actix-website/docs/extractors.md

title
Extractors

import CodeBlock from "@site/src/components/code_block";

Type-safe Request Information Extraction

Actix Web provides a facility for type-safe request information access called extractors (i.e., impl FromRequest). There are lots of built-in extractor implementations (see implementors).

An extractor can be accessed as an argument to a handler function. Actix Web supports up to 12 extractors per handler function.

In most cases, argument position does not matter. However, if an extractor takes the body (i.e., it reads any bytes from the request body stream), then only the first extractor will succeed. If you need fallback behavior such as "read body as JSON or just give me the raw bytes if that fails", then use the Either extractor (e.g., Either<Json<T>, Bytes>).

Some other specific use cases where request bodies need reading twice can be supported:

• For body (any extractor) + it's hash/digest, see the actix-hash crate.
• For body (any extractor) + custom request signature scheme: see the RequestSignature extractor from actix-web-lab.

Simple example showing extraction of two positional dynamic path segments and a JSON body:

Path

Path provides information that is extracted from the request's path. Parts of the path that are extractable are called "dynamic segments" and are marked with curly braces. You can deserialize any variable segment from the path.

For instance, for resource that registered for the /users/{user_id}/{friend} path, two segments could be deserialized, user_id and friend. These segments could be extracted as a tuple in the order they are declared (e.g., Path<(u32, String)>).

It is also possible to extract path information to a type that implements the Deserialize trait from serde by matching dynamic segment names with field names. Here is an equivalent example that uses a deserialization struct using serde (make sure to enable its derive feature) instead of a tuple type.

As a non-type-safe alternative, it's also possible to query (see match_info docs) the request for path parameters by name within a handler:

Query

The Query<T> type provides extraction functionality for the request's query parameters. Underneath it uses serde_urlencoded crate.

JSON

Json<T> allows deserialization of a request body into a struct. To extract typed information from a request's body, the type T must implement serde::Deserialize.

Some extractors provide a way to configure the extraction process. To configure an extractor, pass its configuration object to the resource's .app_data() method. In the case of Json extractor it returns a JsonConfig. You can configure the maximum size of the JSON payload as well as a custom error handler function.

The following example limits the size of the payload to 4kb and uses a custom error handler.

URL-Encoded Forms

A URL-encoded form body can be extracted to a struct, much like Json<T>. This type must implement serde::Deserialize.

FormConfig allows configuring the extraction process.

Other

Actix Web also provides many other extractors, here's a few important ones:

• Data - For accessing pieces of application state.
• HttpRequest - HttpRequest is itself an extractor, in case you need access to other parts of the request.
• String - You can convert a request's payload to a String. An example is available in the rustdoc.
• Bytes - You can convert a request's payload into Bytes. An example is available in the rustdoc.
• Payload - Low-level payload extractor primarily for building other extractors. An example is available in the rustdoc.

Application State Extractor

Application state is accessible from the handler with the web::Data extractor; however, state is accessible as a read-only reference. If you need mutable access to state, it must be implemented.

Here is an example of a handler that stores the number of processed requests:

Although this handler will work, data.count will only count the number of requests handled by each worker thread. To count the number of total requests across all threads, one should use shared Arc and atomics.

Note: If you want the entire state to be shared across all threads, use web::Data and app_data as described in Shared Mutable State.

Be careful when using blocking synchronization primitives like Mutex or RwLock within your app state. Actix Web handles requests asynchronously. It is a problem if the critical section in your handler is too big or contains an .await point. If this is a concern, we would advise you to also read Tokio's advice on using blocking Mutex in async code.