Addition of an attribute to a Play request makes a new instance of request?

问题

If, like me, you're coming from the Java Servlet world, Play's support for request attributes (recently introduced in release 2.6) was only too long in coming. Now, that I've had a chance to look at it in some detail though, I wonder if it's usable.

Verbatim from the docs (Scala):

// Create a TypedKey to store a User object
object Attrs {
  val User: TypedKey[User] = TypedKey.apply[User]("user")
}
// Get the User object from the request
val user: User = req.attrs(Attrs.User)
// Put a User object into the request
val newReq = req.addAttr(Attrs.User, newUser)

I get and appreciate functional idea of immutable objects, but if this snippet is to be interpreted as that the act of adding an attribute to a request leaves me holding a new request, then what am I to do with it!? In the reactive world of callbacks, isn't the idea that Play passes the requests to my code, not the other way around? If I can't attach an attribute in an action and then inspect it in the error handler, what good is this? Hopefully, I am just missing something here...

回答1:

You attach attributes to a request through a filter. https://www.playframework.com/documentation/2.6.x/ScalaHttpFilters

The filter can then pass along the new request to the framework, which will then give it to your handler.

EDIT:

Passing the modified (new) request back to the framework:

...
nextFilter(requestHeader.addAttr(Attrs.User, newUser))
...

来源：https://stackoverflow.com/questions/49742712/addition-of-an-attribute-to-a-play-request-makes-a-new-instance-of-request