java - How to use JDO persistence manager? -

i have 2 questions regarding how create / use jdo persistence manager (pm, hereafter).

say, in java web application, if have 10 entities, can logically grouped 2 groups (for example, 5 user related entities , 5 business related entities)

1. should need 2 different pms manage these 2 groups or 1 pm enough?
2. regarding initialization, shall use singleton instance of pm (which shared user's using app @ given point of time) or should create pm each , every session?

according jdo documentation create 1 persistencemanagerfactory per datastore. if using jdo access databases via sql , have more 1 database, need 1 persistencemanagerfactory per database (since need specify jdbc url, user name , password when create persistencemanagerfactory).

for simple use cases, can create persistencemanager when need , close in finally clause (see the persistence manager documentation).

if use transactions, , code updating entities can spread across multiple methods or objects, recommend creating persistencemanager on demand , storing in threadlocal (or request-scoped object if use guice or spring). make sure code updates participates in current transaction. make sure close persistencemanager @ end of request.

if need 1 persistence manager factory, can do:

public class datastore {   private static persistencemanagerfactory pmf;   private static final threadlocal<persistencemanager> per_thread_pm       = new threadlocal<persistencemanager>();    public static void initialize() {      if (pmf != null) {        throw new illegalstateexception("initialize() called");      }      pmf = jdohelper.getpersistencemanagerfactory("jdo.properties");   }    public static persistencemanager getpersistencemanager() {     persistencemanager pm = per_thread_pm.get();     if (pm == null) {       pm = pmf.getpersistencemanager();       per_thread_pm.set(pm);     }     return pm;   }    public static void finishrequest() {     persistencemanager pm = per_thread_pm.get();     if (pm != null) {       per_thread_pm.remove();       transaction tx = pm.currenttransaction();       if (tx.isactive()) {          tx.rollback();       }       pm.close();     }   } } 

any code needs persistence manager can call datastore.getpersistencemanager()

note: used static methods make simple purposes of answering question. if using dependency-injection framework guice, make methods non-static , bind datastore singleton.

you call finishrequest in servlet filter:

public class persistencemanagerfilter implements javax.servlet.filter {    public init(filterconfig filterconfig) {     datastore.initialize();   }    public void dofilter(servletrequest request,        servletresponse response, filterchain chain)       throws ioexception, servletexception {     try {       chain.dofilter(request, response);     } {       datastore.finishrequest();     }       } }