One more fix wrt #72, to distinguish between empty String, null, when…

… reading; only former will cause coercion of empty String into `null`, at streaming parser level (higher-level mapping may still occur).

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

@@ -643,7 +643,7 @@ public void writeNull() throws IOException
         _verifyValueWrite("write null value");
         if (!_skipValue) {
             if (_arraySeparator >= 0) {
-                _addToArray(_schema.getNullValue());
+                _addToArray(_schema.getNullValueOrEmpty());
             } else if (_writeContext.inRoot()) { // as per [#69]
                 // or, to write 'empty Object' (for common case), would
                 // write single null, then finish row, like so:

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

@@ -84,6 +84,8 @@ public class CsvSchema
 
     protected final static int DEFAULT_ENCODING_FEATURES = 0;
 
+    protected final static char[] NO_CHARS = new char[0];
+
     /*
     /**********************************************************************
     /* Constants, default settings
@@ -100,13 +102,20 @@ public class CsvSchema
      * semicolon.
      */
     public final static char DEFAULT_ARRAY_ELEMENT_SEPARATOR = ';';
-    
+
     public final static char DEFAULT_QUOTE_CHAR = '"';
 
     /**
-     * By default, nulls are written as empty Strings ("")
+     * By default, nulls are written as empty Strings (""); and no coercion
+     * is performed from any String (higher level databind may, however,
+     * coerce Strings into Java nulls).
+     * To use automatic coercion on reading, null value must be set explicitly
+     * to empty String ("").
+     *<p>
+     * NOTE: before 2.6, this value default to empty <code>char[]</code>; changed
+     * to Java null in 2.6.
      */
-    public final static char[] DEFAULT_NULL_VALUE = new char[0];
+    public final static char[] DEFAULT_NULL_VALUE = null;
 
     /**
      * By default, no escape character is used -- this is denoted by
@@ -549,7 +558,7 @@ public Builder setNullValue(String nvl) {
         }
 
         public Builder setNullValue(char[] nvl) {
-            _nullValue = (nvl == null) ? DEFAULT_NULL_VALUE : nvl;
+            _nullValue = nvl;
             return this;
         }
 
@@ -888,7 +897,7 @@ public CsvSchema withNullValue(String nvl) {
         return new CsvSchema(_columns, _features,
                 _columnSeparator, _quoteChar, _escapeChar, _lineSeparator,
                 _arrayElementSeparator,
-                (nvl == null) ? DEFAULT_NULL_VALUE : nvl.toCharArray(),
+                (nvl == null) ? null : nvl.toCharArray(),
                 _columnsByName);
     }
 
@@ -982,21 +991,36 @@ public String getSchemaType() {
     public char[] getLineSeparator() { return _lineSeparator; }
 
     /**
+     * @return Null value defined, as char array, if one is defined to be recognized; Java null
+     *    if not.
+     * 
      * @since 2.5
      */
     public char[] getNullValue() { return _nullValue; }
 
+    /**
+     * Same as {@link #getNullValue()} except that undefined null value (one that remains as <code>null</code>,
+     * or explicitly set as such) will be returned as empty <code>char[]</code>
+     *
+     * @since 2.6
+     */
+    public char[] getNullValueOrEmpty() {
+        if (_nullValue == null) {
+            return NO_CHARS;
+        }
+        return _nullValue;
+    }
+
     /**
      * @since 2.6
      */
     public String getNullValueString() {
         String str = _nullValueAsString;
         if (str == null) {
-            if (_nullValue == null || _nullValue.length == 0) {
-                str = "";
-            } else {
-                str = new String(_nullValue);
+            if (_nullValue == null) {
+                return null;
             }
+            str = (_nullValue.length == 0) ? "" : new String(_nullValue);
             _nullValueAsString = str;
         }
         return str;

src/main/java/com/fasterxml/jackson/dataformat/csv/impl/CsvEncoder.java

@@ -171,7 +171,7 @@ public CsvEncoder(IOContext ctxt, int csvFeatures, Writer out, CsvSchema schema)
         _cfgQuoteCharacter = schema.getQuoteChar();
         _cfgLineSeparator = schema.getLineSeparator();
         _cfgLineSeparatorLength = (_cfgLineSeparator == null) ? 0 : _cfgLineSeparator.length;
-        _cfgNullValue = schema.getNullValue();
+        _cfgNullValue = schema.getNullValueOrEmpty();
 
         _columnCount = schema.size();
 
@@ -198,7 +198,7 @@ public CsvEncoder(CsvEncoder base, CsvSchema newSchema)
         _cfgQuoteCharacter = newSchema.getQuoteChar();
         _cfgLineSeparator = newSchema.getLineSeparator();
         _cfgLineSeparatorLength = _cfgLineSeparator.length;
-        _cfgNullValue = newSchema.getNullValue();
+        _cfgNullValue = newSchema.getNullValueOrEmpty();
         _cfgMinSafeChar = _calcSafeChar();
         _columnCount = newSchema.size();
     }  

src/test/java/com/fasterxml/jackson/dataformat/csv/deser/NullReadTest.java

@@ -50,19 +50,19 @@ public void testReadNullValueFromEmptyString() throws Exception
         String csv = MAPPER.writer(schemaWithDefault).writeValueAsString(new IdDesc("id", null));
         assertEquals("id,\n", csv);
 
-        // but read back
+        // but read back. Note: no null coercion unless explicitly defined
 
         ObjectReader r = MAPPER.readerFor(IdDesc.class).with(schemaWithDefault);
 
         IdDesc result = r.readValue(csv);
         assertNotNull(result);
         assertEquals("id", result.id);
-        assertNull(result.desc);
+        assertEquals("", result.desc);
 
         // also try the other combination
         result = r.readValue(",Whatevs\n");
         assertNotNull(result);
-        assertNull(result.id);
+        assertEquals("", result.id);
         assertEquals("Whatevs", result.desc);
 
         // And then with explicit Empty String
@@ -80,7 +80,7 @@ public void testReadNullValueFromEmptyString() throws Exception
         assertEquals("id", result.id);
         assertNull(result.desc);
 
-        // and finally with explicit `null`
+        // and finally with explicit `null`, which once again disables coercion
         CsvSchema schemaWithExplicitNull = CsvSchema.builder()
                 .setNullValue((String) null)
                 .addColumn("id")
@@ -93,6 +93,6 @@ public void testReadNullValueFromEmptyString() throws Exception
         result = r.readValue(csv);
         assertNotNull(result);
         assertEquals("id", result.id);
-        assertNull(result.desc);
+        assertEquals("", result.desc);
     }
 }