Good way of managing database entity manager

Question

Following recommendations given to me in this thread

I am making my DAO accessors @Singleton, @Startup, @LocalBean

My question is on the good way to implement the entityManager that is supposed to execute the various queries.

Knowing that there are several Dao's, but they shall all point to the same entityManager (also created from a single entityManagerFactory), what's the best practice ?

This is so far my implementation, but I have a feeling that it is reeeeealy bad :)

Dao's

@Singleton
@LocalBean
@Startup
public class AircraftTypeDaoImpl extends DatabaseAccessor implements
        AircraftTypeDao {

    /** JPQL query to find a {@link AircraftType} given its OACI name */
    private static final String JPQL_FIND_BY_TYPE = "SELECT a FROM AircraftType a WHERE a.typeOACI=:typeOACI";

    /** JPQL query to find all {@link AircraftType} */
    private static final String JPQL_FIND_ALL = "SELECT a FROM AircraftType a";

    @Override
    public AircraftType find(String typeOACI) throws DAOException {
        AircraftType aircraftType = null;
        try {
            begin();
            Query findQuery = em.createQuery(JPQL_FIND_BY_TYPE).setParameter(
                    "typeOACI", typeOACI);
            aircraftType = (AircraftType) findQuery.getSingleResult();
        } catch (NoResultException e) {
            aircraftType = null;
            rollback();
        } catch (Exception e) {
            rollback();
            throw new DAOException(e);
        } finally {
            commit();
        }
        return aircraftType;
    }

    @Override
    public List<AircraftType> findAll() throws DAOException {
        List<AircraftType> types = null;
        try {
            begin();
            TypedQuery<AircraftType> findQuery = em.createQuery(JPQL_FIND_ALL,
                    AircraftType.class);
            types = findQuery.getResultList();
        } catch (NoResultException e) {
            types = new ArrayList<AircraftType>();
            rollback();
        } catch (Exception e) {
            rollback();
            throw new DAOException(e);
        } finally {
            commit();
        }
        return types;
    }

}

@Singleton
@LocalBean
@Startup
public class VariantDaoImpl extends DatabaseAccessor implements VariantDao {

    /** JPQL query to find a {@link Variant} given its variant name */
    private static final String JPQL_FIND_BY_NAME = "SELECT v FROM Variant v WHERE v.variantName=:variantName";

    /**
     * JPQL query to find all {@link Variant} given their associated
     * {@link AircraftType}
     */
    private static final String JPQL_FIND_BY_AC_TYPE = "SELECT v FROM Variant v WHERE v.type.typeOACI=:typeOACI";

    @Override
    public Variant find(String variantName) throws DAOException {
        Variant variant = null;
        try {
            begin();
            Query findQuery = em.createQuery(JPQL_FIND_BY_NAME).setParameter(
                    "variantName", variantName);
            variant = (Variant) findQuery.getSingleResult();
        } catch (NoResultException e) {
            variant = null;
            rollback();
        } catch (Exception e) {
            rollback();
            throw new DAOException(e);
        } finally {
            commit();
        }
        return variant;
    }

    @Override
    public List<Variant> find(AircraftType aircraftType) throws DAOException {
        List<Variant> variants = null;
        try {
            begin();
            TypedQuery<Variant> findQuery = em.createQuery(
                    JPQL_FIND_BY_AC_TYPE, Variant.class).setParameter(
                    "typeOACI", aircraftType.getTypeOACI());
            variants = findQuery.getResultList();
        } catch (NoResultException e) {
            variants = new ArrayList<Variant>();
            rollback();
        } catch (Exception e) {
            rollback();
            throw new DAOException(e);
        } finally {
            commit();
        }
        return variants;
    }

}

"mother class"

public abstract class DatabaseAccessor {

    protected EntityManager em;

    private EntityTransaction tx;

    public DatabaseAccessor() {
        em = Persistence.createEntityManagerFactory("database")
                .createEntityManager();
    }

    public void begin() {
        tx = em.getTransaction();
        tx.begin();
    }

    public void commit() {
        tx.commit();
    }

    public void rollback() {
        tx.rollback();
    }

}

EDIT

Okay, I simplified the mess

ALl my DAO's are now like that

@Singleton
@LocalBean
@Startup
public class AircraftTypeDaoImpl implements AircraftTypeDao {

    @PersistenceContext
    protected EntityManager em;

    /** JPQL query to find a {@link AircraftType} given its OACI name */
    private static final String JPQL_FIND_BY_TYPE = "SELECT a FROM AircraftType a WHERE a.typeOACI=:typeOACI";

    /** JPQL query to find all {@link AircraftType} */
    private static final String JPQL_FIND_ALL = "SELECT a FROM AircraftType a";

    @Override
    public AircraftType find(String typeOACI) throws DAOException {
        AircraftType aircraftType = null;
        try {
            Query findQuery = em.createQuery(JPQL_FIND_BY_TYPE).setParameter(
                    "typeOACI", typeOACI);
            aircraftType = (AircraftType) findQuery.getSingleResult();
        } catch (NoResultException e) {
            aircraftType = null;
        } catch (Exception e) {
            throw new DAOException(e);
        }
        return aircraftType;
    }

    @Override
    public List<AircraftType> findAll() throws DAOException {
        List<AircraftType> types = null;
        try {
            TypedQuery<AircraftType> findQuery = em.createQuery(JPQL_FIND_ALL,
                    AircraftType.class);
            types = findQuery.getResultList();
        } catch (NoResultException e) {
            types = new ArrayList<AircraftType>();
        } catch (Exception e) {
            throw new DAOException(e);
        }
        return types;
    }

}

My persistence.xml is like that

<?xml version="1.0" encoding="UTF-8"?>
<persistence version="2.0" xmlns="http://java.sun.com/xml/ns/persistence" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://java.sun.com/xml/ns/persistence http://java.sun.com/xml/ns/persistence/persistence_2_0.xsd">
    <persistence-unit name="flightfaq">
        <provider>org.hibernate.ejb.HibernatePersistence</provider>
        <properties>
            <property name="hibernate.archive.autodetection" value="class, hbm"/>
            <property name="hibernate.connection.username" value="root"/>
            <property name="hibernate.connection.password" value="password"/>
            <property name="hibernate.connection.url" value="jdbc:mysql://localhost:3306/flightfaq"/>
            <property name="hibernate.connection.driver_class" value="com.mysql.jdbc.Driver"/>
            <property name="hibernate.hbm2ddl.auto" value="update"/>
            <property name="hibernate.c3p0.min_size" value="5"/>
            <property name="hibernate.c3p0.max_size" value="20"/>
            <property name="hibernate.c3p0.timeout" value="300"/>
            <property name="hibernate.c3p0.max_statements" value="50"/>
            <property name="hibernate.c3p0.idle_test_period" value="3000"/>
            <property name="hibernate.show_sql" value="true"/>
            <property name="hibernate.format_sql" value="true"/>
            <property name="use_sql_comments" value="true"/>
            <property name="jadira.usertype.autoRegisterUserTypes" value="true"/>
        </properties>
    </persistence-unit>
</persistence>

When deploying the App on TomEE, I get

26 févr. 2013 21:01:41 org.apache.openejb.config.ConfigurationFactory configureApplication
INFO: Configuring enterprise application: /Users/valentine/jbossworkspace/.metadata/.plugins/org.eclipse.wst.server.core/tmp0/wtpwebapps/FlightFAQ
26 févr. 2013 21:01:43 org.apache.openejb.config.InitEjbDeployments deploy
INFO: Auto-deploying ejb CustomerDaoImpl: EjbDeployment(deployment-id=CustomerDaoImpl)
26 févr. 2013 21:01:43 org.apache.openejb.config.InitEjbDeployments deploy
INFO: Auto-deploying ejb AircraftTypeDaoImpl: EjbDeployment(deployment-id=AircraftTypeDaoImpl)
26 févr. 2013 21:01:43 org.apache.openejb.config.InitEjbDeployments deploy
INFO: Auto-deploying ejb VariantDaoImpl: EjbDeployment(deployment-id=VariantDaoImpl)
26 févr. 2013 21:01:43 org.apache.openejb.config.InitEjbDeployments deploy
INFO: Auto-deploying ejb FlightFaqUserDaoImpl: EjbDeployment(deployment-id=FlightFaqUserDaoImpl)
26 févr. 2013 21:01:43 org.apache.openejb.config.InitEjbDeployments deploy
INFO: Auto-deploying ejb AirportDaoImpl: EjbDeployment(deployment-id=AirportDaoImpl)
26 févr. 2013 21:01:43 org.apache.openejb.config.InitEjbDeployments deploy
INFO: Auto-deploying ejb LegDaoImpl: EjbDeployment(deployment-id=LegDaoImpl)
26 févr. 2013 21:01:43 org.apache.openejb.config.InitEjbDeployments deploy
INFO: Auto-deploying ejb MissionDaoImpl: EjbDeployment(deployment-id=MissionDaoImpl)
26 févr. 2013 21:01:43 org.apache.openejb.config.AutoConfig deploy
INFO: Configuring PersistenceUnit(name=flightfaq, provider=org.hibernate.ejb.HibernatePersistence)
26 févr. 2013 21:01:43 org.apache.openejb.config.AutoConfig setJtaDataSource
INFO: Adjusting PersistenceUnit flightfaq <jta-data-source> to Resource ID 'My DataSource' from 'null'
26 févr. 2013 21:01:43 org.apache.openejb.config.AutoConfig setNonJtaDataSource
INFO: Adjusting PersistenceUnit flightfaq <non-jta-data-source> to Resource ID 'My Unmanaged DataSource' from 'null'
26 févr. 2013 21:01:43 org.apache.openejb.config.AppInfoBuilder build
INFO: Enterprise application "/Users/valentine/jbossworkspace/.metadata/.plugins/org.eclipse.wst.server.core/tmp0/wtpwebapps/FlightFAQ" loaded.
26 févr. 2013 21:01:43 org.apache.openejb.assembler.classic.Assembler createApplication
INFO: Assembling app: /Users/valentine/jbossworkspace/.metadata/.plugins/org.eclipse.wst.server.core/tmp0/wtpwebapps/FlightFAQ
26 févr. 2013 21:01:44 org.hibernate.annotations.common.Version <clinit>
INFO: HCANN000001: Hibernate Commons Annotations {4.0.1.Final}
26 févr. 2013 21:01:44 org.hibernate.Version logVersion
INFO: HHH000412: Hibernate Core {4.1.9.Final}
26 févr. 2013 21:01:44 org.hibernate.cfg.Environment <clinit>
INFO: HHH000206: hibernate.properties not found
26 févr. 2013 21:01:44 org.hibernate.cfg.Environment buildBytecodeProvider
INFO: HHH000021: Bytecode provider name : javassist
26 févr. 2013 21:01:44 org.hibernate.ejb.Ejb3Configuration configure
INFO: HHH000204: Processing PersistenceUnitInfo [
    name: flightfaq
    ...]
26 févr. 2013 21:01:44 org.hibernate.service.jdbc.connections.internal.ConnectionProviderInitiator instantiateExplicitConnectionProvider
INFO: HHH000130: Instantiating explicit connection provider: org.hibernate.ejb.connection.InjectedDataSourceConnectionProvider
26 févr. 2013 21:02:05 com.sun.faces.mgbean.BeanManager addBean
ATTENTION: JSF1074 : Le bean géré nommé «startup» a déjà été enregistré.  Remplacement du type de classe du bean géré existant java.util.Date par java.util.Date.
26 févr. 2013 21:02:05 com.sun.faces.mgbean.BeanManager addBean
ATTENTION: JSF1074 : Le bean géré nommé «now» a déjà été enregistré.  Remplacement du type de classe du bean géré existant java.util.Date par java.util.Date.
26 févr. 2013 21:02:07 org.richfaces.application.InitializationListener logWarningWhenConnectionFactoryPresent
ATTENTION: JMS API was found on the classpath; if you want to enable RichFaces Push JMS integration, set context-param 'org.richfaces.push.jms.enabled' in web.xml
26 févr. 2013 21:02:07 org.apache.catalina.session.StandardManager doLoad
GRAVE: "ClassNotFoundException" lors du chargement de sessions persistantes: java.lang.ClassNotFoundException: com.flightfaq.dao.impl.AircraftTypeDaoImpl$LocalBeanProxy
java.lang.ClassNotFoundException: com.flightfaq.dao.impl.AircraftTypeDaoImpl$LocalBeanProxy
    at org.apache.catalina.loader.WebappClassLoader.loadClass(WebappClassLoader.java:1714)
    at org.apache.catalina.loader.WebappClassLoader.loadClass(WebappClassLoader.java:1559)
    at org.apache.tomee.catalina.LazyStopWebappClassLoader.loadClass(LazyStopWebappClassLoader.java:98)
    at java.lang.Class.forName0(Native Method)
