From e35e184262e369311635f47bbf8cb88036e669c5 Mon Sep 17 00:00:00 2001
From: Attila Szegedi <attila@twitter.com>
Date: Thu, 22 Sep 2022 18:05:00 +0000
Subject: [PATCH] scrooge: Allow null values for nullable fields in
 StructBuilder

Problem
-------
`c.t.scrooge.StructBuilder` methods `setField` and `setAllFields` don't accept
`null` as a valid value for nullable fields, making it impossible to use
`StructBuilder` to construct otherwise valid instances that have `null` as a value
of one or more such fields.

Solution
--------
Enhance `StructBuilder` so it allows setting `null` values for nullable fields.

JIRA Issues: CSL-12312

Differential Revision: https://phabricator.twitter.biz/D976737
---
 CHANGELOG.rst                                 |  2 +
 .../com/twitter/scrooge/StructBuilder.scala   | 41 +++++++++++++-
 .../gold/thriftscala/AnotherException.scala   | 10 ++--
 .../test/gold/thriftscala/CollectionId.scala  | 10 ++--
 .../test/gold/thriftscala/GoldService.scala   | 56 +++++++++++--------
 .../thriftscala/OverCapacityException.scala   | 10 ++--
 .../gold/thriftscala/PlatinumService.scala    | 24 ++++----
 .../test/gold/thriftscala/Recursive.scala     | 10 ++--
 .../test/gold/thriftscala/Request.scala       | 30 +++++-----
 .../gold/thriftscala/RequestException.scala   | 14 +++--
 .../test/gold/thriftscala/Response.scala      | 14 +++--
 .../scrooge/backend/ScalaGeneratorSpec.scala  | 31 ++++++++++
 .../main/resources/scalagen/struct.mustache   | 14 +++--
 .../twitter/scrooge/backend/Generator.scala   | 21 ++++---
 .../scrooge/backend/StructTemplate.scala      |  3 +-
 15 files changed, 197 insertions(+), 93 deletions(-)

diff --git a/CHANGELOG.rst b/CHANGELOG.rst
index c826b7f2..362f7e95 100644
--- a/CHANGELOG.rst
+++ b/CHANGELOG.rst
@@ -39,6 +39,8 @@ Runtime Behavior Changes
 * scrooge: Use the util-mock for testing which depends on the 3.12.4 "org.mockito", remove the
   "org.jmock" dependencies. ``PHAB_ID=D973946``
 
+* scrooge: `c.t.scrooge.StructBuilder` now accepts `null` values for non-primitive default-required fields. ``PHAB_ID=D976737``
+
 22.7.0
 ------
 
diff --git a/scrooge-core/src/main/scala/com/twitter/scrooge/StructBuilder.scala b/scrooge-core/src/main/scala/com/twitter/scrooge/StructBuilder.scala
index 7e900cdf..41a6c923 100644
--- a/scrooge-core/src/main/scala/com/twitter/scrooge/StructBuilder.scala
+++ b/scrooge-core/src/main/scala/com/twitter/scrooge/StructBuilder.scala
@@ -1,6 +1,7 @@
 package com.twitter.scrooge
 
 import com.twitter.util.Memoize
+import org.apache.thrift.protocol.TType
 import scala.reflect.ClassTag
 
 /**
@@ -8,9 +9,12 @@ import scala.reflect.ClassTag
  * ThriftStruct or statically from T.
  *
  * We pass in a list of [[ClassTag]]s which describe each of the struct's field types
- * so that we can validate the values we are setting at runtime.
+ * so that we can validate the values we are setting at runtime. We also pass in a set
+ * of indices of non-primitive default-required fields; those can have null as valid value.
  */
-abstract class StructBuilder[T <: ThriftStruct](fieldTypes: IndexedSeq[ClassTag[_]]) {
+abstract class StructBuilder[T <: ThriftStruct](
+  fieldTypes: IndexedSeq[ClassTag[_]],
+  nullableIndices: Set[Int]) {
   protected val fieldArray: Array[Any] = new Array[Any](fieldTypes.size)
 
   /**
@@ -23,6 +27,7 @@ abstract class StructBuilder[T <: ThriftStruct](fieldTypes: IndexedSeq[ClassTag[
   private[this] def addOrUpdateFieldArray[A](index: Int, v: Any)(implicit tag: ClassTag[A]): Unit =
     v match {
       case inputValue: A => fieldArray(index) = inputValue
+      case null if nullableIndices(index) => fieldArray(index) = StructBuilder.ExplicitNull
       case _ => throw new IllegalArgumentException(s"value at index $index must be of type $tag")
     }
 
@@ -84,12 +89,44 @@ abstract class StructBuilder[T <: ThriftStruct](fieldTypes: IndexedSeq[ClassTag[
  * This object provides operations to obtain `StructBuilder` instances.
  */
 object StructBuilder {
+  // Used as an explicitly set null value for default-required fields
+  private[StructBuilder] object ExplicitNull
+
+  def unwrapExplicitNull(v: Any): Any =
+    v match {
+      case ExplicitNull => null
+      case _ => v
+    }
+
   private[this] val memoizeBuilderMethod: Class[_] => () => Any = Memoize.classValue { clazz =>
     val thriftCodec = ThriftStructCodec.forStructClass(clazz.asSubclass(classOf[ThriftStruct]))
     val m = thriftCodec.getClass.getMethod("newBuilder")
     () => m.invoke(thriftCodec)
   }
 
+  /**
+   * Given a sequence of struct field information, returns a set of indices of non-primitive
+   * default-required fields.
+   * @param fieldInfos a sequence of field informations
+   * @return a set of indices for non-primitive default-required fields
+   */
+  final def nullableIndices(fieldInfos: Seq[ThriftStructFieldInfo]): Set[Int] = {
+    fieldInfos.zipWithIndex.collect {
+      case (f, i) if isNullable(f) => i
+    }.toSet
+  }
+
+  private[this] def isNullable(f: ThriftStructFieldInfo): Boolean = {
+    if (f.isOptional || f.isRequired) {
+      false
+    } else {
+      f.tfield.`type` match {
+        case TType.BOOL | TType.BYTE | TType.I16 | TType.I32 | TType.I64 | TType.DOUBLE => false
+        case _ => true
+      }
+    }
+  }
+
   /**
    * For a given scrooge-generated thrift struct or union class, returns its StructBuilder.
    * This can be used for building a new ThriftStruct object.
diff --git a/scrooge-generator-tests/src/test/resources/gold_file_output_scala/com/twitter/scrooge/test/gold/thriftscala/AnotherException.scala b/scrooge-generator-tests/src/test/resources/gold_file_output_scala/com/twitter/scrooge/test/gold/thriftscala/AnotherException.scala
index f82fdb3c..5fe9c532 100644
--- a/scrooge-generator-tests/src/test/resources/gold_file_output_scala/com/twitter/scrooge/test/gold/thriftscala/AnotherException.scala
+++ b/scrooge-generator-tests/src/test/resources/gold_file_output_scala/com/twitter/scrooge/test/gold/thriftscala/AnotherException.scala
@@ -51,6 +51,8 @@ object AnotherException extends ValidatingThriftStructCodec3[AnotherException] w
   )
 
