为什么杰克逊多态序列化在列表中不起作用？

杰克逊正在做一些真正奇怪的事情，我找不到任何解释。我正在进行多态序列化，当一个对象独立时它可以很好地工作。但是，如果将相同的对象放入列表并对列表进行序列化，则会删除类型信息。

Jackson is doing something truly bizarre and I cannot find any explanation for it. I'm doing polymorphic serialization and it works perfectly when an object is on its own. But if you put the same object into a list and serialize the list instead, it erases the type information.

它丢失类型信息的事实会导致人们怀疑类型擦除。但是在序列化列表的内容期间会发生这种情况;杰克逊所要做的就是检查它正在序列化的当前对象以确定它的类型。

The fact that it's losing type info would lead one to suspect type erasure. But this is happening during serialization of the contents of the list; all Jackson has to do is inspect the current object it's serializing to determine its type.

我用Jackson 2.5.1创建了一个例子：

I've created an example using Jackson 2.5.1:

import com.fasterxml.jackson.annotation.JsonIgnoreProperties;
import com.fasterxml.jackson.annotation.JsonSubTypes;
import com.fasterxml.jackson.annotation.JsonSubTypes.Type;
import com.fasterxml.jackson.annotation.JsonTypeInfo;
import com.fasterxml.jackson.annotation.JsonTypeName;
import com.fasterxml.jackson.core.JsonProcessingException;
import com.fasterxml.jackson.databind.ObjectMapper;

import java.util.ArrayList;
import java.util.List;

public class Test {

  @JsonIgnoreProperties(ignoreUnknown = true)
  @JsonTypeInfo(use = JsonTypeInfo.Id.NAME, include = JsonTypeInfo.As.PROPERTY)
  @JsonSubTypes({
    @Type(value = Dog.class, name = "dog"),
    @Type(value = Cat.class, name = "cat")})
  public interface Animal {}

  @JsonTypeName("dog")
  public static class Dog implements Animal {
    private String name;

    public String getName() {
      return name;
    }

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

  @JsonTypeName("cat")
  public static class Cat implements Animal {
    private String name;

    public String getName() {
      return name;
    }

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

  public static void main(String[] args) throws JsonProcessingException {
    List<Cat> list = new ArrayList<>();
    list.add(new Cat());
    System.out.println(new ObjectMapper().writeValueAsString(list));
    System.out.println(new ObjectMapper().writeValueAsString(list.get(0)));
  }
}

这是输出：

[{"name":null}]
{"@type":"cat","name":null}

如您所见，当对象在列表中时，Jackson不会添加类型信息。有谁知道为什么会发生这种情况？

As you can see, Jackson is not adding the type information when the object is in a list. Does anyone know why this is happening?