    at java.lang.Class.forName(Class.java:249)
    at org.apache.catalina.util.CustomObjectInputStream.resolveClass(CustomObjectInputStream.java:76)
    at java.io.ObjectInputStream.readNonProxyDesc(ObjectInputStream.java:1572)
    at java.io.ObjectInputStream.readClassDesc(ObjectInputStream.java:1493)
    at java.io.ObjectInputStream.readOrdinaryObject(ObjectInputStream.java:1729)
    at java.io.ObjectInputStream.readObject0(ObjectInputStream.java:1326)
    at java.io.ObjectInputStream.defaultReadFields(ObjectInputStream.java:1950)
    at java.io.ObjectInputStream.readSerialData(ObjectInputStream.java:1874)
    at java.io.ObjectInputStream.readOrdinaryObject(ObjectInputStream.java:1756)
    at java.io.ObjectInputStream.readObject0(ObjectInputStream.java:1326)
    at java.io.ObjectInputStream.readObject(ObjectInputStream.java:348)
    at java.util.HashMap.readObject(HashMap.java:1030)
    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:969)
    at java.io.ObjectInputStream.readSerialData(ObjectInputStream.java:1852)
    at java.io.ObjectInputStream.readOrdinaryObject(ObjectInputStream.java:1756)
    at java.io.ObjectInputStream.readObject0(ObjectInputStream.java:1326)
    at java.io.ObjectInputStream.defaultReadFields(ObjectInputStream.java:1950)
    at java.io.ObjectInputStream.readSerialData(ObjectInputStream.java:1874)
    at java.io.ObjectInputStream.readOrdinaryObject(ObjectInputStream.java:1756)
    at java.io.ObjectInputStream.readObject0(ObjectInputStream.java:1326)
    at java.io.ObjectInputStream.readArray(ObjectInputStream.java:1664)
    at java.io.ObjectInputStream.readObject0(ObjectInputStream.java:1320)
    at java.io.ObjectInputStream.readObject(ObjectInputStream.java:348)
    at java.util.HashMap.readObject(HashMap.java:1030)
    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:969)
    at java.io.ObjectInputStream.readSerialData(ObjectInputStream.java:1852)
    at java.io.ObjectInputStream.readOrdinaryObject(ObjectInputStream.java:1756)
    at java.io.ObjectInputStream.readObject0(ObjectInputStream.java:1326)
    at java.io.ObjectInputStream.readArray(ObjectInputStream.java:1664)
    at java.io.ObjectInputStream.readObject0(ObjectInputStream.java:1320)
    at java.io.ObjectInputStream.readObject(ObjectInputStream.java:348)
    at java.util.HashMap.readObject(HashMap.java:1030)
    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:969)
    at java.io.ObjectInputStream.readSerialData(ObjectInputStream.java:1852)
    at java.io.ObjectInputStream.readOrdinaryObject(ObjectInputStream.java:1756)
    at java.io.ObjectInputStream.readObject0(ObjectInputStream.java:1326)
    at java.io.ObjectInputStream.readObject(ObjectInputStream.java:348)
    at java.util.HashMap.readObject(HashMap.java:1030)
    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:969)
    at java.io.ObjectInputStream.readSerialData(ObjectInputStream.java:1852)
    at java.io.ObjectInputStream.readOrdinaryObject(ObjectInputStream.java:1756)
    at java.io.ObjectInputStream.readObject0(ObjectInputStream.java:1326)
    at java.io.ObjectInputStream.readObject(ObjectInputStream.java:348)
    at org.apache.catalina.session.StandardSession.readObject(StandardSession.java:1595)
    at org.apache.catalina.session.StandardSession.readObjectData(StandardSession.java:1060)
    at org.apache.catalina.session.StandardManager.doLoad(StandardManager.java:284)
    at org.apache.catalina.session.StandardManager.load(StandardManager.java:204)
    at org.apache.catalina.session.StandardManager.startInternal(StandardManager.java:491)
    at org.apache.catalina.util.LifecycleBase.start(LifecycleBase.java:150)
    at org.apache.catalina.core.StandardContext.startInternal(StandardContext.java:5294)
    at org.apache.catalina.util.LifecycleBase.start(LifecycleBase.java:150)
    at org.apache.catalina.core.ContainerBase$StartChild.call(ContainerBase.java:1559)
    at org.apache.catalina.core.ContainerBase$StartChild.call(ContainerBase.java:1549)
    at java.util.concurrent.FutureTask$Sync.innerRun(FutureTask.java:303)
    at java.util.concurrent.FutureTask.run(FutureTask.java:138)
    at java.util.concurrent.ThreadPoolExecutor$Worker.runTask(ThreadPoolExecutor.java:895)
    at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:918)
    at java.lang.Thread.run(Thread.java:680)
26 févr. 2013 21:02:07 org.apache.catalina.session.StandardManager startInternal
GRAVE: Exception au chargement des sessions depuis le stockage persistant (persistent storage)
java.lang.ClassNotFoundException: com.flightfaq.dao.impl.AircraftTypeDaoImpl$LocalBeanProxy
