[FasterXML#106] added code block to CsvGenerator writeNull method to process null values when processing nulls deeper than the root level.

[FasterXML#106] added unit test code to verify writeNull behavior for list elements.

Author: morbrian

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

@@ -706,15 +706,30 @@ public void writeNull() throws IOException
         if (!_skipValue) {
             if (!_arraySeparator.isEmpty()) {
                 _addToArray(_schema.getNullValueOrEmpty());
-            } else if (!_writeContext.inObject()) { // as per [#69]
+            } else if (!_writeContext.inObject()) {
+
+                // per issue 106:
+                // this checks to see if this null item is at a level deeper
+                // than the root level list of items.
+                // Example Snippet:
+                //      List mainList = Arrays.asList(item1, item2, item3);
+                // If one of the item1..3 are null, it will not be processed.
+                // but if this null is part of a sub-list of item1..3,
+                // then the null will be processed.
+                if (_writeContext.getParent() != null
+                        && _writeContext.getParent().getParent() != null) {
+                    _writer.writeNull(_columnIndex());
+                }
+
+                // the initial 'inObject()' check is done 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/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.*;
@@ -78,4 +80,43 @@ public void testCustomNullValue() throws Exception
         // MUST use doubling for quotes!
         assertEquals("id,n/a\n", result);
     }
+
+    public void testNullFieldsOfListsContainedByMainLevelListIssue106() throws Exception {
+        CsvMapper mapper = mapperForCsv();
+
+        CsvSchema schema = CsvSchema.builder().build();
+
+        List row1 = Arrays.asList("d0", null, "d2");
+        List row2 = Arrays.asList(null, "d1", "d2");
+        List row3 = Arrays.asList("d0", "d1", null);
+
+        List dataList = Arrays.asList(row1, row2, row3);
+
+        String result = mapper.writer(schema).writeValueAsString(dataList);
+        assertEquals("d0,,d2\n,d1,d2\nd0,d1,\n", result);
+
+        schema = schema.withNullValue("n/a");
+        result = mapper.writer(schema).writeValueAsString(dataList);
+        assertEquals("d0,n/a,d2\nn/a,d1,d2\nd0,d1,n/a\n", result);
+    }
+
+    public void testNullElementsOfMainLevelListIssue106() throws Exception {
+        CsvMapper mapper = mapperForCsv();
+        CsvSchema schema = CsvSchema.builder().build();
+
+        List row1 = Arrays.asList("d0", null, "d2");
+        List row2 = Arrays.asList(null, "d1", "d2");
+        List row3 = Arrays.asList("d0", "d1", null);
+
+        // when serialized, the added root level nulls at index 1 and 3
+        // should be absent from the output
+        List dataList = Arrays.asList(row1, null, row2, null, row3);
+
+        String result = mapper.writer(schema).writeValueAsString(dataList);
+        assertEquals("d0,,d2\n,d1,d2\nd0,d1,\n", result);
+
+        schema = schema.withNullValue("n/a");
+        result = mapper.writer(schema).writeValueAsString(dataList);
+        assertEquals("d0,n/a,d2\nn/a,d1,d2\nd0,d1,n/a\n", result);
+    }
 }