 
+  lazy val nullableIndices: Set[Int] = StructBuilder.nullableIndices(fieldInfos)
+
   val structAnnotations: immutable$Map[String, String] =
     immutable$Map.empty[String, String]
 
@@ -111,7 +113,7 @@ object AnotherException extends ValidatingThriftStructCodec3[AnotherException] w
     )
   }
 
-  def newBuilder(): StructBuilder[AnotherException] = new AnotherExceptionStructBuilder(_root_.scala.None, fieldTypes)
+  def newBuilder(): StructBuilder[AnotherException] = new AnotherExceptionStructBuilder(_root_.scala.None, fieldTypes, nullableIndices)
 
   override def encode(_item: AnotherException, _oproto: TProtocol): Unit = {
     _item.write(_oproto)
@@ -348,11 +350,11 @@ class AnotherException(
       flags
     )
 
-  def newBuilder(): StructBuilder[AnotherException] = new AnotherExceptionStructBuilder(_root_.scala.Some(this), fieldTypes)
+  def newBuilder(): StructBuilder[AnotherException] = new AnotherExceptionStructBuilder(_root_.scala.Some(this), fieldTypes, nullableIndices)
 }
 
-private[thriftscala] class AnotherExceptionStructBuilder(instance: _root_.scala.Option[AnotherException], fieldTypes: IndexedSeq[ClassTag[_]])
-    extends StructBuilder[AnotherException](fieldTypes) {
+private[thriftscala] class AnotherExceptionStructBuilder(instance: _root_.scala.Option[AnotherException], fieldTypes: IndexedSeq[ClassTag[_]], nullableIndices: Set[Int])
+    extends StructBuilder[AnotherException](fieldTypes, nullableIndices) {
 
   def build(): AnotherException = {
     val _fieldArray = fieldArray // shadow variable
diff --git a/scrooge-generator-tests/src/test/resources/gold_file_output_scala/com/twitter/scrooge/test/gold/thriftscala/CollectionId.scala b/scrooge-generator-tests/src/test/resources/gold_file_output_scala/com/twitter/scrooge/test/gold/thriftscala/CollectionId.scala
index e1cb90b0..f5914715 100644
--- a/scrooge-generator-tests/src/test/resources/gold_file_output_scala/com/twitter/scrooge/test/gold/thriftscala/CollectionId.scala
+++ b/scrooge-generator-tests/src/test/resources/gold_file_output_scala/com/twitter/scrooge/test/gold/thriftscala/CollectionId.scala
@@ -54,6 +54,8 @@ object CollectionId extends ValidatingThriftStructCodec3[CollectionId] with Stru
   )
 
 
+  lazy val nullableIndices: Set[Int] = StructBuilder.nullableIndices(fieldInfos)
+
   val structAnnotations: immutable$Map[String, String] =
     immutable$Map.empty[String, String]
 
@@ -114,7 +116,7 @@ object CollectionId extends ValidatingThriftStructCodec3[CollectionId] with Stru
     )
   }
 
-  def newBuilder(): StructBuilder[CollectionId] = new CollectionIdStructBuilder(_root_.scala.None, fieldTypes)
+  def newBuilder(): StructBuilder[CollectionId] = new CollectionIdStructBuilder(_root_.scala.None, fieldTypes, nullableIndices)
 
   override def encode(_item: CollectionId, _oproto: TProtocol): Unit = {
     _item.write(_oproto)
@@ -444,11 +446,11 @@ trait CollectionId
 
   def _codec: ValidatingThriftStructCodec3[CollectionId] = CollectionId
 
-  def newBuilder(): StructBuilder[CollectionId] = new CollectionIdStructBuilder(_root_.scala.Some(this), fieldTypes)
+  def newBuilder(): StructBuilder[CollectionId] = new CollectionIdStructBuilder(_root_.scala.Some(this), fieldTypes, nullableIndices)
 }
 
-private[thriftscala] class CollectionIdStructBuilder(instance: _root_.scala.Option[CollectionId], fieldTypes: IndexedSeq[ClassTag[_]])
-    extends StructBuilder[CollectionId](fieldTypes) {
+private[thriftscala] class CollectionIdStructBuilder(instance: _root_.scala.Option[CollectionId], fieldTypes: IndexedSeq[ClassTag[_]], nullableIndices: Set[Int])
+    extends StructBuilder[CollectionId](fieldTypes, nullableIndices) {
 
   def build(): CollectionId = {
     val _fieldArray = fieldArray // shadow variable
diff --git a/scrooge-generator-tests/src/test/resources/gold_file_output_scala/com/twitter/scrooge/test/gold/thriftscala/GoldService.scala b/scrooge-generator-tests/src/test/resources/gold_file_output_scala/com/twitter/scrooge/test/gold/thriftscala/GoldService.scala
index 46b881a4..4f7e0c2a 100644
--- a/scrooge-generator-tests/src/test/resources/gold_file_output_scala/com/twitter/scrooge/test/gold/thriftscala/GoldService.scala
+++ b/scrooge-generator-tests/src/test/resources/gold_file_output_scala/com/twitter/scrooge/test/gold/thriftscala/GoldService.scala
@@ -329,6 +329,8 @@ object GoldService extends _root_.com.twitter.finagle.thrift.GeneratedThriftServ
       )
     
     
+      lazy val nullableIndices: Set[Int] = StructBuilder.nullableIndices(fieldInfos)
+    
       val structAnnotations: immutable$Map[String, String] =
         immutable$Map.empty[String, String]
     
@@ -425,7 +427,7 @@ object GoldService extends _root_.com.twitter.finagle.thrift.GeneratedThriftServ
         )
       }
     
-      def newBuilder(): StructBuilder[Args] = new ArgsStructBuilder(_root_.scala.None, fieldTypes)
+      def newBuilder(): StructBuilder[Args] = new ArgsStructBuilder(_root_.scala.None, fieldTypes, nullableIndices)
     
       override def encode(_item: Args, _oproto: TProtocol): Unit = {
         _item.write(_oproto)
@@ -589,27 +591,27 @@ object GoldService extends _root_.com.twitter.finagle.thrift.GeneratedThriftServ
     
       def _codec: ValidatingThriftStructCodec3[Args] = Args
     
-      def newBuilder(): StructBuilder[Args] = new ArgsStructBuilder(_root_.scala.Some(this), fieldTypes)
+      def newBuilder(): StructBuilder[Args] = new ArgsStructBuilder(_root_.scala.Some(this), fieldTypes, nullableIndices)
     }
     
