JUnit unit testing framework: The best way to test exceptions

The JUnit unit testing framework provides three methods for testing exceptions: 1. Expected exception assertion, which allows you to specify the type of exception expected to be thrown; 2. Exception message assertion, which can verify whether the exception has the expected message; 3. Exception cause assertion, Used to verify the root cause of an exception.

JUnit Unit Testing Framework: The Best Way to Test Exceptions

JUnit is a unit testing framework widely used by Java developers. It provides various assertion methods to verify the expected results of the test. For testing exceptions, JUnit provides a set of specialized methods.

1. Expected exception assertion

@Test(expected = ExceptionClass.class) Annotations allow us to specify specific exceptions that should be thrown in the test method type. If the expected exception is not thrown, the test will fail.

@Test(expected = NullPointerException.class)
public void testNullPointerException() {
    String str = null;
    str.toUpperCase();
}

2. Exception message assertion

Using the assertThrowWithMessage method, we can not only verify whether an exception is raised, but also verify whether its message As expected.

@Test
public void testExceptionMessage() {
    Exception exception = assertThrows(Exception.class, () -> {
        throw new Exception("Custom Exception");
    });
    assertEquals("Custom Exception", exception.getMessage());
}

3. Exception cause assertion

Using the assertCause method, we can verify the root cause of the exception (if any).

@Test
public void testExceptionCause() {
    Exception cause = new Exception("Cause Exception");
    Exception exception = new Exception("Actual Exception", cause);
    Exception actualCause = assertThrows(Exception.class, () -> { throw exception; }).getCause();
    assertEquals(cause, actualCause);
}

Practical case

In the following example, we use JUnit to test a method that may throw an ArithmeticException exception:

public class Calculator {

    public int divide(int numerator, int denominator) {
        if (denominator == 0) {
            throw new ArithmeticException("Cannot divide by zero");
        }
        return numerator / denominator;
    }
}

@ExtendWith(SpringExtension.class)
public class CalculatorTest {

    @Test(expected = ArithmeticException.class)
    public void testDivideByZero() {
        Calculator calculator = new Calculator();
        calculator.divide(10, 0);
    }
}

Tip

• When testing exceptions, it is important to use explicit exception types.
• Exception message assertions ensure that exceptions have the expected error message text.
• Exception cause assertions are useful for validating complex exception hierarchies.

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