How to resolve Object Manager has been closed error? - google-app-engine

Question

I'll appreciate if someone can point me to a tutorial or best practice on how to close
JDO connection.
I constantly get javax.jdo.JDOUserException: Object Manager has been closed error whenever I include the finally block.
My code is below:
public static List<AgentEntity> findAgentEntityByString(String id) {
List<AgentEntity> agententity = new ArrayList<AgentEntity>();
if (id == null) {
return null;
}
try {
Query q = pm.newQuery("select id from " + AgentEntity.class.getName());
agententity = (List<AgentEntity>) q.execute();
} catch(Exception ex) {
log.warning(ex.getMessage());
}
return agententity;
}
Regards

Answer 1

One possible solution to avoid this lazy loading problem is to use the size() method forcing the PersistenceManager object to load the result list from datastore before being closed.
public static List<AgentEntity> findAgentEntityByString(String id) {
List<AgentEntity> agententity = new ArrayList<AgentEntity>();
if (id == null) {
return null;
}
try {
Query q = pm.newQuery("select id from " + AgentEntity.class.getName());
agententity = (List<AgentEntity>) q.execute();
agententity.size() //Should populate the returned list
return agententity;
} finally {
pm.close();
}
}
Reference here.

Answer 2

Why do you want to close your PersistenceManager here ?
If you want to close the Query you should use either
javax.jdo.Query.closeAll() or javax.jdo.Query.close(Object result).
So you can do either a transient copy of the result and than close the query and its result:
public static List<AgentEntity> findAgentEntityByString(String id) {
if (id == null) {
return null;
}
Query q = null;
try {
q = pm.newQuery("select id from " + AgentEntity.class.getName());
return new ArrayList<AgentEntity>((List<AgentEntity>) q.execute());
} finally {
if(q!= null){
q.closeAll();
}
}
}
or you can close the result later explicitly:
public static List<AgentEntity> findAgentEntityByString(String id) {
if (id == null) {
return null;
}
Query q = pm.newQuery("select id from " + AgentEntity.class.getName());
return (List<AgentEntity>) q.execute();
}
}
....
List agents = X.findAgentEntityByString("Foobar");
....
pm.close(agents);

Answer 3

Try setting the fetch plan just after getting the PM, then setting it to all before you perform your query:
import javax.jdo.FetchPlan;
pm = PMF.get().getPersistenceManager();
FetchPlan fp = pm.getFetchPlan();
fp.setGroup(FetchPlan.ALL);

Answer 4

Actually, what fixed this for me is answered here:
Why do I get "Persistence Manager has been closed" exception
I had an instance reference to the persistence manager, I just made a local instance and all my errors fixed