直接从作者那里得到了Jackson谷歌组的答案。
要理解的关键是**JsonDeserializer只被初始化/上下文化一次**，它们只在那一刻接收完整的类型和其他信息。为了获得这些信息，反序列化器需要实现ContextualDeserializer。它的createContextual方法被调用来初始化一个反序列化器示例，并且可以访问BeanProperty，它也提供了完整的JavaType。
所以最后看起来可能是这样的：

public class MapDeserializer extends JsonDeserializer implements ContextualDeserializer {

    private JavaType type;

    public MapDeserializer() {
    }

    public MapDeserializer(JavaType type) {
        this.type = type;
    }

    @Override
    public JsonDeserializer<?> createContextual(DeserializationContext deserializationContext, BeanProperty beanProperty) throws JsonMappingException {
        //beanProperty is null when the type to deserialize is the top-level type or a generic type, not a type of a bean property
        JavaType type = deserializationContext.getContextualType() != null 
            ? deserializationContext.getContextualType()
            : beanProperty.getMember().getType();            
        return new MapDeserializer(type);
    }

    @Override
    public Map deserialize(JsonParser jsonParser, DeserializationContext deserializationContext) throws IOException {
        //use this.type as needed
    }

    ...
}

注册并正常使用：

ObjectMapper mapper = new ObjectMapper();
SimpleModule module = new SimpleModule();
module.addDeserializer(Map.class, new MapDeserializer());
mapper.registerModule(module);