StefanRRichter commented on a change in pull request #6707: [FLINK-10157] 
[State TTL] Allow `null` user values in map state with TTL
URL: https://github.com/apache/flink/pull/6707#discussion_r219101008
 
 

 ##########
 File path: 
flink-core/src/main/java/org/apache/flink/api/java/typeutils/runtime/NullableSerializer.java
 ##########
 @@ -56,33 +59,51 @@
 public class NullableSerializer<T> extends TypeSerializer<T> {
        private static final long serialVersionUID = 3335569358214720033L;
 
+       @Nonnull
        private final TypeSerializer<T> originalSerializer;
 
-       private NullableSerializer(TypeSerializer<T> originalSerializer) {
+       private NullableSerializer(@Nonnull TypeSerializer<T> 
originalSerializer) {
                Preconditions.checkNotNull(originalSerializer, "The original 
serializer cannot be null");
                this.originalSerializer = originalSerializer;
        }
 
        /**
-        * This method tries to serialize null value with the {@code 
originalSerializer}
+        * This method tries to serialize {@code null} value with the {@code 
originalSerializer}
         * and wraps it in case of {@link NullPointerException}, otherwise it 
returns the {@code originalSerializer}.
+        *
+        * @param originalSerializer serializer to wrap and add {@code null} 
support
+        * @return serializer which supports {@code null} values
         */
-       public static <T> TypeSerializer<T> 
wrapIfNullIsNotSupported(TypeSerializer<T> originalSerializer) {
+       public static <T> TypeSerializer<T> wrapIfNullIsNotSupported(@Nonnull 
TypeSerializer<T> originalSerializer) {
                return checkIfNullSupported(originalSerializer) ? 
originalSerializer : wrap(originalSerializer);
        }
 
-       private static <T> boolean checkIfNullSupported(TypeSerializer<T> 
originalSerializer) {
+       /**
+        * This method checks if {@code serializer} supports {@code null} value.
+        *
+        * @param serializer serializer to check
+        */
+       public static <T> boolean checkIfNullSupported(@Nonnull 
TypeSerializer<T> serializer) {
                try {
-                       originalSerializer.serialize(null, new 
DataOutputSerializer(1));
-                       
Preconditions.checkArgument(originalSerializer.copy(null) == null);
-               } catch (NullPointerException | IOException e) {
+                       int length = serializer.getLength() > 0 ? 
serializer.getLength() : 1;
+                       DataOutputSerializer dos = new 
DataOutputSerializer(length);
+                       serializer.serialize(null, dos);
+                       DataInputDeserializer dis = new 
DataInputDeserializer(dos.getSharedBuffer());
+                       Preconditions.checkArgument(serializer.deserialize(dis) 
== null);
 
 Review comment:
   Both precondition checks will just be caught by the following `catch` block. 
Depending on what you intend to do, the check should go outside the block, or 
rather result in `false` value directly (I assume the second case is what you 
want).

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