
}

Test a variety bag of inputs

Feed these to JUnit's Parameterized tests for thorough cross-examinations:

@ParameterizedTest
@ValueSource(strings = {"input1", "input2", "input3"}) // "Eat this, and this, and this..." 🍎🥦🍔
public void testWithMultipleInputs(String input) {
    Assertions.assertDoesNotThrow(() -> targetMethod(input));
}

This effectively runs our assertDoesNotThrow() test with different meal courses.

Playing catch with specific exceptions

Catching Exception is like using a fishing net for a goldfish—always be specific:

@Test(expected = SpecificException.class) 
public void testForSpecificException() {
    methodThatThrowsSpecificException(); // "Catch me if you can!" 🐠
}