Catcher is used to handle how to display page when response's code is error.

pub trait Catcher: Send + Sync + 'static {
    fn catch(&self, req: &Request, res: &mut Response) -> bool;

A web application can specify several different Catchers to handle errors. They are stored in the Service field:

pub struct Service {
    pub(crate) router: Arc<Router>,
    pub(crate) catchers: Arc<Vec<Box<dyn Catcher>>>,
    pub(crate) allowed_media_types: Arc<Vec<Mime>>,

They can be set via the with_catchers function of Server:

struct Handle404;
impl Catcher for Handle404 {
    fn catch(&self, _req: &Request, _depot: &Depot, res: &mut Response) -> bool {
        if let Some(StatusCode::NOT_FOUND) = res.status_code() {
            res.render("Custom 404 Error Page");
        } else {
async fn main() {
    let router = Router::new().get(hello_world);
    let catchers: Vec<Box<dyn Catcher>> = vec![Box::new(Handle404)];
    let service = Service::new(router).with_catchers(catchers);

When there is an error in the website request result, first try to set the error page through the Catcher set by the user. If the Catcher catches the error, it will return true.

If your custom catchers does not capture this error, then the system uses the default CatcherImpl to capture processing errors and send the default error page. The default error implementation CatcherImpl supports sending error pages in XML, JSON, HTML, Text formats.