Practical PHP Testing Patterns: Garbage-Collected Teardown

In order to perform tests, you create fixtures such as the System Under Test and input or output data. Usually this fixtures are just objects.

Garbage collection, a mechanism present in many languages, deletes objects and variables (and thus fixtures) when there are no references for them anymore in the available scopes.

The logic behind garbage collection is that objects which are not reachable anymore by any other object in the local or global scope are as good as garbage. Their code can't possibly be executed as you cannot call a method on them without a reference.

Basically, if you do not put your fixtures in strange places, they will be automatically deleted with the test case. While a setUp() method is very common, a tearDown() method is usually not necessary.

Implementation

References like local variables or fields on $this (at the Testcase Object level) are deleted with the test case object. Currently, the PHPUnit manual say that the gabage collection of Testcase Objects is not predictable, so you may have to perform an unset() over $this->field to have the object removed.

Static references are special: they are always reachable, as they are kept on classes (the current or other ones) instead of objects, and won't be deleted. Even if an object is pointed by another object in a static reference, it won't be garbage-collected for transitivity.

Also singletons are a big problem (which derives from the static fields where their instances are kept): if you put something in Zend_Registry or in a singleton, it won't be deleted after a test. That's one reason singletons are dangerous: it's not only a matter of performance, but of them being capable of bringing objects from one test to another, spoiling our pursue of test isolation.

PHP garbage collection

Up to PHP 5.2, objects with mutual references won't usually be collected, even if neither of them was reachable from the other scopes. This is not a problem for a script that produces a web page and exits, but it can be for a test suite which runs for some minutes when at full capacity.

In fact, an example of problematic behavior is the ezComponents test suite, which is cited on php.net. The suite would require several gigabytes of memory to run, due to the leaks of the long-running PHP process.

From PHP 5.3, if zend.enable_gc is enabled (by default, it is) cycles will be collected when the table of roots is full. So this particular issue (circular references) won't worry you anymore.

In case you encounter erratic behavior, see if you can change the php.ini directive memory_limit to detect the problem, and use the native function memory_get_usage() to gather statistics.

Examples

In this code sample, I show you tests that create fixtures which are automatically collected. How do we prove that these objects are really deleted? With a simple  __destruct() method.

Keep in mind that explicit teardown is usually not necessary: you should adopt it only if you have an heavy fixture, which you want to be gargabe-collected before the end of the PHP process.

Running this test case:

<?php
class GarbageCollectedTeardownTest extends PHPUnit_Framework_TestCase
{
    private $deletedFixture;
    private $abandonedFixture;

    public function setUp()
    {
        $this->deletedFixture = new SomeFixture(1);
        $this->abandonedFixture = new SomeFixture(2);
    }

    public function testFirst()
    {
    }

    public function testSecond()
    {
    }

    public function tearDown()
    {
        // this fixture will be garbage-collected at the end of each test
        unset($this->deletedFixture);
        // since we do not touch $this->abandonedFixture, its collection
        // is not predictable. It can happen at any time after the tests execution.
    }
}

class SomeFixture
{
    private $number;
    public function __construct($number)
    {
        $this->number = $number;
    }

    public function __destruct()
    {
        echo "Cleaning up fixture $this->number.\n";
    }
}

[13:11:15][giorgio@Desmond:~]$ phpunit code/practical-php-testing-patterns/GarbageCollectedTeardownTest.php
PHPUnit 3.5.5 by Sebastian Bergmann.

Cleaning up fixture 1.
.Cleaning up fixture 1.
.

Time: 0 seconds, Memory: 4.75Mb

OK (2 tests, 0 assertions)
Cleaning up fixture 2.
Cleaning up fixture 2.