How To Test For Exceptions With PHPUnit?

Installing and Configuring PHPUnit

To test exceptions with PHPUnit, you need to install and set it up. Here's how:

• Install PHPUnit:

  1. Use Composer to install PHPUnit in your project:

     composer require --dev phpunit/phpunit

  2. Run PHPUnit from the command line:

     ./vendor/bin/phpunit

• Configure for exception testing:

  1. Create a phpunit.xml file in your project root:

     <?xml version="1.0" encoding="UTF-8"?>
     <phpunit bootstrap="vendor/autoload.php">
     <testsuites>
        <testsuite name="YourTestSuite">
            <directory>tests</directory>
        </testsuite>
     </testsuites>
     </phpunit>

  2. This configuration shows PHPUnit where to find your tests and sets up autoloading.

3. For exception testing, no extra configuration is needed. PHPUnit's methods for exception testing work as is.

With PHPUnit installed and configured, you can start writing exception tests for your PHP code.

Using expectException() Method

The expectException() method tests for exceptions in PHPUnit. Here's how to use it:

• Syntax: $this->expectException(ExceptionClassName::class);
• Usage: Call this method before the code that should throw the exception.

Example of expecting a specific exception:

public function testDivisionByZero()
{
    $this->expectException(DivisionByZeroError::class);
    $calculator = new Calculator();
    $calculator->divide(10, 0);
}

In this example, we expect a DivisionByZeroError when we try to divide by zero.

public function testComplexException()
{
    $this->expectException(CustomException::class);
    $this->expectExceptionMessage('Invalid operation');
    $this->expectExceptionCode(500);

    $complexOperation = new ComplexOperation();
    $complexOperation->execute();
}

Alternative: setExpectedException() for Older PHPUnit Versions

For older versions of PHPUnit (before 5.2), use the setExpectedException() method:

• How to use: $this->setExpectedException(ExceptionClassName::class);
• Call this method before the code that should throw the exception.

Example:

public function testInvalidArgument()
{
    $this->setExpectedException(InvalidArgumentException::class);
    $validator = new Validator();
    $validator->validate('invalid input');
}

Differences from expectException():

• setExpectedException() is deprecated in newer PHPUnit versions.
• It allows setting the expected exception message and code in one method call.
• expectException() is part of a set of methods for more detailed exception testing.

Both methods test for exceptions, but expectException() is the current standard and offers more options in newer PHPUnit versions.

Best Practices for Exception Test Cases

When writing exception tests in PHPUnit, follow these practices:

• Isolating exception-throwing code:
  • Test one exception per test method.
  • Focus the test on the code that throws the exception.
  • Use a separate test method for each exception scenario.

Example of isolating exception-throwing code:

public function testDivisionByZero()
{
    $calculator = new Calculator();
    $this->expectException(DivisionByZeroError::class);
    $calculator->divide(10, 0);
}

• Testing for exception messages and codes:
  • Use expectExceptionMessage() to check exception messages.
  • Use expectExceptionCode() to verify the exception code.

Example of testing exception details:

public function testInvalidInput()
{
    $validator = new Validator();
    $this->expectException(InvalidArgumentException::class);
    $this->expectExceptionMessage('Input must be a positive number');
    $this->expectExceptionCode(400);
    $validator->validatePositiveNumber(-5);
}

/**
 * @dataProvider invalidInputProvider
 */
public function testInvalidInputs($input, $expectedMessage)
{
    $validator = new Validator();
    $this->expectException(InvalidArgumentException::class);
    $this->expectExceptionMessage($expectedMessage);
    $validator->validateInput($input);
}

public function invalidInputProvider()
{
    return [
        'negative number' => [-5, 'Input must be a positive number'],
        'zero' => [0, 'Input must be greater than zero'],
        'non-numeric' => ['abc', 'Input must be a number'],
    ];
}

Common Pitfalls to Avoid

When testing exceptions, avoid these mistakes:

• Over-testing exceptions:
  • Don't test exceptions in every scenario.
  • Test exceptions that are part of your code's contract.
  • Avoid testing exceptions in language constructs or tested libraries.

Example of avoiding over-testing:

// Unnecessary exception test
public function testArrayAccessOutOfBounds()
{
    $array = [1, 2, 3];
    $this->expectException(OutOfBoundsException::class);
    $value = $array[5]; // This is not necessary to test
}

• Ignoring exception details:
  • Test the exception's type, message, and code.
  • Check if the exception has the expected information.

Example of testing exception details:

public function testFileNotFound()
{
    $fileReader = new FileReader();
    $this->expectException(FileNotFoundException::class);
    $this->expectExceptionMessage('File "nonexistent.txt" not found');
    $fileReader->readFile('nonexistent.txt');
}

By following these practices and avoiding pitfalls, you can write better exception tests in PHPUnit.