Grails throws a 404 error when canceling a page request with a filter

I have a filter:

class MyFilters {
    def filters = {
        before = {
            render(view: "/test")
            return false
        }
    }
}

This works great on pages where I'm using a controller to handle the request, showing the contents of test.gsp instead of the page I requested. However, when I try to access a page that maps directly to a GSP file, I get a 404 error.

Changing the render to simply render "test" produces the same results, as does commenting it out and just leaving the return false.

Grails is a MVC framework. If you want to map an URL directly to a GSP (without redirection through a controller and action) you need to explain this to grails within your UrlMappings.groovy. There you can define your "shortcuts". E.g.:

static mappings = {
    "/$viewName"(view:"/index") {
        constraints {
            viewName([some constraints])
        }
    }
}

Which will render views/index.gsp without going through a controller. If you do NOT define a controller mapping (or at least a view mapping) for those URLs, you canNOT use grails filters:

If you really want to intercept ALL requests, you can add a servlet filter to your grails application like this:

import javax.servlet.*

import org.springframework.web.context.support.WebApplicationContextUtils;

class TestFilter implements Filter {

    def applicationContext

    void init(FilterConfig config) throws ServletException {
        applicationContext = WebApplicationContextUtils.getWebApplicationContext(config.servletContext)
    }

    void destroy() {
    }

    void doFilter(ServletRequest request, ServletResponse response, FilterChain chain) throws IOException, ServletException {

        System.out.println("this filter has been called");
    }
}

In here you can do your redirections or renderings based on the applicationcontext and the current request.

You need to add this filter to your web.xml. On how to do this, have a look at: How do i use a servlet in my grails app?