Groovy, Callable and ExecutorService

Suppose you want submit job to ExecutorService. The Baroque versionYou could create a class that implements Callable:class MyJob implements Callable<Integer>{ @Override Integer call() throws Exception { return 42 }}and give it to …

Suppose you want submit job to ExecutorService.

The Baroque version

You could create a class that implements Callable:

class MyJob implements Callable<Integer>{
    @Override
    Integer call() throws Exception {
        return 42
    }
}

and give it to the executor service:

def 'submit callable as MyJob object'() {
    expect:
    executorService.submit(new MyJob()).get() == 42
}

The response is, as expected, 42.

Map as Callable version

You want to use this job only in one place so why not inline this class:

def 'submit callable as map'() {
    expect:
        executorService.submit([call: { 42 }] as Callable).get() == 42
}

The response is again 42.

Groovy closure version

Why not use closure instead of map?

def 'submit callable as closure'(){
    expect:
        executorService.submit { 42 }.get() == 42
}

The response is … null.

Condition not satisfied:
executorService.submit { 42 }.get() == 42
|               |             |      |
|               |             |      false
|               |             null
|               java.util.concurrent.FutureTask@21de60b4
java.util.concurrent.Executors$FinalizableDelegatedExecutorService@1700915

 

Why? It is because Groovy treats this closure as Runnable, not Callable and Future#get returns null when task is complete.

Groovy closure version with cast

We have to cast our closure before submiting to executor service:

def 'submit callable as closure with cast'() {
    when:
        int result = executorService.submit({ return 42 } as Callable<Integer>).get()
    then:
        result == 42
}

The response is, as expected, again 42.

What interesting, the same test with inlined result variable fails… Strange… It could be Spock framework error.

Source code is available here.

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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 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!