1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
use crate::extract::Request;
use crate::extract::{rejection::*, FromRequest};
use async_trait::async_trait;
use axum_core::response::{IntoResponse, Response};
use bytes::{BufMut, Bytes, BytesMut};
use http::{
    header::{self, HeaderMap, HeaderValue},
    StatusCode,
};
use serde::{de::DeserializeOwned, Serialize};

/// JSON Extractor / Response.
///
/// When used as an extractor, it can deserialize request bodies into some type that
/// implements [`serde::de::DeserializeOwned`]. The request will be rejected (and a [`JsonRejection`] will
/// be returned) if:
///
/// - The request doesn't have a `Content-Type: application/json` (or similar) header.
/// - The body doesn't contain syntactically valid JSON.
/// - The body contains syntactically valid JSON, but it couldn't be deserialized into the target
/// type.
/// - Buffering the request body fails.
///
/// ⚠️ Since parsing JSON requires consuming the request body, the `Json` extractor must be
/// *last* if there are multiple extractors in a handler.
/// See ["the order of extractors"][order-of-extractors]
///
/// [order-of-extractors]: crate::extract#the-order-of-extractors
///
/// See [`JsonRejection`] for more details.
///
/// # Extractor example
///
/// ```rust,no_run
/// use axum::{
///     extract,
///     routing::post,
///     Router,
/// };
/// use serde::Deserialize;
///
/// #[derive(Deserialize)]
/// struct CreateUser {
///     email: String,
///     password: String,
/// }
///
/// async fn create_user(extract::Json(payload): extract::Json<CreateUser>) {
///     // payload is a `CreateUser`
/// }
///
/// let app = Router::new().route("/users", post(create_user));
/// # let _: Router = app;
/// ```
///
/// When used as a response, it can serialize any type that implements [`serde::Serialize`] to
/// `JSON`, and will automatically set `Content-Type: application/json` header.
///
/// # Response example
///
/// ```
/// use axum::{
///     extract::Path,
///     routing::get,
///     Router,
///     Json,
/// };
/// use serde::Serialize;
/// use uuid::Uuid;
///
/// #[derive(Serialize)]
/// struct User {
///     id: Uuid,
///     username: String,
/// }
///
/// async fn get_user(Path(user_id) : Path<Uuid>) -> Json<User> {
///     let user = find_user(user_id).await;
///     Json(user)
/// }
///
/// async fn find_user(user_id: Uuid) -> User {
///     // ...
///     # unimplemented!()
/// }
///
/// let app = Router::new().route("/users/:id", get(get_user));
/// # let _: Router = app;
/// ```
#[derive(Debug, Clone, Copy, Default)]
#[cfg_attr(docsrs, doc(cfg(feature = "json")))]
#[must_use]
pub struct Json<T>(pub T);

#[async_trait]
impl<T, S> FromRequest<S> for Json<T>
where
    T: DeserializeOwned,
    S: Send + Sync,
{
    type Rejection = JsonRejection;

    async fn from_request(req: Request, state: &S) -> Result<Self, Self::Rejection> {
        if json_content_type(req.headers()) {
            let bytes = Bytes::from_request(req, state).await?;
            Self::from_bytes(&bytes)
        } else {
            Err(MissingJsonContentType.into())
        }
    }
}

fn json_content_type(headers: &HeaderMap) -> bool {
    let content_type = if let Some(content_type) = headers.get(header::CONTENT_TYPE) {
        content_type
    } else {
        return false;
    };

    let content_type = if let Ok(content_type) = content_type.to_str() {
        content_type
    } else {
        return false;
    };

    let mime = if let Ok(mime) = content_type.parse::<mime::Mime>() {
        mime
    } else {
        return false;
    };

    let is_json_content_type = mime.type_() == "application"
        && (mime.subtype() == "json" || mime.suffix().map_or(false, |name| name == "json"));

    is_json_content_type
}

axum_core::__impl_deref!(Json);

impl<T> From<T> for Json<T> {
    fn from(inner: T) -> Self {
        Self(inner)
    }
}

impl<T> Json<T>
where
    T: DeserializeOwned,
{
    /// Construct a `Json<T>` from a byte slice. Most users should prefer to use the `FromRequest` impl
    /// but special cases may require first extracting a `Request` into `Bytes` then optionally
    /// constructing a `Json<T>`.
    pub fn from_bytes(bytes: &[u8]) -> Result<Self, JsonRejection> {
        let deserializer = &mut serde_json::Deserializer::from_slice(bytes);

        let value = match serde_path_to_error::deserialize(deserializer) {
            Ok(value) => value,
            Err(err) => {
                let rejection = match err.inner().classify() {
                    serde_json::error::Category::Data => JsonDataError::from_err(err).into(),
                    serde_json::error::Category::Syntax | serde_json::error::Category::Eof => {
                        JsonSyntaxError::from_err(err).into()
                    }
                    serde_json::error::Category::Io => {
                        if cfg!(debug_assertions) {
                            // we don't use `serde_json::from_reader` and instead always buffer
                            // bodies first, so we shouldn't encounter any IO errors
                            unreachable!()
                        } else {
                            JsonSyntaxError::from_err(err).into()
                        }
                    }
                };
                return Err(rejection);
            }
        };

