Joomla Plugin Development

Joomla 5/6 Plugin Development: Complete Guide

1. Introduction

Plugins in Joomla 5/6 represent the most modern evolution of the framework. This knowledge set provides everything you need to create robust, efficient, and maintainable extensions following current patterns.

1.1 What You Will Learn

• Modern Joomla 5/6 plugin architecture
• SubscriberInterface and getSubscribedEvents() implementation
• System, content, and user event handling
• Event Classes with type hints
• Dependency injection
• PSR-4 namespaces and automatic autoloading
• Security and performance best practices
• Practical and complete examples

1.2 Prerequisites

• Working Joomla 5 or 6 installation
• Intermediate PHP knowledge
• Familiarity with namespaces and composition
• Administrator access to Joomla
• Code editor (VS Code, PhpStorm, etc.)

2. Modern Plugin Structure

2.1 Standard Directory Tree

Every modern plugin follows this structure:

plg_system_myexample/
├── manifest.xml              # Configuration and installation
├── services/
│   └── provider.php          # Dependency injection
├── src/
│   ├── Extension/
│   │   └── MyExample.php     # Main class
│   ├── Event/                # (Optional) Custom event classes
│   └── Helper/               # (Optional) Helper classes
└── language/
    └── en-GB/
        ├── plg_system_myexample.ini      # Frontend translations
        └── plg_system_myexample.sys.ini  # System translations

Critical naming conventions:

• Prefix: plg_ (always plugin)
• Group: system, content, user, etc.
• Name: lowercase without spaces
• Example: plg_content_shortcodes = content plugin named "shortcodes"

2.2 manifest.xml File

The manifest is the installation entry point:

<?xml version="1.0" encoding="utf-8"?>
<extension type="plugin" group="system">
    <name>PLG_SYSTEM_MYEXAMPLE</name>
    <author>Your Name</author>
    <creationDate>2025-03-06</creationDate>
    <copyright>Copyright 2025</copyright>
    <license>GNU General Public License version 2 or later</license>
    <version>1.0.0</version>
    <description>PLG_SYSTEM_MYEXAMPLE_DESCRIPTION</description>
    <targetPlatform version="5.0" />

    <!-- CRITICAL: Defines PSR-4 namespace -->
    <namespace path="src">MyCompany\Plugin\System\MyExample</namespace>

    <!-- File declarations -->
    <files>
        <file>manifest.xml</file>
        <folder plugin="myexample">services</folder>
        <folder>src</folder>
        <folder>language</folder>
    </files>

    <!-- Parameter configuration (optional) -->
    <config>
        <fields name="params">
            <fieldset name="basic">
                <field
                    name="enabled"
                    type="checkbox"
                    label="Enable plugin"
                    default="1"
                />
            </fieldset>
        </fields>
    </config>
</extension>

Critical elements:

• <namespace path="src">: Defines the namespace prefix. MUST match exactly in provider.php and Extension
• <folder plugin="myexample">: The attribute must be the plugin name (matches manifest.xml)
• type="plugin" and group="...": Identifies type and category

2.3 Service Provider (services/provider.php)

This file registers your plugin in the dependency injection container:

<?php
namespace MyCompany\Plugin\System\MyExample;

use Joomla\CMS\Extension\PluginInterface;
use Joomla\CMS\Plugin\PluginHelper;
use Joomla\DI\Container;
use Joomla\DI\ServiceProviderInterface;

class ServiceProvider implements ServiceProviderInterface
{
    public function register(Container $container)
    {
        $container->set(
            PluginInterface::class,
            function (Container $c) {
                $plugin = new Extension(
                    $c->get('dispatcher'),
                    (array) PluginHelper::getPlugin('system', 'myexample')
                );
                return $plugin;
            }
        );
    }
}

Key points:

• The namespace MUST match exactly with manifest.xml
• PluginHelper::getPlugin() retrieves the plugin configuration
• The dispatcher is Joomla's event manager
• The Extension class is registered as PluginInterface

3. Extension Class: The Heart of the Plugin

The main class implements SubscriberInterface:

<?php
namespace MyCompany\Plugin\System\MyExample;

use Joomla\CMS\Plugin\CMSPlugin;
use Joomla\Event\SubscriberInterface;
use Joomla\CMS\Event\System\AfterInitialiseEvent;

class Extension extends CMSPlugin implements SubscriberInterface
{
    protected $autoloadLanguage = true;
    protected $allowLegacyListeners = false;

    /**
     * Declares subscribed events
     * This static method is MANDATORY
     */
    public static function getSubscribedEvents(): array
    {
        return [
            'onAfterInitialise' => 'onAfterInitialise',
        ];
    }

    /**
     * Handles the onAfterInitialise event
     */
    public function onAfterInitialise(AfterInitialiseEvent $event)
    {
        // Your logic here
    }
}

Important properties:

• $autoloadLanguage = true: Automatically loads .ini language files
• $allowLegacyListeners = false: Improves performance, disables Reflection-based lookup
• Extends CMSPlugin which provides $params, $app, etc.

4. SubscriberInterface and getSubscribedEvents()

4.1 Modern vs Legacy Pattern

Legacy (Joomla 3-4):

public function onContentPrepare($context, &$article, &$params, $page = 0)
{
    // Logic
}

Modern (Joomla 5+):

public static function getSubscribedEvents(): array
{
    return ['onContentPrepare' => 'onContentPrepare'];
}

public function onContentPrepare(ContentPrepareEvent $event)
{
    $article = $event->getArgument('0');
    // Logic with type hints
}

4.2 getSubscribedEvents() Formats

// Basic format
public static function getSubscribedEvents(): array
{
    return [
        'onContentPrepare' => 'onContentPrepare',
        'onContentAfterTitle' => 'onContentAfterTitle',
    ];
}

// With priorities (lower number = executes first)
public static function getSubscribedEvents(): array
{
    return [
        'onContentPrepare' => ['onContentPrepare', 5],
        'onContentAfterTitle' => 'onContentAfterTitle',
    ];
}

// Multiple listeners for one event
public static function getSubscribedEvents(): array
{
    return [
        'onContentPrepare' => [
            ['primaryHandler', 0],
            ['secondaryHandler', 10],
        ],
    ];
}

5. Main Event Types

5.1 System Events

Triggered on every page load:

5.2 Content Events

Triggered during article read/write cycle:

5.3 User Events

User management:

6. Event Classes (Joomla 5.2+)

Event Classes provide type safety and better structure:

// Legacy
public function onContentPrepare($context, &$article, &$params, $page = 0)
{
    $text = $article->text;
}

// Modern with Event Class
use Joomla\CMS\Event\Content\ContentPrepareEvent;

public function onContentPrepare(ContentPrepareEvent $event)
{
    // Access by index
    $article = $event->getArgument('0');

    // Or use specific methods (if available)
    $article = $event->getArticle();

    // Modify
    $event->setArgument('0', $modifiedArticle);
}

Event Classes location:

• \Joomla\CMS\Event\Content\* for content events
• \Joomla\CMS\Event\System\* for system events
• \Joomla\CMS\Event\User\* for user events

7. Dependency Injection

7.1 Accessing Basic Services

<?php
namespace MyCompany\Plugin\Content\Example;

use Joomla\CMS\Plugin\CMSPlugin;
use Joomla\Event\SubscriberInterface;
use Joomla\DI\Traits\ContainerAwareTrait;
use Joomla\Database\DatabaseInterface;

class Extension extends CMSPlugin implements SubscriberInterface
{
    use ContainerAwareTrait;

    public static function getSubscribedEvents(): array
    {
        return ['onContentPrepare' => 'onContentPrepare'];
    }

    public function onContentPrepare($event)
    {
        // Access the container
        $container = $this->getContainer();

        // Get the database
        $db = $container->get(DatabaseInterface::class);

        // Execute query
        $query = $db->getQuery(true)
            ->select('*')
            ->from($db->quoteName('#__articles'));

        $db->setQuery($query);
        $results = $db->loadObjectList();
    }
}

7.2 In Service Provider

<?php
namespace MyCompany\Plugin\Content\Example;

use Joomla\CMS\Extension\PluginInterface;
use Joomla\CMS\Plugin\PluginHelper;
use Joomla\DI\Container;
use Joomla\DI\ServiceProviderInterface;
use Joomla\Database\DatabaseInterface;

class ServiceProvider implements ServiceProviderInterface
{
    public function register(Container $container)
    {
        $container->set(
            PluginInterface::class,
            function (Container $c) {
                $plugin = new Extension(
                    $c->get('dispatcher'),
                    (array) PluginHelper::getPlugin('content', 'example')
                );

                // Inject the container
                $plugin->setContainer($c);

                return $plugin;
            }
        );
    }
}

8. Your First Plugin: "Hello World"

8.1 Minimal Structure

plg_system_helloworld/
├── manifest.xml
├── services/
│   └── provider.php
├── src/
│   └── Extension/
│       └── Helloworld.php
└── language/
    └── en-GB/
        ├── plg_system_helloworld.ini
        └── plg_system_helloworld.sys.ini

8.2 manifest.xml

<?xml version="1.0" encoding="utf-8"?>
<extension type="plugin" group="system">
    <name>PLG_SYSTEM_HELLOWORLD</name>
    <author>Your Name</author>
    <creationDate>2025-03-06</creationDate>
    <copyright>Copyright 2025</copyright>
    <license>GNU General Public License version 2 or later</license>
    <version>1.0.0</version>
    <description>PLG_SYSTEM_HELLOWORLD_DESCRIPTION</description>
    <targetPlatform version="5.0" />

    <namespace path="src">MyCompany\Plugin\System\Helloworld</namespace>

    <files>
        <file>manifest.xml</file>
        <folder plugin="helloworld">services</folder>
        <folder>src</folder>
        <folder>language</folder>
    </files>
</extension>

8.3 services/provider.php

<?php
namespace MyCompany\Plugin\System\Helloworld;

use Joomla\CMS\Extension\PluginInterface;
use Joomla\CMS\Plugin\PluginHelper;
use Joomla\DI\Container;
use Joomla\DI\ServiceProviderInterface;

class ServiceProvider implements ServiceProviderInterface
{
    public function register(Container $container)
    {
        $container->set(
            PluginInterface::class,
            function (Container $c) {
                $plugin = new Extension(
                    $c->get('dispatcher'),
                    (array) PluginHelper::getPlugin('system', 'helloworld')
                );
                return $plugin;
            }
        );
    }
}

8.4 src/Extension/Helloworld.php

<?php
namespace MyCompany\Plugin\System\Helloworld;

use Joomla\CMS\Plugin\CMSPlugin;
use Joomla\Event\SubscriberInterface;
use Joomla\CMS\Event\System\AfterInitialiseEvent;

class Extension extends CMSPlugin implements SubscriberInterface
{
    protected $autoloadLanguage = true;
    protected $allowLegacyListeners = false;

    public static function getSubscribedEvents(): array
    {
        return [
            'onAfterInitialise' => 'onAfterInitialise',
        ];
    }

    public function onAfterInitialise(AfterInitialiseEvent $event)
    {
        $app = $this->getApplication();
        $app->getLogger()->info('Hello World! The plugin is working.');
    }
}

8.5 Language Files

language/en-GB/plg_system_helloworld.ini:

PLG_SYSTEM_HELLOWORLD="Hello World Plugin"

language/en-GB/plg_system_helloworld.sys.ini:

PLG_SYSTEM_HELLOWORLD="Hello World Plugin"
PLG_SYSTEM_HELLOWORLD_DESCRIPTION="A sample plugin that displays 'Hello World'"

8.6 Installation

1. Create folder: plugins/system/helloworld
2. Copy all files
3. Go to Control Panel > Extensions > Plugins
4. Search for "Hello World Plugin"
5. Enable it
6. Verify in logs: logs/joomla.log

9. Advanced Plugin: Content Shortcodes

9.1 Manifest.xml with Parameters

<?xml version="1.0" encoding="utf-8"?>
<extension type="plugin" group="content" method="upgrade">
    <name>PLG_CONTENT_SHORTCODES</name>
    <author>Your Name</author>
    <creationDate>2025-03-06</creationDate>
    <copyright>Copyright 2025</copyright>
    <license>GNU General Public License version 2 or later</license>
    <version>1.0.0</version>
    <description>PLG_CONTENT_SHORTCODES_DESCRIPTION</description>
    <targetPlatform version="5.0" />

    <namespace path="src">MyCompany\Plugin\Content\Shortcodes</namespace>

    <files>
        <file>manifest.xml</file>
        <folder plugin="shortcodes">services</folder>
        <folder>src</folder>
        <folder>language</folder>
    </files>

    <config>
        <fields name="params">
            <fieldset name="basic">
                <field
                    name="process_shortcodes"
                    type="checkbox"
                    label="PLG_CONTENT_SHORTCODES_ENABLED"
                    default="1"
                />
            </fieldset>
        </fields>
    </config>
</extension>

9.2 Complete Extension Class

<?php
namespace MyCompany\Plugin\Content\Shortcodes;

use Joomla\CMS\Plugin\CMSPlugin;
use Joomla\Event\SubscriberInterface;
use Joomla\CMS\Event\Content\ContentPrepareEvent;
use Joomla\CMS\Factory;

class Extension extends CMSPlugin implements SubscriberInterface
{
    protected $autoloadLanguage = true;
    protected $allowLegacyListeners = false;

    public static function getSubscribedEvents(): array
    {
        return [
            'onContentPrepare' => 'onContentPrepare',
        ];
    }

    public function onContentPrepare(ContentPrepareEvent $event)
    {
        // Check if enabled
        if (!$this->params->get('process_shortcodes', true)) {
            return;
        }

        $article = $event->getArgument('0');

        if (!isset($article) || !isset($article->text)) {
            return;
        }

        // Process shortcodes
        $article->text = $this->processShortcodes($article->text);
    }

    protected function processShortcodes($text)
    {
        $config = Factory::getConfig();

        // Replace {sitename}
        $text = str_replace(
            '{sitename}',
            $config->get('sitename'),
            $text
        );

        // Replace {siteurl}
        $text = str_replace(
            '{siteurl}',
            $config->get('live_site'),
            $text
        );

        // Replace {year}
        $text = str_replace(
            '{year}',
            date('Y'),
            $text
        );

        return $text;
    }
}

9.3 Service Provider

<?php
namespace MyCompany\Plugin\Content\Shortcodes;

use Joomla\CMS\Extension\PluginInterface;
use Joomla\CMS\Plugin\PluginHelper;
use Joomla\DI\Container;
use Joomla\DI\ServiceProviderInterface;

class ServiceProvider implements ServiceProviderInterface
{
    public function register(Container $container)
    {
        $container->set(
            PluginInterface::class,
            function (Container $c) {
                $plugin = new Extension(
                    $c->get('dispatcher'),
                    (array) PluginHelper::getPlugin('content', 'shortcodes')
                );
                return $plugin;
            }
        );
    }
}

9.4 Language Files

language/en-GB/plg_content_shortcodes.ini:

PLG_CONTENT_SHORTCODES="Content Shortcodes"
PLG_CONTENT_SHORTCODES_ENABLED="Process Shortcodes"

language/en-GB/plg_content_shortcodes.sys.ini:

PLG_CONTENT_SHORTCODES="Content Shortcodes"
PLG_CONTENT_SHORTCODES_DESCRIPTION="Replace shortcodes like {sitename} with site configuration values"

10. Best Practices

10.1 Security

// ALWAYS validate input
use Joomla\CMS\Filter\InputFilter;

$filter = InputFilter::getInstance();
$safe_input = $filter->clean($_GET['data'], 'STRING');

// ALWAYS escape output
use Joomla\CMS\HTML\HTMLHelper;

echo HTMLHelper::_('common.escape', $user_content);

// Check permissions
$user = Factory::getUser();
if (!$user->authorise('core.manage', 'com_example')) {
    return;
}

10.2 Performance

// Use cache
$cache = Factory::getCache('_system');
$key = 'plugin_example_data_' . $article_id;

if ($data = $cache->get($key)) {
    return $data;
}

// Processing
$data = $this->expensiveOperation();

// Store in cache (3600 seconds = 1 hour)
$cache->store($data, $key, '_system', 3600);

// Be selective with events - DO NOT subscribe to all of them
public static function getSubscribedEvents(): array
{
    return [
        'onContentPrepare' => 'onContentPrepare',
        // NO: 'onAfterRender', 'onBeforeRender', etc.
    ];
}

10.3 Namespace and PSR-4

CORRECT:

• manifest.xml: <namespace path="src">MyCompany\Plugin\Content\Shortcodes</namespace>
• services/provider.php: namespace MyCompany\Plugin\Content\Shortcodes;
• src/Extension/Shortcodes.php: namespace MyCompany\Plugin\Content\Shortcodes;

INCORRECT:

• Mismatching namespaces
• Not including path="src" in manifest
• Using incorrect paths in files

11. Common Troubleshooting

11.1 Plugin does not appear in the list

Solutions:

1. Verify that manifest.xml is in the root of the plugin folder
2. Check that the XML is valid (no special characters)
3. Clear cache: Control Panel > System > Cache > Clear Cache
4. Verify folder permissions (755)

11.2 Event does not fire

Solutions:

1. Verify that getSubscribedEvents() correctly declares the event
2. Make sure the plugin is enabled
3. Check that $allowLegacyListeners = false
4. Check logs at logs/joomla.log

11.3 Namespace error

Solutions:

1. Match EXACTLY the namespace in manifest, provider, and Extension
2. Use semicolon at the end of namespace
3. Verify path in <folder plugin="pluginname">services</folder>
4. Clear autoload cache: administrator/cache/autoload_psr4.php

12. Installation Verification

After installing a plugin, verify:

1. Control Panel > Extensions > Plugins
2. Search for the plugin by name
3. Verify it appears in the list
4. Enable it (green status)
5. Check logs at logs/joomla.log for messages
6. Test functionality according to the plugin type

13. Additional Resources

• Official Joomla Manual Documentation
• Joomla Event Classes Documentation
• API Reference
• Joomla Community - Forum