Issue when serializing Map.Entry using jackson - serialization

Question

If I try to deserialize below type stored as String:
List<Entry<String, String>> entryList;
where entryList contains:
[{"dummyKey1":"dummyValue1"}]
I get the following errors
Caused by: com.fasterxml.jackson.databind.JsonMappingException: Can not construct instance of java.util.Map$Entry, problem: abstract types either need to be mapped to concrete types, have custom deserializer, or be instantiated with additional type information.
I get above error while running a test case in junit, but if I remove the test case, then after deploying everything runs fine :
Above error comes while running junit test case because of absence of NoArgsConstructor in Entry. So, I created a DummyEntry with NoArgsConstructor that calls Entry with arguments as null.
DummyEntry<K, V> extends SimpleEntry<K, V>
After making this change, above error didn't come but I started getting below error after changes are deployed.
Caused by: com.fasterxml.jackson.databind.exc.UnrecognizedPropertyException:
Unrecognized field "dummyKey1", not marked as ignorable (2 known properties: "value", "key"]).
What is the reason that one way doesn't works for junit, but in production it works while other does work in junit but not in production.
Also, I noticed one additional thing: In production, Map.Entry is serialized to
{'dummyKey1':'dummyValue1'}
whereas, test case in junit serializes the same string as
{'key':'dummyKey1', 'value':'dummyValue1'}
What is the reason about this weird behavior ? How can I make this thing work for both ?

Answer 1

I suspect you might be encountering an issue with different serialisation strategies for Map.Entry.
In v2.5.0 (IIRC) of jackson-databind Map.Entry was supported as a 'known type'. Prior to this version, the key and value attributes of Map.Entry would appear in a serialised Map.Entry. After this version, that's no longer the case.
Here are some example test cases showing what I mean:
#Test
public void mapSerialisationPreJackson2_5_0() throws IOException {
Map<String, String> aMap = Maps.newHashMap();
aMap.put("dummyKey1", "dummyValue1");
Set<Map.Entry<String, String>> incoming = aMap.entrySet();
ObjectMapper objectMapper = new ObjectMapper();
String serialised = objectMapper.writeValueAsString(incoming);
// prints: [{"key":"dummyKey1","value":"dummyValue1"}]
System.out.println(serialised);
Set<Map.Entry<String, String>> deserialised = objectMapper.readValue(serialised, Set.class);
// prints: [{key=dummyKey1, value=dummyValue1} (just like you posted in your question) whereas for versions > 2.5.0 the serialised form is ]
System.out.println(deserialised);
}
#Test
public void mapSerialisationPostJackson2_5_0() throws IOException {
Map<String, String> aMap = Maps.newHashMap();
aMap.put("dummyKey1", "dummyValue1");
Set<Map.Entry<String, String>> incoming = aMap.entrySet();
ObjectMapper objectMapper = new ObjectMapper();
String serialised = objectMapper.writeValueAsString(incoming);
// prints: [{"dummyKey1":"dummyValue1"}]
System.out.println(serialised);
Set<Map.Entry<String, String>> deserialised = objectMapper.readValue(serialised, Set.class);
// prints: [{dummyKey1=dummyValue1}]
System.out.println(deserialised);
}
Prior to v2.5.0 a Map.Entry would be serialised to {key=dummyKey1, value=dummyValue1} (just like you posted in your question) whereas for versions > 2.5.0 the serialised form is {dummyKey1=dummyValue1}.
I think you are using a version of jackson-databind in your test context which is < 2.5.0 and a version of jackson-databind in your production context which is > 2.5.0

Answer 2

In order to be able to deserialize [{"dummyKey1":"dummyValue1"}] into a List<Entry<String, String>> variable you can:
Use Jackson's parameter names module. Read more here. It basically allows non-annotated, non default constructors with parameters to be used for deserialization of a class. In this case the constructors of the various implementations of Map.Entry. A perfectly straightforward solution if you use Java 8 anyway.
If you can't use the parameter names module (e.g. Java 7), you can look into using mixins to annotate a constructor of a class without modifying it's source code. I had a go at that and it's tricky. For HashMap for instance the implementation of Map.Entry is Node which has package private visibility.