Classloader problem with Java 7 and WebServices in Grails

Our Grails 2.1 application communicates with external SOAP WebServices. It worked fine as we follow Software Guy’s advices from this blog post. Recently, our client required new functionality – export to Excel. We’ve used Apache POI libraries for expor…

Our Grails 2.1 application communicates with external SOAP WebServices. It worked fine as we follow Software Guy’s advices from this blog post.

Recently, our client required new functionality – export to Excel. We’ve used Apache POI libraries for export. And our web service communication died. All it gave us was:

Caused by: java.lang.LinkageError: loader constraint violation: when resolving field "DATETIME" the class loader (instance of org/codehaus/groovy/grails/cli/support/GrailsRootLoader) of the referring class, javax/xml/datatype/DatatypeConstants, and the class loader (instance of <bootloader>) for the field's resolved type, ants, have different Class objects for that type</bootloader>

This message is strange, LinkageError can give you creeps. But read carefully: resolved type, ants? Something is definitely wrong here. After many searches it turned out that Java 7 already contains some conflicting classes from stax-api.jar. To solve this problem there are two thing you need to do:

  1. Ensure that your jaxws-rt dependency is runtime, not compile! // http://asoftwareguy.com/2012/02/25/web-service-clients-where-grails-lost-its-mojo/
    // Do not remove this dependency. Web services need this to work flawlessly.
    runtime (‘com.sun.xml.ws:jaxws-rt:2.1.4’)
  2. Create dependency report (grails dependency-report), search and exclude all stax-api dependencies other than jaxws-rt like this example:
    compile ('org.apache.poi:poi-ooxml-3.7') {
        excludes 'stax-api'
    }
    compile ('org.apache.poi:poi-ooxml-schemas:3.7') {
        excludes 'stax-api'
    }
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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!