xml-rs — A streaming XML library

xml-rs, an XML library for Rust

Documentation

xml-rs is an XML library for Rust programming language. It is heavily inspired by Java Streaming API for XML (StAX).

This library currently contains pull parser much like StAX event reader. It provides iterator API, so you can leverage Rust's existing iterators library features.

It also provides a streaming document writer much like StAX event writer. This writer consumes its own set of events, but reader events can be converted to writer events easily, and so it is possible to write XML transformation chains in a pretty clean manner.

This parser is mostly full-featured, however, there are limitations:

• no other encodings but UTF-8 are supported yet, because no stream-based encoding library is available now; when (or if) one will be available, I'll try to make use of it;
• DTD validation is not supported, <!DOCTYPE> declarations are completely ignored; thus no support for custom entities too; internal DTD declarations are likely to cause parsing errors;
• attribute value normalization is not performed, and end-of-line characters are not normalized too.

Other than that the parser tries to be mostly XML-1.0-compliant.

Writer is also mostly full-featured with the following limitations:

• no support for encodings other than UTF-8, for the same reason as above;
• no support for emitting <!DOCTYPE> declarations;
• more validations of input are needed, for example, checking that namespace prefixes are bounded or comments are well-formed.

What is planned (highest priority first, approximately):

0. missing features required by XML standard (e.g. aforementioned normalization and proper DTD parsing);
1. miscellaneous features of the writer;
2. parsing into a DOM tree and its serialization back to XML text;
3. SAX-like callback-based parser (fairly easy to implement over pull parser);
4. DTD validation;
5. (let's dream a bit) XML Schema validation.

Building and using

xml-rs uses Cargo, so just add a dependency section in your project's manifest:

[dependencies]
xml-rs = "0.8"

The package exposes a single crate called xml:

extern crate xml;

Reading XML documents

xml::reader::EventReader requires a Read instance to read from. When a proper stream-based encoding library is available, it is likely that xml-rs will be switched to use whatever character stream structure this library would provide, but currently it is a Read.

Using EventReader is very straightforward. Just provide a Read instance to obtain an iterator over events:

extern crate xml;

use std::fs::File;
use std::io::BufReader;

use xml::reader::{EventReader, XmlEvent};

fn indent(size: usize) -> String {
    const INDENT: &'static str = "    ";
    (0..size).map(|_| INDENT)
             .fold(String::with_capacity(size*INDENT.len()), |r, s| r + s)
}

fn main() {
    let file = File::open("file.xml").unwrap();
    let file = BufReader::new(file);

    let parser = EventReader::new(file);
    let mut depth = 0;
    for e in parser {
        match e {
            Ok(XmlEvent::StartElement { name, .. }) => {
                println!("{}+{}", indent(depth), name);
                depth += 1;
            }
            Ok(XmlEvent::EndElement { name }) => {
                depth -= 1;
                println!("{}-{}", indent(depth), name);
            }
            Err(e) => {
                println!("Error: {}", e);
                break;
            }
            _ => {}
        }
    }
}

EventReader implements IntoIterator trait, so you can just use it in a for loop directly. Document parsing can end normally or with an error. Regardless of exact cause, the parsing process will be stopped, and iterator will terminate normally.

You can also have finer control over when to pull the next event from the parser using its own next() method:

match parser.next() {
    ...
}

Upon the end of the document or an error the parser will remember that last event and will always return it in the result of next() call afterwards. If iterator is used, then it will yield error or end-of-document event once and will produce None afterwards.

It is also possible to tweak parsing process a little using xml::reader::ParserConfig structure. See its documentation for more information and examples.

You can find a more extensive example of using EventReader in src/analyze.rs, which is a small program (BTW, it is built with cargo build and can be run after that) which shows various statistics about specified XML document. It can also be used to check for well-formedness of XML documents - if a document is not well-formed, this program will exit with an error.

Writing XML documents

xml-rs also provides a streaming writer much like StAX event writer. With it you can write an XML document to any Write implementor.

extern crate xml;

use std::fs::File;
use std::io::{self, Write};

use xml::writer::{EventWriter, EmitterConfig, XmlEvent, Result};

fn handle_event<W: Write>(w: &mut EventWriter<W>, line: String) -> Result<()> {
    let line = line.trim();
    let event: XmlEvent = if line.starts_with("+") && line.len() > 1 {
        XmlEvent::start_element(&line[1..]).into()
    } else if line.starts_with("-") {
        XmlEvent::end_element().into()
    } else {
        XmlEvent::characters(&line).into()
    };
    w.write(event)
}

fn main() {
    let mut file = File::create("output.xml").unwrap();

    let mut input = io::stdin();
    let mut output = io::stdout();
    let mut writer = EmitterConfig::new().perform_indent(true).create_writer(&mut file);
    loop {
        print!("> "); output.flush().unwrap();
        let mut line = String::new();
        match input.read_line(&mut line) {
            Ok(0) => break,
            Ok(_) => match handle_event(&mut writer, line) {
                Ok(_) => {}
                Err(e) => panic!("Write error: {}", e)
            },
            Err(e) => panic!("Input error: {}", e)
        }
    }
}

The code example above also demonstrates how to create a writer out of its configuration. Similar thing also works with EventReader.

The library provides an XML event building DSL which helps to construct complex events, e.g. ones having namespace definitions. Some examples:

// <a:hello a:param="value" xmlns:a="urn:some:document">
XmlEvent::start_element("a:hello").attr("a:param", "value").ns("a", "urn:some:document")

// <hello b:config="name" xmlns="urn:default:uri">
XmlEvent::start_element("hello").attr("b:config", "value").default_ns("urn:defaul:uri")

// <![CDATA[some unescaped text]]>
XmlEvent::cdata("some unescaped text")

Of course, one can create XmlEvent enum variants directly instead of using the builder DSL. There are more examples in xml::writer::XmlEvent documentation.

The writer has multiple configuration options; see EmitterConfig documentation for more information.

Other things

No performance tests or measurements are done. The implementation is rather naive, and no specific optimizations are made. Hopefully the library is sufficiently fast to process documents of common size. I intend to add benchmarks in future, but not until more important features are added.

Known issues

All known issues are present on GitHub issue tracker: http://github.com/netvl/xml-rs/issues. Feel free to post any found problems there.

License

This library is licensed under MIT license.

Copyright (C) Vladimir Matveev, 2014-2020