Dropwizard cookie-based flash scope

A flash scope for Dropwizard that avoids the need for any server-side state. Useful when you're running multiple nodes but don't want the complexity of sticky sessions or an external session store.

Note: the flash cookie is not encrypted or signed in anyway, so don't use it to store any confidential data, or anything where a user modifying the contents of the cookie might cause problems.

Adding to your application

Default configuration

If you're happy for the flash scope cookie to be created with the default configuration, add this to your Application class:

@Override
public void initialize(Bootstrap<MyAppConfig> bootstrap) {
    bootstrap.addBundle(new FlashScopeBundle<MyAppConfig>());
}

This will cause the flash scope to be managed in a cookie called DW_FLASH with a 5 second timeout, a path of /, on the request domain.

Configured

If you want to override any of these settings you'll need to add an instance of FlashScopeConfig to your Dropwizard config class e.g.

@JsonProperty
private FlashScopeConfig flashScope;

public FlashScopeConfig getFlashScope() {
    return flashScope;
}

Add the relevant config to your YAML file:

flashScope:
  cookieName: MY_FLASH_SCOPE
  cookiePath: /my-app
  cookieMaxAge: 20s
  cookieDomain: flashexample.com

And initialise it in your application like this:

@Override
public void initialize(Bootstrap<MyAppConfig> bootstrap) {
    bootstrap.addBundle(new FlashScopeBundle<MyAppConfig>() {
        @Override
        protected FlashScopeConfig getFlashScopeConfig(MyAppConfig configuration) {
            return configuration.getFlashScope();
        }
    });
}

Using in your app

The flash scope is made available to your web resources via an injected Flash parameter. The Flash class has generic get and set methods which return/take any object serialisable by Jackson (using Dropwizard's ObjectMapper instance).

e.g. to define an action that populates the flash and redirects to a view:

@Path("/action")
@POST
public Response doSomething(@FlashScope Flash flash, @FormParam("message") String message) {
    flash.set(new FlashMessage(message));
    return Response.seeOther(URI.create("/result")).build();
}

@Path("/result")
@GET
@Produces("text/plain")
public String getResult(@FlashScope Flash flash) {
    Optional<FlashMessage> contents = flash.get(FlashMessage.class);
    return (contents.or(new FlashMessage("OK"))).getMessage();
}