通过忽略fastXml Jackson Java中的null元素/属性来序列化空列表

Question

When I am trying to serialize a class, Jackson giving empty list as list with one empty element.

....
@JacksonXmlElementWrapper(localName = "users")
@JacksonXmlProperty(localName = "user")
private List<User> users;
....

Xml Output:

<response>
   <users>
        <user/>
   </users>
</response>

Json Output:

{
    "users": null
}

But I think, I should get empty list as below (without empty user element inside users):

<response>
   <users/>
</response>

How to fix this ?

Answer 1

Just returning empty list from getUsers() gives desired result.

@JacksonXmlRootElement(localName = "response")
public class Response {

    @JacksonXmlElementWrapper(localName = "users")
    @JacksonXmlProperty(localName = "user")
    private List<User> users;

    public List<User> getUsers() {

        if(null == users) {
            return Collections.emptyList();
        }
        return users;
    }

    public void setUsers(List<User> users) {
        this.users = users;
    }
}

Main Method

ObjectMapper mapper = new XmlMapper();
Response response = new Response();
String s = mapper.writerWithDefaultPrettyPrinter().writeValueAsString(response);
System.out.println(s);

response.setUsers(Arrays.asList(new User("1", "Hemant")));
s = mapper.writerWithDefaultPrettyPrinter().writeValueAsString(response);
System.out.println(s);

Output

<response>
  <users/>
</response>

<response>
  <users>
    <user>
      <id>1</id>
      <name>Hemant</name>
    </user>
  </users>
</response>

One more thing, when getUsers() is unchanged

public List<User> getUsers() {  
    /*if(null == users) {
        return Collections.emptyList();
    }*/
    return users;
}

Output is

<response/>

Answer 2

If you use the same model for Hibernate/DB and Jackson/XML you need to load appropriate Hibernate module to properly serialize persisted collections. For more information see jackson-datatype-hibernate .