(Quick Reference) # 17.4 Authentication Version: 6.0.0 ## 17.4 Authentication Grails has no default mechanism for authentication as it is possible to implement authentication in many different ways. It is however, easy to implement a simple authentication mechanism using interceptors. This is sufficient for simple use cases but it’s highly preferable to use an established security framework, for example by using the Spring Security or the Shiro plugin. Interceptors let you apply authentication across all controllers or across a URI space. For example you can create a new set of filters in a class called `grails-app/controllers/SecurityInterceptor.groovy` by running:
    grails create-interceptor security
and implement your interception logic there:
    class SecurityInterceptor {

        SecurityInterceptor() {
            matchAll()
            .except(controller:'user', action:'login')
        }

        boolean before() {
            if (!session.user && actionName != "login") {
                redirect(controller: "user", action: "login")
                return false
            }
            return true
        }

    }
Here the interceptor intercepts execution before all actions except `login` are executed, and if there is no user in the session then redirect to the `login` action. The `login` action itself is simple too:
    def login() {
        if (request.get) {
            return // render the login view
        }

        def u = User.findByLogin(params.login)
        if (u) {
            if (u.password == params.password) {
                session.user = u
                redirect(action: "home")
            }
            else {
                render(view: "login", model: [message: "Password incorrect"])
            }
        }
        else {
            render(view: "login", model: [message: "User not found"])
        }
    }
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