1. Overview
In this tutorial, we’ll explore how to use JUnit fail assertion for common testing scenarios.
We’ll also see fail() method differences between JUnit 4 and JUnit 5.
2. Using fail Assertion
The fail assertion fails a test throwing an AssertionError unconditionally.
When writing unit tests, we can use fail to explicitly create a failure under desired testing conditions. Let’s see some cases where this can be helpful.
2.1. Incomplete Test
We can fail a test when it is incomplete or not yet implemented:
@Test
public void incompleteTest() {
fail("Not yet implemented");
}
2.2. Expected Exception
We can also do it when we think an exception will happen:
@Test
public void expectedException() {
try {
methodThrowsException();
fail("Expected exception was not thrown");
} catch (Exception e) {
assertNotNull(e);
}
}
2.3. Unexpected Exception
Failing the test when an exception is not expected to be thrown is another option:
@Test
public void unexpectedException() {
try {
safeMethod();
// more testing code
} catch (Exception e) {
fail("Unexpected exception was thrown");
}
}
2.4. Testing Condition
We can call fail() when a result doesn’t meet some desired condition:
@Test
public void testingCondition() {
int result = randomInteger();
if(result > Integer.MAX_VALUE) {
fail("Result cannot exceed integer max value");
}
// more testing code
}
2.5. Returning Before
Finally, we can fail a test when the code doesn’t return/break when expected:
@Test
public void returnBefore() {
int value = randomInteger();
for (int i = 0; i < 5; i++) {
// returns when (value + i) is an even number
if ((i + value) % 2 == 0) {
return;
}
}
fail("Should have returned before");
}
3. JUnit 5 vs JUnit 4
All assertions in JUnit 4 are part of org.junit.Assert class. For JUnit 5 these were moved to org.junit.jupiter.api.Assertions.
When we call fail in JUnit 5 and get an exception, we receive an AssertionFailedError instead of AssertionError found in JUnit 4.
Along with fail() and fail(String message), JUnit 5 includes some useful overloads:
- fail(Throwable cause)
- fail(String message, Throwable cause)
- fail(Supplier<String> messageSupplier)
In addition, all forms of fail are declared as public static <V> V fail() in JUnit 5. The generic return type V, allows these methods to be used as single-statement in lambda expressions:
Stream.of().map(entry -> fail("should not be called"));
4. Conclusion
In this article, we covered some practical use cases for the fail assertion in JUnit. See JUnit Assertions for all available assertions in JUnit 4 and JUnit 5.
We also highlighted the main differences between JUnit 4 and JUnit 5, and some useful enhancements of the fail method.
As always, the full source code of the article is available over on GitHub.
