ProvisionException using PlayFramework 2.4.2

问题

I am migrating a project from Play 2.2.4 to 2.4.2 and I am getting an exception which I am not able to understand and solve.

Unexpected exception

ProvisionException: Unable to provision, see the following errors:

1) Error injecting constructor, java.lang.NullPointerException
  at controllers.Application.<init>(Application.java:33)
  while locating controllers.Application
    for parameter 1 at router.Routes.<init>(Routes.scala:36)
  while locating router.Routes
  while locating play.api.inject.RoutesProvider
  while locating play.api.routing.Router

1 error

It occurs since I added the dependency injection for the WS API like:

public class Application extends Controller {

    @Inject
    WSClient ws;

    WSRequest request = ws.url("https://...");

    ...
}

The build.sbt file includes the necessary configuration

libraryDependencies ++= Seq(
  javaJdbc,
  cache,
  javaWs
)

// Play provides two styles of routers, one expects its actions to be injected, the
// other, legacy style, accesses its actions statically.
routesGenerator := InjectedRoutesGenerator

What could be missing or be done wrong?

回答1:

This line causes Nullpointer: ws.url("https://..."); ws is null by the time when Guice instantiates Application class. More over, having a request as controller field is not thread safe. Change your code to following:

public class Application extends Controller {

    private WSClient ws;

    @Inject
    public Application(WSClient ws) {

        this.ws = ws;
        WSRequest request = this.ws.url("https://...");

        ...
    }
}

来源：https://stackoverflow.com/questions/31610945/provisionexception-using-playframework-2-4-2