[英]Parsing JSON Array with numbers as keys using Jackson?
如何使用Jackson解析以下种类的JSON数组并保留内容的顺序:
{
"1": {
"title": "ABC",
"category": "Video",
},
"2": {
"title": "DEF",
"category": "Audio",
},
"3": {
"title": "XYZ",
"category": "Text",
}
}
一个简单的解决方案:与其直接将其反序列化为数组/列表,反而将其反序列化为SortedMap<Integer, Value>
,然后在其上调用values()
以按顺序获取值。 有点混乱,因为它公开了模型对象中JSON处理的详细信息,但这是最少的实现工作。
@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,如果您只想在模型中包含Collection<Value>
并将其隐藏起来,则可以创建一个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”:它将需要知道集合中对象的类型。 (尽管只有一种情况,您可以对它进行硬编码,但是我觉得这有什么意思呢?)
@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);
}
}
(请注意,为便于阅读,省略了hashCode()
, equals(x)
等的定义)
最后是解串器实现:
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();
}
}
至少涵盖了这种简单情况:诸如集合内容为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的来源。
另外,如果未提供上下文,则可以将UntypedObjectDeserializer用作默认值,而不是阻塞。
最后,如果您希望解串器返回保留索引的List,则可以修改上面的内容,然后插入TreeMap的一些后处理:
int capacity = values.lastKey() + 1;
Object[] objects = new Object[capacity];
values.forEach((key, value) -> objects[key] = value);
return Arrays.asList(objects);
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