    at org.apache.catalina.loader.WebappClassLoader.loadClass(WebappClassLoader.java:1714)
    at org.apache.catalina.loader.WebappClassLoader.loadClass(WebappClassLoader.java:1559)
    at org.apache.tomee.catalina.LazyStopWebappClassLoader.loadClass(LazyStopWebappClassLoader.java:98)
    at java.lang.Class.forName0(Native Method)
    at java.lang.Class.forName(Class.java:249)
    at org.apache.catalina.util.CustomObjectInputStream.resolveClass(CustomObjectInputStream.java:76)
    at java.io.ObjectInputStream.readNonProxyDesc(ObjectInputStream.java:1572)
    at java.io.ObjectInputStream.readClassDesc(ObjectInputStream.java:1493)
    at java.io.ObjectInputStream.readOrdinaryObject(ObjectInputStream.java:1729)
    at java.io.ObjectInputStream.readObject0(ObjectInputStream.java:1326)
    at java.io.ObjectInputStream.defaultReadFields(ObjectInputStream.java:1950)
    at java.io.ObjectInputStream.readSerialData(ObjectInputStream.java:1874)
    at java.io.ObjectInputStream.readOrdinaryObject(ObjectInputStream.java:1756)
    at java.io.ObjectInputStream.readObject0(ObjectInputStream.java:1326)
    at java.io.ObjectInputStream.readObject(ObjectInputStream.java:348)
    at java.util.HashMap.readObject(HashMap.java:1030)
    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:969)
    at java.io.ObjectInputStream.readSerialData(ObjectInputStream.java:1852)
    at java.io.ObjectInputStream.readOrdinaryObject(ObjectInputStream.java:1756)
    at java.io.ObjectInputStream.readObject0(ObjectInputStream.java:1326)
    at java.io.ObjectInputStream.defaultReadFields(ObjectInputStream.java:1950)
    at java.io.ObjectInputStream.readSerialData(ObjectInputStream.java:1874)
    at java.io.ObjectInputStream.readOrdinaryObject(ObjectInputStream.java:1756)
    at java.io.ObjectInputStream.readObject0(ObjectInputStream.java:1326)
    at java.io.ObjectInputStream.readArray(ObjectInputStream.java:1664)
    at java.io.ObjectInputStream.readObject0(ObjectInputStream.java:1320)
    at java.io.ObjectInputStream.readObject(ObjectInputStream.java:348)
    at java.util.HashMap.readObject(HashMap.java:1030)
    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:969)
    at java.io.ObjectInputStream.readSerialData(ObjectInputStream.java:1852)
    at java.io.ObjectInputStream.readOrdinaryObject(ObjectInputStream.java:1756)
    at java.io.ObjectInputStream.readObject0(ObjectInputStream.java:1326)
    at java.io.ObjectInputStream.readArray(ObjectInputStream.java:1664)
    at java.io.ObjectInputStream.readObject0(ObjectInputStream.java:1320)
    at java.io.ObjectInputStream.readObject(ObjectInputStream.java:348)
    at java.util.HashMap.readObject(HashMap.java:1030)
    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:969)
    at java.io.ObjectInputStream.readSerialData(ObjectInputStream.java:1852)
    at java.io.ObjectInputStream.readOrdinaryObject(ObjectInputStream.java:1756)
    at java.io.ObjectInputStream.readObject0(ObjectInputStream.java:1326)
    at java.io.ObjectInputStream.readObject(ObjectInputStream.java:348)
    at java.util.HashMap.readObject(HashMap.java:1030)
    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:969)
    at java.io.ObjectInputStream.readSerialData(ObjectInputStream.java:1852)
    at java.io.ObjectInputStream.readOrdinaryObject(ObjectInputStream.java:1756)
    at java.io.ObjectInputStream.readObject0(ObjectInputStream.java:1326)
    at java.io.ObjectInputStream.readObject(ObjectInputStream.java:348)
    at org.apache.catalina.session.StandardSession.readObject(StandardSession.java:1595)
    at org.apache.catalina.session.StandardSession.readObjectData(StandardSession.java:1060)
    at org.apache.catalina.session.StandardManager.doLoad(StandardManager.java:284)
    at org.apache.catalina.session.StandardManager.load(StandardManager.java:204)
    at org.apache.catalina.session.StandardManager.startInternal(StandardManager.java:491)
    at org.apache.catalina.util.LifecycleBase.start(LifecycleBase.java:150)
    at org.apache.catalina.core.StandardContext.startInternal(StandardContext.java:5294)
    at org.apache.catalina.util.LifecycleBase.start(LifecycleBase.java:150)
    at org.apache.catalina.core.ContainerBase$StartChild.call(ContainerBase.java:1559)
    at org.apache.catalina.core.ContainerBase$StartChild.call(ContainerBase.java:1549)
    at java.util.concurrent.FutureTask$Sync.innerRun(FutureTask.java:303)
    at java.util.concurrent.FutureTask.run(FutureTask.java:138)
    at java.util.concurrent.ThreadPoolExecutor$Worker.runTask(ThreadPoolExecutor.java:895)
    at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:918)
    at java.lang.Thread.run(Thread.java:680)

