Override Core Joomla! Classes

config.php

<?php
/**
 * @package     Joomla.Plugin
 * @subpackage  System.Overrides
 *
 * @copyright   Copyright (C) 2012 Don Gilbert. All rights reserved.
 * @license     GNU General Public License version 2 or later; see LICENSE
 */
define('OVERRIDES', dirname(__FILE__).'/overrides');

// Use JLoader to register all the classes you want to override
JLoader::register('JClassToOverride', OVERRIDES.'/libraries/joomla/class/to/override.php', true);

index.html

silence is golden

overrides.php

<?php
/**
 * @package     Joomla.Plugin
 * @subpackage  System.Overrides
 *
 * @copyright   Copyright (C) 2012 Don Gilbert. All rights reserved.
 * @license     GNU General Public License version 2 or later; see LICENSE
 */

defined('JPATH_BASE') or die;

/**
 * System plugin to override core classes terms.
 *
 * @package     Joomla.Plugin
 * @subpackage  System.Overrides
 * @since       2.5
 */
class PlgSystemOverrides extends JPlugin
{
	/* We do our thing in the __construct method
	 * so that our overridden classes will be
	 * available everywhere
	 */
	public function __construct(&$subject, $config)
	{
		parent::__construct($subject, $config);
		
		include_once 'config.php';
	}
}

overrides.xml

<?xml version="1.0" encoding="utf-8"?>
<extension version="2.5" type="plugin" group="system" method="upgrade">
	<name>plg_system_overrides</name>
	<author>Don  Gilbert</author>
	<creationDate>Aug 2012</creationDate>
	<copyright>(C) 2012 Don Gilbert. All rights reserved.</copyright>
	<license>GNU General Public License version 2 or later; see LICENSE.txt</license>
	<authorEmail>don@electriceasel.com</authorEmail>
	<authorUrl>www.electriceasel.com</authorUrl>
	<version>2.5.6</version>
	<description>This plugin will override classes contained in the included config file.</description>
	<files>
		<filename plugin="overrides">overrides.php</filename>
		<filename>config.php</filename>
		<filename>index.html</filename>
		<folder>overrides</folder>
	</files>
</extension>

This is very useful. Thanks very much.

A tip for anyone wanting to override one class from a larger namespace: use the whole namespace with the name of the class in the register() definition in config.php. For example I wanted to override Community Builder's UserTable class only,

JLoader::register('CB\Database\Table\UserTable', OVERRIDES.'/libraries/CBLib/CB/Database/Table/UserTable.php', true);

hello noob question... with this plugin is posible to override JApplicationSite? i tried and didn't work ..

this is what i did

class PlgSystemOverrides extends JPlugin
{
/* We do our thing in the __construct method
* so that our overridden classes will be
* available everywhere
*/

public function __construct(&$subject, $config)
{
	
	parent::__construct($subject, $config);
	
	//Load config file only at frontend
	$app = JFactory::getApplication(); 

	if($app->isSite()) {
		// Use JLoader to register all the classes you want to override
		JLoader::register('JApplicationSite', '/baseappi_core/libraries/cms/application/site.php', true);
	}
}

Plugin folder structure is the following:

overrides
---baseappi_core
------- libraries
---------- cms
-------------application
----------------site.php
---install.script.php
---index.html
---overrides.php
---overrides.xml