Inconsistent Dependency Injection to domains with Grails

byTomasz Kalkosiński
October 18, 2012
2 minute read

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 constructAndExamineBooksFinished constructor Book() Service libraryService is injected? -> eu.spoonman.refaktor.LibraryService@2affcce2Finished constructor Book()Finished constructor Book(String author) Service libraryService is injected? -> eu.spoonman.refaktor.LibraryService@2affcce2Finished constructor Book(String author, String title) Service libraryService is injected? -> nullFinished constructor Book(String author, String title, int pageCount) Service libraryService is injected? -> nullFinished 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!

Constructors experiment

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

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:

Analysis

Output looks like this:

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!

Tomasz Kalkosiński

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DDevelopment & Design

Log4j and MDC in Grails

byRafał Nowak
November 5, 2013

Log4j provides very useful feature: MDC - mapped diagnostic context. It can be used to store data in context of current thread. It may sound scary a bit but idea is simple.

My post is based on post http://burtbeckwith.com/blog/?p=521 from Burt Beckwith's excellent blog, it's definitely worth checking if you are interested in Grails.

Short background story...

Suppose we want to do logging our brand new shopping system and we want to have in each log customer's shopping basket number. And our system can be used at once by many users who can perform many transactions, actions like adding items and so on. How can we achieve that? Of course we can add basket number in every place where we do some logging but this task would be boring and error-prone.

Instead of this we can use MDC to store variable with basket number in map.

In fact MDC can be treated as map of custom values for current thread that can be used by logger.

How to do that with Grails?

Using MDC with Grails is quite simple. All we need to do is to create our own custom filter which works for given urls and puts our data in MDC.

Filters in Grails are classes in directory grails-app/conf/* which names end with *Filters.groovy postfix. We can create this class manually or use Grails command:

grails create-filters info.rnowak.App.Basket

In result class named BasketFilters will be created in grails-app/conf/info/rnowak/UberApp.

Initially filter class looks a little bit empty:

class BasketFilters {
    def filters = {
        all(controller:'*', action:'*') {
            before = {

            }
            after = { Map model ->

            }
            afterView = { Exception e ->

            }
        }
    }
}

All we need to do is fill empty closures, modify filter properties and put some data into MDC.

all is the general name of our filter, as class BasketFilters (plural!) can contain many various filters. You can name it whatever you want, for this post let assume it will be named basketFilter.

Another thing is change of filter parameters. According to official documentation (link) we can customize our filter in many ways. You can specify controller to be filtered, its actions, filtered urls and so on. In our example you can stay with default option where filter is applied to every action of every controller. If you are interested in filtering only some urls, use uri parameter with expression describing desired urls to be filtered.

Three closures that are already defined in template have their function and they are started in these conditions:

before - as name says, it is executed before filtered action takes place
after - similarly, it is called after the action
afterView - called after rendering of the actions view

Ok, so now we know what are these mysterious methods and when they are called. But what can be done within them? In official Grails docs (link again) under section 7.6.3 there is a list of properties that are available to use in filter.

With that knowledge, we can proceed to implementing filter.

Putting something into MDC in filter

What we want to do is quite easy: we want to retrieve basket number from parameters and put it into MDC in our filter:

class BasketFilters {
    def filters = {
        basketFilter(controller:'*', action:'*') {
            before = {
                MDC.put("basketNumber", params.basketNumber ?: "")
            }
            after = { Map model ->
                MDC.remove("basketNumber")
            }
        }
    }
}

We retrieve basket number from Grails params map and then we put in map under specified key ("basketNumber" in this case), which will be later used in logger conversion pattern. It is important to remove custom value after processing of action to avoid leaks.

So we are putting something into MDC. But how make use of it in logs?

We can refer to custom data in MDC in conversion patter using syntax: %X{key}, where key is our key we used in filter to put data, like:

def conversionPattern = "%d{yyyy-MM-dd HH:mm:ss} %-5p %t [%c{1}] %X{basketNumber} - %m%n"

And that's it :) We've put custom data in log4j MDC and successfully used it in logs to display interesting values.