java - PersistenceUnit [...] 的部署失败。关闭此 PersistenceUnit 的所有工厂

Question

我在调用 hentMeldinger() 时不断收到此错误；

public class Kontroll {
    private EntityManagerFactory emf;

    Kontroll() {
        emf = Persistence.createEntityManagerFactory("meldingsTing");
    }

    public List<Melding> hentMeldinger() {
        EntityManager em = emf.createEntityManager();

        TypedQuery<Melding> query = em.createQuery("SELECT o FROM Melding o",
        Melding.class);// FIKS
        List<Melding> meldinger = query.getResultList();

        em.close();

        return meldinger;
    }
}

异常报告:

message Exception [EclipseLink-28019] (Eclipse Persistence Services - 2.5.2.v20140319-9ad6abd): org.eclipse.persistence.exceptions.EntityManagerSetupException

description The server encountered an internal error that prevented it from fulfilling this request.

exception

javax.persistence.PersistenceException: Exception [EclipseLink-28019] (Eclipse Persistence Services - 2.5.2.v20140319-9ad6abd): org.eclipse.persistence.exceptions.EntityManagerSetupException
Exception Description: Deployment of PersistenceUnit [meldingsTing] failed. Close all factories for this PersistenceUnit.
Internal Exception: Exception [EclipseLink-0] (Eclipse Persistence Services - 2.5.2.v20140319-9ad6abd): org.eclipse.persistence.exceptions.IntegrityException
Descriptor Exceptions: 
---------------------------------------------------------

Exception [EclipseLink-63] (Eclipse Persistence Services - 2.5.2.v20140319-9ad6abd): org.eclipse.persistence.exceptions.DescriptorException
Exception Description: The instance creation method [no.hib.dat104.oblig1.Melding.<Default Constructor>], with no parameters, does not exist, or is not accessible.
Internal Exception: java.lang.NoSuchMethodException: no.hib.dat104.oblig1.Melding.<init>()
Descriptor: RelationalDescriptor(no.hib.dat104.oblig1.Melding --> [DatabaseTable(MELDING)])

Runtime Exceptions: 
---------------------------------------------------------

    org.eclipse.persistence.internal.jpa.EntityManagerSetupImpl.createDeployFailedPersistenceException(EntityManagerSetupImpl.java:820)
    org.eclipse.persistence.internal.jpa.EntityManagerSetupImpl.deploy(EntityManagerSetupImpl.java:760)
    org.eclipse.persistence.internal.jpa.EntityManagerFactoryDelegate.getAbstractSession(EntityManagerFactoryDelegate.java:204)
    org.eclipse.persistence.internal.jpa.EntityManagerFactoryDelegate.createEntityManagerImpl(EntityManagerFactoryDelegate.java:304)
    org.eclipse.persistence.internal.jpa.EntityManagerFactoryImpl.createEntityManagerImpl(EntityManagerFactoryImpl.java:336)
    org.eclipse.persistence.internal.jpa.EntityManagerFactoryImpl.createEntityManager(EntityManagerFactoryImpl.java:302)
    no.hib.dat104.oblig1.Kontroll.lagreMelding(Kontroll.java:32)
    no.hib.dat104.oblig1.MeldingServlet.doPost(MeldingServlet.java:83)
    javax.servlet.http.HttpServlet.service(HttpServlet.java:646)
    javax.servlet.http.HttpServlet.service(HttpServlet.java:727)
    org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:52)

根本原因:

Exception [EclipseLink-28019] (Eclipse Persistence Services - 2.5.2.v20140319-9ad6abd): org.eclipse.persistence.exceptions.EntityManagerSetupException
Exception Description: Deployment of PersistenceUnit [meldingsTing] failed. Close all factories for this PersistenceUnit.
Internal Exception: Exception [EclipseLink-0] (Eclipse Persistence Services - 2.5.2.v20140319-9ad6abd): org.eclipse.persistence.exceptions.IntegrityException
Descriptor Exceptions: 
---------------------------------------------------------

Exception [EclipseLink-63] (Eclipse Persistence Services - 2.5.2.v20140319-9ad6abd): org.eclipse.persistence.exceptions.DescriptorException
Exception Description: The instance creation method [no.hib.dat104.oblig1.Melding.<Default Constructor>], with no parameters, does not exist, or is not accessible.
Internal Exception: java.lang.NoSuchMethodException: no.hib.dat104.oblig1.Melding.<init>()
Descriptor: RelationalDescriptor(no.hib.dat104.oblig1.Melding --> [DatabaseTable(MELDING)])

