title: Extractors menu: docs_basics weight: 170

Type-safe information extraction

Actix-web provides a facility for type-safe request information access called extractors (ie, impl FromRequest). By default, actix-web provides several extractor implementations.

An extractor can be accessed as an argument to a handler function. Actix-web supports up to 10 extractors per handler function. Argument position does not matter.

{{< include-example example=”extractors” file=”main.rs” section=”option-one” >}}

Path

Path provides information that can be extracted from the Request’s path. 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 into a tuple, i.e. Path<(u32, String)> or any structure that implements the Deserialize trait from the serde crate.

{{< include-example example=”extractors” file=”path_one.rs” section=”path-one” >}}

It is also possible to extract path information to a specific type that implements the Deserialize trait from serde. Here is an equivalent example that uses serde instead of a tuple type.

{{< include-example example=”extractors” file=”path_two.rs” section=”path-two” >}}

It is also possible to get or query the request for path parameters by name:

{{< include-example example=”extractors” file=”path_three.rs” section=”path-three” >}}

Query

The Query type provides extraction functionality for the request’s query parameters. Underneath it uses serde_urlencoded crate.

{{< include-example example=”extractors” file=”query.rs” section=”query” >}}

Json

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

{{< include-example example=”extractors” file=”json_one.rs” section=”json-one” >}}

Some extractors provide a way to configure the extraction process. Json extractor JsonConfig type for configuration. To configure an extractor, pass its configuration object to the resource’s .data() method. In case of a 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.

{{< include-example example=”extractors” file=”json_two.rs” section=”json-two” >}}

Form

At the moment, only url-encoded forms are supported. The url-encoded body could be extracted to a specific type. This type must implement the Deserialize trait from the serde crate.

FormConfig allows configuring the extraction process.

{{< include-example example=”extractors” file=”form.rs” section=”form” >}}

Other

Actix-web also provides several other extractors:

  • Data - If you need access to an application state.
  • HttpRequest - HttpRequest itself is an extractor which returns self, in case you need access to the request.
  • String - You can convert a request’s payload to a String. Example is available in doc strings.
  • bytes::Bytes - You can convert a request’s payload into Bytes. Example is available in doc strings.
  • Payload - You can access a request’s payload. Example

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.

Beware, actix creates multiple copies of the application state and the handlers. It creates one copy for each thread.

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

{{< include-example example=”request-handlers” file=”main.rs” section=”data” >}}

Although this handler will work, self.0 will be different depending on the number of threads and number of requests processed per thread. A proper implementation would use Arc and AtomicUsize.

{{< include-example example=”request-handlers” file=”handlers_arc.rs” section=”arc” >}}

Be careful with synchronization primitives like Mutex or RwLock. The actix-web framework handles requests asynchronously. By blocking thread execution, all concurrent request handling processes would block. If you need to share or update some state from multiple threads, consider using the tokio synchronization primitives.