Flash

The post/redirect/get pattern avoids duplicate form submissions: after a POST you redirect to a GET page instead of rendering directly, so a page refresh can't resubmit the form. Flash messages carry one-time feedback across that redirect, things like "Saved successfully" or validation errors, without stuffing the text into the URL as a query parameter.

Flash data lives in its own JWT cookie, separate from the session cookie. Values survive exactly one subsequent request, and then the cookie is cleared automatically. In Freemarker templates, the built-in flash object exposes success, warning, and error slots, plus arbitrary keys via put().

Typical flow: set flash in the POST handler, Response.redirect(...), then read flash in the GET template or controller.

package controllers;

import io.mangoo.routing.Response;
import io.mangoo.routing.bindings.Flash;

public class FlashController {
    public Response save(Flash flash) {
        flash.setSuccess("Saved");
        flash.setWarning("Check the address");
        flash.setError("Could not send email");
        flash.put("foo", "bar");
        return Response.redirect("/");
    }
}

setSuccess, setWarning, and setError store under the fixed keys success, warning, and error. Reach for put when you need a custom key instead.

Keys and values must not contain spaces, |, :, or &.

Templates

Flash is available in Freemarker automatically, with no need to pass it from the controller:

${flash.success}
${flash.warning}
${flash.error}
${flash.foo}

Cookie name, key, and secret are configured under flash.cookie.*. The Secure flag follows session.cookie.secure rather than having its own setting. See Configuration.