问题描述
我有一个应用程序,它使用 Jackson 将我的复杂对象编组到 JSON 中,在 DynamoDB 中存储一些数据.
I've got an application that stores some data in DynamoDB using Jackson to marshall my complex object into a JSON.
例如,我正在编组的对象可能如下所示:
For example the object I'm marshalling might look like this:
private String aString;
private List<SomeObject> someObjectList;
SomeObject 可能如下所示:
Where SomeObject might look like this:
private int anInteger;
private SomeOtherObject;
和 SomeOtherObject 可能看起来像这样:
and SomeOtherObject might look like this:
private long aLong;
private float aFloat;
这很好,对象被编组没有问题并作为 JSON 字符串存储在数据库中.
This is fine an the object gets marshalled no problem and stored in the DB as a JSON string.
当需要从 DynamoDB 检索数据时,Jackson 会自动检索 JSON 并将其转换回......除了someObjectList"作为 List
返回,而不是作为 >List
!这是杰克逊的标准行为,发生这种情况并非错误.
When it comes time to retrieve the data from DynamoDB Jackson automatically retrieves the JSON and converts it back... EXCEPT that 'someObjectList' is returned as a List<LinkedHashMap>
not as a List<SomeObject>
! This is standard behaviour for Jackson, its not a mistake that this is happening.
所以现在这导致了一个问题.我的代码库认为它处理的是 List
但实际情况是它处理的是 List
!我的问题是如何将 LinkedHashMap 恢复为SomeObject".显然这是一个手动过程,但我的意思是我什至无法提取值.
So now this leads to a problem. My code base thinks its dealing with a List<SomeObject>
but the reality is that its handling a List<LinkedHashMap>
! My question is how do I get my LinkedHashMap back into a 'SomeObject'. Obviously this is a manual process but what I mean is I can't even extract the values.
如果我这样做:
for (LinkedHashMap lhm : someObjectList) {
// Convert the values back
}
我收到一个编译错误,告诉我 someObjectList 的类型是SomeObject"而不是 LinkedHashMap.
I get a compile error telling me that someObjectList is of type 'SomeObject' not LinkedHashMap.
如果我这样做:
for (SomeObject lhm : someObjectList) {
// Convert the values back
}
我收到一个运行时错误,告诉我 LinkedHashMap 无法转换为SomeObject".
I get a runtime error telling me that LinkedHashMap cannot be cast to 'SomeObject'.
推荐答案
您可以使用 ObjectMapper.convertValue()
,一个值一个值,甚至整个列表.但是你需要知道要转换成的类型:
You can use ObjectMapper.convertValue()
, either value by value or even for the whole list. But you need to know the type to convert to:
POJO pojo = mapper.convertValue(singleObject, POJO.class);
// or:
List<POJO> pojos = mapper.convertValue(listOfObjects, new TypeReference<List<POJO>>() { });
这在功能上和你一样:
byte[] json = mapper.writeValueAsBytes(singleObject);
POJO pojo = mapper.readValue(json, POJO.class);
但避免将数据实际序列化为 JSON,而是使用内存中的事件序列作为中间步骤.
but avoids actual serialization of data as JSON, instead using an in-memory event sequence as the intermediate step.
这篇关于将 LinkedHashMap 转换为复杂对象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!