xquery4j in action

byRafal Rusin
January 20, 2010
2 minute read

In my previous article, I introduced a wrapper library for Saxon, xquery4j http://github.com/rafalrusin/xquery4j.
Here, I will explain how to use it to create an article generator in Java and XQuery for XHTML, called Article. You can download it here: http://github.com/rafalrusin/Article. It’s a simple DSL for article generation.

I think it is something worth noticing, because the whole project took me just a while to implement and has interesting features. Those are:

embedded code syntax highlighting for a lot of programming languages (using external program highlight),
creating href entries for links, so you don’t need to type URL twice
it integrates natively with XHTML constructs

This is an example of an input it takes:

<a:article xmlns='http://www.w3.org/1999/xhtml' xmlns:a="urn:article">
Some text
<a:code lang="xml"><![CDATA[

]]>

It generates XHTML output for it, using command

./run <input.xml >output.xhtml

The interesting thing is that XQuery expression for this transformation is very simple to do in Saxon. This is the complete code of it:

declare namespace a="urn:article";
declare default element namespace "http://www.w3.org/1999/xhtml";

declare function a:processLine($l) {
for $i in $l/node()
return
typeswitch ($i)
case element(a:link, xs:untyped) return <a href=“{$i/text()}”>{$i/text()}
default return $i
};

declare function a:articleItem($i) {
typeswitch ($i)
case element(a:l, xs:untyped) return (a:processLine($i),
)

case element(a:code, xs:untyped) return
( a:highlight($i/text(), $i/@lang)/body/* ,
)

default return “error;”
};

<html xmlns=“http://www.w3.org/1999/xhtml”>

<br /> a.xml

Inside this expression, there is bound a:highlight Java function, which takes two strings on input (a code and a language) and returns DOM Node containing XHTML output from highlight command.
Since there is not much trouble with manipulating DOM using xquery4j, we can get as simple solution as this for a:highlight function:

public static class Mod {
        public static Node highlight(final String code, String lang) throws Exception {
            Validate.notNull(lang);
            final Process p = new ProcessBuilder("highlight", "-X", "--syntax", lang).start();
            Thread t = new Thread(new Runnable() {

public void run() {
try {
OutputStream out = p.getOutputStream();
IOUtils.write(code, out);
out.flush();
out.close();
} catch (IOException e) {
throw new RuntimeException(e);
}
}
});
t.start();
String result = IOUtils.toString(p.getInputStream());
t.join();
return DOMUtils.parse(result).getDocumentElement();
}
}

Please note that creating a separate thread for feeding input into highlight command is required, since Thread’s output queue is limited and potentially might lead to dead lock. So we need to concurrently collect output from spawned Process.
However at the end, when we need to convert a String to DOM and we use xquery4j’s DOMUtils.parse(result), so it’s a very simple construct.

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Rafal Rusin

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I've encountered strange behavior with a domain class in my project: services that should be injected were null. I've became suspicious as why is that? Services are injected properly in other domain classes so why this one is different?

Constructors experiment

I've created an experiment. I've created empty LibraryService that should be injected and Book domain class like this:

class Book {
    def libraryService

    String author
    String title
    int pageCount

    Book() {
        println("Finished constructor Book()")
    }

    Book(String author) {
        this()
        this.@author = author
        println("Finished constructor Book(String author)")
    }

    Book(String author, String title) {
        super()
        this.@author = author
        this.@title = title
        println("Finished constructor Book(String author, String title)")
    }

    Book(String author, String title, int pageCount) {
        this.@author = author
        this.@title = title
        this.@pageCount = pageCount
        println("Finished constructor Book(String author, String title, int pageCount)")
    }

    void logInjectedService() {
        println("    Service libraryService is injected? -> $libraryService")
    }
}

class LibraryService {
    def serviceMethod() {
    }
}

Book has 4 explicit constructors. I want to check which constructor is injecting dependecies. This is my method that constructs Book objects and I called it in controller:

class BookController {
    def index() {
        constructAndExamineBooks()
    }

    static constructAndExamineBooks() {
        println("Started constructAndExamineBooks")
        Book book1 = new Book().logInjectedService()
        Book book2 = new Book("foo").logInjectedService()
        Book book3 = new Book("foo", 'bar').logInjectedService()
        Book book4 = new Book("foo", 'bar', 100).logInjectedService()
        Book book5 = new Book(author: "foo", title: 'bar')
        println("Finished constructor Book(Map params)")
        book5.logInjectedService()
    }
}

Analysis

Output looks like this:

Started constructAndExamineBooks
Finished constructor Book()
    Service libraryService is injected? -> eu.spoonman.refaktor.LibraryService@2affcce2
Finished constructor Book()
Finished constructor Book(String author)
    Service libraryService is injected? -> eu.spoonman.refaktor.LibraryService@2affcce2
Finished constructor Book(String author, String title)
    Service libraryService is injected? -> null
Finished constructor Book(String author, String title, int pageCount)
    Service libraryService is injected? -> null
Finished constructor Book()
Finished constructor Book(Map params)
    Service libraryService is injected? -> eu.spoonman.refaktor.LibraryService@2affcce2

What do we see?

✔
Empty constructor injects dependencies.
✔
Constructor that invokes empty constructor explicitly injects dependencies.
✘
Constructor that invokes parent's constructor explicitly does not inject dependencies.
✘
Constructor without any explicit call declared does not call empty constructor thus it does not inject dependencies.
✔
Constructor provied by Grails with a map as a parameter invokes empty constructor and injects dependencies.

Conclusion

Always explicitily invoke empty constructor in your Grail domain classes to ensure Dependency Injection! I didn't know until today either!

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