        Ok(Json(value))
    }
}

impl<T> IntoResponse for Json<T>
where
    T: Serialize,
{
    fn into_response(self) -> Response {
        // Use a small initial capacity of 128 bytes like serde_json::to_vec
        // https://docs.rs/serde_json/1.0.82/src/serde_json/ser.rs.html#2189
        let mut buf = BytesMut::with_capacity(128).writer();
        match serde_json::to_writer(&mut buf, &self.0) {
            Ok(()) => (
                [(
                    header::CONTENT_TYPE,
                    HeaderValue::from_static(mime::APPLICATION_JSON.as_ref()),
                )],
                buf.into_inner().freeze(),
            )
                .into_response(),
            Err(err) => (
                StatusCode::INTERNAL_SERVER_ERROR,
                [(
                    header::CONTENT_TYPE,
                    HeaderValue::from_static(mime::TEXT_PLAIN_UTF_8.as_ref()),
                )],
                err.to_string(),
            )
                .into_response(),
        }
    }
}

#[cfg(test)]
mod tests {
    use super::*;
    use crate::{routing::post, test_helpers::*, Router};
    use serde::Deserialize;
    use serde_json::{json, Value};

    #[crate::test]
    async fn deserialize_body() {
        #[derive(Debug, Deserialize)]
        struct Input {
            foo: String,
        }

        let app = Router::new().route("/", post(|input: Json<Input>| async { input.0.foo }));

        let client = TestClient::new(app);
        let res = client.post("/").json(&json!({ "foo": "bar" })).await;
        let body = res.text().await;

        assert_eq!(body, "bar");
    }

    #[crate::test]
    async fn consume_body_to_json_requires_json_content_type() {
        #[derive(Debug, Deserialize)]
        struct Input {
            foo: String,
        }

        let app = Router::new().route("/", post(|input: Json<Input>| async { input.0.foo }));

        let client = TestClient::new(app);
        let res = client.post("/").body(r#"{ "foo": "bar" }"#).await;

        let status = res.status();

        assert_eq!(status, StatusCode::UNSUPPORTED_MEDIA_TYPE);
    }

    #[crate::test]
    async fn json_content_types() {
        async fn valid_json_content_type(content_type: &str) -> bool {
            println!("testing {content_type:?}");

            let app = Router::new().route("/", post(|Json(_): Json<Value>| async {}));

            let res = TestClient::new(app)
                .post("/")
                .header("content-type", content_type)
                .body("{}")
                .await;

            res.status() == StatusCode::OK
        }

        assert!(valid_json_content_type("application/json").await);
        assert!(valid_json_content_type("application/json; charset=utf-8").await);
        assert!(valid_json_content_type("application/json;charset=utf-8").await);
        assert!(valid_json_content_type("application/cloudevents+json").await);
        assert!(!valid_json_content_type("text/json").await);
    }

    #[crate::test]
    async fn invalid_json_syntax() {
        let app = Router::new().route("/", post(|_: Json<serde_json::Value>| async {}));

        let client = TestClient::new(app);
        let res = client
            .post("/")
            .body("{")
            .header("content-type", "application/json")
            .await;

        assert_eq!(res.status(), StatusCode::BAD_REQUEST);
    }

    #[derive(Deserialize)]
    struct Foo {
        #[allow(dead_code)]
        a: i32,
        #[allow(dead_code)]
        b: Vec<Bar>,
    }

    #[derive(Deserialize)]
    struct Bar {
        #[allow(dead_code)]
        x: i32,
        #[allow(dead_code)]
        y: i32,
    }

    #[crate::test]
    async fn invalid_json_data() {
        let app = Router::new().route("/", post(|_: Json<Foo>| async {}));

        let client = TestClient::new(app);
        let res = client
            .post("/")
            .body("{\"a\": 1, \"b\": [{\"x\": 2}]}")
            .header("content-type", "application/json")
            .await;

        assert_eq!(res.status(), StatusCode::UNPROCESSABLE_ENTITY);
        let body_text = res.text().await;
        assert_eq!(
            body_text,
            "Failed to deserialize the JSON body into the target type: b[0]: missing field `y` at line 1 column 23"
        );
    }
}