GAE, JDO, Jcache: can't put a list of entities to the cache

Question

I have a query which returns a list of entities. When I try to put this list to the cache, I get the familiar java.lang.IllegalArgumentException.

However... I can put every item inside the list to the cache with no problems. Even when I create a new LinkedList, copy entities from the fetched list to this new one using for cycle and try to put it to the cache, it also works.

List<MyEntity> a = (List<MyEntity>) q.execute(getKey());

List<MyEntity> b = new LinkedList<MyEntity>();
for (MyEntity e : a)
        b.add(e);

cache.put(key, b); // this works
cache.put(key, a); // this doesn't

So what I am missing? Caching other queries works, I don't know why this one is differen.

Answer 1

Most probably List returned by JDO query is not serializable, because it's a dynamic proxy.

To make a copy you can simply use

List<MyEntity> b = new ArrayList<MyEntity>(a);

Note: this makes a new list a from elements of b, but is does not make a copy of elements, so it's pretty lightweight.