Breaking: #90799 - Dependency injection with non-public properties has been removed

Description

In TYPO3 v9, the (dependency) injection via @Inject has been marked as deprecated for non-public properties. The reason was to avoid having the core use the PHP reflection api to make non-public properties writable from outside the class scope. Since there are other methods for dependency injection (constructor/setter injection), injection into non-public properties has now been removed.

Impact

Non-public properties with @Inject annotations will no longer trigger extbase dependency injection. Those properties will have their default state after object instantiation.

Affected Installations

All installations that use non-public properties for extbase dependency injection as seen in this example:

class Foo
{
    /**
     * @var Service
     * @TYPO3\CMS\Extbase\Annotation\Inject
     */
    private $service;
}

Migration

When not using constructor/setter injection instead, switch to inject methods (recommended for compatibility with symfony dependency injection) or mark the property public (works with extbase dependency injection only):

class Foo
{
    /**
     * @var Service
     * @TYPO3\CMS\Extbase\Annotation\Inject
     */
    public $service;
}