Add an allowExtraKeys setting in ProductFormats

src/main/boilerplate/spray/json/ProductFormatsInstances.scala.template

@@ -16,14 +16,21 @@
 
 package spray.json
 
+import scala.collection.mutable.HashSet
+
 trait ProductFormatsInstances { self: ProductFormats with StandardFormats =>
+  def allowExtraKeys: Boolean = true
 [#  // Case classes with 1 parameters
 
   def jsonFormat1[[#P1 :JF#], T <: Product :ClassManifest](construct: ([#P1#]) => T): RootJsonFormat[T] = {
     val Array([#p1#]) = extractFieldNames(classManifest[T])
     jsonFormat(construct, [#p1#])
   }
   def jsonFormat[[#P1 :JF#], T <: Product](construct: ([#P1#]) => T, [#fieldName1: String#]): RootJsonFormat[T] = new RootJsonFormat[T]{
+    val knownFields = HashSet[String]()
+    [# knownFields.add(fieldName1)#
+    ]
+
     def write(p: T) = {
       val fields = new collection.mutable.ListBuffer[(String, JsValue)]
       fields.sizeHint(1 * 2)
@@ -34,6 +41,14 @@ trait ProductFormatsInstances { self: ProductFormats with StandardFormats =>
     def read(value: JsValue) = {
       [#val p1V = fromField[P1](value, fieldName1)#
       ]
+      if (!allowExtraKeys) {
+        val jsObject = value.asJsObject()
+        val keySet = jsObject.fields.keys.toSet
+        val keySetDiff = keySet.diff(knownFields)
+        if (!keySetDiff.isEmpty) {
+          throw new DeserializationException(s"${keySetDiff.head} is not a known key", null, keySetDiff.toList)
+        }
+      }
       construct([#p1V#])
     }
   }#

src/main/scala/spray/json/ProductFormats.scala

@@ -37,7 +37,7 @@ trait ProductFormats extends ProductFormatsInstances {
     }
 
   // helpers
-  
+
   protected def productElement2Field[T](fieldName: String, p: Product, ix: Int, rest: List[JsField] = Nil)
                                        (implicit writer: JsonWriter[T]): List[JsField] = {
     val value = p.productElement(ix).asInstanceOf[T]
@@ -152,3 +152,14 @@ trait NullOptions extends ProductFormats {
     (fieldName, writer.write(value)) :: rest
   }
 }
+
+/**
+ * This trait changes the behavior for reading JSON values.
+ * If you mix in this trait into your custom JsonProtocol, JSON deserialization
+ * will throw an error if the input contains keys that are not a field of the
+ * target case class.
+ */
+trait ExtraKeysOptions extends ProductFormats {
+  this: StandardFormats =>
+  override def allowExtraKeys = false
+}

src/test/scala/spray/json/ProductFormatsSpec.scala

@@ -44,6 +44,8 @@ class ProductFormatsSpec extends Specification {
   object TestProtocol1 extends DefaultJsonProtocol with TestProtocol
   object TestProtocol2 extends DefaultJsonProtocol with TestProtocol with NullOptions
 
+  object TestProtocol3 extends DefaultJsonProtocol with TestProtocol with ExtraKeysOptions
+
   "A JsonFormat created with `jsonFormat`, for a case class with 2 elements," should {
     import TestProtocol1._
     val obj = Test2(42, Some(4.2))
@@ -61,6 +63,9 @@ class ProductFormatsSpec extends Specification {
     "not require the presence of optional fields for deserialization" in {
       JsObject("a" -> JsNumber(42)).convertTo[Test2] mustEqual Test2(42, None)
     }
+    "allow extra fields during deserialization" in {
+      JsObject("a" -> JsNumber(42), "extra_key" -> JsNumber(43)).convertTo[Test2] mustEqual Test2(42, None)
+    }
     "not render `None` members during serialization" in {
       Test2(42, None).toJson mustEqual JsObject("a" -> JsNumber(42))
     }
@@ -92,6 +97,15 @@ class ProductFormatsSpec extends Specification {
     }
   }
 
+  "A JsonProtocol mixing in ExtraKeysOptions" should {
+    "not allow extra keys to be read" in {
+      import TestProtocol3._
+      JsObject("a" -> JsNumber(42), "extra_key" -> JsNumber(43)).convertTo[Test2] must throwA[DeserializationException].like {
+        case DeserializationException(_, _, fieldNames) => fieldNames mustEqual "extra_key" :: Nil
+      }
+    }
+  }
+
   "A JsonFormat for a generic case class and created with `jsonFormat`" should {
     import TestProtocol1._
     val obj = Test3(42 :: 43 :: Nil, "x" :: "y" :: "z" :: Nil)