425 lines
12 KiB
Markdown
425 lines
12 KiB
Markdown
Serde Rust Serialization Framework
|
|
==================================
|
|
|
|
[](https://travis-ci.org/serde-rs/serde)
|
|
[](https://crates.io/crates/serde)
|
|
|
|
Serde is a powerful framework that enables serialization libraries to
|
|
generically serialize Rust data structures without the overhead of runtime type
|
|
information. In many situations, the handshake protocol between serializers and
|
|
serializees can be completely optimized away, leaving Serde to perform roughly
|
|
the same speed as a hand written serializer for a specific type.
|
|
|
|
Documentation is available at http://serde-rs.github.io/serde/serde
|
|
|
|
Making a Type Serializable
|
|
==========================
|
|
|
|
The simplest way to make a type serializable is to use the `serde_macros`
|
|
syntax extension, which comes with a `#[derive(Serialize, Deserialize)]`
|
|
annotation, which automatically generates implementations of
|
|
[Serialize](http://serde-rs.github.io/serde/serde/ser/trait.Serialize.html)
|
|
and
|
|
[Deserialize](http://serde-rs.github.io/serde/serde/de/trait.Deserialize.html)
|
|
for the annotated type:
|
|
|
|
```rust
|
|
#![feature(custom_derive, plugin)]
|
|
#![plugin(serde_macros)]
|
|
|
|
extern crate serde;
|
|
|
|
...
|
|
|
|
#[derive(Serialize, Deserialize)]
|
|
struct Point {
|
|
x: i32,
|
|
y: i32,
|
|
}
|
|
```
|
|
|
|
Serde bundles a high performance JSON serializer and deserializer,
|
|
[serde_json](http://serde-rs.github.io/serde/serde_json/index.html),
|
|
which comes with the helper functions
|
|
[to_string](http://serde-rs.github.io/serde/serde/json/ser/fn.to_string.html)
|
|
and
|
|
[from_str](http://serde-rs.github.io/serde/serde/json/de/fn.from_str.html)
|
|
that make it easy to go to and from JSON:
|
|
|
|
```rust
|
|
use serde_json;
|
|
|
|
...
|
|
|
|
let point = Point { x: 1, y: 2 };
|
|
let serialized_point = json::to_string(&point).unwrap();
|
|
|
|
println!("{}", serialized_point); // prints: {"x":1,"y":2}
|
|
|
|
let deserialize_point: Point = json::from_str(&serialized_point).unwrap();
|
|
```
|
|
|
|
[serde_json](http://serde-rs.github.io/serde/serde_json/index.html) also
|
|
supports a generic
|
|
[Value](http://serde-rs.github.io/serde/serde/json/value/enum.Value.html)
|
|
type, which can represent any JSON value. Also, any
|
|
[Serialize](http://serde-rs.github.io/serde/serde/ser/trait.Serialize.html)
|
|
and
|
|
[Deserialize](http://serde-rs.github.io/serde/serde/de/trait.Deserialize.html)
|
|
can be converted into a
|
|
[Value](http://serde-rs.github.io/serde/serde/json/value/enum.Value.html)
|
|
with the methods
|
|
[to_value](http://serde-rs.github.io/serde/serde/json/value/fn.to_value.html)
|
|
and
|
|
[from_value](http://serde-rs.github.io/serde/serde/json/value/fn.from_value.html):
|
|
|
|
```rust
|
|
let point = Point { x: 1, y: 2 };
|
|
let point_value = json::to_value(&point).unwrap();
|
|
|
|
println!("{}", point_value.find("x")); // prints: Some(1)
|
|
|
|
let deserialize_point: Point = json::from_value(point_value).unwrap();
|
|
```
|
|
|
|
Serialization without Macros
|
|
============================
|
|
|
|
Under the covers, Serde extensively uses the Visitor pattern to thread state
|
|
between the
|
|
[Serializer](http://serde-rs.github.io/serde/serde/ser/trait.Serializer.html)
|
|
and
|
|
[Serialize](http://serde-rs.github.io/serde/serde/ser/trait.Serialize.html)
|
|
without the two having specific information about each other's concrete type.