Runtime Exceptions: 
---------------------------------------------------------

    org.eclipse.persistence.exceptions.EntityManagerSetupException.deployFailed(EntityManagerSetupException.java:238)
    org.eclipse.persistence.internal.jpa.EntityManagerSetupImpl.createDeployFailedPersistenceException(EntityManagerSetupImpl.java:820)
    org.eclipse.persistence.internal.jpa.EntityManagerSetupImpl.deploy(EntityManagerSetupImpl.java:760)
    org.eclipse.persistence.internal.jpa.EntityManagerFactoryDelegate.getAbstractSession(EntityManagerFactoryDelegate.java:204)
    org.eclipse.persistence.internal.jpa.EntityManagerFactoryDelegate.createEntityManagerImpl(EntityManagerFactoryDelegate.java:304)
    org.eclipse.persistence.internal.jpa.EntityManagerFactoryImpl.createEntityManagerImpl(EntityManagerFactoryImpl.java:336)
    org.eclipse.persistence.internal.jpa.EntityManagerFactoryImpl.createEntityManager(EntityManagerFactoryImpl.java:302)
    no.hib.dat104.oblig1.Kontroll.lagreMelding(Kontroll.java:32)
    no.hib.dat104.oblig1.MeldingServlet.doPost(MeldingServlet.java:83)
    javax.servlet.http.HttpServlet.service(HttpServlet.java:646)
    javax.servlet.http.HttpServlet.service(HttpServlet.java:727)
    org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:52)
root cause

Exception [EclipseLink-0] (Eclipse Persistence Services - 2.5.2.v20140319-9ad6abd): org.eclipse.persistence.exceptions.IntegrityException
Descriptor Exceptions: 
---------------------------------------------------------

Exception [EclipseLink-63] (Eclipse Persistence Services - 2.5.2.v20140319-9ad6abd): org.eclipse.persistence.exceptions.DescriptorException
Exception Description: The instance creation method [no.hib.dat104.oblig1.Melding.<Default Constructor>], with no parameters, does not exist, or is not accessible.
Internal Exception: java.lang.NoSuchMethodException: no.hib.dat104.oblig1.Melding.<init>()
Descriptor: RelationalDescriptor(no.hib.dat104.oblig1.Melding --> [DatabaseTable(MELDING)])

Runtime Exceptions: 
---------------------------------------------------------

    org.eclipse.persistence.internal.sessions.DatabaseSessionImpl.initializeDescriptors(DatabaseSessionImpl.java:696)
    org.eclipse.persistence.internal.sessions.DatabaseSessionImpl.initializeDescriptors(DatabaseSessionImpl.java:632)
    org.eclipse.persistence.internal.sessions.DatabaseSessionImpl.initializeDescriptors(DatabaseSessionImpl.java:568)
    org.eclipse.persistence.internal.sessions.DatabaseSessionImpl.postConnectDatasource(DatabaseSessionImpl.java:799)
    org.eclipse.persistence.internal.sessions.DatabaseSessionImpl.loginAndDetectDatasource(DatabaseSessionImpl.java:743)
    org.eclipse.persistence.internal.jpa.EntityManagerFactoryProvider.login(EntityManagerFactoryProvider.java:239)
    org.eclipse.persistence.internal.jpa.EntityManagerSetupImpl.deploy(EntityManagerSetupImpl.java:685)
    org.eclipse.persistence.internal.jpa.EntityManagerFactoryDelegate.getAbstractSession(EntityManagerFactoryDelegate.java:204)
    org.eclipse.persistence.internal.jpa.EntityManagerFactoryDelegate.createEntityManagerImpl(EntityManagerFactoryDelegate.java:304)
    org.eclipse.persistence.internal.jpa.EntityManagerFactoryImpl.createEntityManagerImpl(EntityManagerFactoryImpl.java:336)
    org.eclipse.persistence.internal.jpa.EntityManagerFactoryImpl.createEntityManager(EntityManagerFactoryImpl.java:302)
    no.hib.dat104.oblig1.Kontroll.lagreMelding(Kontroll.java:32)
    no.hib.dat104.oblig1.MeldingServlet.doPost(MeldingServlet.java:83)
    javax.servlet.http.HttpServlet.service(HttpServlet.java:646)
    javax.servlet.http.HttpServlet.service(HttpServlet.java:727)
    org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:52)

我的问题是，在查看此错误报告时，我不知道从哪里开始。我现在已经尝试了几个小时，但这对我来说毫无意义。如果有人能从中理解并解释，那就太棒了，但解决问题将是最好的。

Answer 1

Exception Description: The instance creation method [no.hib.dat104.oblig1.Melding.<Default Constructor>], with no parameters, does not exist, or is not accessible.

对于JPA实体，至少必须包含默认构造函数(无参数构造函数)，

public Melding() {
}