# Schemars
[](https://github.com/GREsau/schemars/actions)
[](https://crates.io/crates/schemars)
[](https://docs.rs/schemars)
[](https://blog.rust-lang.org/2022/04/07/Rust-1.60.0.html)
Generate JSON Schema documents from Rust code
## Basic Usage
If you don't really care about the specifics, the easiest way to generate a JSON schema for your types is to `#[derive(JsonSchema)]` and use the `schema_for!` macro. All fields of the type must also implement `JsonSchema` - Schemars implements this for many standard library types.
```rust
use schemars::{schema_for, JsonSchema};
#[derive(JsonSchema)]
pub struct MyStruct {
pub my_int: i32,
pub my_bool: bool,
pub my_nullable_enum: Option<MyEnum>,
}
#[derive(JsonSchema)]
pub enum MyEnum {
StringNewType(String),
StructVariant { floats: Vec<f32> },
}
let schema = schema_for!(MyStruct);
println!("{}", serde_json::to_string_pretty(&schema).unwrap());
```
<details>
<summary>Click to see the output JSON schema...</summary>
```json
{
"$schema": "http://json-schema.org/draft-07/schema#",
"title": "MyStruct",
"type": "object",
"required": ["my_bool", "my_int"],
"properties": {
"my_bool": {
"type": "boolean"
},
"my_int": {
"type": "integer",
"format": "int32"
},
"my_nullable_enum": {
"anyOf": [
{
"$ref": "#/definitions/MyEnum"
},
{
"type": "null"
}
]
}
},
"definitions": {
"MyEnum": {
"anyOf": [
{
"type": "object",
"required": ["StringNewType"],
"properties": {
"StringNewType": {
"type": "string"
}
},
"additionalProperties": false
},
{
"type": "object",
"required": ["StructVariant"],
"properties": {
"StructVariant": {
"type": "object",
"required": ["floats"],
"properties": {
"floats": {
"type": "array",
"items": {
"type": "number",
"format": "float"
}
}
}
}
},
"additionalProperties": false
}
]
}
}
}
```
</details>
### Serde Compatibility
One of the main aims of this library is compatibility with [Serde](https://github.com/serde-rs/serde). Any generated schema _should_ match how [serde_json](https://github.com/serde-rs/json) would serialize/deserialize to/from JSON. To support this, Schemars will check for any `#[serde(...)]` attributes on types that derive `JsonSchema`, and adjust the generated schema accordingly.
```rust
use schemars::{schema_for, JsonSchema};
use serde::{Deserialize, Serialize};
#[derive(Deserialize, Serialize, JsonSchema)]
#[serde(rename_all = "camelCase", deny_unknown_fields)]
pub struct MyStruct {
#[serde(rename = "myNumber")]
pub my_int: i32,
pub my_bool: bool,
#[serde(default)]
pub my_nullable_enum: Option<MyEnum>,
}
#[derive(Deserialize, Serialize, JsonSchema)]
#[serde(untagged)]
pub enum MyEnum {
StringNewType(String),
StructVariant { floats: Vec<f32> },
}
let schema = schema_for!(MyStruct);
println!("{}", serde_json::to_string_pretty(&schema).unwrap());
```
<details>
<summary>Click to see the output JSON schema...</summary>
```json
{
"$schema": "http://json-schema.org/draft-07/schema#",
"title": "MyStruct",
"type": "object",
"required": ["myBool", "myNumber"],
"properties": {
"myBool": {
"type": "boolean"
},
"myNullableEnum": {
"default": null,
"anyOf": [
{
"$ref": "#/definitions/MyEnum"
},
{
"type": "null"
}
]
},
"myNumber": {
"type": "integer",
"format": "int32"
}
},
"additionalProperties": false,
"definitions": {
"MyEnum": {
"anyOf": [
{
"type": "string"
},
{
"type": "object",
"required": ["floats"],
"properties": {
"floats": {
"type": "array",
"items": {
"type": "number",
"format": "float"
}
}
}
}
]
}
}
}
```
</details>
`#[serde(...)]` attributes can be overriden using `#[schemars(...)]` attributes, which behave identically (e.g. `#[schemars(rename_all = "camelCase")]`). You may find this useful if you want to change the generated schema without affecting Serde's behaviour, or if you're just not using Serde.
