map null转空字符串

按照方法调用，没有效果
Map < String , Object > jsonMap = new HashMap< String , Object>();
jsonMap.put("a",1);
jsonMap.put("b","");
jsonMap.put("c",null);
jsonMap.put("d","wuzhuti.cn");

String str = JSONObject.toJSONString(jsonMap,SerializerFeature.WriteNullStringAsEmpty);
System.out.println(str);
//输出结果:{"d":"wuzhuti.cn","b":"","a":1}

将 SerializerFeature.WriteNullStringAsEmpty 改为 SerializerFeature.WriteMapNullValue