I have these two classes:
@JsonIdentityInfo(generator = ObjectIdGenerators.PropertyGenerator.class, property = "id",scope = Rol.class)
public class Rol extends MyEntity implements Serializable {
private Integer id;
private String rolName;
public Rol(Integer id, String rolName) {
this.id = id;
this.rolName = rolName;
}
...
}
@JsonIdentityInfo(generator = ObjectIdGenerators.PropertyGenerator.class, property = "id",scope = User.class)
public class User extends MyEntity implements Serializable {
private Integer id;
private String name;
private List<Rol> rolList;
public User(Integer id, String name, List<Rol> rolList) {
this.id = id;
this.name = name;
this.rolList = rolList;
}
...
}
and I try to serialize and deserialize the user object as following
Rol rol1 = new Rol(1, "MyRol");
Rol rol2 = new Rol(1, "MyRol");
List<Rol> rolList = new ArrayList();
rolList.add(rol1);
rolList.add(rol2);
user = new User(1, "MyUser", rolList);
ObjectMapper mapper = new ObjectMapper();
String jsonString = mapper.writeValueAsString(user);
User userJson = mappe.readValue(jsonString, User.class);
and the JsonMappingException: Already had POJO for id is produced. Why?
When I review the json result of the serialization I see that the result is
{"id": 1,"name": "MyName","rolList": [{"id": 1,"rolName": "MyRol"},{"id": 1,"rolName": "MyRol"}]}
when the result should be
{"id": 1,"name": "MyName","rolList": [{"id": 1,"rolName": "MyRol"},1]}
because rol1 and rol2 are different instances of the same POJO identifier with id 1.
How can I avoid the JsonMappingException? In my project I have some different instances of the same POJO. I can guarantee that if the id's are equal -> objects are equal.
Excuse me for my bad English.
For anyone returning to this question, it looks like there's option to do this with a custom ObjectIdResolver in Jackson. You can specify this on the @JsonIdentityInfo annotation, eg :
@JsonIdentityInfo(generator = ObjectIdGenerators.PropertyGenerator.class, property = "name",
resolver = CustomObjectIdResolver.class)
Then perhaps wrap the normal SimpleObjectIdResolver class to get going and customise bindItem().
In my case I wanted to avoid overlapping objectIds, so cleared down the references when I started a new Something:
public class CustomObjectIdResolver implements ObjectIdResolver {
private ObjectIdResolver objectIdResolver;
public CustomObjectIdResolver() {
clearReferences();
}
@Override
public void bindItem(IdKey id, Object pojo) {
// Time to drop the references?
if (pojo instanceof Something)
clearReferences();
objectIdResolver.bindItem(id, pojo);
}
@Override
public Object resolveId(IdKey id) {
return objectIdResolver.resolveId(id);
}
@Override
public boolean canUseFor(ObjectIdResolver resolverType) {
return resolverType.getClass() == getClass();
}
@Override
public ObjectIdResolver newForDeserialization(Object context) {
return new CustomObjectIdResolver();
}
private void clearReferences() {
objectIdResolver = new SimpleObjectIdResolver();
}
}
Jackson expects in this case different id
for different class instances. There has been a previous discussion at github here . Overriding hashCode
and equals
will not help. Object references must match for equal id
.
Options
Rol
instances instead of making new ones with equal fields. As a bonus you will also save memory.@JsonIdentityInfo
The technical post webpages of this site follow the CC BY-SA 4.0 protocol. If you need to reprint, please indicate the site URL or the original address.Any question please contact:yoyou2525@163.com.