如何编写 Jackson 转换器来处理通用集合类型？

Question

我创建了一个按升序排序的元素，以便生成的 JSON 数组按排序顺序输出，而不是给定实现使用的任意迭代顺序。它按照文档的建议扩展了 Jackson 中的标准类。Jackson ConverterSetSetStdConverterConverter

注意：强烈鼓励实现者扩展StdConverter而不是直接实现Converter，因为这可以帮助典型样板代码的默认实现。

public class OrderedSetConverter
        extends StdConverter<Set<DayOfWeek>, Set<DayOfWeek>> {
    @Override
    public Set<DayOfWeek> convert(Set<DayOfWeek> value) {
        return value == null ? null : value.stream()
                .sorted(Comparator.nullsLast(Comparator.naturalOrder()))
                .collect(Collectors.toCollection(LinkedHashSet::new));
    }
}

public class MyType {
    @JsonSerialize(converter = OrderedSetConverter.class)
    private Set<DayOfWeek> myValues;
}

Converter当要转换特定类型的元素时（在本例中） ，这很有效DayOfWeek。但是，此实现中没有任何特定于特定类型的内容；它对于任何类型都同样有效Comparable。因此，我更希望有一个可用于任何Set可比较的转换器的通用实现。

我尝试通过使用泛型的简单用法来实现这一点：

public class OrderedSetConverter<E extends Comparable<? super E>>
        extends StdConverter<Set<E>, Set<E>> {
    @Override
    public Set<E> convert(Set<E> value) {
        return value == null ? null : value.stream()
                .sorted(Comparator.nullsLast(Comparator.naturalOrder()))
                .collect(Collectors.toCollection(LinkedHashSet::new));
    }
}

这并不像我希望的那样工作，因为 Jackson 尝试（但失败）将集合的元素类型转换为Comparable，而不是实际类型（例如DayOfWeek）。

Exception in thread "main" com.fasterxml.jackson.databind.exc.InvalidDefinitionException: Cannot construct instance of `java.lang.Comparable` (no Creators, like default constructor, exist): abstract types either need to be mapped to concrete types, have custom deserializer, or contain additional type information
 at [Source: (String)"{"daysOfWeek":["MONDAY","TUESDAY","WEDNESDAY","THURSDAY","FRIDAY","SATURDAY","SUNDAY"]}"; line: 1, column: 16] (through reference chain: com.example.MyType["daysOfWeek"]->java.util.HashSet[0])
    at com.fasterxml.jackson.databind.exc.InvalidDefinitionException.from(InvalidDefinitionException.java:67)
    at com.fasterxml.jackson.databind.DeserializationContext.reportBadDefinition(DeserializationContext.java:1764)
    at com.fasterxml.jackson.databind.DatabindContext.reportBadDefinition(DatabindContext.java:400)
    at com.fasterxml.jackson.databind.DeserializationContext.handleMissingInstantiator(DeserializationContext.java:1209)
    at com.fasterxml.jackson.databind.deser.AbstractDeserializer.deserialize(AbstractDeserializer.java:274)
    at com.fasterxml.jackson.databind.deser.std.CollectionDeserializer._deserializeFromArray(CollectionDeserializer.java:347)
    at com.fasterxml.jackson.databind.deser.std.CollectionDeserializer.deserialize(CollectionDeserializer.java:244)
    at com.fasterxml.jackson.databind.deser.std.CollectionDeserializer.deserialize(CollectionDeserializer.java:28)
    at com.fasterxml.jackson.databind.deser.std.StdDelegatingDeserializer.deserialize(StdDelegatingDeserializer.java:175)
    at com.fasterxml.jackson.databind.deser.impl.MethodProperty.deserializeAndSet(MethodProperty.java:129)
    at com.fasterxml.jackson.databind.deser.BeanDeserializer.vanillaDeserialize(BeanDeserializer.java:324)
    at com.fasterxml.jackson.databind.deser.BeanDeserializer.deserialize(BeanDeserializer.java:187)
    at com.fasterxml.jackson.databind.deser.DefaultDeserializationContext.readRootValue(DefaultDeserializationContext.java:322)
    at com.fasterxml.jackson.databind.ObjectMapper._readMapAndClose(ObjectMapper.java:4593)
    at com.fasterxml.jackson.databind.ObjectMapper.readValue(ObjectMapper.java:3548)
    at com.fasterxml.jackson.databind.ObjectMapper.readValue(ObjectMapper.java:3516)

