无法使用杰克逊反序列化和序列化Java List <>

Question

I am trying to deserializer and then serialize a java object. I got an object like this one-

public class Blas{
private Integer blasRootId;

private List<Bla> blaList = new ArrayList<>();

public Blas() {}

/region g & s
getter and setters ..
//endregion
}

And the object -

public class Bla{
private String fileName;
private String description;
private Integer id;

public Bla() {}

//region g & s
getter and setters ..
//endregion
}

I deserialize the object with

ObjectMapper mapper = new ObjectMapper();
mapper.configure(SerializationFeature.FAIL_ON_EMPTY_BEANS, false);

String jsonString = mapper.writeValueAsString(Blas);

And the created json is like

{
"Blas": {
    "blasRootId": 2840,
    "blaList": [
      "java.util.ArrayList",
      [
        {
          "fileName": "RegularPayload",
          "description": "",
          "id": 2260
        }
      ]
    ]
  }
}

So when I try to serialize the created json the following error accord -

Can not instantiate value of type [simple type, class Bla] from String value        ('java.util.ArrayList'); no single-String constructor/factory method

Who can i make the deserializer to write the list as it is ,without the addition "java.util.ArrayList" list, or how can i read it right?

Update : It was my mistake, I added in the "mapper.configure" a parameter (That i don't recall which) that caused the serializer to add the "java.util.ArrayList". My code example should work fine.

Answer 1

As prsmax asked, it depends how the code were you try to deserialize blas looks like, it seems like you are trying to take the string of blas and deserialize like this:

mapper.readValue(blasStr, Bla.class)

If you want only to deserialize a list of Bla you can do this:

    JavaType javaType = mapper.getTypeFactory().constructCollectionType(ArrayList.class, ValueType.class);
    List<ValueType> list = mapper.readValue(str, javaType);

If you actually need the wrapper object Blas, then you should have a constructor marked with @JsonCreator accepting the a List<Bla> marked with @JsonProperty (There are other ways, but this is a fail safe way and makes the code readable)