问题描述
如何使用Jackson解析以下类型的JSON数组并保留内容的顺序：
How to parse following kind of JSON Array using Jackson with preserving order of the content:
{
"1": {
"title": "ABC",
"category": "Video",
},
"2": {
"title": "DEF",
"category": "Audio",
},
"3": {
"title": "XYZ",
"category": "Text",
}
}
推荐答案
一个简单的解决方案：而不是直接将其反序列化为数组/列表，将其反序列化为 SortedMap< Integer，Value> ，然后只需调用 values（）就可以按顺序获取值。有点混乱，因为它暴露了模型对象中JSON处理的细节，但这是最不实现的工作。
One simple solution: rather than deserializing it directly as an array/list, deserialize it to a SortedMap<Integer, Value> and then just call values() on that to get the values in order. A bit messy, since it exposes details of the JSON handling in your model object, but this is the least work to implement.
@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;
}
}
但是如果你想要一个<$ c您的模型中的$ c> Collection< Value> ，并隐藏此详细信息，您可以创建自定义反序列化器来执行此操作。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。 （虽然你可以硬编码，如果你只有一个案例，我想，但那里的乐趣在哪里？）
But if you want to just have a Collection<Value> in your model, and hide this detail away, you can create a custom deserializer to do that. Note that you need to implement "contextualisation" for the deserializer: it will need to be aware of what the type of the objects in your collection are. (Although you could hardcode this if you only have one case of it, I guess, but where's the fun in that?)
@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的定义（），等于（x）等因为可读性而被省略。）
(note definitions of hashCode(), equals(x) etc. are all omitted for readability)
最后是反序列化器实现：
And finally here comes the deserializer implementation:
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();
}
}
这至少涵盖了这个简单的案例：像作为多态类型的集合的内容可能需要更多处理：请参阅Jackson自己的CollectionDeserializer的源代码。
This covers at least this simple case: things like the contents of the collection being polymorphic types may require more handling: see the source of Jackson's own CollectionDeserializer.
此外，如果没有上下文，您可以使用UntypedObjectDeserializer作为默认值而不是choking给出。
Also, you could use UntypedObjectDeserializer as a default instead of choking if no context is given.
最后，如果你想让反序列化器返回一个保留了索引的List，你可以修改上面的内容并插入一些后处理TreeMap：
Finally, if you want the deserializer to return a List with the indices preserved, you can modify the above and just insert a bit of post-processing of the TreeMap:
int capacity = values.lastKey() + 1;
Object[] objects = new Object[capacity];
values.forEach((key, value) -> objects[key] = value);
return Arrays.asList(objects);
这篇关于使用Jackson解析带有数字作为键的JSON数组？的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持！