-    private[thriftscala] class ArgsStructBuilder(instance: _root_.scala.Option[Args], fieldTypes: IndexedSeq[ClassTag[_]])
-        extends StructBuilder[Args](fieldTypes) {
+    private[thriftscala] class ArgsStructBuilder(instance: _root_.scala.Option[Args], fieldTypes: IndexedSeq[ClassTag[_]], nullableIndices: Set[Int])
+        extends StructBuilder[Args](fieldTypes, nullableIndices) {
     
       def build(): Args = {
         val _fieldArray = fieldArray // shadow variable
         if (instance.isDefined) {
           val instanceValue = instance.get
           Args(
-            if (_fieldArray(0) == null) instanceValue.request else _fieldArray(0).asInstanceOf[com.twitter.scrooge.test.gold.thriftscala.Request],
-            if (_fieldArray(1) == null) instanceValue.unionRequest else _fieldArray(1).asInstanceOf[com.twitter.scrooge.test.gold.thriftscala.RequestUnion],
-            if (_fieldArray(2) == null) instanceValue.exceptionRequest else _fieldArray(2).asInstanceOf[com.twitter.scrooge.test.gold.thriftscala.RequestException]
+            if (_fieldArray(0) == null) instanceValue.request else StructBuilder.unwrapExplicitNull(_fieldArray(0)).asInstanceOf[com.twitter.scrooge.test.gold.thriftscala.Request],
+            if (_fieldArray(1) == null) instanceValue.unionRequest else StructBuilder.unwrapExplicitNull(_fieldArray(1)).asInstanceOf[com.twitter.scrooge.test.gold.thriftscala.RequestUnion],
+            if (_fieldArray(2) == null) instanceValue.exceptionRequest else StructBuilder.unwrapExplicitNull(_fieldArray(2)).asInstanceOf[com.twitter.scrooge.test.gold.thriftscala.RequestException]
           )
         } else {
           if (genericArrayOps(_fieldArray).contains(null)) throw new InvalidFieldsException(structBuildError("Args"))
           Args(
-            _fieldArray(0).asInstanceOf[com.twitter.scrooge.test.gold.thriftscala.Request],
-            _fieldArray(1).asInstanceOf[com.twitter.scrooge.test.gold.thriftscala.RequestUnion],
-            _fieldArray(2).asInstanceOf[com.twitter.scrooge.test.gold.thriftscala.RequestException]
+            StructBuilder.unwrapExplicitNull(_fieldArray(0)).asInstanceOf[com.twitter.scrooge.test.gold.thriftscala.Request],
+            StructBuilder.unwrapExplicitNull(_fieldArray(1)).asInstanceOf[com.twitter.scrooge.test.gold.thriftscala.RequestUnion],
+            StructBuilder.unwrapExplicitNull(_fieldArray(2)).asInstanceOf[com.twitter.scrooge.test.gold.thriftscala.RequestException]
           )
         }
       }
@@ -655,6 +657,8 @@ object GoldService extends _root_.com.twitter.finagle.thrift.GeneratedThriftServ
       )
     
     
+      lazy val nullableIndices: Set[Int] = StructBuilder.nullableIndices(fieldInfos)
+    
       val structAnnotations: immutable$Map[String, String] =
         immutable$Map.empty[String, String]
     
@@ -739,7 +743,7 @@ object GoldService extends _root_.com.twitter.finagle.thrift.GeneratedThriftServ
         )
       }
     
-      def newBuilder(): StructBuilder[Result] = new ResultStructBuilder(_root_.scala.None, fieldTypes)
+      def newBuilder(): StructBuilder[Result] = new ResultStructBuilder(_root_.scala.None, fieldTypes, nullableIndices)
     
       override def encode(_item: Result, _oproto: TProtocol): Unit = {
         _item.write(_oproto)
@@ -886,11 +890,11 @@ object GoldService extends _root_.com.twitter.finagle.thrift.GeneratedThriftServ
     
       def _codec: ValidatingThriftStructCodec3[Result] = Result
     
-      def newBuilder(): StructBuilder[Result] = new ResultStructBuilder(_root_.scala.Some(this), fieldTypes)
+      def newBuilder(): StructBuilder[Result] = new ResultStructBuilder(_root_.scala.Some(this), fieldTypes, nullableIndices)
     }
     
-    private[thriftscala] class ResultStructBuilder(instance: _root_.scala.Option[Result], fieldTypes: IndexedSeq[ClassTag[_]])
-        extends StructBuilder[Result](fieldTypes) {
+    private[thriftscala] class ResultStructBuilder(instance: _root_.scala.Option[Result], fieldTypes: IndexedSeq[ClassTag[_]], nullableIndices: Set[Int])
+        extends StructBuilder[Result](fieldTypes, nullableIndices) {
     
       def build(): Result = {
         val _fieldArray = fieldArray // shadow variable
@@ -971,6 +975,8 @@ object GoldService extends _root_.com.twitter.finagle.thrift.GeneratedThriftServ
       )
     
     
+      lazy val nullableIndices: Set[Int] = StructBuilder.nullableIndices(fieldInfos)
+    
       val structAnnotations: immutable$Map[String, String] =
         immutable$Map.empty[String, String]
     
@@ -1035,7 +1041,7 @@ object GoldService extends _root_.com.twitter.finagle.thrift.GeneratedThriftServ
         )
       }
     
-      def newBuilder(): StructBuilder[Args] = new ArgsStructBuilder(_root_.scala.None, fieldTypes)
+      def newBuilder(): StructBuilder[Args] = new ArgsStructBuilder(_root_.scala.None, fieldTypes, nullableIndices)
     
       override def encode(_item: Args, _oproto: TProtocol): Unit = {
         _item.write(_oproto)
@@ -1156,23 +1162,23 @@ object GoldService extends _root_.com.twitter.finagle.thrift.GeneratedThriftServ
     
       def _codec: ValidatingThriftStructCodec3[Args] = Args
     
-      def newBuilder(): StructBuilder[Args] = new ArgsStructBuilder(_root_.scala.Some(this), fieldTypes)
+      def newBuilder(): StructBuilder[Args] = new ArgsStructBuilder(_root_.scala.Some(this), fieldTypes, nullableIndices)
     }
     
-    private[thriftscala] class ArgsStructBuilder(instance: _root_.scala.Option[Args], fieldTypes: IndexedSeq[ClassTag[_]])
-        extends StructBuilder[Args](fieldTypes) {
+    private[thriftscala] class ArgsStructBuilder(instance: _root_.scala.Option[Args], fieldTypes: IndexedSeq[ClassTag[_]], nullableIndices: Set[Int])
+        extends StructBuilder[Args](fieldTypes, nullableIndices) {
     
       def build(): Args = {
         val _fieldArray = fieldArray // shadow variable
         if (instance.isDefined) {
           val instanceValue = instance.get
           Args(
-            if (_fieldArray(0) == null) instanceValue.request else _fieldArray(0).asInstanceOf[com.twitter.scrooge.test.gold.thriftscala.Request]
+            if (_fieldArray(0) == null) instanceValue.request else StructBuilder.unwrapExplicitNull(_fieldArray(0)).asInstanceOf[com.twitter.scrooge.test.gold.thriftscala.Request]
           )
         } else {
           if (genericArrayOps(_fieldArray).contains(null)) throw new InvalidFieldsException(structBuildError("Args"))
           Args(
-            _fieldArray(0).asInstanceOf[com.twitter.scrooge.test.gold.thriftscala.Request]
+            StructBuilder.unwrapExplicitNull(_fieldArray(0)).asInstanceOf[com.twitter.scrooge.test.gold.thriftscala.Request]
           )
         }
       }
@@ -1204,6 +1210,8 @@ object GoldService extends _root_.com.twitter.finagle.thrift.GeneratedThriftServ
       )
     
