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

Book(String author, String title) {
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() {

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)")


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?

  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.


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

Atom Feeds with Spring MVC

How to add feeds (Atom) to your web application with just two classes?
How about Spring MVC?

Here are my assumptions:
  • you are using Spring framework
  • you have some entity, say “News”, that you want to publish in your feeds
  • your "News" entity has creationDate, title, and shortDescription
  • you have some repository/dao, say "NewsRepository", that will return the news from your database
  • you want to write as little as possible
  • you don't want to format Atom (xml) by hand
You actually do NOT need to use Spring MVC in your application already. If you do, skip to step 3.

Step 1: add Spring MVC dependency to your application
With maven that will be:

Step 2: add Spring MVC DispatcherServlet
With web.xml that would be:
Notice, I set the url-pattern to “/feed” which means I don't want Spring MVC to handle any other urls in my app (I'm using a different web framework for the rest of the app). I also give it a brand new contextConfigLocation, where only the mvc configuration is kept.

Remember that, when you add a DispatcherServlet to an app that already has Spring (from ContextLoaderListener for example), your context is inherited from the global one, so you should not create beans that exist there again, or include xml that defines them. Watch out for Spring context getting up twice, and refer to spring or servlet documentation to understand what's happaning.

Step 3. add ROME – a library to handle Atom format
With maven that is:

Step 4. write your very simple controller
public class FeedController {
    static final String LAST_UPDATE_VIEW_KEY = "lastUpdate";
    static final String NEWS_VIEW_KEY = "news";
    private NewsRepository newsRepository;
    private String viewName;

    protected FeedController() {} //required by cglib

    public FeedController(NewsRepository newsRepository, String viewName) {
        notNull(newsRepository); hasText(viewName);
        this.newsRepository = newsRepository;
        this.viewName = viewName;

    @RequestMapping(value = "/feed", method = RequestMethod.GET)        
    public ModelAndView feed() {
        ModelAndView modelAndView = new ModelAndView();
        List<News> news = newsRepository.fetchPublished();
        modelAndView.addObject(NEWS_VIEW_KEY, news);
        modelAndView.addObject(LAST_UPDATE_VIEW_KEY, getCreationDateOfTheLast(news));
        return modelAndView;

    private Date getCreationDateOfTheLast(List<News> news) {
        if(news.size() > 0) {
            return news.get(0).getCreationDate();
        return new Date(0);
And here's a test for it, in case you want to copy&paste (who doesn't?):
public class FeedControllerShould {
    @Mock private NewsRepository newsRepository;
    private Date FORMER_ENTRY_CREATION_DATE = new Date(1);
    private Date LATTER_ENTRY_CREATION_DATE = new Date(2);
    private ArrayList<News> newsList;
    private FeedController feedController;

    public void prepareNewsList() {
        News news1 = new News().title("title1").creationDate(FORMER_ENTRY_CREATION_DATE);
        News news2 = new News().title("title2").creationDate(LATTER_ENTRY_CREATION_DATE);
        newsList = newArrayList(news2, news1);

    public void prepareFeedController() {
        feedController = new FeedController(newsRepository, "viewName");

    public void returnViewWithNews() {
        ModelAndView modelAndView = feedController.feed();
                .includes(entry(FeedController.NEWS_VIEW_KEY, newsList));

    public void returnViewWithLastUpdateTime() {

        ModelAndView modelAndView = feedController.feed();

                .includes(entry(FeedController.LAST_UPDATE_VIEW_KEY, LATTER_ENTRY_CREATION_DATE));

    public void returnTheBeginningOfTimeAsLastUpdateInViewWhenListIsEmpty() {
        given(newsRepository.fetchPublished()).willReturn(new ArrayList<News>());

        ModelAndView modelAndView = feedController.feed();

                .includes(entry(FeedController.LAST_UPDATE_VIEW_KEY, new Date(0)));
Notice: here, I'm using fest-assert and mockito. The dependencies are:

Step 5. write your very simple view
Here's where all the magic formatting happens. Be sure to take a look at all the methods of Entry class, as there is quite a lot you may want to use/fill.
import org.springframework.web.servlet.view.feed.AbstractAtomFeedView;

public class AtomFeedView extends AbstractAtomFeedView {
    private String feedId = "tag:yourFantastiSiteName";
    private String title = "yourFantastiSiteName: news";
    private String newsAbsoluteUrl = ""; 

    protected void buildFeedMetadata(Map<String, Object> model, Feed feed, HttpServletRequest request) {
        setUpdatedIfNeeded(model, feed);

    private void setUpdatedIfNeeded(Map<String, Object> model, Feed feed) {
        Date lastUpdate = (Date)model.get(FeedController.LAST_UPDATE_VIEW_KEY);
        if (feed.getUpdated() == null || lastUpdate != null || lastUpdate.compareTo(feed.getUpdated()) > 0) {

    protected List<Entry> buildFeedEntries(Map<String, Object> model, HttpServletRequest request, HttpServletResponse response) throws Exception {
        List<News> newsList = (List<News>)model.get(FeedController.NEWS_VIEW_KEY);
        List<Entry> entries = new ArrayList<Entry>();
        for (News news : newsList) {
            addEntry(entries, news);
        return entries;

    private void addEntry(List<Entry> entries, News news) {
        Entry entry = new Entry();
        entry.setId(feedId + ", " + news.getId());
        entry = setSummary(news, entry);
        entry = setLink(news, entry);

    private Entry setSummary(News news, Entry entry) {
        Content summary = new Content();
        return entry;

    private Entry setLink(News news, Entry entry) {
        Link link = new Link();
        link.setHref(newsAbsoluteUrl + news.getId()); //because I have a different controller to show news at
        return entry;


Step 6. add your classes to your Spring context
I'm using xml approach. because I'm old and I love xml. No, seriously, I use xml because I may want to declare FeedController a few times with different views (RSS 1.0, RSS 2.0, etc.).

So this is the forementioned spring-mvc.xml

<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns=""

    <bean class="org.springframework.web.servlet.view.ContentNegotiatingViewResolver">
        <property name="mediaTypes">
                <entry key="atom" value="application/atom+xml"/>
                <entry key="html" value="text/html"/>
        <property name="viewResolvers">
                <bean class="org.springframework.web.servlet.view.BeanNameViewResolver"/>

    <bean class="eu.margiel.pages.confitura.feed.FeedController">
        <constructor-arg index="0" ref="newsRepository"/>
        <constructor-arg index="1" value="atomFeedView"/>

    <bean id="atomFeedView" class="eu.margiel.pages.confitura.feed.AtomFeedView"/>

And you are done.

I've been asked a few times before to put all the working code in some public repo, so this time it's the other way around. I've describe things that I had already published, and you can grab the commit from the bitbucket.

Hope that helps.
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