Jackson 常见问题及解决方法

本文翻译自：https://www.baeldung.com/jackson-exception

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1，“JsonMappingException: Can not construct instance of”

造成这种异常的原因是 Jackson 无法创建抽象类和接口的实例。

如下面的例子所示：

public class Zoo {
    public Animal animal;
     
    public Zoo() { }
}
 
abstract class Animal {
    public String name;
     
    public Animal() { }
}
 
class Cat extends Animal {
    public int lives;
     
    public Cat() { }
}

Zoo 类中含有1个 抽象类 Animal。
当我们将1个 JSON 字符串反序列化成 Zoo 实例的时候，就会抛出JsonMappingException: Can not construct instance of异常：

@Test(expected = JsonMappingException.class)
public void givenAbstractClass_whenDeserializing_thenException() 
  throws IOException {
    String json = "{"animal":{"name":"lacy"}}";
    ObjectMapper mapper = new ObjectMapper();
 
    mapper.reader().forType(Zoo.class).readValue(json);
}

异常内容为：

com.fasterxml.jackson.databind.JsonMappingException: 
Can not construct instance of org.baeldung.jackson.exception.Animal,
  problem: abstract types either need to be mapped to concrete types, 
  have custom deserializer, 
  or be instantiated with additional type information
  at 
[Source: {"animal":{"name":"lacy"}}; line: 1, column: 2] 
(through reference chain: org.baeldung.jackson.exception.Zoo["animal"])
    at c.f.j.d.JsonMappingException.from(JsonMappingException.java:148)