Trait Responder

pub trait Responder<'r, 'o: 'r> {
    // Required method
    fn respond_to(self, request: &'r Request<'_>) -> Result<'o>;
}

Trait implemented by types that generate responses for clients.

Any type that implements Responder can be used as the return type of a handler:

// This works for any `T` that implements `Responder`.
#[get("/")]
fn index() -> T { /* ... */ }

Deriving

This trait can, and largely should, be automatically derived. The derive can handle all simple cases and most complex cases, too. When deriving Responder, the first field of the annotated structure (or of each variant if an enum) is used to generate a response while the remaining fields are used as response headers:

use rocket::http::ContentType;
use rocket::serde::{Serialize, json::Json};

#[derive(Responder)]
enum Error<T> {
    #[response(status = 400)]
    Unauthorized(Json<T>),
    #[response(status = 404)]
    NotFound(Template, ContentType),
}

For full details on deriving Responder, see the Responder derive.

Provided Implementations

Rocket implements Responder for several standard library types. Their behavior is documented here. Note that the Result implementation is overloaded, allowing for two Responders to be used at once, depending on the variant.

• &str

  Sets the Content-Type to text/plain. The string is used as the body of the response, which is fixed size and not streamed. To stream a raw string, use Stream::from(Cursor::new(string)).

• String

  Sets the Content-Type to text/plain. The string is used as the body of the response, which is fixed size and not streamed. To stream a string, use Stream::from(Cursor::new(string)).

• &[u8]

  Sets the Content-Type to application/octet-stream. The slice is used as the body of the response, which is fixed size and not streamed. To stream a slice of bytes, use Stream::from(Cursor::new(data)).

• Vec<u8>

  Sets the Content-Type to application/octet-stream. The vector’s data is used as the body of the response, which is fixed size and not streamed. To stream a vector of bytes, use Stream::from(Cursor::new(vec)).

• File

  Responds with a streamed body containing the data in the File. No Content-Type is set. To automatically have a Content-Type set based on the file’s extension, use NamedFile.

• ()

  Responds with an empty body. No Content-Type is set.

• Option<T>

  If the Option is Some, the wrapped responder is used to respond to the client. Otherwise, an Err with status 404 Not Found is returned and a warning is printed to the console.

• Result<T, E>

  If the Result is Ok, the wrapped Ok responder is used to respond to the client. If the Result is Err, the wrapped Err responder is used to respond to the client.

Return Value

A Responder returns a Result<Response, Status>.

• An Ok(Response) indicates success. The Response will be written out to the client.

• An Err(Status) indicates an error. The error catcher for Status will be invoked to generate a response.

Implementation Tips

This section describes a few best practices to take into account when implementing Responder.

1. Avoid Manual Implementations

   The Responder derive is a powerful mechanism that eliminates the need to implement Responder in almost all cases. We encourage you to explore using the derive before attempting to implement Responder directly. It allows you to leverage existing Responder implementations through composition, decreasing the opportunity for mistakes or performance degradation.

2. Joining and Merging

   When chaining/wrapping other Responders, start with Response::build_from() and/or use the merge() or join() methods on the Response or ResponseBuilder struct. Ensure that you document merging or joining behavior appropriately.

3. Inspecting Requests

   While tempting, a Responder that varies its functionality based on the incoming request sacrifices its functionality being understood based purely on its type. By implication, gleaning the functionality of a handler from its type signature also becomes more difficult. You should avoid varying responses based on the Request value as much as possible.

4. Perform async I/O in Constructors

   The Responder trait is not an async trait. This is not an oversight. Instead of performing async operations in what would be the respond_to method, perform them in a constructor for the responder. As an example, see NamedFile::open(), which performs the requisite I/O for NamedFile’s responder.

Lifetimes

Responder has two lifetimes: Responder<'r, 'o: 'r>.

• 'r bounds the reference to the &'r Request.

• 'o bounds the returned Response<'o> to values that live at least as long as the request.

  This includes borrows from the Request itself (where 'o would be 'r as in impl<'r> Responder<'r, 'r>) as well as 'static data (where 'o would be 'static as in impl<'r> Responder<'r, 'static>).

