alibaba的JSON.toString会把值为null的字段去掉，谨记

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Map<String,Object> map = new HashMap<>();
map.put("a", "1");
map.put("b", "2");
map.put("c", null);
String jsonString = JSON.toJSONString(map);
System.out.println(jsonString);结果：{"a":"1","b":"2"}Map<String,Object> map = new HashMap<>();
map.put("a", "1");
map.put("b", "2");
map.put("c", "");
String jsonString = JSON.toJSONString(map);
System.out.println(jsonString);结果：{"a":"1","b":"2","c":""}