偶然用到fastjson转换json 在前台用js解析居然某些字段没有，以前用过gson。联想到是不是类似gson默认将null值不显示了，找了下资料果真如此

直接上代码吧

import java.util.HashMap;
import java.util.Map;

import com.alibaba.fastjson.JSONObject;
import com.alibaba.fastjson.serializer.SerializerFeature;


public class Test2 {

    /** * @param args */
    public static void main(String[] args) {

        /* * QuoteFieldNames———-输出key时是否使用双引号,默认为true WriteMapNullValue——–是否输出值为null的字段,默认为false WriteNullNumberAsZero—-数值字段如果为null,输出为0,而非null WriteNullListAsEmpty—–List字段如果为null,输出为[],而非null WriteNullStringAsEmpty—字符类型字段如果为null,输出为”“,而非null WriteNullBooleanAsFalse–Boolean字段如果为null,输出为false,而非null */



        Map < String,Object > jsonMap = new HashMap< String,Object>();  
        jsonMap.put("xyw",1);  
        jsonMap.put("123","");  
        jsonMap.put("xuyw",null);  
        jsonMap.put("xywa","css");  

        String str = JSONObject.toJSONString(jsonMap);  
        System.out.println(str);

        String str2 = JSONObject.toJSONString(jsonMap,SerializerFeature.WriteMapNullValue);  
        System.out.println(str2);
    }

}

输出结果

{"123":"","xyw":1,"xywa":"css"}
{"123":"","xuyw":null,"xywa":"css"}

指定序列化字段
SimplePropertyPreFilter filter = new SimplePropertyPreFilter(Map.class,"code","name");  
JSON.toJSONString(list,filter)