Per [FasterXML#106] added allowNullFields to provide the option of pr…

…ocessing null elements of lists as empty fields or the nullCharacter during serialization instead of the default behavior which skips null elements.

src/main/java/com/fasterxml/jackson/dataformat/csv/CsvGenerator.java

@@ -706,15 +706,20 @@ public void writeNull() throws IOException
         if (!_skipValue) {
             if (!_arraySeparator.isEmpty()) {
                 _addToArray(_schema.getNullValueOrEmpty());
-            } else if (!_writeContext.inObject()) { // as per [#69]
+            } else if (!_writeContext.inObject()) {
+                // [#106] write single null when allow null fields enabled
+                if (_schema.allowNullFields()) {
+                    _writer.writeNull(_columnIndex());
+                }
+
+                // as per [#69]
                 // note: 'root' not enough, for case of wrap-as array, or serialize List
-
                 // or, to write 'empty Object' (for common case), would
                 // write single null, then finish row, like so:
                 /*
-                _writer.writeNull(_columnIndex());
                 finishRow();
-*/
+                */
+
             } else {
                 _writer.writeNull(_columnIndex());
             }

src/main/java/com/fasterxml/jackson/dataformat/csv/CsvSchema.java

@@ -51,6 +51,11 @@
  *   the header row (if header row handling enabled): if true, they must be and
  *   an exception if thrown if order differs: if false, no verification is performed.
  *  </li>
+ *  <li>allowNullFields (boolean) [default: false] (added in Jackson 2.7): whether to
+ *    treat null values and objects as a field when serializing data. When enabled null
+ *    objects as a row may result in empty rows, and null objects as a field may result
+ *    in an empty character or the nullValue written between separators.
+ *  </li>
  * </ul>
  *<p>
  * Note that schemas without any columns are legal, but if no columns
@@ -92,6 +97,7 @@ public class CsvSchema
     protected final static int ENCODING_FEATURE_ALLOW_COMMENTS = 0x0004;
     protected final static int ENCODING_FEATURE_REORDER_COLUMNS = 0x0008;
     protected final static int ENCODING_FEATURE_STRICT_HEADERS = 0x0010;
+    protected final static int ENCODING_FEATURE_ALLOW_NULL_FIELDS = 0x0020;
 
     protected final static int DEFAULT_ENCODING_FEATURES = 0;
 
@@ -583,6 +589,11 @@ public Builder setAllowComments(boolean b) {
             _feature(ENCODING_FEATURE_ALLOW_COMMENTS, b);
             return this;
         }
+
+        public Builder setAllowNullFields(boolean b) {
+            _feature(ENCODING_FEATURE_ALLOW_NULL_FIELDS, b);
+            return this;
+        }
 
         protected final void _feature(int feature, boolean state) {
             _encodingFeatures = state ? (_encodingFeatures | feature) : (_encodingFeatures & ~feature);
@@ -995,6 +1006,14 @@ public CsvSchema withoutComments() {
         return _withFeature(ENCODING_FEATURE_ALLOW_COMMENTS, false);
     }
 
+    public CsvSchema withAllowNullFields() {
+        return _withFeature(ENCODING_FEATURE_ALLOW_NULL_FIELDS, true);
+    }
+
+    public CsvSchema withoutAllowNullFields() {
+        return _withFeature(ENCODING_FEATURE_ALLOW_NULL_FIELDS, false);
+    }
+
     protected CsvSchema _withFeature(int feature, boolean state) {
         int newFeatures = state ? (_features | feature) : (_features & ~feature);
         return (newFeatures == _features) ? this : new CsvSchema(this, newFeatures);
@@ -1157,6 +1176,7 @@ public String getSchemaType() {
     public boolean skipsFirstDataRow() { return (_features & ENCODING_FEATURE_SKIP_FIRST_DATA_ROW) != 0; }
     public boolean allowsComments() { return (_features & ENCODING_FEATURE_ALLOW_COMMENTS) != 0; }
     public boolean strictHeaders() { return (_features & ENCODING_FEATURE_STRICT_HEADERS) != 0; }
+    public boolean allowNullFields() { return (_features & ENCODING_FEATURE_ALLOW_NULL_FIELDS) != 0; }
 
     /**
      * @deprecated Use {@link #usesHeader()} instead
@@ -1314,7 +1334,8 @@ public String toString()
         sb.append(", header? ").append(usesHeader());
         sb.append(", skipFirst? ").append(skipsFirstDataRow());
         sb.append(", comments? ").append(allowsComments());
-
+        sb.append(", allowNullFields? ").append(allowNullFields());
+
         sb.append(']');
         return sb.toString();
     }

src/test/java/com/fasterxml/jackson/dataformat/csv/ser/NullWritingTest.java

@@ -2,6 +2,8 @@
 
 import java.io.ByteArrayOutputStream;
 import java.io.StringWriter;
+import java.util.Arrays;
+import java.util.List;
 
 import com.fasterxml.jackson.databind.*;
 import com.fasterxml.jackson.dataformat.csv.*;
@@ -67,15 +69,58 @@ public void testNullToStream() throws Exception {
     // [dataformat-csv#53]
     public void testCustomNullValue() throws Exception
     {
-        ObjectMapper mapper = mapperForCsv();
+        CsvMapper mapper = mapperForCsv();
         CsvSchema schema = CsvSchema.builder()
                 .setNullValue("n/a")
                 .addColumn("id")
                 .addColumn("desc")
                 .build();
-
         String result = mapper.writer(schema).writeValueAsString(new IdDesc("id", null));
         // MUST use doubling for quotes!
         assertEquals("id,n/a\n", result);
     }
+
+    public void testCustomNullValueInObjectListIssue106() throws Exception
+    {
+        ObjectMapper mapper = mapperForCsv();
+        CsvSchema schema = CsvSchema.builder()
+            .setArrayElementSeparator(",")
+            .setNullValue("n/a")
+            .setAllowNullFields(true)
+            .build();
+
+        List list = Arrays.asList("d0", null, "d2");
+
+        String result = mapper.writer(schema).writeValueAsString(list);
+        assertEquals("d0,n/a,d2\n", result);
+
+        list = Arrays.asList(null, "d1", "d2");
+        result = mapper.writer(schema).writeValueAsString(list);
+        assertEquals("n/a,d1,d2\n", result);
+
+        list = Arrays.asList("d0", "d1", null);
+        result = mapper.writer(schema).writeValueAsString(list);
+        assertEquals("d0,d1,n/a\n", result);
+    }
+
+    public void testDefaultNullValueInObjectListIssue106() throws Exception
+    {
+        CsvMapper mapper = mapperForCsv();
+        CsvSchema schema = CsvSchema.builder()
+            .setArrayElementSeparator(",")
+            .setAllowNullFields(true)
+            .build();
+
+        List list = Arrays.asList("d0", null, "d2");
+        String result = mapper.writer(schema).writeValueAsString(list);
+        assertEquals("d0,,d2\n", result);
+
+        list = Arrays.asList(null, "d1", "d2");
+        result = mapper.writer(schema).writeValueAsString(list);
+        assertEquals(",d1,d2\n", result);
+
+        list = Arrays.asList("d0", "d1", null);
+        result = mapper.writer(schema).writeValueAsString(list);
+        assertEquals("d0,d1,\n", result);
+    }
 }