### Schema from Example Value
If you want a schema for a type that can't/doesn't implement `JsonSchema`, but does implement `serde::Serialize`, then you can generate a JSON schema from a value of that type. However, this schema will generally be less precise than if the type implemented `JsonSchema` - particularly when it involves enums, since schemars will not make any assumptions about the structure of an enum based on a single variant.
```rust
use schemars::schema_for_value;
use serde::Serialize;
#[derive(Serialize)]
pub struct MyStruct {
pub my_int: i32,
pub my_bool: bool,
pub my_nullable_enum: Option<MyEnum>,
}
#[derive(Serialize)]
pub enum MyEnum {
StringNewType(String),
StructVariant { floats: Vec<f32> },
}
let schema = schema_for_value!(MyStruct {
my_int: 123,
my_bool: true,
my_nullable_enum: Some(MyEnum::StringNewType("foo".to_string()))
});
println!("{}", serde_json::to_string_pretty(&schema).unwrap());
```
<details>
<summary>Click to see the output JSON schema...</summary>
```json
{
"$schema": "http://json-schema.org/draft-07/schema#",
"title": "MyStruct",
"examples": [
{
"my_bool": true,
"my_int": 123,
"my_nullable_enum": {
"StringNewType": "foo"
}
}
],
"type": "object",
"properties": {
"my_bool": {
"type": "boolean"
},
"my_int": {
"type": "integer"
},
"my_nullable_enum": true
}
}
```
</details>
## Feature Flags
- `derive` (enabled by default) - provides `#[derive(JsonSchema)]` macro
- `impl_json_schema` - implements `JsonSchema` for Schemars types themselves
- `preserve_order` - keep the order of struct fields in `Schema` and `SchemaObject`
- `raw_value` - implements `JsonSchema` for `serde_json::value::RawValue` (enables the serde_json `raw_value` feature)
Schemars can implement `JsonSchema` on types from several popular crates, enabled via feature flags (dependency versions are shown in brackets):
- `chrono` - [chrono](https://crates.io/crates/chrono) (^0.4)
- `indexmap1` - [indexmap](https://crates.io/crates/indexmap) (^1.2)
- `indexmap2` - [indexmap](https://crates.io/crates/indexmap) (^2.0)
- `either` - [either](https://crates.io/crates/either) (^1.3)
- `uuid08` - [uuid](https://crates.io/crates/uuid) (^0.8)
- `uuid1` - [uuid](https://crates.io/crates/uuid) (^1.0)
- `smallvec` - [smallvec](https://crates.io/crates/smallvec) (^1.0)
- `arrayvec05` - [arrayvec](https://crates.io/crates/arrayvec) (^0.5)
- `arrayvec07` - [arrayvec](https://crates.io/crates/arrayvec) (^0.7)
- `url` - [url](https://crates.io/crates/url) (^2.0)
- `bytes` - [bytes](https://crates.io/crates/bytes) (^1.0)
- `enumset` - [enumset](https://crates.io/crates/enumset) (^1.0)
- `rust_decimal` - [rust_decimal](https://crates.io/crates/rust_decimal) (^1.0)
- `bigdecimal03` - [bigdecimal](https://crates.io/crates/bigdecimal) (^0.3)
- `bigdecimal04` - [bigdecimal](https://crates.io/crates/bigdecimal) (^0.4)
- `smol_str` - [smol_str](https://crates.io/crates/smol_str) (^0.1.17)
- `semver` - [semver](https://crates.io/crates/semver) (^1.0.9)
For example, to implement `JsonSchema` on types from `chrono`, enable it as a feature in the `schemars` dependency in your `Cargo.toml` like so:
```toml
[dependencies]
schemars = { version = "0.8", features = ["chrono"] }
```