|
|
This has many of the same benefits as frameworks that use runtime type
|
|
information without the overhead. In fact, when compiling with optimizations,
|
|
Rust is able to remove most or all the visitor state, and generate code that's
|
|
nearly as fast as a hand written serializer format for a specific type.
|
|
|
|
To see it in action, lets look at how a simple type like `i32` is serialized.
|
|
The
|
|
[Serializer](http://serde-rs.github.io/serde/serde/ser/trait.Serializer.html)
|
|
is threaded through the type:
|
|
|
|
```rust
|
|
impl serde::Serialize for i32 {
|
|
fn serialize<S>(&self, serializer: &mut S) -> Result<(), S::Error>
|
|
where S: serde::Serializer,
|
|
{
|
|
serializer.visit_i32(*self)
|
|
}
|
|
}
|
|
```
|
|
|
|
As you can see it's pretty simple. More complex types like `BTreeMap` need to
|
|
pass a
|
|
[MapVisitor](http://serde-rs.github.io/serde/serde/ser/trait.MapVisitor.html)
|
|
to the
|
|
[Serializer](http://serde-rs.github.io/serde/serde/ser/trait.Serializer.html)
|
|
in order to walk through the type:
|
|
|
|
```rust
|
|
impl<K, V> Serialize for BTreeMap<K, V>
|
|
where K: Serialize + Ord,
|
|
V: Serialize,
|
|
{
|
|
#[inline]
|
|
fn serialize<S>(&self, serializer: &mut S) -> Result<(), S::Error>
|
|
where S: Serializer,
|
|
{
|
|
serializer.visit_map(MapIteratorVisitor::new(self.iter(), Some(self.len())))
|
|
}
|
|
}
|
|
|
|
pub struct MapIteratorVisitor<Iter> {
|
|
iter: Iter,
|
|
len: Option<usize>,
|
|
}
|
|
|
|
impl<K, V, Iter> MapIteratorVisitor<Iter>
|
|
where Iter: Iterator<Item=(K, V)>
|
|
{
|
|
#[inline]
|
|
pub fn new(iter: Iter, len: Option<usize>) -> MapIteratorVisitor<Iter> {
|
|
MapIteratorVisitor {
|
|
iter: iter,
|
|
len: len,
|
|
}
|
|
}
|
|
}
|
|
|
|
impl<K, V, I> MapVisitor for MapIteratorVisitor<I>
|
|
where K: Serialize,
|
|
V: Serialize,
|
|
I: Iterator<Item=(K, V)>,
|
|
{
|
|
#[inline]
|
|
fn visit<S>(&mut self, serializer: &mut S) -> Result<Option<()>, S::Error>
|
|
where S: Serializer,
|
|
{
|
|
match self.iter.next() {
|
|
Some((key, value)) => {
|
|
let value = try!(serializer.visit_map_elt(key, value));
|
|
Ok(Some(value))
|
|
}
|
|
None => Ok(None)
|
|
}
|
|
}
|
|
|
|
#[inline]
|
|
fn len(&self) -> Option<usize> {
|
|
self.len
|
|
}
|
|
}
|
|
```
|
|
|
|
Serializing structs follow this same pattern. In fact, structs are represented
|
|
as a named map. Its visitor uses a simple state machine to iterate through all
|
|
the fields:
|
|
|
|
```rust
|
|
struct Point {
|
|
x: i32,
|
|
y: i32,
|
|
}
|
|
|
|
impl serde::Serialize for Point {
|
|
fn serialize<S>(&self, serializer: &mut S) -> Result<(), S::Error>
|
|
where S: serde::Serializer
|
|
{
|
|
serializer.visit_struct("Point", PointMapVisitor {
