java - 使用 Jackson 解析以数字为键的 JSON 数组？

Question

如何在保留内容顺序的情况下使用 Jackson 解析以下类型的 JSON 数组:

{
  "1": {
    "title": "ABC",
    "category": "Video",
  },
  "2": {
    "title": "DEF",
    "category": "Audio",
  },
  "3": {
    "title": "XYZ",
    "category": "Text",
  }
}

Answer 1

一个简单的解决方案:与其将其直接反序列化为数组/列表，不如将其反序列化为 SortedMap<Integer, Value>。然后调用values()在此基础上按顺序获取值。有点困惑，因为它在您的模型对象中公开了 JSON 处理的细节，但这是实现起来最少的工作。

@Test
public void deserialize_object_keyed_on_numbers_as_sorted_map() throws Exception {
    ObjectMapper mapper = new ObjectMapper();
    SortedMap<Integer, Value> container = mapper
            .reader(new TypeReference<SortedMap<Integer, Value>>() {})
            .with(JsonParser.Feature.ALLOW_SINGLE_QUOTES)
            .with(JsonParser.Feature.ALLOW_UNQUOTED_FIELD_NAMES)
            .readValue(
                    "{ 1: { title: 'ABC', category: 'Video' }, 2: { title: 'DEF', category: 'Video' }, 3: { title: 'XYZ', category: 'Video' } }");
    assertThat(container.values(),
            contains(new Value("ABC", "Video"), new Value("DEF", "Video"), new Value("XYZ", "Video")));
}


public static final class Value {
    public final String title;
    public final String category;

    @JsonCreator
    public Value(@JsonProperty("title") String title, @JsonProperty("category") String category) {
        this.title = title;
        this.category = category;
    }
}

但是如果你只想拥有一个 Collection<Value>在你的模型中，隐藏这个细节，你可以创建一个自定义反序列化器来做到这一点。请注意，您需要为反序列化器实现“上下文化”:它需要知道集合中对象的类型。 (虽然如果你只有一个案例，你可以硬编码，我猜，但那有什么乐趣呢？)

@Test
public void deserialize_object_keyed_on_numbers_as_ordered_collection() throws Exception {
    ObjectMapper mapper = new ObjectMapper();
    CollectionContainer container = mapper
            .reader(CollectionContainer.class)
            .with(JsonParser.Feature.ALLOW_SINGLE_QUOTES)
            .with(JsonParser.Feature.ALLOW_UNQUOTED_FIELD_NAMES)
            .readValue(
                    "{ values: { 1: { title: 'ABC', category: 'Video' }, 2: { title: 'DEF', category: 'Video' }, 3: { title: 'XYZ', category: 'Video' } } }");
    assertThat(
            container,
            equalTo(new CollectionContainer(ImmutableList.of(new Value("ABC", "Video"), new Value("DEF", "Video"),
                    new Value("XYZ", "Video")))));
}


public static final class CollectionContainer {
    @JsonDeserialize(using = CustomCollectionDeserializer.class)
    public final Collection<Value> values;

    @JsonCreator
    public CollectionContainer(@JsonProperty("values") Collection<Value> values) {
        this.values = ImmutableList.copyOf(values);
    }
}

(注意 hashCode() 、 equals(x) 等的定义，为了便于阅读，都被省略了)

最后是反序列化器实现:

public static final class CustomCollectionDeserializer extends StdDeserializer<Collection<?>> implements
        ContextualDeserializer {
    private JsonDeserializer<Object> contentDeser;

    public CustomCollectionDeserializer() {
        super(Collection.class);
    }

    public CustomCollectionDeserializer(JavaType collectionType, JsonDeserializer<Object> contentDeser) {
        super(collectionType);
        this.contentDeser = contentDeser;
    }

    @Override
    public JsonDeserializer<?> createContextual(DeserializationContext ctxt, BeanProperty property)
            throws JsonMappingException {
        if (!property.getType().isCollectionLikeType()) throw ctxt
                .mappingException("Can only be contextualised for collection-like types (was: "
                        + property.getType() + ")");
        JavaType contentType = property.getType().getContentType();
        return new CustomCollectionDeserializer(property.getType(), ctxt.findContextualValueDeserializer(
                contentType, property));
    }

    @Override
    public Collection<?> deserialize(JsonParser p, DeserializationContext ctxt) throws IOException,
            JsonProcessingException {
        if (contentDeser == null) throw ctxt.mappingException("Need context to produce elements of collection");
        SortedMap<Integer, Object> values = new TreeMap<>();
        for (JsonToken t = p.nextToken(); t != JsonToken.END_OBJECT; t = p.nextToken()) {
            if (t != JsonToken.FIELD_NAME) throw ctxt.wrongTokenException(p, JsonToken.FIELD_NAME,
                    "Expected index field");
            Integer index = Integer.valueOf(p.getText());
            p.nextToken();
            Object value = contentDeser.deserialize(p, ctxt);
            values.put(index, value);
        }
        return values.values();
    }
}

这至少涵盖了这种简单的情况:诸如集合内容是多态类型之类的事情可能需要更多处理:请参阅 Jackson 自己的 CollectionDeserializer 的源代码。

此外，如果没有给出上下文，您可以使用 UntypedObjectDeserializer 作为默认值而不是阻塞。

最后，如果您希望反序列化器返回一个保留索引的列表，您可以修改上面的内容并插入一些 TreeMap 的后处理:

        int capacity = values.lastKey() + 1;
        Object[] objects = new Object[capacity];
        values.forEach((key, value) -> objects[key] = value);
        return Arrays.asList(objects);