(Quick Reference) # 18.5 Hooking into Runtime Configuration Version: 3.2.8 ## 18.5 Hooking into Runtime Configuration Grails provides a number of hooks to leverage the different parts of the system and perform runtime configuration by convention. #### Hooking into the Grails Spring configuration First, you can hook in Grails runtime configuration overriding the `doWithSpring` method from the Plugin class and returning a closure that defines additional beans. For example the following snippet is from one of the core Grails plugins that provides i18n support:
    import org.springframework.web.servlet.i18n.CookieLocaleResolver
    import org.springframework.web.servlet.i18n.LocaleChangeInterceptor
    import org.springframework.context.support.ReloadableResourceBundleMessageSource
    import grails.plugins.*

    class I18nGrailsPlugin extends Plugin {

        def version = "0.1"

        Closure doWithSpring() {{->
            messageSource(ReloadableResourceBundleMessageSource) {
                basename = "WEB-INF/grails-app/i18n/messages"
            }
            localeChangeInterceptor(LocaleChangeInterceptor) {
                paramName = "lang"
            }
            localeResolver(CookieLocaleResolver)
        }}
    }
This plugin configures the Grails `messageSource` bean and a couple of other beans to manage Locale resolution and switching. It using the Spring Bean Builder syntax to do so. #### Customizing the Servlet Environment In previous versions of Grails it was possible to dynamically modify the generated `web.xml`. In Grails 3.x there is no `web.xml` file and it is not possible to programmatically modify the `web.xml` file anymore. However, it is possible to perform the most commons tasks of modifying the Servlet environment in Grails 3.x. #### Adding New Servlets If you want to add a new Servlet instance the simplest way is simply to define a new Spring bean in the `doWithSpring` method:
    Closure doWithSpring() {{->
      myServlet(MyServlet)
    }}
If you need to customize the servlet you can use Spring Boot’s ServletRegistrationBean:
    Closure doWithSpring() {{->
      myServlet(ServletRegistrationBean, new MyServlet(), "/myServlet/*") {
        loadOnStartup = 2
      }
    }}
#### Adding New Servlet Filters Just like Servlets, the simplest way to configure a new filter is to simply define a Spring bean:
    Closure doWithSpring() {{->
      myFilter(MyFilter)
    }}
However, if you want to control the order of filter registrations you will need to use Spring Boot’s FilterRegistrationBean:
    myFilter(FilterRegistrationBean) {
        filter = bean(MyFilter)
        urlPatterns = ['/*']
        order = Ordered.HIGHEST_PRECEDENCE
    }
| Grails' internal registered filters (`GrailsWebRequestFilter`, `HiddenHttpMethodFilter` etc.) are defined by incrementing `HIGHEST_PRECEDENCE` by 10 thus allowing several filters to be inserted before or between Grails' filters.
#### Doing Post Initialisation Configuration Sometimes it is useful to be able do some runtime configuration after the Spring ApplicationContext has been built. In this case you can define a `doWithApplicationContext` closure property.
    class SimplePlugin extends Plugin{

        def name = "simple"
        def version = "1.1"

        @Override
        void doWithApplicationContext() {
            def sessionFactory = applicationContext.sessionFactory
            // do something here with session factory
        }
    }
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