为什么fastjson字段为null时不输出空字符串？

为什么fastjson字段为null时不输出空字符串？

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);
System.out.println(str);
//输出结果:{"a":1,"b":"",d:"wuzhuti.cn"}

从输出结果可以看出，null对应的key已经被过滤掉；这明显不是我们想要的结果，这时我们就需要用到fastjson的SerializerFeature序列化属性

也就是这个方法：JSONObject.toJSONString(Object object, SerializerFeature... features)

SerializerFeature有用的一些枚举值

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

在加上

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.WriteMapNullValue);
System.out.println(str);
//输出结果:{"a":1,"b":"","c":null,"d":"wuzhuti.cn"}

如果把WriteNullStringAsEmpty也加进去，为啥不起作用？！？！？

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

对规则的理解：

1. SerializerFeature.WriteMapNullValue 是否输出值为null的字段,默认为false
   也就是说有null时会输出而不是忽略（默认策略是忽略，所以看不到为null的字段）

2. WriteNullStringAsEmpty—字符类型字段如果为null,输出为”“,而非null 
   注意是字段是字段是字段，而不是json.put("key",null)，所以用它时，字段为null的可以转换为空字符串。

3. 如果让输出的json中所有为null的字符串都变成空字符串，最简单的做法就是加一个值过滤器，这样就避免了有的字段为null，有的字段为空字符的现象。

贴点示例代码

import java.util.ArrayList;
import java.util.HashMap;
import java.util.HashSet;
import java.util.List;
import java.util.Map;
import java.util.Set;

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

public class Demo1 {

public class Student {
private String name;
private int age;
private boolean isMale;
private Student gf;

public String getName() {
return name;
}

public void setName(String name) {
this.name = name;
}

public int getAge() {
return age;
}

public void setAge(int age) {
this.age = age;
}

public boolean isMale() {
return isMale;
}

public void setMale(boolean isMale) {
this.isMale = isMale;
}

public Student getGf() {
return gf;
}

public void setGf(Student gf) {
this.gf = gf;
}
}

private static ValueFilter filter = new ValueFilter() {
@Override
public Object process(Object obj, String s, Object v) {
if (v == null)
return "";
return v;
}
};

public static void main(String[] args) {
new Demo1().foo();
new Demo1().bar();
}

private void foo() {
System.out.println("foo()---------------------------");
JSONObject j1 = new JSONObject();
j1.put("name", "zhangsan");
j1.put("age", 13);
j1.put("isMale", true);
j1.put("gf", null);
Map<String, Object> fav = new HashMap<String, Object>();
Set<String> books = new HashSet<String>();
books.add("三国");
books.add("史记");
fav.put("history", books);
String[] arts = new String[] {};
fav.put("arts", arts);
String[] musics = new String[] { "北京欢迎你", "画心" };
fav.put("musics", musics);
List<String> sports = new ArrayList<String>();
fav.put("sports", sports);
j1.put("fav", fav);
List<Student> classmates = new ArrayList<Student>();
classmates.add(new Student());
Student lisi = new Student();
lisi.setMale(false);
lisi.setAge(11);
classmates.add(lisi);
Student zhangsan = new Student();
zhangsan.setAge(13);
zhangsan.setName("张三");
zhangsan.setMale(true);
zhangsan.setGf(lisi);
classmates.add(zhangsan);
j1.put("classmates", classmates);
String str = null;
j1.put("str", str);
System.out.println(j1.toString());
System.out
.println(JSON.toJSONString(j1, SerializerFeature.WriteMapNullValue, SerializerFeature.WriteNullStringAsEmpty));
System.out.println(
JSON.toJSONString(j1, filter, SerializerFeature.WriteMapNullValue, SerializerFeature.WriteNullStringAsEmpty));
System.out.println(JSON.toJSONString(j1, SerializerFeature.WriteNullStringAsEmpty));
System.out.println(JSON.toJSONString(j1, filter, SerializerFeature.WriteNullStringAsEmpty));

Map<String, JSONObject> m = new HashMap<String, JSONObject>();
m.put("key", j1);
System.out.println(
JSON.toJSONString(m, SerializerFeature.WriteNonStringKeyAsString, SerializerFeature.WriteNullStringAsEmpty));
System.out.println(JSON.toJSONString(m, filter, SerializerFeature.WriteNonStringKeyAsString,
SerializerFeature.WriteNullStringAsEmpty));

