actix-website/content/docs/extractors.md

---
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.

## Extractors Within Handler Functions

An extractor can be accessed in a few different ways.

Option 1 - passed as a parameter to a handler function:

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

Option 2 - accessed by calling `extract()` on the Extractor

{{< include-example example="extractors" file="main.rs" section="option-two" >}}

# Path

[*Path*](https://docs.rs/actix-web/1.0.2/actix_web/dev/struct.Path.html) 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/{userid}/{friend}` path
two segments could be deserialized, `userid` 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*](https://docs.rs/actix-web/1.0.2/actix_web/web/struct.Query.html)
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*](https://docs.rs/actix-web/1.0.2/actix_web/web/struct.Json.html) allows to deserialize
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*](https://docs.rs/actix-web/1.0.2/actix_web/web/struct.JsonConfig.html) type
for configuration. To configure an extractor, pass it's 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*](https://docs.rs/actix-web/1.0.2/actix_web/web/struct.FormConfig.html) allows
configuring the extraction process.

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

# Multiple extractors

Actix-web provides extractor implementations for tuples (up to 10 elements)
whose elements implement `FromRequest`.

For example we can use a path extractor and a query extractor at the same time.

{{< include-example example="extractors" file="multiple.rs" section="multi" >}}

# Other

Actix-web also provides several other extractors:

* [*Data*](https://docs.rs/actix-web/1.0.2/actix_web/web/struct.Data.html) - 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*](https://docs.rs/actix-web/1.0.2/actix_web/trait.FromRequest.html#example-2)
  is available in doc strings.
* *bytes::Bytes* - You can convert a request's payload into *Bytes*.
  [*Example*](https://docs.rs/actix-web/1.0.2/actix_web/trait.FromRequest.html#example-4)
  is available in doc strings.
* *Payload* - You can access a request's payload.
  [*Example*](https://docs.rs/actix-web/1.0.2/actix_web/web/struct.Payload.html)

# Async Data Access

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,
> unique for each thread. If you run your application in several threads, actix will
> create the same amount as number of threads of application state objects and handler
> objects.

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 [actix](https://actix.github.io/actix/actix/) actor system.