使用JSON.toJSONString(str,SerializerFeature... serial);

SerializerFeature

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

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

public class JavaToJson {
    public static void main(String[] args) {
        Class<?> c;
        Object o;
        try {
            c = Class.forName("com.free.User");
            o = c.newInstance();
            String json = JSON.toJSONString(o,SerializerFeature.WriteMapNullValue,SerializerFeature.WriteNullStringAsEmpty);
            System.out.println(json);
        } catch (Exception e) {
        }
        
       
    }
}