com.github.zherebjatjew:xml-machine

Overview

Stream XML parsers are in general faster than DOM-based parsers and less memory consuming. However, they have significant drawbacks:

• require very complex code to convert stream of tags to target data structure,
• do not allow rewinding to an element.

This library helps to solve at least the first problem. It provides to you an instrument for building a state machine and easily operate with xml nodes respecting document structure.

How to use

There are two main components: state machine builder and xml parser. The parser takes the state machine and runs it against given XML. Parser can use any Stax-based xml reader. In tests, I use the native one, java.xml.stream.XMLStreamReader.

Let's go to the code. Imagine you have an xml:

<library>
    <book language="en">
        <title>American Gods</title>
        <author>Neil Gaiman</author>
    </book>
    <book language="pl">
        <author>Chuck Palahniuk</author>
        <title>Fight Club</title>
    </book>
</library>

and you need to print out authors and titles. With the tool you can do it easily:

try (StringReader reader = new StringReader(xml) {
    StaxParser parser = new StaxParser(XMLInputFactory.newInstance()
        .createXMLStreamReader(reader));
    parser.read(RootHandler.instance(
        "library", r -> r.then("book")
        .or("author", x -> x.text(System.out::println))
        .or("title",  x -> x.text(System.out::println)))
    );
}

it will print out

American Gods
Neil Gaiman
Chuck Palahniuk
Fight Club

Value propagation

You can notice that it is not quite fancy because authors and titles go in a random order. What if you have to prepare a CSV where first column contains authors and second column contains book titles?

To solve that we can take advantage of a powerful feature called propagation. It tells the machine to publish text of a node to its parent.

try (StringReader reader = new StringReader(xml) {
    StaxParser parser = new StaxParser(XMLInputFactory.newInstance()
        .createXMLStreamReader(reader));
    parser.read(RootHandler.instance(
        "library", r -> r.then("book")
        .or("author", x -> Handler::propagate)
        .or("title",  x -> Handler::propagate)
        .close(book -> System.out.println(book.getProperty("author") + ',' + book.getProperty("title"))))
    );
}

Output:

Neil Gaiman,American Gods
Chuck Palahniuk,Fight Club

Values can be propagated further. In this case their names get path reflecting source of the value. E.g., in the snippet above, if you propagated book node, you can address values from library node as book/author and book/title.

Attributes

By default, the parser skips attributes for the sake of performance. You can turn on attribute fetching explicitly for separate nodes.

Attributes can be propagated. Let's filter from our books only English versions. Notice new withAttributes instruction, and that attribute is referenced with '@' prefix.

try (StringReader reader = new StringReader(xml) {
    StaxParser parser = new StaxParser(XMLInputFactory.newInstance()
        .createXMLStreamReader(reader));
    parser.read(RootHandler.instance(
        "library", r -> r.then("book").withAttributes()
        .or("author", x -> Handler::propagate)
        .or("title",  x -> Handler::propagate)
        .close(book -> {
            if ("en".equals(book.getProperty("@language") {
                System.out.println(book.getProperty("author") + ',' + book.getProperty("title"))))
            }
        }
    );
}

Assumptions

The example above can be implemented in more concise way by using assumptions.

try (StringReader reader = new StringReader(xml) {
    StaxParser parser = new StaxParser(XMLInputFactory.newInstance()
        .createXMLStreamReader(reader));
    parser.read(RootHandler.instance(
        "library", r -> r.then("book").withAttributes()
        .assume(f -> "en".equals(f.getProperty("@language"))
        .or("author", x -> Handler::propagate)
        .or("title",  x -> Handler::propagate)
        .close(book -> System.out.println(book.getProperty("author") + ',' + book.getProperty("title"))))
    );
}

Limitations

1. The machine matches nodes by full node name. You cannot use patterns in methods Handler#then, Handler#or.