     
+      lazy val nullableIndices: Set[Int] = StructBuilder.nullableIndices(fieldInfos)
+    
       val structAnnotations: immutable$Map[String, String] =
         immutable$Map.empty[String, String]
     
@@ -1270,7 +1278,7 @@ object GoldService extends _root_.com.twitter.finagle.thrift.GeneratedThriftServ
         )
       }
     
-      def newBuilder(): StructBuilder[Result] = new ResultStructBuilder(_root_.scala.None, fieldTypes)
+      def newBuilder(): StructBuilder[Result] = new ResultStructBuilder(_root_.scala.None, fieldTypes, nullableIndices)
     
       override def encode(_item: Result, _oproto: TProtocol): Unit = {
         _item.write(_oproto)
@@ -1393,11 +1401,11 @@ object GoldService extends _root_.com.twitter.finagle.thrift.GeneratedThriftServ
     
       def _codec: ValidatingThriftStructCodec3[Result] = Result
     
-      def newBuilder(): StructBuilder[Result] = new ResultStructBuilder(_root_.scala.Some(this), fieldTypes)
+      def newBuilder(): StructBuilder[Result] = new ResultStructBuilder(_root_.scala.Some(this), fieldTypes, nullableIndices)
     }
     
-    private[thriftscala] class ResultStructBuilder(instance: _root_.scala.Option[Result], fieldTypes: IndexedSeq[ClassTag[_]])
-        extends StructBuilder[Result](fieldTypes) {
+    private[thriftscala] class ResultStructBuilder(instance: _root_.scala.Option[Result], fieldTypes: IndexedSeq[ClassTag[_]], nullableIndices: Set[Int])
+        extends StructBuilder[Result](fieldTypes, nullableIndices) {
     
       def build(): Result = {
         val _fieldArray = fieldArray // shadow variable
diff --git a/scrooge-generator-tests/src/test/resources/gold_file_output_scala/com/twitter/scrooge/test/gold/thriftscala/OverCapacityException.scala b/scrooge-generator-tests/src/test/resources/gold_file_output_scala/com/twitter/scrooge/test/gold/thriftscala/OverCapacityException.scala
index 91b8bfbc..b50b2956 100644
--- a/scrooge-generator-tests/src/test/resources/gold_file_output_scala/com/twitter/scrooge/test/gold/thriftscala/OverCapacityException.scala
+++ b/scrooge-generator-tests/src/test/resources/gold_file_output_scala/com/twitter/scrooge/test/gold/thriftscala/OverCapacityException.scala
@@ -53,6 +53,8 @@ object OverCapacityException extends ValidatingThriftStructCodec3[OverCapacityEx
   )
 
 
+  lazy val nullableIndices: Set[Int] = StructBuilder.nullableIndices(fieldInfos)
+
   lazy val structAnnotations: immutable$Map[String, String] =
     immutable$Map[String, String](
         ("e.annotation", "true")
@@ -115,7 +117,7 @@ object OverCapacityException extends ValidatingThriftStructCodec3[OverCapacityEx
     )
   }
 
-  def newBuilder(): StructBuilder[OverCapacityException] = new OverCapacityExceptionStructBuilder(_root_.scala.None, fieldTypes)
+  def newBuilder(): StructBuilder[OverCapacityException] = new OverCapacityExceptionStructBuilder(_root_.scala.None, fieldTypes, nullableIndices)
 
   override def encode(_item: OverCapacityException, _oproto: TProtocol): Unit = {
     _item.write(_oproto)
@@ -352,11 +354,11 @@ class OverCapacityException(
       flags
     )
 
-  def newBuilder(): StructBuilder[OverCapacityException] = new OverCapacityExceptionStructBuilder(_root_.scala.Some(this), fieldTypes)
+  def newBuilder(): StructBuilder[OverCapacityException] = new OverCapacityExceptionStructBuilder(_root_.scala.Some(this), fieldTypes, nullableIndices)
 }
 
-private[thriftscala] class OverCapacityExceptionStructBuilder(instance: _root_.scala.Option[OverCapacityException], fieldTypes: IndexedSeq[ClassTag[_]])
-    extends StructBuilder[OverCapacityException](fieldTypes) {
+private[thriftscala] class OverCapacityExceptionStructBuilder(instance: _root_.scala.Option[OverCapacityException], fieldTypes: IndexedSeq[ClassTag[_]], nullableIndices: Set[Int])
+    extends StructBuilder[OverCapacityException](fieldTypes, nullableIndices) {
 
   def build(): OverCapacityException = {
     val _fieldArray = fieldArray // shadow variable
diff --git a/scrooge-generator-tests/src/test/resources/gold_file_output_scala/com/twitter/scrooge/test/gold/thriftscala/PlatinumService.scala b/scrooge-generator-tests/src/test/resources/gold_file_output_scala/com/twitter/scrooge/test/gold/thriftscala/PlatinumService.scala
index d21acba4..ba01dbb2 100644
--- a/scrooge-generator-tests/src/test/resources/gold_file_output_scala/com/twitter/scrooge/test/gold/thriftscala/PlatinumService.scala
+++ b/scrooge-generator-tests/src/test/resources/gold_file_output_scala/com/twitter/scrooge/test/gold/thriftscala/PlatinumService.scala
@@ -336,6 +336,8 @@ object PlatinumService extends _root_.com.twitter.finagle.thrift.GeneratedThrift
       )
     
     
+      lazy val nullableIndices: Set[Int] = StructBuilder.nullableIndices(fieldInfos)
+    
       val structAnnotations: immutable$Map[String, String] =
         immutable$Map.empty[String, String]
     
@@ -400,7 +402,7 @@ object PlatinumService extends _root_.com.twitter.finagle.thrift.GeneratedThrift
         )
       }
     
-      def newBuilder(): StructBuilder[Args] = new ArgsStructBuilder(_root_.scala.None, fieldTypes)
+      def newBuilder(): StructBuilder[Args] = new ArgsStructBuilder(_root_.scala.None, fieldTypes, nullableIndices)
     
       override def encode(_item: Args, _oproto: TProtocol): Unit = {
         _item.write(_oproto)
@@ -521,23 +523,23 @@ object PlatinumService extends _root_.com.twitter.finagle.thrift.GeneratedThrift
     
       def _codec: ValidatingThriftStructCodec3[Args] = Args
     
-      def newBuilder(): StructBuilder[Args] = new ArgsStructBuilder(_root_.scala.Some(this), fieldTypes)
+      def newBuilder(): StructBuilder[Args] = new ArgsStructBuilder(_root_.scala.Some(this), fieldTypes, nullableIndices)
     }
     
