I’ve got an application that stores some data in DynamoDB using Jackson to marshall my complex object into a JSON.
For example the object I’m marshalling might look like this:
private String aString; private List<SomeObject> someObjectList;
Where SomeObject might look like this:
private int anInteger; private SomeOtherObject;
and SomeOtherObject might look like this:
private long aLong; private float aFloat;
This is fine an the object gets marshalled no problem and stored in the DB as a JSON string.
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.
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.
If I do this:
for (LinkedHashMap lhm : someObjectList) {
// Convert the values back
}
I get a compile error telling me that someObjectList is of type ‘SomeObject’ not LinkedHashMap.
If I do this:
for (SomeObject lhm : someObjectList) {
// Convert the values back
}
I get a runtime error telling me that LinkedHashMap cannot be cast to ‘SomeObject’.
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Answer
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>>() { });
this is functionally same as if you did:
byte[] json = mapper.writeValueAsBytes(singleObject); POJO pojo = mapper.readValue(json, POJO.class);
but avoids actual serialization of data as JSON, instead using an in-memory event sequence as the intermediate step.