<form action='auth' method='POST' id='loginForm' class='cssform' autocomplete='off'>
<p>
    <label for='j_username'>Login ID</label>
    <input type='text' class='text_' name='j_username' id='j_username' />
</p>
<p>
    <label for='j_password'>Password</label>
    <input type='password' class='text_' name='j_password' id='j_password' />
</p>
<p>
    <input type='submit' value='Login' />
</p>

class LoginController {

def index()  {
    if (session.user?.name == 'test') {
        render view: '/login/youarealreadyin'
    }
    else {
        render view: '/login/auth'
    }
}

def auth() {
    def loginName = params.j_username?.trim()
    def pass = params.j_password?.trim()

    if (loginName == 'test' && pass == 'TEST' ) {
        session.user = [name: loginName]
        redirect uri: '/'
    }
    else {
        render view: '/login/denied'
    }
}

http://localhost:8080/?j_username=test&j_password=TEST