Importing some values from hashmap to POJO

Question

I want to convert a Hashmap of type to a POJO. I am using jackson to convert it currently, however, since the request is a API request, the user might provide more fields than needed. For example, The hashmap could be :

{
  field1ID:"hi",
  field2ID:["HI","HI","JO"],
  field3ID:"bye"
}

while the pojo is simply

{
  field1ID:"hi",
  field3ID:"bye"
}

When using ObjectMapper.convertValue , unless there is a one to one mapping from hashmap to pojo, a IllegalArguemetException will be throw. What I wan to do is, if the field is there then map the field. Else leave it as null.

Answer 1

As you didn't provide any code in your question, consider, for example, you have a map as shown below:

Map<String, Object> map = new HashMap<>();
map.put("firstName", "John");
map.put("lastName", "Doe");
map.put("emails", Arrays.asList("johndoe@mail.com", "john.doe@mail.com"));
map.put("birthday", LocalDate.of(1990, 1, 1));

And you want to map it to and instance of Contact :

@Data
public class Contact {
    private String firstName;
    private String lastName;
    private List<String> emails;
}

It could be achieved with the following code:

ObjectMapper mapper = new ObjectMapper();
mapper.disable(DeserializationFeature.FAIL_ON_UNKNOWN_PROPERTIES);
Contact contact = mapper.convertValue(map, Contact.class);

In the example above, the map has a birthday key, which cannot be mapped to any field of the Contact class. To prevent the serialization from failing, the FAIL_ON_UNKNOWN_PROPERTIES feature has been disabled.

---

Alternatively, you could annotate the Contact class with @JsonIgnoreProperties and set ignoreUnknown to true :

@Data
@JsonIgnoreProperties(ignoreUnknown = true)
public class Contact {
    ...
}

And then perform the conversion:

ObjectMapper mapper = new ObjectMapper();
Contact contact = mapper.convertValue(map, Contact.class);

---

To convert the Contact instance back to a map, you can use:

ObjectMapper mapper = new ObjectMapper();
Map<String, Object> map = mapper.convertValue(contact, 
        new TypeReference<Map<String, Object>>() {});