Inconsistent Dependency Injection to domains with Grails

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")
	}
	}

view raw Book.groovy hosted with ❤ by GitHub

	class LibraryService {
	def serviceMethod() {
	}
	}

view raw LibraryService.groovy hosted with ❤ by GitHub

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()
	}
	}

view raw BookController.groovy hosted with ❤ by GitHub

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

view raw output.txt hosted with ❤ by GitHub

What do we see?

1. ✔

   Empty constructor injects dependencies.

2. ✔

   Constructor that invokes empty constructor explicitly injects dependencies.

3. ✘

   Constructor that invokes parent's constructor explicitly does not inject dependencies.

4. ✘

   Constructor without any explicit call declared does not call empty constructor thus it does not inject dependencies.

5. ✔

   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!