Deprecation: #67670 - Deprecate custom singleton logic in GeneralUtility::getUserObj

See forge#67670

Description

The functionality of instantiating classes only once by calling GeneralUtility::getUserObj($className) multiple times while having a $className that is prepended with a ampersand ("&") has been marked as deprecated.

An example of the deprecated behaviour in the ext_localconf.php of an extension:

$GLOBALS['TYPO3_CONF_VARS']['EXTCONF']['indexed_search']['pi1_hooks']['getResultRows_SQLpointer'] = '&Acme\\MyExtension\\Hooks\\MysqlFulltextIndexHook';

Impact

Any calls to GeneralUtility::getUserObj() with a prefixed ampersand will throw a deprecation message.

Affected Installations

TYPO3 Instances with extensions that use getUserObj() themselves and/or use hooks built with getUserObj() and use references.

Migration

Check if the classes that hook into certain parts of your custom extensions really need to be referenced / instantiated once. If so, implement the SingletonInterface of the TYPO3 Core, so the underlying function GeneralUtility::makeInstance() will register the SingletonInterface only once.

The modified example from above now looks like this:

$GLOBALS['TYPO3_CONF_VARS']['EXTCONF']['indexed_search']['pi1_hooks']['getResultRows_SQLpointer'] = \Acme\MyExtension\Hooks\MysqlFulltextIndexHook::class;

While the class itself implements the SingletonInterface of the TYPO3 Core to only be instantiated once during a single request:

<?php
namespace \Acme\MyExtension\Hooks;

class MysqlFulltextIndexHook implements \TYPO3\CMS\Core\Core\SingletonInterface {
        ...
}