In practice, you are likely choosing between four signatures:

// If the response contains no borrowed data.
impl<'r> Responder<'r, 'static> for A {
    fn respond_to(self, _: &'r Request<'_>) -> response::Result<'static> {
        todo!()
    }
}

// If the response borrows from the request.
impl<'r> Responder<'r, 'r> for B<'r> {
    fn respond_to(self, _: &'r Request<'_>) -> response::Result<'r> {
        todo!()
    }
}

// If the response is or wraps a borrow that may outlive the request.
impl<'r, 'o: 'r> Responder<'r, 'o> for &'o C {
    fn respond_to(self, _: &'r Request<'_>) -> response::Result<'o> {
        todo!()
    }
}

// If the response wraps an existing responder.
impl<'r, 'o: 'r, R: Responder<'r, 'o>> Responder<'r, 'o> for D<R> {
    fn respond_to(self, _: &'r Request<'_>) -> response::Result<'o> {
        todo!()
    }
}

Example

Say that you have a custom type, Person:

struct Person {
    name: String,
    age: u16
}

You’d like to use Person as a Responder so that you can return a Person directly from a handler:

#[get("/person/<id>")]
fn person(id: usize) -> Option<Person> {
    Person::from_id(id)
}

You want the Person responder to set two header fields: X-Person-Name and X-Person-Age as well as supply a custom representation of the object (Content-Type: application/x-person) in the body of the response. The following Responder implementation accomplishes this:

use std::io::Cursor;

use rocket::request::Request;
use rocket::response::{self, Response, Responder};
use rocket::http::ContentType;

impl<'r> Responder<'r, 'static> for Person {
    fn respond_to(self, req: &'r Request<'_>) -> response::Result<'static> {
        let string = format!("{}:{}", self.name, self.age);
        Response::build_from(string.respond_to(req)?)
            .raw_header("X-Person-Name", self.name)
            .raw_header("X-Person-Age", self.age.to_string())
            .header(ContentType::new("application", "x-person"))
            .ok()
    }
}

Note that the implementation could have instead been derived if structured in a slightly different manner:

use rocket::http::Header;
use rocket::response::Responder;

#[derive(Responder)]
#[response(content_type = "application/x-person")]
struct Person {
    text: String,
    name: Header<'static>,
    age: Header<'static>,
}

impl Person {
    fn new(name: &str, age: usize) -> Person {
        Person {
            text: format!("{}:{}", name, age),
            name: Header::new("X-Person-Name", name.to_string()),
            age: Header::new("X-Person-Age", age.to_string())
        }
    }
}

Required Methods

fn respond_to(self, request: &'r Request<'_>) -> Result<'o>

Returns Ok if a Response could be generated successfully. Otherwise, returns an Err with a failing Status.

The request parameter is the Request that this Responder is responding to.

When using Rocket’s code generation, if an Ok(Response) is returned, the response will be written out to the client. If an Err(Status) is returned, the error catcher for the given status is retrieved and called to generate a final error response, which is then written out to the client.

Implementations on Foreign Types

impl<'r> Responder<'r, 'static> for ()

Returns an empty, default Response. Always returns Ok.

fn respond_to(self, _: &'r Request<'_>) -> Result<'static>

impl<'r> Responder<'r, 'static> for Arc<str>

Returns a response with Content-Type text/plain and a fixed-size body containing the string self. Always returns Ok.

fn respond_to(self, _: &'r Request<'_>) -> Result<'static>

impl<'r> Responder<'r, 'static> for Arc<[u8]>

Returns a response with Content-Type application/octet-stream and a fixed-size body containing the data in self. Always returns Ok.

fn respond_to(self, _: &'r Request<'_>) -> Result<'static>

impl<'r> Responder<'r, 'static> for File

Returns a response with a sized body for the file. Always returns Ok.

fn respond_to(self, req: &'r Request<'_>) -> Result<'static>

impl<'r> Responder<'r, 'static> for Error

Prints a warning with the error and forwards to the 500 error catcher.

fn respond_to(self, _: &'r Request<'_>) -> Result<'static>

impl<'r> Responder<'r, 'static> for File

Returns a response with a sized body for the file. Always returns Ok.

fn respond_to(self, _: &'r Request<'_>) -> Result<'static>

impl<'r, 'o: 'r> Responder<'r, 'o> for &'o str

Returns a response with Content-Type text/plain and a fixed-size body containing the string self. Always returns Ok.

fn respond_to(self, _: &'r Request<'_>) -> Result<'o>

impl<'r, 'o: 'r> Responder<'r, 'o> for &'o [u8]

Returns a response with Content-Type application/octet-stream and a fixed-size body containing the data in self. Always returns Ok.

fn respond_to(self, _: &'r Request<'_>) -> Result<'o>

impl<'r, 'o: 'r, 't: 'o, 'e: 'o, T, E> Responder<'r, 'o> for Either<T, E>

where T: Responder<'r, 't>, E: Responder<'r, 'e>,

Responds with the wrapped Responder in self, whether it is Left or Right.