-    private[thriftscala] class ArgsStructBuilder(instance: _root_.scala.Option[Args], fieldTypes: IndexedSeq[ClassTag[_]])
-        extends StructBuilder[Args](fieldTypes) {
+    private[thriftscala] class ArgsStructBuilder(instance: _root_.scala.Option[Args], fieldTypes: IndexedSeq[ClassTag[_]], nullableIndices: Set[Int])
+        extends StructBuilder[Args](fieldTypes, nullableIndices) {
     
       def build(): Args = {
         val _fieldArray = fieldArray // shadow variable
         if (instance.isDefined) {
           val instanceValue = instance.get
           Args(
-            if (_fieldArray(0) == null) instanceValue.request else _fieldArray(0).asInstanceOf[com.twitter.scrooge.test.gold.thriftscala.Request]
+            if (_fieldArray(0) == null) instanceValue.request else StructBuilder.unwrapExplicitNull(_fieldArray(0)).asInstanceOf[com.twitter.scrooge.test.gold.thriftscala.Request]
           )
         } else {
           if (genericArrayOps(_fieldArray).contains(null)) throw new InvalidFieldsException(structBuildError("Args"))
           Args(
-            _fieldArray(0).asInstanceOf[com.twitter.scrooge.test.gold.thriftscala.Request]
+            StructBuilder.unwrapExplicitNull(_fieldArray(0)).asInstanceOf[com.twitter.scrooge.test.gold.thriftscala.Request]
           )
         }
       }
@@ -597,6 +599,8 @@ object PlatinumService extends _root_.com.twitter.finagle.thrift.GeneratedThrift
       )
     
     
+      lazy val nullableIndices: Set[Int] = StructBuilder.nullableIndices(fieldInfos)
+    
       val structAnnotations: immutable$Map[String, String] =
         immutable$Map.empty[String, String]
     
@@ -693,7 +697,7 @@ object PlatinumService extends _root_.com.twitter.finagle.thrift.GeneratedThrift
         )
       }
     
-      def newBuilder(): StructBuilder[Result] = new ResultStructBuilder(_root_.scala.None, fieldTypes)
+      def newBuilder(): StructBuilder[Result] = new ResultStructBuilder(_root_.scala.None, fieldTypes, nullableIndices)
     
       override def encode(_item: Result, _oproto: TProtocol): Unit = {
         _item.write(_oproto)
@@ -859,11 +863,11 @@ object PlatinumService extends _root_.com.twitter.finagle.thrift.GeneratedThrift
     
       def _codec: ValidatingThriftStructCodec3[Result] = Result
     
-      def newBuilder(): StructBuilder[Result] = new ResultStructBuilder(_root_.scala.Some(this), fieldTypes)
+      def newBuilder(): StructBuilder[Result] = new ResultStructBuilder(_root_.scala.Some(this), fieldTypes, nullableIndices)
     }
     
-    private[thriftscala] class ResultStructBuilder(instance: _root_.scala.Option[Result], fieldTypes: IndexedSeq[ClassTag[_]])
-        extends StructBuilder[Result](fieldTypes) {
+    private[thriftscala] class ResultStructBuilder(instance: _root_.scala.Option[Result], fieldTypes: IndexedSeq[ClassTag[_]], nullableIndices: Set[Int])
+        extends StructBuilder[Result](fieldTypes, nullableIndices) {
     
       def build(): Result = {
         val _fieldArray = fieldArray // shadow variable
diff --git a/scrooge-generator-tests/src/test/resources/gold_file_output_scala/com/twitter/scrooge/test/gold/thriftscala/Recursive.scala b/scrooge-generator-tests/src/test/resources/gold_file_output_scala/com/twitter/scrooge/test/gold/thriftscala/Recursive.scala
index 913f9eec..8dbb2dad 100644
--- a/scrooge-generator-tests/src/test/resources/gold_file_output_scala/com/twitter/scrooge/test/gold/thriftscala/Recursive.scala
+++ b/scrooge-generator-tests/src/test/resources/gold_file_output_scala/com/twitter/scrooge/test/gold/thriftscala/Recursive.scala
@@ -68,6 +68,8 @@ object Recursive extends ValidatingThriftStructCodec3[Recursive] with StructBuil
   )
 
 
+  lazy val nullableIndices: Set[Int] = StructBuilder.nullableIndices(fieldInfos)
+
   val structAnnotations: immutable$Map[String, String] =
     immutable$Map.empty[String, String]
 
@@ -146,7 +148,7 @@ object Recursive extends ValidatingThriftStructCodec3[Recursive] with StructBuil
     )
   }
 
-  def newBuilder(): StructBuilder[Recursive] = new RecursiveStructBuilder(_root_.scala.None, fieldTypes)
+  def newBuilder(): StructBuilder[Recursive] = new RecursiveStructBuilder(_root_.scala.None, fieldTypes, nullableIndices)
 
   override def encode(_item: Recursive, _oproto: TProtocol): Unit = {
     _item.write(_oproto)
@@ -522,11 +524,11 @@ trait Recursive
 
   def _codec: ValidatingThriftStructCodec3[Recursive] = Recursive
 
-  def newBuilder(): StructBuilder[Recursive] = new RecursiveStructBuilder(_root_.scala.Some(this), fieldTypes)
+  def newBuilder(): StructBuilder[Recursive] = new RecursiveStructBuilder(_root_.scala.Some(this), fieldTypes, nullableIndices)
 }
 
-private[thriftscala] class RecursiveStructBuilder(instance: _root_.scala.Option[Recursive], fieldTypes: IndexedSeq[ClassTag[_]])
-    extends StructBuilder[Recursive](fieldTypes) {
+private[thriftscala] class RecursiveStructBuilder(instance: _root_.scala.Option[Recursive], fieldTypes: IndexedSeq[ClassTag[_]], nullableIndices: Set[Int])
+    extends StructBuilder[Recursive](fieldTypes, nullableIndices) {
 
   def build(): Recursive = {
     val _fieldArray = fieldArray // shadow variable
diff --git a/scrooge-generator-tests/src/test/resources/gold_file_output_scala/com/twitter/scrooge/test/gold/thriftscala/Request.scala b/scrooge-generator-tests/src/test/resources/gold_file_output_scala/com/twitter/scrooge/test/gold/thriftscala/Request.scala
index 837b5410..80867039 100644
--- a/scrooge-generator-tests/src/test/resources/gold_file_output_scala/com/twitter/scrooge/test/gold/thriftscala/Request.scala
+++ b/scrooge-generator-tests/src/test/resources/gold_file_output_scala/com/twitter/scrooge/test/gold/thriftscala/Request.scala
@@ -275,6 +275,8 @@ object Request extends ValidatingThriftStructCodec3[Request] with StructBuilderF
   )
 
 
+  lazy val nullableIndices: Set[Int] = StructBuilder.nullableIndices(fieldInfos)
+
   lazy val structAnnotations: immutable$Map[String, String] =
     immutable$Map[String, String](
         ("s.annotation.one", "something"),
@@ -544,7 +546,7 @@ object Request extends ValidatingThriftStructCodec3[Request] with StructBuilderF
     )
   }
 
