How to create a cache for JPA Entities using an EJB

Question

Hello everybody! I have recently got started with JPA 2.0 (I use eclipseLink) and EJB 3.1 and have a problem to figure out how to best implement a cache for my JPA Entities using an EJB.
In the following I try to describe my problem.. I know it is a bit verbose, but hope somebody will help me.. (I highlighted in bold the core of my problem, in case you want to first decide if you can/want help and in the case spend another couple of minutes to understand the domain)
I have the following JPA Entities:
@Entity Genre{
private String name;
@OneToMany(mappedBy = "genre", cascade={CascadeType.MERGE, CascadeType.PERSIST})
private Collection<Novel> novels;
@Entity
class Novel{
@ManyToOne(cascade={CascadeType.MERGE, CascadeType.PERSIST})
private Genre genre;
private String titleUnique;
@OneToMany(mappedBy="novel", cascade={CascadeType.MERGE, CascadeType.PERSIST})
private Collection<NovelEdition> editions;
@Entity
class NovelEdition{
private String publisherNameUnique;
private String year;
@ManyToOne(optional=false, cascade={CascadeType.PERSIST, CascadeType.MERGE})
private Novel novel;
@ManyToOne(optional=false, cascade={CascadeType.MERGE, CascadeType.PERSIST})
private Catalog appearsInCatalog;
@Entity
class Catalog{
private String name;
@OneToMany(mappedBy = "appearsInCatalog", cascade = {CascadeType.MERGE, CascadeType.PERSIST})
private Collection<NovelEdition> novelsInCatalog;
The idea is to have several Novels, belonging each to a specific Genre, for which can exist more than an edition (different publisher, year, etc). For semplicity a NovelEdition can belong to just one Catalog, being such a Catalog represented by such a text file:
FILE 1:
Catalog: Name Of Catalog 1
"Title of Novel 1", "Genre1 name","Publisher1 Name", 2009
"Title of Novel 2", "Genre1 name","Pulisher2 Name", 2010
FILE 2:
Catalog: Name Of Catalog 2
"Title of Novel 1", "Genre1 name","Publisher2 Name", 2011
"Title of Novel 2", "Genre1 name","Pulisher1 Name", 2011
Each entity has associated a Stateless EJB that acts as a DAO, using a Transaction Scoped EntityManager. For example:
@Stateless
public class NovelDAO extends AbstractDAO<Novel> {
@PersistenceContext(unitName = "XXX")
private EntityManager em;
protected EntityManager getEntityManager() {
return em;
public NovelDAO() {
super(Novel.class);
//NovelDAO Specific methods
I am interested at when the catalog files are parsed and the corresponding entities are built (I usually read a whole batch of Catalogs at a time).
Being the parsing a String-driven procedure, I don't want to repeat actions like novelDAO.getByName("Title of Novel 1") so I would like to use a centralized cache for mappings of type String-Identifier->Entity object.
Currently I use +3 Objects+:
1) The file parser, which does something like:
final CatalogBuilder catalogBuilder = //JNDI Lookup
//for each file:
String catalogName = parseCatalogName(file);
catalogBuilder.setCatalogName(catalogName);
//For each novel edition
String title= parseNovelTitle();
String genre= parseGenre();
catalogBuilder.addNovelEdition(title, genre, publisher, year);
//End foreach
catalogBuilder.build();
2) The CatalogBuilder is a Stateful EJB which uses the Cache and gets re-initialized every time a new Catalog file is parsed and gets "removed" after a catalog is persisted.
@Stateful
public class CatalogBuilder {
@PersistenceContext(unitName = "XXX", type = PersistenceContextType.EXTENDED)
private EntityManager em;
@EJB
private Cache cache;
private Catalog catalog;
@PostConstruct
public void initialize() {
catalog = new Catalog();
catalog.setNovelsInCatalog(new ArrayList<NovelEdition>());
public void addNovelEdition(String title, String genreStr, String publisher, String year){
Genre genre = cache.findGenreCreateIfAbsent(genreStr);//##
Novel novel = cache.findNovelCreateIfAbsent(title, genre);//##
NovelEdition novEd = new NovelEdition();
novEd.setNovel(novel);
//novEd.set publisher year catalog
catalog.getNovelsInCatalog().add();
public void setCatalogName(String name) {
catalog.setName(name);
@Remove
public void build(){
em.merge(catalog);
3) Finally, the problematic bean: Cache. For CatalogBuilder I used an EXTENDED persistence context (which I need as the Parser executes several succesive transactions) together with a Stateful EJB; but in this case I am not really sure what I need. In fact, the cache:
Should stay in memory until the parser is finished with its job, but not longer (should not be a singleton) as the parsing is just a very particular activity which happens rarely.
Should keep all of the entities in context, and should return managed entities form mehtods marked with ##, otherwise the attempt to persist the catalog should fail (duplicated INSERTs)..
Should use the same persistence context as the CatalogBuilder.
What I have now is :
@Stateful
public class Cache {
@PersistenceContext(unitName = "XXX", type = PersistenceContextType.EXTENDED)
private EntityManager em;
@EJB
private sessionbean.GenreDAO genreDAO;
//DAOs for other cached entities
Map<String, Genre> genreName2Object=new TreeMap<String, Genre>();
@PostConstruct
public void initialize(){
for (Genre g: genreDAO.findAll()) {
genreName2Object.put(g.getName(), em.merge(g));
public Genre findGenreCreateIfAbsent(String genreName){
if (genreName2Object.containsKey(genreName){
return genreName2Object.get(genreName);
Genre g = new Genre();
g.setName();
g.setNovels(new ArrayList<Novel>());
genreDAO.persist(t);
genreName2Object.put(t.getIdentifier(), em.merge(t));
return t;
But honestly I couldn't find a solution which satisfies these 3 points at the same time. For example, using another stateful bean with an extended persistence context (PC) would work for the 1st parsed file, but I have no idea what should happen from the 2nd file on.. Indeed, for the 1st file the PC will be created and propagated from CatalogBuilder to Cache, which will then use the same PC. But after build() returns, the PC of CatalogBuilder should (I guess) be removed and re-created during the succesive parsing, although the PC of Cache should stay "alive": shouldn't in this case an exception being thrown? Another problem is what to do when the Cache bean is passivated. Currently I get the exception:
"passivateEJB(), Exception caught ->
java.io.IOException: java.io.IOException
at com.sun.ejb.base.io.IOUtils.serializeObject(IOUtils.java:101)
at com.sun.ejb.containers.util.cache.LruSessionCache.saveStateToStore(LruSessionCache.java:501)"
Hence, I have no Idea how to implement my cache.. Can you please tell me how would you solve the problem?
Many thanks!
Bye

Answer 1

Hi Chris,
thanks for your reply!
I've tried to add the following into persistence.xml (although I've read that eclipseLink uses L2 cache by default..):
<shared-cache-mode>ALL</shared-cache-mode>
Then I replaced the Cache bean with a stateless bean which has methods like
Genre findGenreCreateIfAbsent(String genreName){
Genre genre = genreDAO.findByName(genreName);
if (genre!=null){
return genre;
genre = //Build new genre object
genreDAO.persist(genre);
return genre;
As far as I undestood, the shared cache should automatically store the genre and avoid querying the DB multiple times for the same genre, but unfortunately this is not the case: if I use a FINE logging level, I see really a lot of SELECT queries, which I didn't see with my "home made" Cache...
I am really confused.. :(
Thanks again for helping + bye