stackoverflow error when converting hibernate object to json

Question

I have a user object with the following hibernate mapping which is a many to many self join:

    <hibernate-mapping>
        <class name="User" table="User">
            <id name="id" type="int">
            <column name="userId" />
                <generator class="native" />
            </id>
            <set name="friends" table="User_Friend" 
                    inverse="false" lazy="true" cascade="all">
                <key column="userId"/>
                <many-to-many column="friendId" class="User" />
            </set>
            <set name="cars" table="Car" inverse="true" fetch="select" lazy="true">
            <key>
                <column name="userId" not-null="true" />
            </key>
            <one-to-many class="Car" />
        </set>
        </class>
    </hibernate-mapping>

The car mapping looks like the following:

<hibernate-mapping>
    <class name="Car" table="Car">
        <id name="id" type="int">
            <column name="carId" />
            <generator class="native" />
        </id>
        <set name="carStatuses" table="Car_Status" 
                inverse="true" lazy="true" fetch="select">
            <key>
                <column name="carId" not-null="true" />
            </key>
            <one-to-many class="CarStatus" />
        </set>
        <many-to-one name="user" 
        column="userId"
        not-null="true"/>

    </class>
</hibernate-mapping>

I retrieve the user object and then attempt to return it as a Restlet JSON Representation with this method:

public Representation getJSONRepresentationFromObject(User object) {


    JSONArray ja = new JSONArray();
    JSONObject jo = new JSONObject(object);

    ja.put(jo);

    JsonRepresentation jr = new JsonRepresentation(ja);
    jr.setCharacterSet(CharacterSet.UTF_8);

    return jr;
}

The problem is that I get a StackOverflowError :

WARNING: Exception or error caught in resource java.lang.StackOverflowError at sun.reflect.GeneratedMethodAccessor74.invoke(Unknown Source) at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25) at java.lang.reflect.Method.invoke(Method.java:597) at org.json.JSONObject.populateMap(JSONObject.java:988) at org.json.JSONObject.(JSONObject.java:272) at org.json.JSONObject.wrap(JSONObject.java:1587) at org.json.JSONArray.(JSONArray.java:158) at org.json.JSONObject.wrap(JSONObject.java:1569) at org.json.JSONObject.populateMap(JSONObject.java:990) at org.json.JSONObject.(JSONObject.java:272) at org.json.JSONObject.wrap(JSONObject.java:1587) at org.json.JSONArray.(JSONArray.java:158) at org.json.JSONObject.wrap(JSONObject.java:1569) at org.json.JSONObject.populateMap(JSONObject.java:990) at org.json.JSONObject.(JSONObject.java:272)

If I remove the cars set in the user mapping the error goes away and I am able to convert the user to json. Is there something wrong with the car mapping that throws it into an infinite loop?

Answer 1

The problem was that the Car object has a back reference to the User/ Friend object. I solved this problem here:

Hibernate Object not detaching

Answer : pull object from hibernate. Create separate java object. Loop through objects and populate newly created objects with hibernate object values.