Memcached and caching JPA entities

[nobullet]

Does anybody have some recommendations and rules about using relations
in JPA entities and caching these entities with Memcached?

I have entity User and City:

class User extends play.db.jpa.Model implements Serializable {

@ManyToOne(fetch = FetchType.LAZY)
private City city;

}

City is very simple: id and name (with Hibernate's @Index annotation).
When using Memcached to cache entities that has User reference I see
exception in logs:

Could not deserialize
java.lang.ClassCastException: cannot assign instance of
org.hibernate.proxy.pojo.javassist.SerializableProxy to field
models.user.User.city of type models.City in instance of
models.user.User
at java.io.ObjectStreamClass
$FieldReflector.setObjFieldValues(ObjectStreamClass.java:2032)
at java.io.ObjectStreamClass.setObjFieldValues(ObjectStreamClass.java:
1212)
at java.io.ObjectInputStream.defaultReadFields(ObjectInputStream.java:
1953)
at java.io.ObjectInputStream.readSerialData(ObjectInputStream.java:
1871)
at
java.io.ObjectInputStream.readOrdinaryObject(ObjectInputStream.java:
1753)
at java.io.ObjectInputStream.readObject0(ObjectInputStream.java:1329)
at java.io.ObjectInputStream.defaultReadFields(ObjectInputStream.java:
1947)
at java.io.ObjectInputStream.readSerialData(ObjectInputStream.java:
1871)
at
java.io.ObjectInputStream.readOrdinaryObject(ObjectInputStream.java:
1753)
at java.io.ObjectInputStream.readObject0(ObjectInputStream.java:1329)
at java.io.ObjectInputStream.readObject(ObjectInputStream.java:351)
at java.util.LinkedList.readObject(LinkedList.java:964)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at
sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:
39)
at
sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:
25)
at java.lang.reflect.Method.invoke(Method.java:597)
at java.io.ObjectStreamClass.invokeReadObject(ObjectStreamClass.java:
974)
at java.io.ObjectInputStream.readSerialData(ObjectInputStream.java:
1849)
at
java.io.ObjectInputStream.readOrdinaryObject(ObjectInputStream.java:
1753)
at java.io.ObjectInputStream.readObject0(ObjectInputStream.java:1329)
at java.io.ObjectInputStream.defaultReadFields(ObjectInputStream.java:
1947)
at java.io.ObjectInputStream.readSerialData(ObjectInputStream.java:
1871)
at
java.io.ObjectInputStream.readOrdinaryObject(ObjectInputStream.java:
1753)
at java.io.ObjectInputStream.readObject0(ObjectInputStream.java:1329)
at java.io.ObjectInputStream.readObject(ObjectInputStream.java:351)
at java.util.ArrayList.readObject(ArrayList.java:593)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at
sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:
39)
at
sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:
25)
at java.lang.reflect.Method.invoke(Method.java:597)
at java.io.ObjectStreamClass.invokeReadObject(ObjectStreamClass.java:
974)
at java.io.ObjectInputStream.readSerialData(ObjectInputStream.java:
1849)
at
java.io.ObjectInputStream.readOrdinaryObject(ObjectInputStream.java:
1753)
at java.io.ObjectInputStream.readObject0(ObjectInputStream.java:1329)
at java.io.ObjectInputStream.readObject(ObjectInputStream.java:351)
at play.cache.MemcachedImpl$1.deserialize(MemcachedImpl.java:43)
at
net.spy.memcached.transcoders.SerializingTranscoder.decode(SerializingTranscoder.java:
66)
at net.spy.memcached.transcoders.TranscodeService
$1.call(TranscodeService.java:36)
at java.util.concurrent.FutureTask$Sync.innerRun(FutureTask.java:303)
at java.util.concurrent.FutureTask.run(FutureTask.java:138)
at net.spy.memcached.transcoders.TranscodeService
$Task.run(TranscodeService.java:83)
at java.util.concurrent.ThreadPoolExecutor
$Worker.runTask(ThreadPoolExecutor.java:886)
at java.util.concurrent.ThreadPoolExecutor
$Worker.run(ThreadPoolExecutor.java:908)
at java.lang.Thread.run(Thread.java:637)

Do I need to skip using FetchType.LAZY with Memcached?
No problems with Ehcache (when Memcached is disabled).

[GrailsDeveloper]

Hi,
not exactly and answer to your question, but I wouldn't put a JPA-
Object into memcache. I guess it's cheaper to read it again from
database.
But it's only a design decision.

Furthermore I assume you must detach the object from the hibernate
session.

Regards
Niels

[nobullet]

Ehcache works on one JVM, Memcached server can be moved to separate
machine (it's possible to use Play cluster if to follow stateless
architecture).
Play examples advises to use Memcached: http://www.playframework.org/documentation/1.0.3/cache

The possible workaround is described here: https://jira.jboss.org/browse/JBAS-3952
:
You may define your relation as EAGER when possible...

But it's not perfect - joins, big datasets between jvm and database...
Big overhead.

It's possible not to use relation:
store Long city_id instead of City, and have setter and getter:

public void setCity(City city) {
this.city_id = city.getId();
}

public City getCity() {
return City.findById(this.city_id);
}

It works (even multiple calls to getCity() will return object from
Hibernate's session) but it is ugly.

Is there any nicer way?

On Jul 26, 2:09 pm, GrailsDeveloper <opensourc...@googlemail.com>
wrote: