Templates

See How to Override Templates from Third-Party Bundles.

Routing

Routing is never automatically imported in Symfony. If you want to include the routes from any bundle, then they must be manually imported from somewhere in your application (e.g. app/config/routing.yml).

The easiest way to "override" a bundle's routing is to never import it at all. Instead of importing a third-party bundle's routing, copy that routing file into your application, modify it, and import it instead.

Controllers

If the controller is a service, see the next section on how to override it. Otherwise, define a new route + controller with the same path associated to the controller you want to override (and make sure that the new route is loaded before the bundle one).

Services & Configuration

If you want to modify the services created by a bundle, you can use service decoration.

If you want to do more advanced manipulations, like removing services created by other bundles, you must work with service definitions inside a compiler pass.

Entities & Entity Mapping

Overriding entity mapping is only possible if a bundle provides a mapped superclass (such as the User entity in the FOSUserBundle). It's possible to override attributes and associations in this way. Learn more about this feature and its limitations in the Doctrine documentation.

Forms

Existing form types can be modified defining form type extensions.

Validation Metadata

Symfony loads all validation configuration files from every bundle and combines them into one validation metadata tree. This means you are able to add new constraints to a property, but you cannot override them.

To overcome this, the 3rd party bundle needs to have configuration for validation groups. For instance, the FOSUserBundle has this configuration. To create your own validation, add the constraints to a new validation group:

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# src/Acme/UserBundle/Resources/config/validation.yml
FOS\UserBundle\Model\User:
    properties:
        plainPassword:
            - NotBlank:
                groups: [AcmeValidation]
            - Length:
                min: 6
                minMessage: fos_user.password.short
                groups: [AcmeValidation]

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<!-- src/Acme/UserBundle/Resources/config/validation.xml -->
<?xml version="1.0" encoding="UTF-8" ?>
<constraint-mapping xmlns="http://symfony.com/schema/dic/constraint-mapping"
    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xsi:schemaLocation="http://symfony.com/schema/dic/constraint-mapping
        https://symfony.com/schema/dic/constraint-mapping/constraint-mapping-1.0.xsd">

    <class name="FOS\UserBundle\Model\User">
        <property name="plainPassword">
            <constraint name="NotBlank">
                <option name="groups">
                    <value>AcmeValidation</value>
                </option>
            </constraint>

            <constraint name="Length">
                <option name="min">6</option>
                <option name="minMessage">fos_user.password.short</option>
                <option name="groups">
                    <value>AcmeValidation</value>
                </option>
            </constraint>
        </property>
    </class>
</constraint-mapping>

Now, update the FOSUserBundle configuration, so it uses your validation groups instead of the original ones.

Translations

Translations are not related to bundles, but to domains. That means that you can override the translations from any translation file, as long as it is in the correct domain.