-  def newBuilder(): StructBuilder[Request] = new RequestStructBuilder(_root_.scala.None, fieldTypes)
+  def newBuilder(): StructBuilder[Request] = new RequestStructBuilder(_root_.scala.None, fieldTypes, nullableIndices)
 
   override def encode(_item: Request, _oproto: TProtocol): Unit = {
     _item.write(_oproto)
@@ -1799,23 +1801,23 @@ trait Request
 
   def _codec: ValidatingThriftStructCodec3[Request] = Request
 
-  def newBuilder(): StructBuilder[Request] = new RequestStructBuilder(_root_.scala.Some(this), fieldTypes)
+  def newBuilder(): StructBuilder[Request] = new RequestStructBuilder(_root_.scala.Some(this), fieldTypes, nullableIndices)
 }
 
-private[thriftscala] class RequestStructBuilder(instance: _root_.scala.Option[Request], fieldTypes: IndexedSeq[ClassTag[_]])
-    extends StructBuilder[Request](fieldTypes) {
+private[thriftscala] class RequestStructBuilder(instance: _root_.scala.Option[Request], fieldTypes: IndexedSeq[ClassTag[_]], nullableIndices: Set[Int])
+    extends StructBuilder[Request](fieldTypes, nullableIndices) {
 
   def build(): Request = {
     val _fieldArray = fieldArray // shadow variable
     if (instance.isDefined) {
       val instanceValue = instance.get
       Request(
-        if (_fieldArray(0) == null) instanceValue.aList else _fieldArray(0).asInstanceOf[_root_.scala.collection.Seq[String]],
-        if (_fieldArray(1) == null) instanceValue.aSet else _fieldArray(1).asInstanceOf[_root_.scala.collection.Set[Int]],
-        if (_fieldArray(2) == null) instanceValue.aMap else _fieldArray(2).asInstanceOf[_root_.scala.collection.Map[Long, Long]],
+        if (_fieldArray(0) == null) instanceValue.aList else StructBuilder.unwrapExplicitNull(_fieldArray(0)).asInstanceOf[_root_.scala.collection.Seq[String]],
+        if (_fieldArray(1) == null) instanceValue.aSet else StructBuilder.unwrapExplicitNull(_fieldArray(1)).asInstanceOf[_root_.scala.collection.Set[Int]],
+        if (_fieldArray(2) == null) instanceValue.aMap else StructBuilder.unwrapExplicitNull(_fieldArray(2)).asInstanceOf[_root_.scala.collection.Map[Long, Long]],
         if (_fieldArray(3) == null) instanceValue.aRequest else _fieldArray(3).asInstanceOf[_root_.scala.Option[com.twitter.scrooge.test.gold.thriftscala.Request]],
-        if (_fieldArray(4) == null) instanceValue.subRequests else _fieldArray(4).asInstanceOf[_root_.scala.collection.Seq[com.twitter.scrooge.test.gold.thriftscala.Request]],
-        if (_fieldArray(5) == null) instanceValue._default else _fieldArray(5).asInstanceOf[String],
+        if (_fieldArray(4) == null) instanceValue.subRequests else StructBuilder.unwrapExplicitNull(_fieldArray(4)).asInstanceOf[_root_.scala.collection.Seq[com.twitter.scrooge.test.gold.thriftscala.Request]],
+        if (_fieldArray(5) == null) instanceValue._default else StructBuilder.unwrapExplicitNull(_fieldArray(5)).asInstanceOf[String],
         if (_fieldArray(6) == null) instanceValue.noComment else _fieldArray(6).asInstanceOf[_root_.scala.Option[Long]],
         if (_fieldArray(7) == null) instanceValue.doubleSlashComment else _fieldArray(7).asInstanceOf[_root_.scala.Option[Long]],
         if (_fieldArray(8) == null) instanceValue.hashtagComment else _fieldArray(8).asInstanceOf[_root_.scala.Option[Long]],
@@ -1830,12 +1832,12 @@ private[thriftscala] class RequestStructBuilder(instance: _root_.scala.Option[Re
     } else {
       if (genericArrayOps(_fieldArray).contains(null)) throw new InvalidFieldsException(structBuildError("Request"))
       Request(
-        _fieldArray(0).asInstanceOf[_root_.scala.collection.Seq[String]],
-        _fieldArray(1).asInstanceOf[_root_.scala.collection.Set[Int]],
-        _fieldArray(2).asInstanceOf[_root_.scala.collection.Map[Long, Long]],
+        StructBuilder.unwrapExplicitNull(_fieldArray(0)).asInstanceOf[_root_.scala.collection.Seq[String]],
+        StructBuilder.unwrapExplicitNull(_fieldArray(1)).asInstanceOf[_root_.scala.collection.Set[Int]],
+        StructBuilder.unwrapExplicitNull(_fieldArray(2)).asInstanceOf[_root_.scala.collection.Map[Long, Long]],
         _fieldArray(3).asInstanceOf[_root_.scala.Option[com.twitter.scrooge.test.gold.thriftscala.Request]],
-        _fieldArray(4).asInstanceOf[_root_.scala.collection.Seq[com.twitter.scrooge.test.gold.thriftscala.Request]],
-        _fieldArray(5).asInstanceOf[String],
+        StructBuilder.unwrapExplicitNull(_fieldArray(4)).asInstanceOf[_root_.scala.collection.Seq[com.twitter.scrooge.test.gold.thriftscala.Request]],
+        StructBuilder.unwrapExplicitNull(_fieldArray(5)).asInstanceOf[String],
         _fieldArray(6).asInstanceOf[_root_.scala.Option[Long]],
         _fieldArray(7).asInstanceOf[_root_.scala.Option[Long]],
         _fieldArray(8).asInstanceOf[_root_.scala.Option[Long]],
diff --git a/scrooge-generator-tests/src/test/resources/gold_file_output_scala/com/twitter/scrooge/test/gold/thriftscala/RequestException.scala b/scrooge-generator-tests/src/test/resources/gold_file_output_scala/com/twitter/scrooge/test/gold/thriftscala/RequestException.scala
index 968a1594..8c37b10b 100644
--- a/scrooge-generator-tests/src/test/resources/gold_file_output_scala/com/twitter/scrooge/test/gold/thriftscala/RequestException.scala
+++ b/scrooge-generator-tests/src/test/resources/gold_file_output_scala/com/twitter/scrooge/test/gold/thriftscala/RequestException.scala
@@ -53,6 +53,8 @@ object RequestException extends ValidatingThriftStructCodec3[RequestException] w
   )
 
 
+  lazy val nullableIndices: Set[Int] = StructBuilder.nullableIndices(fieldInfos)
+
   val structAnnotations: immutable$Map[String, String] =
     immutable$Map.empty[String, String]
 
@@ -113,7 +115,7 @@ object RequestException extends ValidatingThriftStructCodec3[RequestException] w
     )
   }
 
-  def newBuilder(): StructBuilder[RequestException] = new RequestExceptionStructBuilder(_root_.scala.None, fieldTypes)
+  def newBuilder(): StructBuilder[RequestException] = new RequestExceptionStructBuilder(_root_.scala.None, fieldTypes, nullableIndices)
 
   override def encode(_item: RequestException, _oproto: TProtocol): Unit = {
     _item.write(_oproto)
@@ -358,23 +360,23 @@ class RequestException(
       flags
     )
 
