serde_json crate 解析。
serde_json 提供了 serde_json::Value 类型, 可以用于反序列化 无类型的 JSON
字符串, 后续可以对 Value 使用 map index 操作来获得成员(类型是 &Value):
- 可以使用 &str 字符串来获得 map 类型值, 可以使用整型 index 操作来获得数组成员;
- 如果 index 的 map key 或 array index 不存在, 则返回
Value::Null; - 打印 Value 时, 返回 quota string. 如果不显示字符串 quota 双引号, 则可以调用 Value::as_str() 返回的字符串;
enum Value {
Null,
Bool(bool),
Number(Number), // Number 为 serde_json 定义的通用数字类型
String(String),
Array(Vec<Value>),
Object(Map<String, Value>),
}
use serde_json::{Result, Value};
fn untyped_example() -> Result<()> {
let data = r#"
{
"name": "John Doe",
"age": 43,
"phones": [
"+44 1234567",
"+44 2345678"
]
}"#;
// 返回的值为通用 serde_json::Value 类型, 如果失败则为
// serde_json::Result 类型
let v: Value = serde_json::from_str(data)?;
println!("Please call {} at the number {}", v["name"], v["phones"][0]);
Ok(())
}
serde_json::Value 类型提供了一些列方法，实现了 Index/IndexMut trait：
impl Value
pub fn get<I: Index>(&self, index: I) -> Option<&Value>
pub fn get_mut<I: Index>(&mut self, index: I) -> Option<&mut Value>
pub fn is_object(&self) -> bool
pub fn as_object(&self) -> Option<&Map<String, Value>>
pub fn as_object_mut(&mut self) -> Option<&mut Map<String, Value>>
pub fn is_array(&self) -> bool
pub fn as_array(&self) -> Option<&Vec<Value>>
pub fn as_array_mut(&mut self) -> Option<&mut Vec<Value>>
pub fn is_string(&self) -> bool
pub fn as_str(&self) -> Option<&str>
pub fn is_number(&self) -> bool
pub fn as_number(&self) -> Option<&Number>
pub fn is_i64(&self) -> bool
pub fn is_u64(&self) -> bool
pub fn is_f64(&self) -> bool
pub fn as_i64(&self) -> Option<i64>
pub fn as_u64(&self) -> Option<u64>
pub fn as_f64(&self) -> Option<f64>
pub fn is_boolean(&self) -> bool
pub fn as_bool(&self) -> Option<bool>
pub fn is_null(&self) -> bool
pub fn as_null(&self) -> Option<()>
pub fn pointer(&self, pointer: &str) -> Option<&Value>
pub fn pointer_mut(&mut self, pointer: &str) -> Option<&mut Value>
pub fn take(&mut self) -> Value
impl<I> Index<I> for Value where I: Index,
fn index(&self, index: I) -> &Value
type Output = Value
impl<I> IndexMut<I> for Value where I: Index,
fn index_mut(&mut self, index: I) -> &mut Value
// Index 使用 Index trait 定义
// &str/String/usize 均实现了 Index trait
pub trait Index: Sealed { }
impl Index for str
impl Index for usize
impl Index for String
impl<T> Index for &T where T: ?Sized + Index,
可以使用 json!() 宏快速创建 Value enum variant 值:
let v = json!(null);
let v = json!(true);
let v = json!(12.5);
let v = json!("a string");
let v = json!(["an", "array"]);
let v = json!({ "an": "object" });
对于实现了 Serialize 和 Deserialize trait 的自定义类型, 可以序列化和反序列化对应类型的值:
use serde::{Deserialize, Serialize};
use serde_json::Result;
#[derive(Serialize, Deserialize)]
struct Person {
name: String,
age: u8,
phones: Vec<String>,
}
fn typed_example() -> Result<()> {
let data = r#"
{
"name": "John Doe",
"age": 43,
"phones": [
"+44 1234567",
"+44 2345678"
]
}"#;
let p: Person = serde_json::from_str(data)?;
println!("Please call {} at the number {}", p.name, p.phones[0]);
Ok(())
}
类型实现 Serialize trait 后, 可以使用 serde_json::to_string() 将对象转换为 JSON string :
use serde::{Deserialize, Serialize};
use serde_json::Result;
#[derive(Serialize, Deserialize)]
struct Address {
street: String,
city: String,
}
fn print_an_address() -> Result<()> {
let address = Address {
street: "10 Downing Street".to_owned(),
city: "London".to_owned(),
};
let j = serde_json::to_string(&address)?;
println!("{}", j);
Ok(())
}
使用 json!() 宏从字符串创建 serde_json::Value 对象，它支持变量和表达式插值,
也可以调用其它宏：
use serde_json::json;
fn main() {
let john: serde_json::Value = json!({
"name": "John Doe",
"age": 43,
"phones": [
"+44 1234567",
"+44 2345678"
]
});
println!("first phone number: {}", john["phones"][0]);
println!("{}", john.to_string());
}
let full_name = "John Doe";
let age_last_year = 42;
let john = json!({
"name": full_name,
"age": age_last_year + 1,
"phones": [
format!("+44 {}", random_phone())
]
});
serde_json 提供了 Error 和 Result 类型, 用于在 Serialize 或 Deserialize
出错时返回:
use serde_json::Value;
use std::io::{self, ErrorKind, Read};
use std::process;
struct ReaderThatWillTimeOut<'a>(&'a [u8]);
impl<'a> Read for ReaderThatWillTimeOut<'a> {
fn read(&mut self, buf: &mut [u8]) -> io::Result<usize> {
if self.0.is_empty() {
Err(io::Error::new(ErrorKind::TimedOut, "timed out"))
} else {
self.0.read(buf)
}
}
}
fn main() {
let reader = ReaderThatWillTimeOut(br#" {"k": "#);
let _: Value = match serde_json::from_reader(reader) {
Ok(value) => value,
Err(error) => {
if error.io_error_kind() == Some(ErrorKind::TimedOut) {
// Maybe this application needs to retry certain kinds of errors.