}

private void bar() {
System.out.println("bar()---------------------------");
Student zhangsan = new Student();
zhangsan.setAge(13);
zhangsan.setName("张三");
zhangsan.setMale(true);
Student lisi = new Student();
// lisi.setName("lisi");
lisi.setMale(false);
lisi.setAge(11);
zhangsan.setGf(lisi);
System.out.println(
JSON.toJSONString(zhangsan, SerializerFeature.WriteMapNullValue, SerializerFeature.WriteNullStringAsEmpty));
System.out.println(JSON.toJSONString(zhangsan, SerializerFeature.WriteMapNullValue));
System.out.println(JSON.toJSONString(zhangsan, SerializerFeature.WriteNullStringAsEmpty));
System.out.println(JSON.toJSONString(zhangsan));
System.out.println(JSON.toJSONString(zhangsan, filter));
System.out.println(JSON.toJSONString(zhangsan, filter, SerializerFeature.WriteMapNullValue,
SerializerFeature.WriteNullStringAsEmpty));
}

}

输出结果：

foo()---------------------------
{"isMale":true,"name":"zhangsan","classmates":[{"age":0,"male":false},{"age":11,"male":false},{"age":13,"gf":{"$ref":"$.classmates[1]"},"male":true,"name":"张三"}],"fav":{"sports":[],"musics":["北京欢迎你","画心"],"history":["史记","三国"],"arts":[]},"age":13}
{"str":null,"isMale":true,"name":"zhangsan","classmates":[{"age":0,"gf":null,"male":false,"name":""},{"age":11,"gf":null,"male":false,"name":""},{"age":13,"gf":{"$ref":"$.classmates[1]"},"male":true,"name":"张三"}],"fav":{"sports":[],"musics":["北京欢迎你","画心"],"history":["史记","三国"],"arts":[]},"age":13,"gf":null}
{"str":"","isMale":true,"name":"zhangsan","classmates":[{"age":0,"gf":"","male":false,"name":""},{"age":11,"gf":"","male":false,"name":""},{"age":13,"gf":{"$ref":"$.classmates[1]"},"male":true,"name":"张三"}],"fav":{"sports":[],"musics":["北京欢迎你","画心"],"history":["史记","三国"],"arts":[]},"age":13,"gf":""}
{"isMale":true,"name":"zhangsan","classmates":[{"age":0,"male":false},{"age":11,"male":false},{"age":13,"gf":{"$ref":"$.classmates[1]"},"male":true,"name":"张三"}],"fav":{"sports":[],"musics":["北京欢迎你","画心"],"history":["史记","三国"],"arts":[]},"age":13}
{"str":"","isMale":true,"name":"zhangsan","classmates":[{"age":0,"gf":"","male":false,"name":""},{"age":11,"gf":"","male":false,"name":""},{"age":13,"gf":{"$ref":"$.classmates[1]"},"male":true,"name":"张三"}],"fav":{"sports":[],"musics":["北京欢迎你","画心"],"history":["史记","三国"],"arts":[]},"age":13,"gf":""}
{"key":{"isMale":true,"name":"zhangsan","classmates":[{"age":0,"male":false},{"age":11,"male":false},{"age":13,"gf":{"$ref":"$.key.classmates[1]"},"male":true,"name":"张三"}],"fav":{"sports":[],"musics":["北京欢迎你","画心"],"history":["史记","三国"],"arts":[]},"age":13}}
{"key":{"str":"","isMale":true,"name":"zhangsan","classmates":[{"age":0,"gf":"","male":false,"name":""},{"age":11,"gf":"","male":false,"name":""},{"age":13,"gf":{"$ref":"$.key.classmates[1]"},"male":true,"name":"张三"}],"fav":{"sports":[],"musics":["北京欢迎你","画心"],"history":["史记","三国"],"arts":[]},"age":13,"gf":""}}
bar()---------------------------
{"age":13,"gf":{"age":11,"gf":null,"male":false,"name":""},"male":true,"name":"张三"}
{"age":13,"gf":{"age":11,"gf":null,"male":false,"name":null},"male":true,"name":"张三"}
{"age":13,"gf":{"age":11,"male":false},"male":true,"name":"张三"}
{"age":13,"gf":{"age":11,"male":false},"male":true,"name":"张三"}
{"age":13,"gf":{"age":11,"gf":"","male":false,"name":""},"male":true,"name":"张三"}
{"age":13,"gf":{"age":11,"gf":"","male":false,"name":""},"male":true,"name":"张三"}

转自：https://blog.csdn.net/u012534163/article/details/88741884