Safe.scala

class Hey @Inject() (val app: Application) {
    app.configuration // Guaranteed to work, since we can never get here unless we have enough contructor parameters
}

Unsafe1.scala

class Hey {
  @Inject private var app: Application = _

  println("Before foo")
  val foo = {
    println("Inside foo")
    app.configuration // Guaranteed to fail, since we're still in the class constructor, and the value starts as null (because Guice sets these values after initialization)
  }
  println("After foo")
}

/*
Output:

    Asking for "Hey"
    Before foo
    Inside foo
    [error] application - Unable to provision, see the following errors:

    1) Error injecting constructor, java.lang.NullPointerException
      at controllers.Hey.<init>(Main.scala:43)
      while locating controllers.Hey

    1 error (Error page was displayed to a user.) | Map(dynoName -> JsValueWrapperImpl(null))
    com.google.inject.ProvisionException: Unable to provision, see the following errors:

    1) Error injecting constructor, java.lang.NullPointerException
      at controllers.Hey.<init>(Main.scala:43)
      while locating controllers.Hey

    1 error
            at com.google.inject.internal.InjectorImpl$2.get(InjectorImpl.java:1025) ~[guice-4.0.jar:na]
            at com.google.inject.internal.InjectorImpl.getInstance(InjectorImpl.java:1051) ~[guice-4.0.jar:na]
            at play.api.inject.guice.GuiceInjector.instanceOf(GuiceInjectorBuilder.scala:321) ~[play_2.11-2.4.6.jar:2.4.6]
            at play.api.inject.guice.GuiceInjector.instanceOf(GuiceInjectorBuilder.scala:316) ~[play_2.11-2.4.6.jar:2.4.6]
            at controllers.Main$$anonfun$testInternalServerError$1.apply(Main.scala:206) ~[classes/:2.4.6]
            at controllers.Main$$anonfun$testInternalServerError$1.apply(Main.scala:204) ~[classes/:2.4.6]
            at play.api.mvc.ActionBuilder$$anonfun$apply$16.apply(Action.scala:408) ~[play_2.11-2.4.6.jar:2.4.6]
            at play.api.mvc.ActionBuilder$$anonfun$apply$16.apply(Action.scala:407) ~[play_2.11-2.4.6.jar:2.4.6]
            at controllers.Actions$BaseAction$.invokeBlock(BaseController.scala:207) ~[classes/:2.4.6]
            at controllers.Actions$BaseAction$.invokeBlock(BaseController.scala:203) ~[classes/:2.4.6]
    Caused by: java.lang.NullPointerException: null
            at controllers.Hey.<init>(Main.scala:38) ~[classes/:2.4.6]
            at controllers.Hey$$FastClassByGuice$$e92610e5.newInstance(<generated>) ~[na:na]
            at com.google.inject.internal.cglib.reflect.$FastConstructor.newInstance(FastConstructor.java:40) ~[guice-4.0.jar:na]
            at com.google.inject.internal.DefaultConstructionProxyFactory$1.newInstance(DefaultConstructionProxyFactory.java:61) ~[guice-4.0.jar:na]
            at com.google.inject.internal.ConstructorInjector.provision(ConstructorInjector.java:105) ~[guice-4.0.jar:na]
            at com.google.inject.internal.ConstructorInjector.construct(ConstructorInjector.java:85) ~[guice-4.0.jar:na]
            at com.google.inject.internal.ConstructorBindingImpl$Factory.get(ConstructorBindingImpl.java:267) ~[guice-4.0.jar:na]
            at com.google.inject.internal.InjectorImpl$2$1.call(InjectorImpl.java:1016) ~[guice-4.0.jar:na]
            at com.google.inject.internal.InjectorImpl.callInContext(InjectorImpl.java:1092) ~[guice-4.0.jar:na]
            at com.google.inject.internal.InjectorImpl$2.get(InjectorImpl.java:1012) ~[guice-4.0.jar:na]
*/

Unsafe2.scala

class Hey {
  @Inject private var app: Application = _

  println("Before foo")
  lazy val foo = {
    println("Inside foo")
    app.configuration
  }
  println("After foo")

  val bar = foo.toString // No safety means careless or race-y calculations could cause unexpected failures
}