fn respond_to(self, req: &'r Request<'_>) -> Result<'o>

impl<'r, 'o: 'r, R: Responder<'r, 'o>> Responder<'r, 'o> for (ContentType, R)

fn respond_to(self, req: &'r Request<'_>) -> Result<'o>

impl<'r, 'o: 'r, R: Responder<'r, 'o>> Responder<'r, 'o> for (Status, R)

fn respond_to(self, request: &'r Request<'_>) -> Result<'o>

impl<'r, 'o: 'r, R: ?Sized + ToOwned> Responder<'r, 'o> for Cow<'o, R>

where &'o R: Responder<'r, 'o> + 'o, <R as ToOwned>::Owned: Responder<'r, 'o> + 'r,

Responds with the inner Responder in Cow.

fn respond_to(self, req: &'r Request<'_>) -> Result<'o>

Implementors

impl<'r> Responder<'r, 'static> for Value

Available on crate feature json only.

Serializes the value into JSON. Returns a response with Content-Type JSON and a fixed-size body with the serialized value.

impl<'r> Responder<'r, 'static> for NamedFile

Streams the named file to the client. Sets or overrides the Content-Type in the response according to the file’s extension if the extension is recognized. See ContentType::from_extension() for more information. If you would like to stream a file with a different Content-Type than that implied by its extension, use a File directly.

impl<'r> Responder<'r, 'static> for Status

The response generated by Status depends on the status code itself. The table below summarizes the functionality:

Status Code Range	Response
[400, 599]	Forwards to catcher for given status.
100, [200, 205]	Empty with status of self.
All others.	Invalid. Errors to 500 catcher.

In short, a client or server error status codes will forward to the corresponding error catcher, a successful status code less than 206 or 100 responds with any empty body and the given status code, and all other status code emit an error message and forward to the 500 (internal server error) catcher.

impl<'r> Responder<'r, 'static> for Box<str>

Returns a response with Content-Type text/plain and a fixed-size body containing the string self. Always returns Ok.

impl<'r> Responder<'r, 'static> for Box<[u8]>

Returns a response with Content-Type application/octet-stream and a fixed-size body containing the data in self. Always returns Ok.

impl<'r> Responder<'r, 'static> for String

Returns a response with Content-Type text/plain and a fixed-size body containing the string self. Always returns Ok.

impl<'r> Responder<'r, 'static> for Vec<u8>

Returns a response with Content-Type application/octet-stream and a fixed-size body containing the data in self. Always returns Ok.

impl<'r> Responder<'r, 'static> for NoContent

Sets the status code of the response to 204 No Content.

impl<'r> Responder<'r, 'static> for Redirect

Constructs a response with the appropriate status code and the given URL in the Location header field. The body of the response is empty. If the URI value used to create the Responder is an invalid URI, an error of Status::InternalServerError is returned.

impl<'r, 'o: 'r, 't: 'o, 'e: 'o, T, E> Responder<'r, 'o> for Result<T, E>

where T: Responder<'r, 't>, E: Responder<'r, 'e>,

Responds with the wrapped Responder in self, whether it is Ok or Err.

impl<'r, 'o: 'r, R: Responder<'r, 'o>> Responder<'r, 'o> for Option<R>

If self is Some, responds with the wrapped Responder. Otherwise prints a warning message and returns an Err of Status::NotFound.

impl<'r, 'o: 'r, R: Responder<'r, 'o>> Responder<'r, 'o> for RawCss<R>

Sets the Content-Type of the response then delegates the remainder of the response to the wrapped responder.

impl<'r, 'o: 'r, R: Responder<'r, 'o>> Responder<'r, 'o> for RawHtml<R>

Sets the Content-Type of the response then delegates the remainder of the response to the wrapped responder.

impl<'r, 'o: 'r, R: Responder<'r, 'o>> Responder<'r, 'o> for RawJavaScript<R>

Sets the Content-Type of the response then delegates the remainder of the response to the wrapped responder.

impl<'r, 'o: 'r, R: Responder<'r, 'o>> Responder<'r, 'o> for RawJson<R>

Sets the Content-Type of the response then delegates the remainder of the response to the wrapped responder.

impl<'r, 'o: 'r, R: Responder<'r, 'o>> Responder<'r, 'o> for RawMsgPack<R>

Sets the Content-Type of the response then delegates the remainder of the response to the wrapped responder.

