CREATE TABLE `customer` (
  `id` int(11) NOT NULL,
  `name` varchar(45) DEFAULT NULL,
  `passnr_fk` int(11) DEFAULT NULL,
   PRIMARY KEY (`id`)
)

CREATE TABLE `passport` (
  `pass_id` int(11) NOT NULL,
  `pass_nr` int(11) NOT NULL,
  `color` varchar(45) DEFAULT NULL,
  PRIMARY KEY (`pass_id`),
  UNIQUE KEY `pass_nr_UNIQUE` (`pass_nr`)
)

import hibernate.test.Customer;
import hibernate.test.Passport;
import org.hibernate.SessionFactory;
import org.hibernate.cfg.Configuration;
import org.hibernate.service.ServiceRegistry;
import org.hibernate.service.ServiceRegistryBuilder;

public class HibernateUtil {

    private static final SessionFactory sessionFactory = buildSessionFactory();

    private static SessionFactory buildSessionFactory() {
        try {
           Configuration config = new Configuration();
           config.addAnnotatedClass(Customer.class);
           config.addAnnotatedClass(Passport.class);
           config.configure("/resources/hibernate.cfg.xml");
           ServiceRegistry serviceRegistry = new ServiceRegistryBuilder().applySettings(config.getProperties()).buildServiceRegistry();
            return config.buildSessionFactory(serviceRegistry);
        }
        catch (Throwable ex) {
            // Make sure you log the exception, as it might be swallowed
            System.err.println("Initial SessionFactory creation failed." + ex);
            throw new ExceptionInInitializerError(ex);
        }
    }

    public static SessionFactory getSessionFactory() {
        return sessionFactory;
    }
}

<?xml version='1.0' encoding='utf-8'?>
<!DOCTYPE hibernate-configuration PUBLIC
        "-//Hibernate/Hibernate Configuration DTD 3.0//EN"
        "http://www.hibernate.org/dtd/hibernate-configuration-3.0.dtd">

<hibernate-configuration>

    <session-factory>

        <!-- Database connection settings -->
        <property name="connection.driver_class">com.mysql.jdbc.Driver</property>
        <property name="connection.url">jdbc:mysql://localhost:3306/test</property>
        <property name="connection.username">root</property>
        <property name="connection.password">root</property>

        <!-- JDBC connection pool (use the built-in) -->
        <property name="connection.pool_size">1</property>

        <!-- SQL dialect -->
        <property name="dialect">org.hibernate.dialect.MySQLDialect</property>

        <!-- Enable Hibernate's automatic session context management -->
        <property name="current_session_context_class">thread</property>

        <!-- Disable the second-level cache  -->
        <property name="cache.provider_class">org.hibernate.cache.internal.NoCacheProvider</property>

        <!-- Echo all executed SQL to stdout -->
        <property name="show_sql">true</property>

    </session-factory>

</hibernate-configuration>

import javax.persistence.Entity;
import javax.persistence.Id;
import javax.persistence.JoinColumn;
import javax.persistence.OneToOne;


@Entity
public class Customer {
   
   private Long id;
   private String name;
   private Passport passport;

   @Id
   public Long getId() {
      return id;
   }
   public void setId(Long id) {
      this.id = id;
   }
   public String getName() {
      return name;
   }
   public void setName(String name) {
      this.name = name;
   }
      
   @OneToOne()
   @JoinColumn(name="passnr_fk",referencedColumnName="pass_nr",unique=true)
   public Passport getPassport() {
      return passport;
   }
   public void setPassport(Passport passport) {
      this.passport = passport;
   }
}

import java.io.Serializable;
import javax.persistence.Column;
import javax.persistence.Entity;
import javax.persistence.Id;
import javax.persistence.OneToOne;

@Entity
public class Passport implements Serializable {
   /**
    *
    */
   private static final long serialVersionUID = 1L;
   private Long passid;
   private Long passnr;
   private String color;
   private Customer owner;
   
   @OneToOne(mappedBy="passport")
   public Customer getOwner() {
      return owner;
   }
   public void setOwner(Customer owner) {
      this.owner = owner;
   }
   
   @Id
   @Column(name="pass_id")
   public Long getPassid() {
      return passid;
   }
   public void setPassid(Long passid) {
      this.passid = passid;
   }

   @Column(name="pass_nr")
   public Long getPassnr() {
      return passnr;
   }
   public void setPassnr(Long passnr) {
      this.passnr = passnr;
   }
   public String getColor() {
      return color;
   }
   public void setColor(String color) {
      this.color = color;
   }
}

import hibernate.test.Customer;
import hibernate.util.HibernateUtil;
import org.hibernate.Session;
import org.hibernate.SessionFactory;


public class Test {
   public static void main(String[] args) {
      
      SessionFactory factory = HibernateUtil.getSessionFactory();
      Session session = factory.getCurrentSession();
      
      session.beginTransaction();
      Customer c1 = new Customer();
      session.load(c1,1L);
      session.getTransaction().commit();
      
