quick-xml

High performance xml pull reader/writer.

The reader:

• is almost zero-copy (use of Cow whenever possible)
• is easy on memory allocation (the API provides a way to reuse buffers)
• support various encoding (with encoding feature), namespaces resolution, special characters.

Syntax is inspired by xml-rs.

Example

Reader

use quick_xml::events::Event;
use quick_xml::reader::Reader;

let xml = r#"<tag1 att1 = "test">
                <tag2><!--Test comment-->Test</tag2>
                <tag2>Test 2</tag2>
             </tag1>"#;
let mut reader = Reader::from_str(xml);
reader.trim_text(true);

let mut count = 0;
let mut txt = Vec::new();
let mut buf = Vec::new();

// The `Reader` does not implement `Iterator` because it outputs borrowed data (`Cow`s)
loop {
    // NOTE: this is the generic case when we don't know about the input BufRead.
    // when the input is a &str or a &[u8], we don't actually need to use another
    // buffer, we could directly call `reader.read_event()`
    match reader.read_event_into(&mut buf) {
        Err(e) => panic!("Error at position {}: {:?}", reader.buffer_position(), e),
        // exits the loop when reaching end of file
        Ok(Event::Eof) => break,

        Ok(Event::Start(e)) => {
            match e.name().as_ref() {
                b"tag1" => println!("attributes values: {:?}",
                                    e.attributes().map(|a| a.unwrap().value)
                                    .collect::<Vec<_>>()),
                b"tag2" => count += 1,
                _ => (),
            }
        }
        Ok(Event::Text(e)) => txt.push(e.unescape().unwrap().into_owned()),

        // There are several other `Event`s we do not consider here
        _ => (),
    }
    // if we don't keep a borrow elsewhere, we can clear the buffer to keep memory usage low
    buf.clear();
}

Writer

use quick_xml::events::{Event, BytesEnd, BytesStart};
use quick_xml::reader::Reader;
use quick_xml::writer::Writer;
use std::io::Cursor;

let xml = r#"<this_tag k1="v1" k2="v2"><child>text</child></this_tag>"#;
let mut reader = Reader::from_str(xml);
reader.trim_text(true);
let mut writer = Writer::new(Cursor::new(Vec::new()));
loop {
    match reader.read_event() {
        Ok(Event::Start(e)) if e.name().as_ref() == b"this_tag" => {

            // crates a new element ... alternatively we could reuse `e` by calling
            // `e.into_owned()`
            let mut elem = BytesStart::new("my_elem");

            // collect existing attributes
            elem.extend_attributes(e.attributes().map(|attr| attr.unwrap()));

            // copy existing attributes, adds a new my-key="some value" attribute
            elem.push_attribute(("my-key", "some value"));

            // writes the event to the writer
            assert!(writer.write_event(Event::Start(elem)).is_ok());
        },
        Ok(Event::End(e)) if e.name().as_ref() == b"this_tag" => {
            assert!(writer.write_event(Event::End(BytesEnd::new("my_elem"))).is_ok());
        },
        Ok(Event::Eof) => break,
        // we can either move or borrow the event to write, depending on your use-case
        Ok(e) => assert!(writer.write_event(e).is_ok()),
        Err(e) => panic!("Error at position {}: {:?}", reader.buffer_position(), e),
    }
}

let result = writer.into_inner().into_inner();
let expected = r#"<my_elem k1="v1" k2="v2" my-key="some value"><child>text</child></my_elem>"#;
assert_eq!(result, expected.as_bytes());

Serde

When using the serialize feature, quick-xml can be used with serde's Serialize/Deserialize traits.

Credits

This has largely been inspired by serde-xml-rs. quick-xml follows its convention for deserialization, including the $value special name.

Original quick-xml was developed by @tafia and abandoned around end of 2021.

Parsing the "value" of a tag

If you have an input of the form <foo abc="xyz">bar</foo>, and you want to get at the bar, you can use the special name $value:

struct Foo {
    pub abc: String,
    #[serde(rename = "$value")]
    pub body: String,
}

Unflattening structs into verbose XML

If your XML files look like <root><first>value</first><second>value</second></root>, you can (de)serialize them with the special name prefix $unflatten=:

struct Root {
    #[serde(rename = "$unflatten=first")]
    first: String,
    #[serde(rename = "$unflatten=second")]
    other_field: String,
}

Serializing unit variants as primitives

The $primitive prefix lets you serialize enum variants without associated values (internally referred to as unit variants) as primitive strings rather than self-closing tags. Consider the following definitions:

enum Foo {
    #[serde(rename = "$primitive=Bar")]
    Bar
}

struct Root {
    foo: Foo
}

Serializing Root { foo: Foo::Bar } will then yield <Root foo="Bar"/> instead of <Root><Bar/></Root>.

Performance

Note that despite not focusing on performance (there are several unnecessary copies), it remains about 10x faster than serde-xml-rs.

Features

• encoding: support non utf8 xmls
• serialize: support serde Serialize/Deserialize

Performance

Benchmarking is hard and the results depend on your input file and your machine.

Here on my particular file, quick-xml is around 50 times faster than xml-rs crate.

// quick-xml benches
test bench_quick_xml            ... bench:     198,866 ns/iter (+/- 9,663)
test bench_quick_xml_escaped    ... bench:     282,740 ns/iter (+/- 61,625)
test bench_quick_xml_namespaced ... bench:     389,977 ns/iter (+/- 32,045)

// same bench with xml-rs
test bench_xml_rs               ... bench:  14,468,930 ns/iter (+/- 321,171)

// serde-xml-rs vs serialize feature
test bench_serde_quick_xml      ... bench:   1,181,198 ns/iter (+/- 138,290)
test bench_serde_xml_rs         ... bench:  15,039,564 ns/iter (+/- 783,485)

For a feature and performance comparison, you can also have a look at RazrFalcon's parser comparison table.

Contribute

Any PR is welcomed!

License

MIT