Allow to configure Java null handling.

src/main/java/org/json/JSONObject.java

@@ -332,7 +332,7 @@ public class JSONObject {
         	        throw new NullPointerException("Null key.");
         	    }
                 final Object value = e.getValue();
-                if (value != null) {
+                if (value != null || jsonParserConfiguration.isJavaNullAsJsonNull()) {
                     testValidity(value);
                     this.map.put(String.valueOf(e.getKey()), wrap(value, recursionDepth + 1, jsonParserConfiguration));
                 }

src/main/java/org/json/JSONParserConfiguration.java

@@ -8,6 +8,11 @@ public class JSONParserConfiguration extends ParserConfiguration {
      * Used to indicate whether to overwrite duplicate key or not.
      */
     private boolean overwriteDuplicateKey;
+    
+    /**
+     * Used to indicate whether ignore null values when converting java maps to JSONObject or not.
+     */
+    private boolean javaNullAsJsonNull;
 
     /**
      * Configuration with the default values.
@@ -67,6 +72,21 @@ public class JSONParserConfiguration extends ParserConfiguration {
 
         return clone;
     }
+    
+    /**
+     * Controls the parser's behavior when meeting duplicate keys.
+     * If set to false, the parser will throw a JSONException when meeting a duplicate key.
+     * Or the duplicate key's value will be overwritten.
+     *
+     * @param javaNullAsJsonNull define, if the parser should ignore null values in Java maps
+     * @return The existing configuration will not be modified. A new configuration is returned.
+     */
+    public JSONParserConfiguration withJavaNullAsJsonNull(final boolean javaNullAsJsonNull) {
+        JSONParserConfiguration clone = this.clone();
+        clone.javaNullAsJsonNull = javaNullAsJsonNull;
+
+        return clone;
+    }
 
     /**
      * Sets the strict mode configuration for the JSON parser with default true value
@@ -106,6 +126,17 @@ public class JSONParserConfiguration extends ParserConfiguration {
     public boolean isOverwriteDuplicateKey() {
         return this.overwriteDuplicateKey;
     }
+    
+    /**
+     * The parser's behavior when meeting a null value in a java map, controls whether the parser should ignore 
+     * that map entry or write a JSON entry with a null value.
+     *
+     * @return The <code>javaNullAsJsonNull</code> configuration value.
+     */
+    public boolean isJavaNullAsJsonNull() {
+        return this.javaNullAsJsonNull;
+    }
+    
 
     /**
      * The parser throws an Exception when strict mode is true and tries to parse invalid JSON characters.

src/test/java/org/json/junit/JSONObjectTest.java

@@ -616,6 +616,46 @@ public class JSONObjectTest {
         assertTrue("expected \"doubleKey\":-23.45e67", Double.valueOf("-23.45e67").equals(jsonObject.query("/doubleKey")));
         Util.checkJSONObjectMaps(jsonObject);
     }
+    
+    @Test
+    public void jsonObjectByMapWithNullValueAndParserConfiguration() {
+        Map<String, Object> map = new HashMap<String, Object>();
+        map.put("nullKey", null);
+        
+        // by default, null values are ignored
+        JSONObject obj1 = new JSONObject(map);
+        assertTrue("expected null value to be ignored by default", obj1.isEmpty());
+
+        // if configured, null values are written as such into the JSONObject.
+        JSONParserConfiguration parserConfiguration = new JSONParserConfiguration().withJavaNullAsJsonNull(true);
+        JSONObject obj2 = new JSONObject(map, parserConfiguration);
+        assertFalse("expected null value to accepted when configured", obj2.isEmpty());
+        assertTrue(obj2.has("nullKey"));
+        assertEquals(JSONObject.NULL, obj2.get("nullKey"));
+    }
+    
+    @Test
+    public void jsonObjectByMapWithNestedNullValueAndParserConfiguration() {
+        Map<String, Object> map = new HashMap<String, Object>();
+        Map<String, Object> nestedMap = new HashMap<String, Object>();
+        nestedMap.put("nullKey", null);
+        map.put("nestedMap", nestedMap);
+        List<Map<String, Object>> nestedList = new ArrayList<Map<String,Object>>();
+        nestedList.add(nestedMap);        
+        map.put("nestedList", nestedList);
+        
+        JSONParserConfiguration parserConfiguration = new JSONParserConfiguration().withJavaNullAsJsonNull(true);
+        JSONObject jsonObject = new JSONObject(map, parserConfiguration);
+
+        JSONObject nestedObject = jsonObject.getJSONObject("nestedMap");
+        assertTrue(nestedObject.has("nullKey"));
+        assertEquals(JSONObject.NULL, nestedObject.get("nullKey"));
+        
+        JSONArray nestedArray = jsonObject.getJSONArray("nestedList");
+        assertEquals(1, nestedArray.length());
+        assertTrue(nestedArray.getJSONObject(0).has("nullKey"));
+        assertEquals(JSONObject.NULL, nestedArray.getJSONObject(0).get("nullKey"));
+    }
 
     /**
      * JSONObject built from a bean. In this case all but one of the