使用Play2.4和scala的Google Guice的循环依赖性错误

Question

My application uses Play 2.4 with Scala 2.11 .I started transforming my existing code to make use of Google Guice that comes with Play 2.4 .

When I run my code after making the first set of changes , I found Some DAOs in my code are failing with circular dependency error.

For example I have two DAOs

class BookDAO @Inject()
(protected val personDAO : PersonDAO,
 @NamedDatabase("mysql")protected val dbConfigProvider: DatabaseConfigProvider) extends HasDatabaseConfigProvider[JdbcProfile] {
...
...
val personId = //some id
personDAO.get(personId)
}

class PersonDAO @Inject()
(protected val bookDAO : BookDAO,
 @NamedDatabase("mysql")protected val dbConfigProvider: DatabaseConfigProvider) extends HasDatabaseConfigProvider[JdbcProfile] {
...
...
val bookName= //some id
personDAO.getByName(bookName)
}

I got the below error while trying to access either BookDAO or PersonDAO

Tried proxying schema.BookDAO to support a circular dependency, but it is not an interface.
  at schema.BookDAO.class(Books.scala:52)
  while locating schema.BookDAO

Can someone help me resolving this error .

Thanks in advance

Answer 1

Quick solution

Inject a Provider instead:

class BookDAO @Inject()(personDaoProvider: Provider[PersonDAO], ...)
extends HasDatabaseConfigProvider[JdbcProfile] {
  val personDAO = personDaoProvider.get
  def person = personDAO.get(personId)
}

And the same for BookDAO . This will work out of the box. Guice already "knows" how to inject Providers.

---

Better approach

Decouple the class definition from the implementation. See Mon Calamari's answer.

Answer 2

Define your dependencies as follows and pull up all needed methods from class to trait:

@ImplementedBy(classOf[BookDao])
trait IBookDao {
  // abstract defs
}

class BookDao @Inject()(protected val personDAO: IPersonDao, protected val dbConfigProvider: DatabaseConfigProvider) extends HasDatabaseConfigProvider[JdbcProfile] with IBookDao {
}

@ImplementedBy(classOf[PersonDao])
trait IPersonDao {
  // abstract defs
}

class PersonDao @Inject()(protected val bookDAO: IBookDao, protected val dbConfigProvider: DatabaseConfigProvider) extends HasDatabaseConfigProvider[JdbcProfile] with IPersonDao {
}

As you can see, each dao implements a trait and all dao dependencies are injected by trait . This gives Guice possibility to inject a proxy class and resolve a circular dependency issue.

More details on playframework scala dependency injection here .