是否有可能在杰克逊中对地图本身进行de/serialize多态？

Question

我搜索了很多,只找到有关地图内容的多态反序列化的问题.是否有可能对地图本身进行多态反序列化？

例如,我有一个Book类包含一个Map作为成员变量.

public class Book {
    @JsonProperty
    private Map<String, Object> reviews;

    @JsonCreator
    public Book(Map<String, Object> map) {
        this.reviews = map;
    }
}

另一个类有一个Book类列表.

public class Shelf {

    @JsonProperty
    private List<Book> books = new LinkedList<>();

    public void setBooks(List<Book> books) {
        this.books = books;
    }

    public List<Book> getBooks() {
       return this.books;
    }
}

还有一个测试课.一本书的评论地图是Hashtable,另一本书的评论地图是HashMap.

public class Test {

    private Shelf shelf;

    @BeforeClass
    public void init() {
        Map<String, Object> review1 = new Hashtable<>(); // Hashtable here
        review1.put("test1", "review1");
        Map<String, Object> review2 = new HashMap<>(); // HashMap here
        review2.put("test2", "review2");

        List<Book> books = new LinkedList<>();
        books.add(new Book(review1));
        books.add(new Book(review2));
        shelf = new Shelf();
        shelf.setBooks(books);
    }

    @Test
    public void test() throws IOException{
        ObjectMapper mapper = new ObjectMapper();
        mapper.configure(SerializationFeature.INDENT_OUTPUT, true);
//        mapper.enableDefaultTyping(ObjectMapper.DefaultTyping.NON_FINAL);
        String json = mapper.writeValueAsString(shelf);
        System.out.println(json);

        Shelf sh = mapper.readValue(json, Shelf.class);
        for (Book b : sh.getBooks()) {
            System.out.println(b.getReviews().getClass());
        }
    }
}

测试输出

{
  "name" : "TestShelf",
  "books" : [ {
    "reviews" : {
      "test1" : "review1"
    }
  }, {
    "reviews" : {
      "test2" : "review2"
    }
  } ]
}
class java.util.LinkedHashMap
class java.util.LinkedHashMap

序列化工作正常.但是在反序列化之后,review1和review2都是LinkedHashMap.我希望review1和review2成为他们的实际类型,它们是Hashtable to review1和HashMap to review2.有没有办法实现这个目标？

我不想使用,mapper.enableDefaultTyping(ObjectMapper.DefaultTyping.NON_FINAL);因为它将在json消息中添加所有json属性的类型信息.如果有更好的方法,我也不想使用自定义的反序列化器.提前致谢.

Answer 1

我在 Jackson 用户论坛上发布了这个问题，他们建议自定义TypeResolverBuilder并在 ObjectMapper 实例中设置它。

ObjectMapper.setDefaultTyping(...)

我的定制TypeResolverBuilder如下，它解决了我的问题。

public class MapTypeIdResolverBuilder extends StdTypeResolverBuilder {

    public MapTypeIdResolverBuilder() {
    }

    @Override
    public TypeDeserializer buildTypeDeserializer(DeserializationConfig config,
                                                  JavaType baseType, Collection<NamedType> subtypes) {
        return useForType(baseType) ? super.buildTypeDeserializer(config, baseType, subtypes) : null;
    }

    @Override
    public TypeSerializer buildTypeSerializer(SerializationConfig config,
                                              JavaType baseType, Collection<namedtype> subtypes) {
        return useForType(baseType) ? super.buildTypeSerializer(config, baseType, subtypes) : null;
    }

    /**
     * Method called to check if the default type handler should be
     * used for given type.
     * Note: "natural types" (String, Boolean, Integer, Double) will never
     * use typing; that is both due to them being concrete and final,
     * and since actual serializers and deserializers will also ignore any
     * attempts to enforce typing.
     */
    public boolean useForType(JavaType t) {
        return t.isMapLikeType() || t.isJavaLangObject();
    }
}

该解决方案需要服务器端和客户端都使用定制的TypeResolverBuilder. 我知道这并不理想，但这是我迄今为止找到的最佳解决方案。解决方案的详细信息可以在我的博客上的这篇文章中找到。