impl<'r, 'o: 'r, R: Responder<'r, 'o>> Responder<'r, 'o> for RawText<R>

Sets the Content-Type of the response then delegates the remainder of the response to the wrapped responder.

impl<'r, 'o: 'r, R: Responder<'r, 'o>> Responder<'r, 'o> for RawXml<R>

Sets the Content-Type of the response then delegates the remainder of the response to the wrapped responder.

impl<'r, 'o: 'r, R: Responder<'r, 'o>> Responder<'r, 'o> for Accepted<R>

impl<'r, 'o: 'r, R: Responder<'r, 'o>> Responder<'r, 'o> for BadRequest<R>

impl<'r, 'o: 'r, R: Responder<'r, 'o>> Responder<'r, 'o> for Conflict<R>

impl<'r, 'o: 'r, R: Responder<'r, 'o>> Responder<'r, 'o> for Created<R>

Sets the status code of the response to 201 Created. Sets the Location header to the parameter in the Created::new() constructor.

The optional responder, set via Created::body() or Created::tagged_body() finalizes the response if it exists. The wrapped responder should write the body of the response so that it contains information about the created resource. If no responder is provided, the response body will be empty.

In addition to setting the status code, Location header, and finalizing the response with the Responder, the ETag header is set conditionally if a hashable Responder is provided via Created::tagged_body(). The ETag header is set to a hash value of the responder.

impl<'r, 'o: 'r, R: Responder<'r, 'o>> Responder<'r, 'o> for Custom<R>

Sets the status code of the response and then delegates the remainder of the response to the wrapped responder.

impl<'r, 'o: 'r, R: Responder<'r, 'o>> Responder<'r, 'o> for Forbidden<R>

impl<'r, 'o: 'r, R: Responder<'r, 'o>> Responder<'r, 'o> for NotFound<R>

impl<'r, 'o: 'r, R: Responder<'r, 'o>> Responder<'r, 'o> for Unauthorized<R>

impl<'r, 'o: 'r, R: Responder<'r, 'o>> Responder<'r, 'o> for Flash<R>

Sets the message cookie and then uses the wrapped responder to complete the response. In other words, simply sets a cookie and delegates the rest of the response handling to the wrapped responder. As a result, the Outcome of the response is the Outcome of the wrapped Responder.

impl<'r, 'o: 'r, T: Responder<'r, 'o> + Sized> Responder<'r, 'o> for Box<T>

Returns the response generated by the inner T. Note that this explicitly does not support Box<dyn Responder>.

#[macro_use] extern crate rocket;

use rocket::response::Responder;

#[get("/")]
fn f() -> Box<dyn Responder<'static,'static>> {
    Box::new(())
}

However, this impl allows boxing sized responders:

#[macro_use] extern crate rocket;

#[derive(Responder)]
enum Content {
    Redirect(Box<rocket::response::Redirect>),
    Text(String),
}

#[get("/")]
fn f() -> Option<Box<String>> {
    None
}

#[get("/")]
fn g() -> Content {
    Content::Text("hello".to_string())
}

impl<'r, 'o: 'r, T: Responder<'r, 'o>> Responder<'r, 'o> for Capped<T>

impl<'r, E: Debug> Responder<'r, 'static> for Debug<E>

impl<'r, S> Responder<'r, 'r> for ByteStream<S>

where S: Send + 'r + Stream, S::Item: AsRef<[u8]> + Send + Unpin + 'r,

impl<'r, S> Responder<'r, 'r> for ReaderStream<S>

where S: Send + 'r + Stream, S::Item: AsyncRead + Send,

impl<'r, S> Responder<'r, 'r> for TextStream<S>

where S: Send + 'r + Stream, S::Item: AsRef<str> + Send + Unpin + 'r,

impl<'r, S: Stream<Item = Event> + Send + 'r> Responder<'r, 'r> for EventStream<S>

impl<'r, T: Serialize> Responder<'r, 'static> for Json<T>

Available on crate feature json only.

Serializes the wrapped value into JSON. Returns a response with Content-Type JSON and a fixed-size body with the serialized value. If serialization fails, an Err of Status::InternalServerError is returned.

impl<'r, T: Serialize, const COMPACT: bool> Responder<'r, 'static> for MsgPack<T, COMPACT>

Available on crate feature msgpack only.

Serializes the wrapped value into MessagePack. Returns a response with Content-Type MsgPack and a fixed-size body with the serialization. If serialization fails, an Err of Status::InternalServerError is returned.