How can I create bi-directional mappings with MyBatis 3.0.5? - mybatis

Question

Say I have two domain objects and a mapper interface.
class Person {
int id;
List<Problem> problems = new ArrayList<Problem>();
}
class Problem {
int id;
Person person;
}
interface PersonMapper {
public List<Person> selectAllPersons();
}
And two database tables.
create table person (
id integer not null generated always as identity constraint person_pk primary key,
)
create table problem (
id integer not null generated always as identity constraint problem_pk primary key,
person_id integer not null constraint problem_person_fk references person
)
I can create a mapping file that gets the data I want.
<resultMap id="personMap" type="Person">
<id column="person_id" property="id" />
<collection column="problem_person_id" property="problems"
javaType="ArrayList" ofType="Problem" resultMap="problemMap" />
</resultMap>
<resultMap id="problemMap" type="Problem">
<id column="problem_id" property="id" />
<!-- Adding an association here will cause a circular dependency -->
<!-- The circular dependency results in a StackOverflowException -->
</resultMap>
<select id="selectAllPersons" resultMap="personMap">
select
person.id as person_id,
problem.id as problem_id
from person left outer join problem on person.id = problem.person_id
</select>
However, since MyBatis doesn't do bi-directional mapping, none of the Problem objects in the returned collections will have their Person reference set correctly.
According to this issue, it sounds like I should be able to update my mapper interface and add a custom result handler that can be supplied by the calling class.
interface PersonMapper {
public List<Person> selectAllPersons(ResultHandler handler);
}
class PersonResultHandler implements ResultHandler {
#Override
public void handleResult(ResultContext context) {
System.out.println(context.getResultObject());
}
}
class PersonDAO {
// Get SqlSession sqlSession
sqlSession.getMapper(PersonMapper.class).selectAllPersons(new PersonResultHandler());
}
However, the handleResult method of my ResultHandler never gets called. I've seen this example, but the extra fluff class in there makes it very difficult to understand. Can anyone give me a simple example of using a custom ResultHandler with mapper interfaces? I'm using MyBatis 3.0.5+.
I've also read through the MyBatis mailing list and there are several suggestions of using caching and lazy loading to solve circular dependencies, but I can't find any examples of how to do it.

Answer 1

You should replace your method declaration to:
interface PersonMapper {
public void selectAllPersons(ResultHandler handler);
}
And populate List<Person> inside your PersonResultHandler
class PersonResultHandler implements ResultHandler {
List<Person> persons = new ArrayList<Person>();
#Override
public void handleResult(ResultContext context) {
Object result = context.getResultObject();
if (result instanceof Person) {
Person person = (Person) result;
for (Problem problem : person.getProblems()) {
problem.setPerson(person);
}
persons.add(person);
}
}
public List<Person> getPersons() {
return persons;
}
}