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") }} 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!

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 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.

Conclusion

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

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Spock, Java and Maven

Few months ago I've came across Groovy - powerful language for JVM platform which combines the power of Java with abilities typical for scripting languages (dynamic typing, metaprogramming).

Together with Groovy I've discovered spock framework (https://code.google.com/p/spock/) - specification framework for Groovy (of course you can test Java classes too!). But spock is not only test/specification framework - it also contains powerful mocking tools.

Even though spock is dedicated for Groovy there is no problem with using it for Java classes tests. In this post I'm going to describe how to configure Maven project to build and run spock specifications together with traditional JUnit tests.


Firstly, we need to prepare pom.xml and add necessary dependencies and plugins.

Two obligatory libraries are:
<dependency>
<groupid>org.spockframework</groupId>
<artifactid>spock-core</artifactId>
<version>0.7-groovy-2.0</version>
<scope>test</scope>
</dependency>
<dependency>
<groupid>org.codehaus.groovy</groupId>
<artifactid>groovy-all</artifactId>
<version>${groovy.version}</version>
<scope>test</scope>
</dependency>
Where groovy.version is property defined in pom.xml for more convenient use and easy version change, just like this:
<properties>
<gmaven-plugin.version>1.4</gmaven-plugin.version>
<groovy.version>2.1.5</groovy.version>
</properties>

I've added property for gmaven-plugin version for the same reason ;)

Besides these two dependencies, we can use few additional ones providing extra functionality:
  • cglib - for class mocking
  • objenesis - enables mocking classes without default constructor
To add them to the project put these lines in <dependencies> section of pom.xml:
<dependency>
<groupid>cglib</groupId>
<artifactid>cglib-nodep</artifactId>
<version>3.0</version>
<scope>test</scope>
</dependency>
<dependency>
<groupid>org.objenesis</groupId>
<artifactid>objenesis</artifactId>
<version>1.3</version>
<scope>test</scope>
</dependency>

And that's all for dependencies section. Now we will focus on plugins necessary to compile Groovy classes. We need to add gmaven-plugin with gmaven-runtime-2.0 dependency in plugins section:
<plugin>
<groupid>org.codehaus.gmaven</groupId>
<artifactid>gmaven-plugin</artifactId>
<version>${gmaven-plugin.version}</version>
<configuration>
<providerselection>2.0</providerSelection>
</configuration>
<executions>
<execution>
<goals>
<goal>compile</goal>
<goal>testCompile</goal>
</goals>
</execution>
</executions>
<dependencies>
<dependency>
<groupid>org.codehaus.gmaven.runtime</groupId>
<artifactid>gmaven-runtime-2.0</artifactId>
<version>${gmaven-plugin.version}</version>
<exclusions>
<exclusion>
<groupid>org.codehaus.groovy</groupId>
<artifactid>groovy-all</artifactId>
</exclusion>
</exclusions>
</dependency>
<dependency>
<groupid>org.codehaus.groovy</groupId>
<artifactid>groovy-all</artifactId>
<version>${groovy.version}</version>
</dependency>
</dependencies>
</plugin>

With these configuration we can use spock and write our first specifications. But there is one issue: default settings for maven-surefire plugin demand that test classes must end with "..Test" postfix, which is ok when we want to use such naming scheme for our spock tests. But if we want to name them like CommentSpec.groovy or whatever with "..Spec" ending (what in my opinion is much more readable) we need to make little change in surefire plugin configuration:
<plugin>
<groupid>org.apache.maven.plugins</groupId>
<artifactid>maven-surefire-plugin</artifactId>
<version>2.15</version>
<configuration>
<includes>
<include>**/*Test.java</include>
<include>**/*Spec.java</include>
</includes>
</configuration>
</plugin>

As you can see there is a little trick ;) We add include directive for standard Java JUnit test ending with "..Test" postfix, but there is also an entry for spock test ending with "..Spec". And there is a trick: we must write "**/*Spec.java", not "**/*Spec.groovy", otherwise Maven will not run spock tests (which is strange and I've spent some time to figure out why Maven can't run my specs).

Little update: instead of "*.java" postfix for both types of tests we can write "*.class" what is in my opinion more readable and clean:
<include>**/*Test.class</include>
<include>**/*Spec.class</include>
(thanks to Tomek Pęksa for pointing this out!)

With such configuration, we can write either traditional JUnit test and put them in src/test/java directory or groovy spock specifications and place them in src/test/groovy. And both will work together just fine :) In one of my next posts I'll write something about using spock and its mocking abilities in practice, so stay in tune.