      HibernateUtil.getSessionFactory().close();
      
      System.out.println(c1.getName());
      System.out.println(c1.getPassport().getColor());
   }
}

Hibernate: select customer0_.id as id0_1_, customer0_.name as name0_1_, customer0_.passnr_fk as passnr3_0_1_, passport1_.pass_id as pass1_1_0_, passport1_.color as color1_0_, passport1_.pass_nr as pass3_1_0_ from Customer customer0_ left outer join Passport passport1_ on customer0_.passnr_fk=passport1_.pass_nr where customer0_.id=?
jan 08, 2013 10:34:14 AM org.hibernate.property.BasicPropertyAccessor$BasicGetter get
ERROR: HHH000122: IllegalArgumentException in class: hibernate.test.Passport, getter method of property: passnr
jan 08, 2013 10:34:14 AM org.hibernate.event.internal.DefaultLoadEventListener onLoad
INFO: HHH000327: Error performing load command : org.hibernate.PropertyAccessException: IllegalArgumentException occurred calling getter of hibernate.test.Passport.passnr
Exception in thread "main" org.hibernate.PropertyAccessException: IllegalArgumentException occurred calling getter of hibernate.test.Passport.passnr
   at org.hibernate.property.BasicPropertyAccessor$BasicGetter.get(BasicPropertyAccessor.java:187)
   at org.hibernate.tuple.component.AbstractComponentTuplizer.getPropertyValue(AbstractComponentTuplizer.java:76)
   at org.hibernate.tuple.component.AbstractComponentTuplizer.getPropertyValues(AbstractComponentTuplizer.java:82)
   at org.hibernate.tuple.component.PojoComponentTuplizer.getPropertyValues(PojoComponentTuplizer.java:107)
   at org.hibernate.type.ComponentType.getPropertyValues(ComponentType.java:434)
   at org.hibernate.type.ComponentType.getHashCode(ComponentType.java:258)
   at org.hibernate.engine.spi.EntityUniqueKey.generateHashCode(EntityUniqueKey.java:85)
   at org.hibernate.engine.spi.EntityUniqueKey.<init>(EntityUniqueKey.java:66)
   at org.hibernate.type.EntityType.loadByUniqueKey(EntityType.java:690)
   at org.hibernate.type.EntityType.resolve(EntityType.java:474)
   at org.hibernate.engine.internal.TwoPhaseLoad.doInitializeEntity(TwoPhaseLoad.java:168)
   at org.hibernate.engine.internal.TwoPhaseLoad.initializeEntity(TwoPhaseLoad.java:134)
   at org.hibernate.loader.Loader.initializeEntitiesAndCollections(Loader.java:1103)
   at org.hibernate.loader.Loader.processResultSet(Loader.java:960)
   at org.hibernate.loader.Loader.doQuery(Loader.java:910)
   at org.hibernate.loader.Loader.doQueryAndInitializeNonLazyCollections(Loader.java:341)
   at org.hibernate.loader.Loader.doQueryAndInitializeNonLazyCollections(Loader.java:311)
   at org.hibernate.loader.Loader.loadEntity(Loader.java:2111)
   at org.hibernate.loader.entity.AbstractEntityLoader.load(AbstractEntityLoader.java:82)
   at org.hibernate.loader.entity.AbstractEntityLoader.load(AbstractEntityLoader.java:72)
   at org.hibernate.persister.entity.AbstractEntityPersister.load(AbstractEntityPersister.java:3887)
   at org.hibernate.event.internal.DefaultLoadEventListener.loadFromDatasource(DefaultLoadEventListener.java:458)
   at org.hibernate.event.internal.DefaultLoadEventListener.doLoad(DefaultLoadEventListener.java:427)
   at org.hibernate.event.internal.DefaultLoadEventListener.load(DefaultLoadEventListener.java:204)
   at org.hibernate.event.internal.DefaultLoadEventListener.proxyOrLoad(DefaultLoadEventListener.java:260)
   at org.hibernate.event.internal.DefaultLoadEventListener.onLoad(DefaultLoadEventListener.java:148)
   at org.hibernate.internal.SessionImpl.fireLoad(SessionImpl.java:1078)
   at org.hibernate.internal.SessionImpl.load(SessionImpl.java:962)
   at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
   at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source)
   at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
   at java.lang.reflect.Method.invoke(Unknown Source)
   at org.hibernate.context.internal.ThreadLocalSessionContext$TransactionProtectionWrapper.invoke(ThreadLocalSessionContext.java:352)
   at $Proxy9.load(Unknown Source)
   at Test.main(Test.java:17)
Caused by: java.lang.IllegalArgumentException: object is not an instance of declaring class
   at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
   at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source)
   at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
   at java.lang.reflect.Method.invoke(Unknown Source)
   at org.hibernate.property.BasicPropertyAccessor$BasicGetter.get(BasicPropertyAccessor.java:164)
   ... 34 more