DDevelopment & Design

xquery4j in action

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


<link rel=“stylesheet” type=“text/css” href=“highlight.css”/>


{
for $i in a:article/*
return
a:articleItem($i)
}

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.

— Previous article

Embedding XQuery in Java

Next article —

Git, Beginner to Advanced Survey

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DDevelopment & Design

Grails session timeout without XML

This article shows clean, non hacky way of configuring featureful event listeners for Grails application servlet context. Feat. HttpSessionListener as a Spring bean example with session timeout depending on whether user account is premium or not.

Common approaches

Speaking of session timeout config in Grails, a default approach is to install templates with a command. This way we got direct access to web.xml file. Also more unnecessary files are created. Despite that unnecessary files are unnecessary, we should also remember some other common knowledge: XML is not for humans.

Another, a bit more hacky, way is to create mysterious scripts/_Events.groovy file. Inside of which, by using not less enigmatic closure: eventWebXmlEnd = { filename -> ... }, we can parse and hack into web.xml with a help of XmlSlurper.
Even though lot of Grails plugins do it similar way, still it’s not really straightforward, is it? Besides, where’s the IDE support? Hello!?

Examples of both above ways can be seen on StackOverflow.

Simpler and cleaner way

By adding just a single line to the already generated init closure we have it done:
class BootStrap {


    def init = { servletContext ->    

        servletContext.addListener(OurListenerClass)    

    }    

}

Allrighty, this is enough to avoid XML. Sweets are served after the main course though :)

Listener as a Spring bean

Let us assume we have a requirement. Set a longer session timeout for premium user account.
Users are authenticated upon session creation through SSO.

To easy meet the requirements just instantiate the CustomTimeoutSessionListener as Spring bean at resources.groovy. We also going to need some source of the user custom session timeout. Let say a ConfigService.
beans = {    

    customTimeoutSessionListener(CustomTimeoutSessionListener) {    

        configService = ref('configService')    

    }    

}

With such approach BootStrap.groovy has to by slightly modified. To keep control on listener instantation, instead of passing listener class type, Spring bean is injected by Grails and the instance passed:
class BootStrap {


    def customTimeoutSessionListener


    def init = { servletContext ->    

        servletContext.addListener(customTimeoutSessionListener)

    }    

}

An example CustomTimeoutSessionListener implementation can look like:
import javax.servlet.http.HttpSessionEvent    

import javax.servlet.http.HttpSessionListener    

import your.app.ConfigService    


class CustomTimeoutSessionListener implements HttpSessionListener {    


    ConfigService configService


    @Override    

    void sessionCreated(HttpSessionEvent httpSessionEvent) {    

        httpSessionEvent.session.maxInactiveInterval = configService.sessionTimeoutSeconds

    }    


    @Override    

    void sessionDestroyed(HttpSessionEvent httpSessionEvent) { /* nothing to implement */ }    

}
Having at hand all power of the Spring IoC this is surely a good place to load some persisted user’s account stuff into the session or to notify any other adequate bean about user presence.

Wait, what about the user context?

Honest answer is: that depends on your case. Yet here’s an example of getSessionTimeoutMinutes() implementation using Spring Security:
import org.springframework.security.core.context.SecurityContextHolder    


class ConfigService {


    static final int 3H = 3 * 60 * 60

    static final int QUARTER = 15 * 60


    int getSessionTimeoutSeconds() {    


        String username = SecurityContextHolder.context?.authentication?.principal    

        def account = Account.findByUsername(username)    


        return account?.premium ? 3H : QUARTER

    }    

}
This example is simplified. Does not contain much of defensive programming. Just an assumption that principal is already set and is a String - unique username. Thanks to Grails convention our ConfigService is transactional so the Account domain class can use GORM dynamic finder.
OK, config fetching implementation details are out of scope here anyway. You can get, load, fetch, obtain from wherever you like to. Domain persistence, principal object, role config, external file and so on...

Any gotchas?

There is one. When running grails test command, servletContext comes as some mocked class instance without addListener method. Thus we going to have a MissingMethodException when running tests :(

Solution is typical:
def init = { servletContext ->

    if (Environment.current != Environment.TEST) {    

        servletContext.addListener(customTimeoutSessionListener)    

    }    

}
An unnecessary obstacle if you ask me. Should I submit a Jira issue about that?

TL;DR

Just implement a HttpSessionListener. Create a Spring bean of the listener. Inject it into BootStrap.groovy and call servletContext.addListener(injectedListener).