Error Catchers

Routing may fail for a variety of reasons. These include:

• A request guard returns Failure.
• A handler returns a Responder that fails.
• No matching route was found.

If any of these conditions occur, Rocket returns an error to the client. To do so, Rocket invokes the error catcher corresponding to the error’s status code. A catcher is like a route, except it only handles errors. Catchers are declared via the catch attribute, which takes a single integer corresponding to the HTTP status code to catch. For instance, to declare a catcher for 404 errors, you’d write:

#[catch(404)]
fn not_found(req: &Request) -> String { ... }

As with routes, Rocket needs to know about a catcher before it is used to handle errors. The process is similar to mounting: call the catch method with a list of catchers via the catchers! macro. The invocation to add the 404 catcher declared above looks like:

rocket::ignite().catch(catchers![not_found])

Unlike request handlers, error handlers can only take 0, 1, or 2 parameters of types Request and/or Error. At present, the Error type is not particularly useful, and so it is often omitted. The error catcher example on GitHub illustrates their use in full.

Rocket has a default catcher for all of the standard HTTP error codes including 404, 500, and more.