|
|
value: self,
|
|
state: 0,
|
|
})
|
|
}
|
|
}
|
|
|
|
struct PointMapVisitor<'a> {
|
|
value: &'a Point,
|
|
state: u8,
|
|
}
|
|
|
|
impl<'a> serde::ser::MapVisitor for PointMapVisitor<'a> {
|
|
fn visit<S>(&mut self, serializer: &mut S) -> Result<Option<()>, S::Error>
|
|
where S: serde::Serializer
|
|
{
|
|
match self.state {
|
|
0 => {
|
|
self.state += 1;
|
|
Ok(Some(try!(serializer.visit_map_elt("x", &self.value.x))))
|
|
}
|
|
1 => {
|
|
self.state += 1;
|
|
Ok(Some(try!(serializer.visit_map_elt("y", &self.value.y))))
|
|
}
|
|
_ => {
|
|
Ok(None)
|
|
}
|
|
}
|
|
}
|
|
}
|
|
```
|
|
|
|
Deserialization without Macros
|
|
==============================
|
|
|
|
Deserialization is a little more complicated since there's a bit more error
|
|
handling that needs to occur. Let's start with the simple `i32`
|
|
[Deserialize](http://serde-rs.github.io/serde/serde/de/trait.Deserialize.html)
|
|
implementation. It passes a
|
|
[Visitor](http://serde-rs.github.io/serde/serde/de/trait.Visitor.html) to the
|
|
[Deserializer](http://serde-rs.github.io/serde/serde/de/trait.Deserializer.html).
|
|
The [Visitor](http://serde-rs.github.io/serde/serde/de/trait.Visitor.html)
|
|
can create the `i32` from a variety of different types:
|
|
|
|
```rust
|
|
impl Deserialize for i32 {
|
|
fn deserialize<D>(deserializer: &mut D) -> Result<i32, D::Error>
|
|
where D: serde::Deserializer,
|
|
{
|
|
deserializer.visit(I32Visitor)
|
|
}
|
|
}
|
|
|
|
struct I32Visitor;
|
|
|
|
impl serde::de::Visitor for I32Visitor {
|
|
type Value = i32;
|
|
|
|
fn visit_i16<E>(&mut self, value: i16) -> Result<i16, E>
|
|
where E: Error,
|
|
{
|
|
self.visit_i32(value as i32)
|
|
}
|
|
|
|
fn visit_i32<E>(&mut self, value: i32) -> Result<i32, E>
|
|
where E: Error,
|
|
{
|
|
Ok(value)
|
|
}
|
|
|
|
...
|
|
|
|
```
|
|
|
|
Since it's possible for this type to get passed an unexpected type, we need a
|
|
way to error out. This is done by way of the
|
|
[Error](http://serde-rs.github.io/serde/serde/de/trait.Error.html) trait,
|
|
which allows a
|
|
[Deserialize](http://serde-rs.github.io/serde/serde/de/trait.Deserialize.html)
|
|
to generate an error for a few common error conditions. Here's how it could be used:
|
|
|
|
```rust
|
|
...
|
|
|
|
fn visit_string<E>(&mut self, _: String) -> Result<i32, E>
|
|
where E: Error,
|
|
{
|
|
Err(serde::de::Error::syntax("expect a string"))
|
|
}
|
|
|
|
...
|
|
|
|
```
|
|
|
|
Maps follow a similar pattern as before, and use a
|
|
[MapVisitor](http://serde-rs.github.io/serde/serde/de/trait.MapVisitor.html)
|
|
to walk through the values generated by the
|
|
[Deserializer](http://serde-rs.github.io/serde/serde/de/trait.Deserializer.html).