It seems that the various DAO's a deploying correctly, but then there is a class not found exception on the LocalProxy version of my Dao ? Could you explain ? Is there a problem on the webserver ? How can I check if it is compatible with that kind of manipulation (@PersistenceContext, etc etc)

score 1 · Answer 1 · answered Feb 26 '13 at 19:08

It looks a bit like self rolled partial implementation of something that comes out of the box with JTA transactions and container managed persistence context.

Additionally using Singleton with default concurrency (container managed, WRITE) in this context can also cause serious performance hit, because it limits concurrent execution of methods. Database queries can take relatively long time, and likely there is no reason to execute only one of them at time. As said in the EJB 3.1 specification:

By default, a singleton bean has container managed concurrency demarcation if the concurrency management type is not specified.
...
If the concurrency locking attribute is not specified, it is assumed to be Lock(WRITE). The absence of a concurrency attribute specification on the bean class is equivalent to the specification of Lock(WRITE)on the bean class.
...
If the container invokes a method associated with a Write lock, no other concurrent invocations will be allowed to proceed until the initial Write method’s processing completes.

If there is no specific reason to not to use container managed EntityManager, following excerpt from the Java EE 6 Tutorial explains why there is no need to pass single instance of EntityManager around with container managed EntityManager:

JTA transactions usually involve calls across application components. To complete a JTA transaction, these components usually need access to a single persistence context. This occurs when an EntityManager is injected into the application components by means of the javax.persistence.PersistenceContext annotation. The persistence context is automatically propagated with the current JTA transaction, and EntityManager references that are mapped to the same persistence unit provide access to the persistence context within that transaction. By automatically propagating the persistence context, application components don’t need to pass references to EntityManager instances to each other in order to make changes within a single transaction. The Java EE container manages the lifecycle of container-managed entity managers.

i'd like to point out that with the change described in the EDIT, the entity manager is not able to execute queries like it was before. A simple query such as "SELECT a FROM AircraftType a" returns an empty list — facewindu, Feb 26 '13 at 21:28
I'm closing this thread because it's not reproducible now. I don't know what happened. BUT I created [this thread](http://stackoverflow.com/questions/15106627/persistencecontext-makes-queries-empty) because the queries are all empty now. — facewindu, Feb 27 '13 at 07:45

Good way of managing database entity manager

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