-  def newBuilder(): StructBuilder[RequestException] = new RequestExceptionStructBuilder(_root_.scala.Some(this), fieldTypes)
+  def newBuilder(): StructBuilder[RequestException] = new RequestExceptionStructBuilder(_root_.scala.Some(this), fieldTypes, nullableIndices)
 }
 
-private[thriftscala] class RequestExceptionStructBuilder(instance: _root_.scala.Option[RequestException], fieldTypes: IndexedSeq[ClassTag[_]])
-    extends StructBuilder[RequestException](fieldTypes) {
+private[thriftscala] class RequestExceptionStructBuilder(instance: _root_.scala.Option[RequestException], fieldTypes: IndexedSeq[ClassTag[_]], nullableIndices: Set[Int])
+    extends StructBuilder[RequestException](fieldTypes, nullableIndices) {
 
   def build(): RequestException = {
     val _fieldArray = fieldArray // shadow variable
     if (instance.isDefined) {
       val instanceValue = instance.get
       RequestException(
-        if (_fieldArray(0) == null) instanceValue.message else _fieldArray(0).asInstanceOf[String]
+        if (_fieldArray(0) == null) instanceValue.message else StructBuilder.unwrapExplicitNull(_fieldArray(0)).asInstanceOf[String]
       )
     } else {
       if (genericArrayOps(_fieldArray).contains(null)) throw new InvalidFieldsException(structBuildError("RequestException"))
       RequestException(
-        _fieldArray(0).asInstanceOf[String]
+        StructBuilder.unwrapExplicitNull(_fieldArray(0)).asInstanceOf[String]
       )
     }
   }
diff --git a/scrooge-generator-tests/src/test/resources/gold_file_output_scala/com/twitter/scrooge/test/gold/thriftscala/Response.scala b/scrooge-generator-tests/src/test/resources/gold_file_output_scala/com/twitter/scrooge/test/gold/thriftscala/Response.scala
index 94f4d884..0414a46f 100644
--- a/scrooge-generator-tests/src/test/resources/gold_file_output_scala/com/twitter/scrooge/test/gold/thriftscala/Response.scala
+++ b/scrooge-generator-tests/src/test/resources/gold_file_output_scala/com/twitter/scrooge/test/gold/thriftscala/Response.scala
@@ -68,6 +68,8 @@ object Response extends ValidatingThriftStructCodec3[Response] with StructBuilde
   )
 
 
+  lazy val nullableIndices: Set[Int] = StructBuilder.nullableIndices(fieldInfos)
+
   lazy val structAnnotations: immutable$Map[String, String] =
     immutable$Map[String, String](
         ("com.twitter.scrooge.scala.generateStructProxy", "true")
@@ -146,7 +148,7 @@ object Response extends ValidatingThriftStructCodec3[Response] with StructBuilde
     )
   }
 
-  def newBuilder(): StructBuilder[Response] = new ResponseStructBuilder(_root_.scala.None, fieldTypes)
+  def newBuilder(): StructBuilder[Response] = new ResponseStructBuilder(_root_.scala.None, fieldTypes, nullableIndices)
 
   override def encode(_item: Response, _oproto: TProtocol): Unit = {
     _item.write(_oproto)
@@ -533,11 +535,11 @@ trait Response
 
   def _codec: ValidatingThriftStructCodec3[Response] = Response
 
-  def newBuilder(): StructBuilder[Response] = new ResponseStructBuilder(_root_.scala.Some(this), fieldTypes)
+  def newBuilder(): StructBuilder[Response] = new ResponseStructBuilder(_root_.scala.Some(this), fieldTypes, nullableIndices)
 }
 
-private[thriftscala] class ResponseStructBuilder(instance: _root_.scala.Option[Response], fieldTypes: IndexedSeq[ClassTag[_]])
-    extends StructBuilder[Response](fieldTypes) {
+private[thriftscala] class ResponseStructBuilder(instance: _root_.scala.Option[Response], fieldTypes: IndexedSeq[ClassTag[_]], nullableIndices: Set[Int])
+    extends StructBuilder[Response](fieldTypes, nullableIndices) {
 
   def build(): Response = {
     val _fieldArray = fieldArray // shadow variable
@@ -545,13 +547,13 @@ private[thriftscala] class ResponseStructBuilder(instance: _root_.scala.Option[R
       val instanceValue = instance.get
       Response(
         if (_fieldArray(0) == null) instanceValue.statusCode else _fieldArray(0).asInstanceOf[Int],
-        if (_fieldArray(1) == null) instanceValue.responseUnion else _fieldArray(1).asInstanceOf[com.twitter.scrooge.test.gold.thriftscala.ResponseUnion]
+        if (_fieldArray(1) == null) instanceValue.responseUnion else StructBuilder.unwrapExplicitNull(_fieldArray(1)).asInstanceOf[com.twitter.scrooge.test.gold.thriftscala.ResponseUnion]
       )
     } else {
       if (genericArrayOps(_fieldArray).contains(null)) throw new InvalidFieldsException(structBuildError("Response"))
       Response(
         _fieldArray(0).asInstanceOf[Int],
-        _fieldArray(1).asInstanceOf[com.twitter.scrooge.test.gold.thriftscala.ResponseUnion]
+        StructBuilder.unwrapExplicitNull(_fieldArray(1)).asInstanceOf[com.twitter.scrooge.test.gold.thriftscala.ResponseUnion]
       )
     }
   }
diff --git a/scrooge-generator-tests/src/test/scala/com/twitter/scrooge/backend/ScalaGeneratorSpec.scala b/scrooge-generator-tests/src/test/scala/com/twitter/scrooge/backend/ScalaGeneratorSpec.scala
index fc9f7668..85f74d24 100644
--- a/scrooge-generator-tests/src/test/scala/com/twitter/scrooge/backend/ScalaGeneratorSpec.scala
+++ b/scrooge-generator-tests/src/test/scala/com/twitter/scrooge/backend/ScalaGeneratorSpec.scala
@@ -20,6 +20,7 @@ import com.twitter.scrooge.validation.MissingConstructionRequiredField
 import com.twitter.scrooge.validation.MissingRequiredField
 import com.twitter.util.Await
 import com.twitter.util.Future
+import includes.a.thriftscala.CityState
 import inheritance.aaa.Aaa
 import inheritance.aaa.Box
 import inheritance.bbb.Bbb
@@ -1626,6 +1627,36 @@ class ScalaGeneratorSpec extends JMockSpec with EvalHelper {
             staticBuilder.build()
           }
         }
+
+        "create new struct with nullable fields" in { _ =>
+          val builder = CityState.newBuilder().setField(0, null).setField(1, null)
+          assert(builder.build() == CityState(null, null))
+        }
+
+        "throw an exception if nullable fields aren't explicitly set" in { _ =>
+          val builder = CityState.newBuilder().setField(0, null)
+
+          intercept[InvalidFieldsException] {
+            // Not all fields were set
+            builder.build()
+          }
+        }
+
+        "refuse null in non-nullable fields" in { _ =>
+          val boxBuilder = Box.newBuilder()
+          boxBuilder.setField(0, 42) // can set it to an int value
+          intercept[IllegalArgumentException] {
+            // primitive fields aren't nullable
+            boxBuilder.setField(0, null)
+          }
+
+          val ooeBuilder = OneOfEachOptional.newBuilder()
+          ooeBuilder.setField(6, Some("foo")) // can set it to option-of-string
+          intercept[IllegalArgumentException] {
+            // optional non-primitive fields aren't nullable either
+            ooeBuilder.setField(6, null)
+          }
+        }
       }
     }
 
