如果你有一個變數型態為父類別Parent，但實例是子類別Child的多型物件(Polymorphism)，則將該物件用Gson或Jaskson Databind含式庫是轉成JSON字串時，子類別中的屬性是否也會輸出呢？
請見以下實驗結果，若多型物件子類別的屬性與父類別有相同的屬性名稱時使用Gson轉JSON字串時會出現錯誤。例如下面父類Parent及子類Child都有相同的屬性名稱name，則出現錯誤
java.lang.IllegalArgumentException: [Class] declares multiple JSON fields named [name]。
import com.google.gson.Gson;
public class Main {
public static void main(String[] args) throws Exception {
Parent person = new Child();
Gson gson = new Gson();
System.out.println(gson.toJson(person)); // IllegalArgumentException錯誤
}
}
class Parent {
String name = "David";
//省略getter setter
}
class Child extends Parent {
String name = "Alex";
//省略getter setter
}
但用Jaskon DataBind仍可以正常的輸出子類別的屬性的JSON字串。
import com.fasterxml.jackson.databind.ObjectMapper;
public class Main {
public static void main(String[] args) throws Exception {
Parent person = new Child();
ObjectMapper mapper = new ObjectMapper();
System.out.println(mapper.writeValueAsString(person)); // {"name":"Alex"}
}
}
class Parent {
String name = "David";
//省略getter setter
}
class Child extends Parent {
String name = "Alex";
//省略getter setter
}
如果屬性名稱沒有重複，則Gson和Jaskson Databind兩者則可正常印出父類及子類屬性的JSON字串。
import com.fasterxml.jackson.databind.ObjectMapper;
import com.google.gson.Gson;
public class Main {
public static void main(String[] args) throws Exception {
Parent person = new Child();
Gson gson = new Gson();
System.out.println(gson.toJson(person)); // {"bike":"HONDA NM4-02","car":"BENZ C300"}
ObjectMapper mapper = new ObjectMapper();
System.out.println(mapper.writeValueAsString(person)); // {"car":"BENZ C300","bike":"HONDA NM4-02"}
}
}
class Parent {
String car = "BENZ C300";
//省略getter setter
}
class Child extends Parent {
String bike = "HONDA NM4-02";
//省略getter setter
}
最後要注意的是，Jaskson Databind轉成JSON字串的物件必須要有getter setter，否則會出現錯誤：
com.fasterxml.jackson.databind.exc.InvalidDefinitionException: No serializer found for class [Class] and no properties discovered to create BeanSerializer (to avoid exception, disable SerializationFeature.FAIL_ON_EMPTY_BEANS)
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