1. Overview
In this quick article, we’re going to look at a more advanced use-case of using Jackson – working with Maps that contain null values or null keys.
2. Ignore Nulls Values in a Map
Jackson has a simple but useful way of globally controlling what happens to null values when a Map gets serialized:
ObjectMapper mapper = new ObjectMapper();
mapper.setSerializationInclusion(Include.NON_NULL);
Now any null value in Map object serialized through this mapper is going to be ignored:
@Test
public void givenIgnoringNullValuesInMap_whenWritingMapObjectWithNullValue_thenIgnored()
throws JsonProcessingException {
ObjectMapper mapper = new ObjectMapper();
mapper.setSerializationInclusion(Include.NON_NULL);
MyDto dtoObject1 = new MyDto();
Map<String, MyDto> dtoMap = new HashMap<String, MyDto>();
dtoMap.put("dtoObject1", dtoObject1);
dtoMap.put("dtoObject2", null);
String dtoMapAsString = mapper.writeValueAsString(dtoMap);
assertThat(dtoMapAsString, containsString("dtoObject1"));
assertThat(dtoMapAsString, not(containsString("dtoObject2")));
}
3. Serializing a Map With a Null Key
By default, Jackson doesn’t allow the serialization of a Map with a null key. If you do try to write out such a map, you’ll get the following exception:
c.f.j.c.JsonGenerationException:
Null key for a Map not allowed in JSON (use a converting NullKeySerializer?)
at c.f.j.d.s.i.FailingSerializer.serialize(FailingSerializer.java:36)
The library is however flexible enough that you can define a custom, null key serializer and override the default behavior:
class MyDtoNullKeySerializer extends StdSerializer<Object> {
public MyDtoNullKeySerializer() {
this(null);
}
public MyDtoNullKeySerializer(Class<Object> t) {
super(t);
}
@Override
public void serialize(Object nullKey, JsonGenerator jsonGenerator, SerializerProvider unused)
throws IOException, JsonProcessingException {
jsonGenerator.writeFieldName("");
}
}
Now the Map with the null key will work just fine – and the null key will be written as an empty String:
@Test
public void givenAllowingMapObjectWithNullKey_whenWriting_thenCorrect()
throws JsonProcessingException {
ObjectMapper mapper = new ObjectMapper();
mapper.getSerializerProvider().setNullKeySerializer(new MyDtoNullKeySerializer());
MyDto dtoObject = new MyDto();
dtoObject.setStringValue("dtoObjectString");
Map<String, MyDto> dtoMap = new HashMap<String, MyDto>();
dtoMap.put(null, dtoObject);
String dtoMapAsString = mapper.writeValueAsString(dtoMap);
assertThat(dtoMapAsString, containsString("\"\""));
assertThat(dtoMapAsString, containsString("dtoObjectString"));
}
4. Ignore Null Fields
Besides Maps, Jackson provides a lot of configuration and flexibility for ignoring/working with null fields in general. You can check out this tutorial to see exactly how that works.
5. Conclusion
Serializing a Map object is common enough that we need a library that’s able to handle the nuances of the serialization process well. Jackson provides a few handy customization options to help you shape the output of this serialization process quite well.
It also provides a lot of solid ways to work with collections in a more general sense.
The implementation of all these examples and code snippets can be found in over on GitHub – this is a Maven-based project, so it should be easy to import and run as it is.
