DDevelopment & Design

Using Kotlin extensions in Groovy

Extensions in Kotlin and GroovyKotlin and Groovy have mechanisms for extending existing classes without using inheritance or decorators. In both languages, the mechanisms are called extension methods. Their source code looks different, but generated by…

Using Kotlin extensions in Groovy

Extensions in Kotlin and Groovy

Kotlin and Groovy have mechanisms for extending existing classes without using inheritance or decorators. In both languages, the mechanisms are called extension methods. Their source code looks different, but generated bytecode is quite similar. Thanks to that, Groovy is able to use Kotlin extensions just like its own.

Why would I want to use such extensions in Groovy? The main reason is that I want to test my extensions using the best testing framework available for the JVM – Spock Framework.

Code is available here.

Extensions in Kotlin

There are many types of extensions in Kotlin. I decided to focus only on extension functions and properties.

As an example, I extend the java.lang.String class. First, I create an extension function skipFirst, which skips first N characters:

fun String.skipFirst(n: Int) = if (length > n) this.substring(n) else ""

 

Next, I create an extension property answer, which is the Answer to the Ultimate Question of Life, the Universe, and Everything:

val String.answer
    get() = 42

Both extensions are declared in package com.github.alien11689.extensions, in file called StringExtensions. However, the generated class in target directory is named StringExtensionsKt and this is the name that must be used when accessing from other languages. Specific class name can be forced by annotation @file:JvmName.

Using Kotlin extensions in Groovy

There are two ways for using extensions in Groovy that are supported by good IDEs. First, you can declare scope where the extensions are available by use method:

def "should use extension method"() {
    expect:
        use(StringExtensionsKt) {
            input.skipFirst(n) == expected
        }
    where:
        input  | n | expected
        "abcd" | 3 | "d"
        "abcd" | 6 | ""
        ""     | 3 | ""
}

def "should use extension property"() {
    expect:
        use(StringExtensionsKt) {
            "abcd".answer == 42
        }
}

It is acceptable, but is not very convenient. The second and much better way is to use an extension module definition. The extension module is defined in file org.codehaus.groovy.runtime.ExtensionModule in directory src/main/resources/META-INF/services/. The same directory is monitored by ServiceLoader, but the file format is completely different:

moduleName=string-extension-module
moduleVersion=1.0.0
extensionClasses=com.github.alien11689.extensions.StringExtensionsKt

The tests look much better now:

def "should use extension method"() {
    expect:
        input.skipFirst(n) == expected
    where:
        input  | n | expected
        "abcd" | 3 | "d"
        "abcd" | 6 | ""
        ""     | 3 | ""
}

def "should use extension property"() {
    expect:
        "abcd".answer == 42
}

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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).