|
|
|
|
```rust
|
|
impl<K, V> serde::Deserialize for BTreeMap<K, V>
|
|
where K: serde::Deserialize + Eq + Ord,
|
|
V: serde::Deserialize,
|
|
{
|
|
fn deserialize<D>(deserializer: &mut D) -> Result<BTreeMap<K, V>, D::Error>
|
|
where D: serde::Deserializer,
|
|
{
|
|
deserializer.visit(BTreeMapVisitor::new())
|
|
}
|
|
}
|
|
|
|
pub struct BTreeMapVisitor<K, V> {
|
|
marker: PhantomData<BTreeMap<K, V>>,
|
|
}
|
|
|
|
impl<K, V> BTreeMapVisitor<K, V> {
|
|
pub fn new() -> Self {
|
|
BTreeMapVisitor {
|
|
marker: PhantomData,
|
|
}
|
|
}
|
|
}
|
|
|
|
impl<K, V> serde::de::Visitor for BTreeMapVisitor<K, V>
|
|
where K: serde::de::Deserialize + Ord,
|
|
V: serde::de::Deserialize
|
|
{
|
|
type Value = BTreeMap<K, V>;
|
|
|
|
fn visit_unit<E>(&mut self) -> Result<BTreeMap<K, V>, E>
|
|
where E: Error,
|
|
{
|
|
Ok(BTreeMap::new())
|
|
}
|
|
|
|
fn visit_map<V_>(&mut self, mut visitor: V_) -> Result<BTreeMap<K, V>, V_::Error>
|
|
where V_: MapVisitor,
|
|
{
|
|
let mut values = BTreeMap::new();
|
|
|
|
while let Some((key, value)) = try!(visitor.visit()) {
|
|
values.insert(key, value);
|
|
}
|
|
|
|
try!(visitor.end());
|
|
|
|
Ok(values)
|
|
}
|
|
}
|
|
|
|
```
|
|
|
|
Deserializing structs goes a step further in order to support not allocating a
|
|
`String` to hold the field names. This is done by custom field enum that
|
|
deserializes an enum variant from a string. So for our `Point` example from
|
|
before, we need to generate:
|
|
|
|
```rust
|
|
enum PointField {
|
|
X,
|
|
Y,
|
|
}
|
|
|
|
impl serde::Deserialize for PointField {
|
|
fn deserialize<D>(deserializer: &mut D) -> Result<PointField, D::Error>
|
|
where D: serde::de::Deserializer
|
|
{
|
|
struct FieldVisitor;
|
|
|
|
impl serde::de::Visitor for FieldVisitor {
|
|
type Value = Field;
|
|
|
|
fn visit_str<E>(&mut self, value: &str) -> Result<PointField, E>
|
|
where E: serde::de::Error
|
|
{
|
|
match value {
|
|
"x" => Ok(Field::X),
|
|
"y" => Ok(Field::Y),
|
|
_ => Err(serde::de::Error::syntax("expected x or y")),
|
|
}
|
|
}
|
|
}
|
|
|
|
deserializer.visit(FieldVisitor)
|
|
}
|
|
}
|
|
```
|
|
|
|
This is then used in our actual deserializer:
|
|
|
|
```rust
|
|
impl serde::Deserialize for Point {
|
|
fn deserialize<D>(deserializer: &mut D) -> Result<Point, D::Error>
|
|
where D: serde::de::Deserializer
|
|
{
|
|
deserializer.visit_struct("Point", PointVisitor)
|
|
}
|
|
}
|
|
|
|
struct PointVisitor;
|
|
|
|
impl serde::de::Visitor for PointVisitor {
|
|
type Value = Point;
|
|
|
|
fn visit_map<V>(&mut self, mut visitor: V) -> Result<Point, V::Error>
|
|
where V: serde::de::MapVisitor
|
|
{
|
|
let mut x = None;
|
|
let mut y = None;
|
|
|
|
loop {
|
|
match try!(visitor.visit_key()) {
|
|
Some(Field::X) => { x = Some(try!(visitor.visit_value())); }
|
|
Some(Field::Y) => { y = Some(try!(visitor.visit_value())); }
|
|
None => { break; }
|
|
}
|
|
}
|
|
|
|
let x = match x {
|
|
Some(x) => x,
|
|
None => try!(visitor.missing_field("x")),
|
|
};
|
|
|
|
let y = match y {
|
|
Some(y) => y,
|
|
None => try!(visitor.missing_field("y")),
|
|
};
|
|
|
|
try!(visitor.end());
|
|
|
|
Ok(Point{ x: x, y: y })
|
|
}
|
|
}
|
|
```
|