為什么fastjson字段為null時不輸出空字符串？
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);
//輸出結果:{"a":1,"b":"",d:"wuzhuti.cn"}
從輸出結果可以看出，null對應的key已經被過濾掉；這明顯不是我們想要的結果，這時我們就需要用到fastjson的SerializerFeature序列化屬性
也就是這個方法：JSONObject.toJSONString(Object object, SerializerFeature... features)
SerializerFeature有用的一些枚舉值
QuoteFieldNames———-輸出key時是否使用雙引號,默認為true
WriteMapNullValue——–是否輸出值為null的字段,默認為false
WriteNullNumberAsZero—-數值字段如果為null,輸出為0,而非null
WriteNullListAsEmpty—–List字段如果為null,輸出為[],而非null
WriteNullStringAsEmpty—字符類型字段如果為null,輸出為”“,而非null
WriteNullBooleanAsFalse–Boolean字段如果為null,輸出為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);
//輸出結果:{"a":1,"b":"","c":null,"d":"wuzhuti.cn"}
如果把WriteNullStringAsEmpty也加進去，為啥不起作用？！？！？
String str = JSONObject.toJSONString(jsonMap,SerializerFeature.WriteMapNullValue，SerializerFeature.WriteNullStringAsEmpty);
System.out.println(str);
//輸出結果:{"a":1,"b":"","c":null,"d":"wuzhuti.cn"}
對規則的理解：
-
SerializerFeature.WriteMapNullValue 是否輸出值為null的字段,默認為false
也就是說有null時會輸出而不是忽略（默認策略是忽略，所以看不到為null的字段） -
WriteNullStringAsEmpty—字符類型字段如果為null,輸出為”“,而非null
注意是字段是字段是字段，而不是json.put("key",null)，所以用它時，字段為null的可以轉換為空字符串。 -
如果讓輸出的json中所有為null的字符串都變成空字符串，最簡單的做法就是加一個值過濾器，這樣就避免了有的字段為null，有的字段為空字符的現象。
貼點示例代碼
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("三國");
books.add("史記");
fav.put("history", books);
String[] arts = new String[] {};
fav.put("arts", arts);
String[] musics = new String[] { "北京歡迎你", "畫心" };
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("張三");
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("張三");
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));
}
}
輸出結果：
foo()---------------------------
{"isMale":true,"name":"zhangsan","classmates":[{"age":0,"male":false},{"age":11,"male":false},{"age":13,"gf":{"$ref":"$.classmates[1]"},"male":true,"name":"張三"}],"fav":{"sports":[],"musics":["北京歡迎你","畫心"],"history":["史記","三國"],"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":"張三"}],"fav":{"sports":[],"musics":["北京歡迎你","畫心"],"history":["史記","三國"],"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":"張三"}],"fav":{"sports":[],"musics":["北京歡迎你","畫心"],"history":["史記","三國"],"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":"張三"}],"fav":{"sports":[],"musics":["北京歡迎你","畫心"],"history":["史記","三國"],"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":"張三"}],"fav":{"sports":[],"musics":["北京歡迎你","畫心"],"history":["史記","三國"],"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":"張三"}],"fav":{"sports":[],"musics":["北京歡迎你","畫心"],"history":["史記","三國"],"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":"張三"}],"fav":{"sports":[],"musics":["北京歡迎你","畫心"],"history":["史記","三國"],"arts":[]},"age":13,"gf":""}}
bar()---------------------------
{"age":13,"gf":{"age":11,"gf":null,"male":false,"name":""},"male":true,"name":"張三"}
{"age":13,"gf":{"age":11,"gf":null,"male":false,"name":null},"male":true,"name":"張三"}
{"age":13,"gf":{"age":11,"male":false},"male":true,"name":"張三"}
{"age":13,"gf":{"age":11,"male":false},"male":true,"name":"張三"}
{"age":13,"gf":{"age":11,"gf":"","male":false,"name":""},"male":true,"name":"張三"}
{"age":13,"gf":{"age":11,"gf":"","male":false,"name":""},"male":true,"name":"張三"}
轉自：https://blog.csdn.net/u012534163/article/details/88741884