diff --git a/scrooge-generator/src/main/resources/scalagen/struct.mustache b/scrooge-generator/src/main/resources/scalagen/struct.mustache
index 707f347b..af045c1e 100644
--- a/scrooge-generator/src/main/resources/scalagen/struct.mustache
+++ b/scrooge-generator/src/main/resources/scalagen/struct.mustache
@@ -95,6 +95,8 @@ object {{StructName}} extends ValidatingThriftStructCodec3[{{StructName}}] with
   )
 {{/hasFields}}
 
+  lazy val nullableIndices: Set[Int] = StructBuilder.nullableIndices(fieldInfos)
+
   {{#structAnnotations}}lazy {{/structAnnotations}}val structAnnotations: immutable$Map[String, String] =
 {{#structAnnotations}}
     immutable$Map[String, String](
@@ -211,7 +213,7 @@ object {{StructName}} extends ValidatingThriftStructCodec3[{{StructName}}] with
     )
   }
 
-  def newBuilder(): StructBuilder[{{StructName}}] = new {{StructName}}StructBuilder(_root_.scala.None, fieldTypes)
+  def newBuilder(): StructBuilder[{{StructName}}] = new {{StructName}}StructBuilder(_root_.scala.None, fieldTypes, nullableIndices)
 
   override def encode(_item: {{StructName}}, _oproto: TProtocol): Unit = {
     _item.write(_oproto)
@@ -931,11 +933,11 @@ class {{StructName}}(
     )
 {{/hasFailureFlags}}
 
-  def newBuilder(): StructBuilder[{{StructName}}] = new {{StructName}}StructBuilder(_root_.scala.Some(this), fieldTypes)
+  def newBuilder(): StructBuilder[{{StructName}}] = new {{StructName}}StructBuilder(_root_.scala.Some(this), fieldTypes, nullableIndices)
 }
 
-private[{{packageName}}] class {{StructName}}StructBuilder(instance: _root_.scala.Option[{{StructName}}], fieldTypes: IndexedSeq[ClassTag[_]])
-    extends StructBuilder[{{StructName}}](fieldTypes) {
+private[{{packageName}}] class {{StructName}}StructBuilder(instance: _root_.scala.Option[{{StructName}}], fieldTypes: IndexedSeq[ClassTag[_]], nullableIndices: Set[Int])
+    extends StructBuilder[{{StructName}}](fieldTypes, nullableIndices) {
 
   def build(): {{StructName}} = {
 {{#hasFields}}
@@ -944,14 +946,14 @@ private[{{packageName}}] class {{StructName}}StructBuilder(instance: _root_.scal
       val instanceValue = instance.get
       {{StructName}}(
 {{#fields}}
-        if (_fieldArray({{index}}) == null) instanceValue.{{fieldName}}{{#constructionRequired}}.get{{/constructionRequired}} else _fieldArray({{index}}).asInstanceOf[{{>constructionOptionalType}}]
+        if (_fieldArray({{index}}) == null) instanceValue.{{fieldName}}{{#constructionRequired}}.get{{/constructionRequired}} else {{#nullValid}}StructBuilder.unwrapExplicitNull({{/nullValid}}_fieldArray({{index}}){{#nullValid}}){{/nullValid}}.asInstanceOf[{{>constructionOptionalType}}]
 {{/fields|,}}
       )
     } else {
       if (genericArrayOps(_fieldArray).contains(null)) throw new InvalidFieldsException(structBuildError("{{StructName}}"))
       {{StructName}}(
 {{#fields}}
-        _fieldArray({{index}}).asInstanceOf[{{>constructionOptionalType}}]
+        {{#nullValid}}StructBuilder.unwrapExplicitNull({{/nullValid}}_fieldArray({{index}}){{#nullValid}}){{/nullValid}}.asInstanceOf[{{>constructionOptionalType}}]
 {{/fields|,}}
       )
     }
diff --git a/scrooge-generator/src/main/scala/com/twitter/scrooge/backend/Generator.scala b/scrooge-generator/src/main/scala/com/twitter/scrooge/backend/Generator.scala
index 45a96c1f..84e5eb4f 100644
--- a/scrooge-generator/src/main/scala/com/twitter/scrooge/backend/Generator.scala
+++ b/scrooge-generator/src/main/scala/com/twitter/scrooge/backend/Generator.scala
@@ -270,14 +270,19 @@ abstract class TemplateGenerator(val resolvedDoc: ResolvedDocument)
 
   def quote(str: String): String = "\"" + str + "\""
   def quoteKeyword(str: String): String
-  def isNullableType(t: FieldType, isOptional: Boolean = false): Boolean = {
-    !isOptional && (
-      t match {
-        case TBool | TByte | TI16 | TI32 | TI64 | TDouble => false
-        case _ => true
-      }
-    )
-  }
+
+  def isNullableType(f: Field): Boolean =
+    !(f.requiredness.isOptional || isPrimitiveField(f))
+
+  // True if null is a valid value for the field
+  def isNullValid(f: Field): Boolean =
+    f.requiredness.isDefault && !isPrimitiveField(f)
+
+  private[this] def isPrimitiveField(f: Field): Boolean =
+    f.fieldType match {
+      case TBool | TByte | TI16 | TI32 | TI64 | TDouble => true
+      case _ => false
+    }
 
   def getServiceParentID(parent: ServiceParent): Identifier = {
     val identifier: Identifier = parent.filename match {
diff --git a/scrooge-generator/src/main/scala/com/twitter/scrooge/backend/StructTemplate.scala b/scrooge-generator/src/main/scala/com/twitter/scrooge/backend/StructTemplate.scala
index c64406f2..09d93b92 100644
--- a/scrooge-generator/src/main/scala/com/twitter/scrooge/backend/StructTemplate.scala
+++ b/scrooge-generator/src/main/scala/com/twitter/scrooge/backend/StructTemplate.scala
@@ -442,7 +442,8 @@ trait StructTemplate { self: TemplateGenerator =>
           ),
           "required" -> v(field.requiredness.isRequired),
           "optional" -> v(field.requiredness.isOptional),
-          "nullable" -> v(isNullableType(field.fieldType, field.requiredness.isOptional)),
+          "nullable" -> v(isNullableType(field)),
+          "nullValid" -> v(isNullValid(field)),
           "constructionOptional" -> v(
             !isConstructionRequiredField(field) && field.requiredness.isOptional
           ),