我能想到的最好的办法就是使用它作为抽象超类，并Converter为实际使用的每种类型提供每个特定的子类型：

public abstract class OrderedSetConverter<E extends Comparable<? super E>>
        extends StdConverter<Set<E>, Set<E>> {
    @Override
    public Set<E> convert(Set<E> value) {
        return value == null ? null : value.stream()
                .sorted(Comparator.nullsLast(Comparator.naturalOrder()))
                .collect(Collectors.toCollection(LinkedHashSet::new));
    }
}

public class DayOfWeekSetConverter extends OrderedSetConverter<DayOfWeek> { }

public class MyType {
    @JsonSerialize(converter = DayOfWeekSetConverter.class)
    private Set<DayOfWeek> myValues;
}

如何为通用集合类型编写转换器并让 Jackson 找出元素类型？

Answer 1

OrderedSetConverter不知道在反序列化过程中必须实例化哪种类型。我们需要指示反序列化过程使用哪种类型。如果您想保持转换器通用，则需要在创建转换器时提供此信息。要扩展默认行为，您需要实现自定义com.fasterxml.jackson.databind.cfg.HandlerInstantiator：

class ConverterHandlerInstantiator extends HandlerInstantiator {

    @Override
    public Converter<?, ?> converterInstance(MapperConfig<?> config, Annotated annotated, Class<?> implClass) {
        if (config instanceof DeserializationConfig) {
            JsonDeserialize jsonDeserialize = annotated.getAnnotation(JsonDeserialize.class);
            Class contentAs = jsonDeserialize.contentAs();
            if (contentAs != Void.class) {
                return new OrderedSetConverter<>(contentAs);
            }
        }
        return super.converterInstance(config, annotated, implClass);
    }

    @Override
    public JsonDeserializer<?> deserializerInstance(DeserializationConfig config, Annotated annotated, Class<?> deserClass) {
        return null;
    }

    @Override
    public KeyDeserializer keyDeserializerInstance(DeserializationConfig config, Annotated annotated, Class<?> keyDeserClass) {
        return null;
    }

    @Override
    public JsonSerializer<?> serializerInstance(SerializationConfig config, Annotated annotated, Class<?> serClass) {
        return null;
    }

    @Override
    public TypeResolverBuilder<?> typeResolverBuilderInstance(MapperConfig<?> config, Annotated annotated, Class<?> builderClass) {
        return null;
    }

    @Override
    public TypeIdResolver typeIdResolverInstance(MapperConfig<?> config, Annotated annotated, Class<?> resolverClass) {
        return null;
    }
}

正如您注意到的，上面的代码使用注释contentAs中的属性JsonDeserialize。我们需要在你的类中指定它：

class MyType {
    @JsonSerialize(converter = OrderedSetConverter.class)
    @JsonDeserialize(converter = OrderedSetConverter.class, contentAs = DayOfWeek.class)
    private Set<DayOfWeek> myValues;
}

当我们序列化对象时不需要这个值，因此，我们可以跳过它。现在，我们需要注册我们的自定义实例化器：

ObjectMapper mapper = new ObjectMapper();
mapper.setHandlerInstantiator(new ConverterHandlerInstantiator());

最后，我们的通用转换器：

class MyType {
    @JsonSerialize(converter = OrderedSetConverter.class)
    @JsonDeserialize(converter = OrderedSetConverter.class, contentAs = DayOfWeek.class)
    private Set<DayOfWeek> myValues;
}