} else {
eprintln!("error: {}", error);
process::exit(1);
}
}
};
}
serde_json module 提供的反序列化函数（均返回一个 T 类型对象）：
// 下面返回的 Result 是 serde_json::Result, 它的 Err 为 serde_json::Error 类型
// 从实现 std::io::Read trait 的 reader 读取
pub fn from_reader<R, T>(rdr: R) -> Result<T> where R: Read, T: DeserializeOwned
// 从 &[u8] 读取
pub fn from_slice<'a, T>(v: &'a [u8]) -> Result<T> where T: Deserialize<'a>
// 从 &str 读取
pub fn from_str<'a, T>(s: &'a str) -> Result<T> where T: Deserialize<'a>
// 从 serde_json::Value 读取
pub fn from_value<T>(value: Value) -> Result<T, Error> where T: DeserializeOwned
// 示例
use serde::Deserialize;
use std::error::Error;
use std::fs::File;
use std::io::BufReader;
use std::path::Path;
#[derive(Deserialize, Debug)]
struct User {
fingerprint: String,
location: String,
}
fn read_user_from_file<P: AsRef<Path>>(path: P) -> Result<User, Box<dyn Error>> {
let file = File::open(path)?;
let reader = BufReader::new(file);
let u = serde_json::from_reader(reader)?;
Ok(u)
}
fn main() {
let u = read_user_from_file("test.json").unwrap();
println!("{:#?}", u);
}
use serde::Deserialize;
use serde_json::json;
#[derive(Deserialize, Debug)]
struct User {
fingerprint: String,
location: String,
}
fn main() {
let j = json!({
"fingerprint": "0xF9BA143B95FF6D82",
"location": "Menlo Park, CA"
});
let u: User = serde_json::from_value(j).unwrap();
println!("{:#?}", u);
}
从 Deserializer 创建的 StreamDeserializer，可以实现 流式反序列化 ：
impl<'de, R, T> StreamDeserializer<'de, R, T>
where
R: Read<'de>,
T: Deserialize<'de>,
// 可以从各种 serde_json::Read 来流式解码数据，例如：
// Deserializer::from_str(…).into_iter()
// Deserializer::from_slice(…).into_iter()
// Deserializer::from_reader(…).into_iter()
pub fn new(read: R) -> Self
pub fn byte_offset(&self) -> usize
// Returns the number of bytes so far deserialized into a successful T.
// StreamDeserializer 实现了 Iterator，每次迭代返回 T 对象
impl<'de, R, T> Iterator for StreamDeserializer<'de, R, T>
where
R: Read<'de>,
T: Deserialize<'de>,
type Item = Result<T, Error>
Deserializer 的 into_iter::<T>() 方法创建一个 StreamDeserializer；
- 待解码的数据可以是任何 JSON Value 序列（而不仅限于一个类型）；
- 每一次迭代，可以使用 stream.byte_offset() 来获得 bufer 中偏移量；
// 例子
let data = b"[0] [1] [";
let de = serde_json::Deserializer::from_slice(data);
let mut stream = de.into_iter::<Vec<i32>>();
assert_eq!(0, stream.byte_offset());
println!("{:?}", stream.next()); // [0]
assert_eq!(3, stream.byte_offset());
println!("{:?}", stream.next()); // [1]
assert_eq!(7, stream.byte_offset());
println!("{:?}", stream.next()); // error
assert_eq!(8, stream.byte_offset());
// If err.is_eof(), can join the remaining data to new data and continue.
let remaining = &data[stream.byte_offset()..];
// 另一个例子
use serde_json::{Deserializer, Value};
fn main() {
let data = "{\"k\": 3}1\"cool\"\"stuff\" 3{} [0, 1, 2]";
let stream = Deserializer::from_str(data).into_iter::<Value>();
for value in stream {
println!("{}", value.unwrap());
}
}
serde_json module 提供的序列化函数：
// 下面返回的 Result 是 serde_json::Result, 它的 Err 为 serde_json::Error 类型
pub fn to_value<T>(value: T) -> Result<Value, Error> where T: Serialize
// 序列化为 String
pub fn to_string<T>(value: &T) -> Result<String> where T: ?Sized + Serialize
pub fn to_string_pretty<T>(value: &T) -> Result<String> where T: ?Sized + Serialize
// 序列化为 Vec<u8>
pub fn to_vec<T>(value: &T) -> Result<Vec<u8>> where T: ?Sized + Serialize
pub fn to_vec_pretty<T>(value: &T) -> Result<Vec<u8>> where T: ?Sized + Serialize
// 序列化为 UTF-8 字节流, 然后写入 writer
pub fn to_writer<W, T>(writer: W, value: &T) -> Result<()> where W: Write, T: ?Sized + Serialize
pub fn to_writer_pretty<W, T>(writer: W, value: &T) -> Result<()> where W: Write, T: ?Sized + Serialize
// 示例
use serde::Serialize;
use serde_json::json;
use std::error::Error;
#[derive(Serialize)]
struct User {
fingerprint: String,
location: String,
}
fn compare_json_values() -> Result<(), Box<dyn Error>> {
let u = User {
fingerprint: "0xF9BA143B95FF6D82".to_owned(),
location: "Menlo Park, CA".to_owned(),
};
let expected: serde_json::Value = json!({
"fingerprint": "0xF9BA143B95FF6D82",
"location": "Menlo Park, CA",
});
let v = serde_json::to_value(u).unwrap();
assert_eq!(v, expected);
Ok(())
}